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The microstructure, mineral composition, total organic carbon content, etc., of gas shale are crucial parameters for shale reservoirs, which can directly/indirectly affect shale brittleness, fracturing effect, adsorption ability and production efficiency. The study proposed a workflow to characterize the physical and mechanical parameters of Lower Silurian Longmaxi shale outcrop samples extracted from the favorable block in Changning, Sichuan, southwest China. This study elaborated on the influence of these physical and mechanical characteristics and proposed a corresponding brittleness index on shale extraction. In addition, it put forward corresponding suggestions for development and risk control. For a better understanding the mechanisms of shale gas storage and production, XRD, XRF, SEM, low temperature Nitrogen adsorption method, nuclear magnetic resonance and other measurements were employed to analyze and study the mineral composition, microstructure, and adsorption performance of shale. The results demonstrated that the pores of shale are mainly slit pores; there are diverse pore types in shale, mainly including intergranular pores, mineral particle dissolution pores, and internal pores of organic matter; The samples with relatively low porosity also noticeably exhibit ultra-low permeability, and the nanopore structure is remarkably significant, with distribution primarily in range of 5-237 nm. Finally, a brittleness index considering the influence of water content and the mechanical properties was proposed, and the coupling interaction of various minerals components and mechanical properties on the brittleness index can more objectively reflect the brittleness characteristics of deep shale formation. [ABSTRACT FROM AUTHOR]

Tight glutenite reservoirs characterization and effective hydrocarbon-bearing formation identification faced great challenge due to ultra-low porosity, ultra-low permeability and complicated pore structure. Fracturing fracture-building technique always needed to obtain deliverability because of poor natural productive capacity. Pore structure characterization and friability prediction were essential in improving such type of reservoir evaluation. In this study, fractured tight glutenite reservoirs in Permian Jiamuhe Formation that located in northwest margin of Junggar Basin, northwest China, were chosen as an example, and 25 typical core samples were drilled and simultaneously applied for mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR) and whole-rock mineral X-ray diffraction experiments. A novel method of synthetizing pseudo-pore-throat radius (Rc) distribution from porosity frequency spectra was established to characterize fractured formation pore structure. Quartz and calcite were considered as the fragile mineral, and rock mineral component ratio method was used to predict brittleness index. Meanwhile, the statistical model raised by Jin et al. (SPE J 20:518–526, 2015) was used to predict two types of fracture toughness. And then, brittleness index and fracture toughness were combined to characterize tight glutenite reservoirs friability. Combining with maximal pore-throat radius (Rmax, reflected rock pore structure) and friability, our target formations were classified into four clusters. In addition, relationships among pore structure, friability and daily hydrocarbon production per meter (DI) were analyzed, and a model to predict DI from well-logging data was established. Comparison of predicted DI with the extracted results from drill stem test (DST) data illustrated the reliability of our raised models. This would be valuable in determining optimal hydrocarbon production intervals and formulating reasonable developed plans. [ABSTRACT FROM AUTHOR]

The time-dependent characteristics of reservoir performance, where deep shale (vertical depth > 3500 m) interacts with fracturing fluid at high temperature and pressure (HTHP) during or after hydraulic fracturing, are closely related to the optimal design of fracturing in adjacent sections. However, the available information on the effect of hydration under HTHP on shale properties is insufficient, especially since the interaction between fracturing fluid and shales with different mineral compositions has rarely been considered comprehensively. In this study, fluid–shale interactions under HTHP were investigated for typical marine shales with different mineral contents (clay-rich, quartz-rich, and carbonate-rich) collected from the Wufeng–Longmaxi Formation in the southern Sichuan Basin, China. The time-dependent characteristics of the mechanical, brittleness, and fracture behaviors of immersion-treated samples at different soaking times were characterized by triaxial compression tests, and X-ray microcomputed tomography (CT) scans of post-test samples. The results show that the evolution of the mechanical parameters with soaking time is different for the three types of shales. The effect of hydration under HTHP on the strength of clay-rich shale and carbonate-rich shale except for the 15-day soaking case ranges from 39.24% to 54.88% and 24.05% to 33.99%, respectively, while the effect on quartz-rich shale is relatively slight, within 20%. The effect of hydration on the shale brittleness was evaluated based on a three-part energy-based brittleness index, in descending order, clay-rich shale > carbonate-rich shale > quartz-rich shale. The crack initiation and propagation during the deformation and failure of the soaked sample are more likely to extend along the direction of the hydration crack. For cases exhibiting quasi-ductile characteristics, hydration-induced microcracks may extend over almost the entire sample, allowing locally concentrated stresses in the immersed sample to be dispersed into each crack during deformation. The experimental results can provide data support and theoretical guidance for fracturing design in deep shale gas exploration in southern Sichuan. Highlights: The evolution of the mechanical and brittleness parameters of THMC-treated shale with soaking time was characterized quantitatively. A three-part energy-based brittleness index, where each part has a well-defined physical meaning related to brittle failure, was used to characterize the shale brittleness at different soaking times. The effect of soaking time and mineral composition on the hydration-induced deterioration of shale was investigated. The deterioration mechanism of THMC-treated shales with different mineral compositions (clay-rich, carbonate-rich, and quartz-rich) was revealed. [ABSTRACT FROM AUTHOR]

Mineralogy-based brittleness index is the simplest and most widely used index in characterizing the brittle property of shale gas/oil reservoirs. Traditionally, mineral compositions are usually determined by laboratory X-ray diffraction (XRD) analysis or interpreted from element logging, among which the former is time-consuming and non-continuous while the latter is costly and not timely. In this paper, a new rapid and economical method is proposed to obtain the mineral composition of shale from a statistical perspective, in the case of the Wufeng-Longmaxi shale, from the southeastern Sichuan Basin, China. The targeted shale cores from one scientific exploration well were scanned in situ to obtain the elemental data by using a non-destructive portable X-ray fluorescence (XRF) and sampled at each scanned site for laboratory XRD analysis to acquire the mineral composition data. Then we established the conversion models for shale with various sub-lithologies (including calcareous shale, sandy shale, and carbonaceous shale) based on a series of multiple regression analyses. These models were verified with the good consistency between predicted and tested mineral compositions of shale cores from another scientific exploration well. The mineralogy-based brittleness characterization results also indicated small differences between the predicted and tested brittleness index values, with the brittleness index curves matching well with the variations of experimental total organic carbon (TOC) with depth and conventional geophysical logging curves. The findings shed light on the application of portable XRF in determining mineral compositions and further the brittleness index of shale, which is essential for fracability evaluation and hydraulic fracturing design of shale gas/oil reservoirs. Highlights. Highlights: Conversion models from elements to minerals for various sub-lithologic shales were established. Brittleness indexes based on converted mineral compositions were closely approximate to the experimentally tested values. A rapid method to determine the brittleness of shale by using a non-destructive portable X-ray fluorescence was proposed. The method from elements to minerals to brittleness was successfully applied in the Wufeng-Longmaxi shale from the southeastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

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At present, hydrofracture technology is mainly used for shale gas exploration and exploitation, and the effect of volume fracturing is tightly linked to the mechanical properties (including anisotropy and brittleness) of shale. Meanwhile, Sichuan Basin is also an important area for unconventional natural gas exploration and development in China. This work analyzed mineral composition and microstructure of outcrop shale matrix in Lower Silurian Longmaxi Formation through XRD test, slice observation, and scanning electron microscope (SEM). How anisotropy affects shale deformation, strength and failure characteristics at different bedding angles (0°, 30°, 45°, 60°, and 90°) under diverse confining pressures (0 MPa, 25 MPa, 50 MPa, 75 MPa, and 100 MPa) in conventional uniaxial/triaxial experiments were discussed. The acoustic emission (AE) evolution was introduced to reveal the cracks closure and propagation of Longmaxi Formation shale and qualitatively characterized the brittleness of the shale. What's more, based on the existing brittleness index, this study proposed an indicator BInew that considers the shape of the stress–strain curve before and after the peak stress to evaluate the brittleness of the Longmaxi Formation shale. The index BInew can better describe the changes of brittleness with confining pressure and bedding angle. Finally, this work elaborately explained the relationship between AE characteristics, brittleness, and failure modes of shale. AE characteristics can not only qualitatively characterize shale brittleness but also show the failure process and modes of shale samples. Change of failure modes rests with the difference in brittleness and also reflects the level of brittleness. The change of brittleness with failure modes can be expressed as shear along bedding plane (Sh-Al) > splitting through bedding plane (Sp-Th) > conjugate shear failure (Co-Sh) > shear along bedding plane (Sh-Al) > shear through bedding plane (Sh-Th). Highlights: Uniaxial/triaxial compression and acoustic emission (AE) monitoring of Longmaxi formation shale were carried out. Influence of anisotropy of shale on mechanical properties under different confining pressures was analyzed. Different brittleness indexes were used to evaluate the brittleness of shale, and a new brittleness index was established. Relationship between AE characteristics, brittleness, and failure modes of shale was discussed in detail. [ABSTRACT FROM AUTHOR]

Highlights: 1. How to quantitative evaluation of mechanical properties of shale in Da'anzhai section combined with triaxial mechanical experiments? The compressive strength of dry rock samples and soaked rock samples were tested by triaxial mechanical experiments. According to the stress-strain curves, the changes of compressive strength, elastic modulus and Poisson's ratio before and after hydration were analyzed. 2. How to establish a mathematical model of shale fracability that comprehensively considers brittleness index, horizontal differential stress, and fracture toughness is proposed? Fracability index is proportional to brittleness index and horizontal differential stress, and inversely proportional to fracture toughness. Young 's modulus, Poisson's ratio and tensile strength in fracability index model are related to rock water saturation, so the fracturing index is also related to rock hydration. 3. What is the effect of shale hydration on fracability and fracture pressure? As the water saturation rate of the rock gradually decreases, the FI of the formation rock de-creases Combined with the variation law of water saturation rate, it shows that after the shale around the well is hydrated, the fracture pressure of the formation decreases Accurate evaluation of shale oil reservoir fracability helps avoid blind fracturing and ensures efficient fracturing. However, the current evaluation of the fracability index rarely considers the impact of hydration caused by drilling fluid invasion during drilling. The results of a rock triaxial mechanical test conducted to evaluate the mechanical properties of shale oil reservoirs are reported in this paper. Based on the results, we developed a comprehensive evaluation method of shale oil reservoir fracability that considers hydration; the effects of the brittleness index, horizontal difference stress, and fracture toughness; and the law of water phase intrusion into shale oil reservoirs. The research results show that the average compressive strength decreased by 37.99%, the average elastic modulus decreased by 53.36%, and the average Poisson's ratio increased by 68.75% after being soaked for 48.00 h at 80 °C and 30.00 MPa. The water saturation rate at the borehole wall was the highest; with the extension to the stratum, it gradually decreases to the original water saturation rate of the formation, while the fluctuation radius gradually increases with time. The Young's modulus and fracture toughness decrease, the Poisson's ratio increases, and the fracability index reaches a maximum value at the wellbore (i.e., the highest water saturation rate), indicating that the strength of the hydrated rock decreases and it can be easily fractured. The case analysis shows that the optimal fracturing position of the Da'anzhai Section of Well NC2H is around 2600 m deep. After the hydration occurs, the fracture initiation pressure of the formation is reduced from the original value of 72.31 MPa to 66.80 MPa. This indicates that when hydration decreases, the formation fracture pressure also increases. The research presented in this paper can be used to optimize fracture location and set a reasonable fracturing pressure. [ABSTRACT FROM AUTHOR]

The generation of dynamic fractures during the process of water injection in ultra-low permeability reservoirs aggravates the heterogeneity of the reservoir, resulting in a rapid rise of water cut and directional flooding of the producers, which affects waterflood sweep efficiency and recovery. A dynamic fracture, as the geological feature of ultra-low permeability reservoirs, has a complex genetic mechanism and is difficult to characterize. Taking the Chang 6 reservoir of the Triassic Yanchang Formation in the Ordos Basin of central China as an example, this paper presents the characterization method and workflow of dynamic fractures. On the one hand, through the analysis of triaxial-compression rock-mechanic experiments and the mineral composition of the core, we evaluated rock brittleness in order to identify the lithology that can easily generate new fractures. On the other hand, beginning with the ancient tectonic stress field and combining the fracture characteristics of core and geological outcrop, the multi-fractal method and the probabilistic neural network were applied to identify the natural fractures and to quantitatively predict the intensity of natural fractures. Based on the rock brittleness evaluation and the natural fracture feature, the intensity of dynamic fractures was characterized by integrating the analysis of the bottom hole pressure, fracture pressure, and production response characteristics. A dynamic fracture is a "double-edged sword" during the waterflooding development of ultra-low permeability reservoirs. The premature activation and generation of dynamic fractures could lead to a worse development status. Nevertheless, the rational control and utilization of dynamic fractures play a positive role in improving oil recovery. Dynamic fractures are of great significance to the optimization and adjustment of well patterns for ultra-low permeability reservoirs. This can provide a reference for similar reservoirs. [ABSTRACT FROM AUTHOR]

The evaluation of caprocks' sealing capacity is exceedingly important for depleted gas reservoirs to be reconstructed into gas storage. In this paper, based on the physical sealing mechanism of caprock, four aspects of ten indexes of caprock quality evaluation were firstly selected, and the related classification standards were established. Secondly, based on the rock mechanical sealing mechanism, elastic and plastic indexes were selected to characterize the mechanical brittleness of caprock, and a brittleness evaluation method of caprock based on complete stress-strain curves was established. Then, a systematic comprehensive evaluation model (including 5 aspects and 12 evaluation indexes) for the sealing capacity of gas storage caprock was proposed, and the analytic hierarchy process (AHP) was used to determine the weight of the 12 indexes in the evaluation model, and the formula for calculating the suitability of the caprock sealing capacity was established. Finally, the geological data, laboratory, and field test data, including X-ray diffraction, poro-permeability test, displacement pressure, and tri-axial compression test, were used for the caprock sealing capacity evaluation of the X9 depleted gas reservoir, and the result from this model showed that the caprock quality is suitable for underground gas storage. [ABSTRACT FROM AUTHOR]

The precipitation of authigenic quartz plays a significant role to reduce the reservoir characteristics and enhance the stiffness of the rock. The Es1 sandstone of Shahejie Formation is acting as a significant hydrocarbon producing rock in the Nanpu Sag. Various methods like thin section petrography, cathodoluminescence (CL), scanning electron microscope (SEM, with EDS), and electron microprobe analysis has been used to reveal the origin of quartz cement as well as to evaluate the effect of quartz cement on reservoir quality. The studied sandstone is classified as immature to mature feldspathic litharenite and lithic arkose and consists of quartz, feldspar, rock fragments and micas. Petrographic studies and SEM analysis shows that the authigenic quartz is acting a significant cement that reduces the reservoir quality. Whereas clay minerals (kaolinite and mixed layer illite to smectite) are dominant in the Es1 sandstone, that can reduce the reservoir quality. SEM, CL and thin section analysis reveal that there are two stages of quartz cement in the studied samples; that are pore filling authigenic cement and quartz overgrowth cement. Fluid inclusion homogenization temperatures shows that stages of quartz cement were developed with continuous process from 70°C to 130°C. Quartz cements were generally originated from I/S reaction, feldspar dissolution, conversion of rock fragments and pressure solution. Feldspar dissolution (K‐feldspar) and kaolinite to illite reaction is an insignificant silica source for the silica cement which is internally precipitated in a close system with diffusion transporting mechanism. Overall, quartz cement significantly enhance the rock strengthen and brittleness effectively as well as it reduce the overall reservoir quality. [ABSTRACT FROM AUTHOR]

There are abundant shale gas resources in the Lower Paleozoic marine organic shale strata of the upper Yangtze region, South China. However, affected by multi-period tectonic activities, leading to poor exploitation of the Lower Paleozoic marine shale gas in the complex tectonic zone. Therefore, this study proposes fracturability index (FI) based on shale brittleness, fracture toughness, and stress decay rate. Therefore, this study combines logging data and rock mechanics experiments to obtain the rock mechanics parameters of the Fenggang block shale reservoir. Based on Young's modulus, fracture toughness and critical strain energy release rate, combined with the analytic hierarchy process, the shale fracturability index is proposed to guide the exploration and development of shale gas. The fracturability index of the Longmaxi Formation shale in the study area is 0.34 ~ 0.72. The fracturability of the middle and lower part of the Longmaxi Formation shale is obviously better than that of the upper part. In general, shale reservoirs with high brittleness index, low fracture toughness, and low critical strain energy release rate have high fracturability. [ABSTRACT FROM AUTHOR]

To quantitatively study the influence of temperature and strain rate on the brittleness of sandstone, the mechanical parameters of sandstone under different temperatures and strain rates are collected from the previous literature, and two empirical equations for calculating rock brittleness are used to quantitatively calculate and evaluate the brittleness of sandstone. The results show that both BI1 and BI2 can characterize the brittleness of sandstone, but the applicable conditions are different. The BI1 method is more accurate in calculating the variation in the sandstone brittleness with a strain rate, while the BI2 method is more accurate in calculating its variation with temperature. The brittleness of sandstone increases with the increase in the strain rate, especially when the strain rate exceeds 100 s-1. Under low-temperature conditions, the strength and brittleness of rocks increase due to the strengthening of ice. Under the condition of high temperature, the thermal damage to sandstone is intensified after 400°C, and the quartz phase changes after 600°C, which leads to the increase in microcrack density and the decrease in brittleness of sandstone. The conditions of low temperature and high strain rate are beneficial to the enhancement of sandstone brittleness. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing technology has become an effective technical method of developing tight oil reservoirs, such as an oilfield in Mahu, China. However, numerical simulation of the actual fracture seam network remains problematic. In this paper, we have simulated hydraulic fractures in the Urho Group of the Mahu target layer and analyzed the characterization of the rock mechanics parameters. The results show that Young's modulus of the Wuerhe domain ranges between 18 and 58.5 GPa, with an average of 32.4 Gpa, the Poisson's ratio is between 0.21 and 0.38, with an average of 0.31, the brittleness index is between 21.0 and 89.0, with an average of 44.3, and the hydraulic fracturing can form a multi-branch crack modification. The designed direction of horizontal wells in the area is north-south, and the horizontal stress difference is between 4.2 and 9.8 MPa, which facilitates easy fracturing of the reservoir and reforming of a complex seam network. Simulation of the artificial seam network helps to optimize the reasonable parameters of fracturing and the development parameters of horizontal wells in the Mahu 1 well area. [ABSTRACT FROM AUTHOR]

Brittleness index is an important mechanical index to evaluate the fracturability of conventional oil and gas reservoirs. However, the brittleness index of an organic coal reservoir is more complex. In this study, based on array sonic logging and density logging data, coal brittleness index is calculated using an elastic parameter method in the Mabidong coalbed methane (CBM) block in southern Qinshui Basin. In combination with coal-body structure observation, the maceral analysis, and proximate data of coal cores, a comprehensive study of geological influencing factors of coal brittleness index has been carried out. According to coal fragmentation degree, the coal-body structure of coal seams can be divided into primary, cataclastic, and granulate structure. The average interval of brittleness index of primary, cataclastic, and granulate structural coals is 63.3-71.48, 73.01-74.85, and 77.41-82.77, respectively. The results indicate that generally the order of brittleness index is primary structural coal < cataclastic structural coal < granulate structural coal. Young's modulus and the brittleness index have a good positive correlation. Poisson's ratio and the brittleness index are negatively correlated in No. 3 coal seam but are positive correlated in No. 15 coal seam. The vitrinite content is positively correlated with brittleness index, and the inertinite content is negatively correlated. For the primary and cataclastic structure coal, the ash and volatile is positively correlated with the brittleness index. The correlation of brittleness index, macerals, and coal quality parameters in the primary structure coal is better than that of the cataclastic and granulate structure coal. The research results are helpful to guide the coal brittleness index and coal-body structure prediction in fracturing of CBM wells. [ABSTRACT FROM AUTHOR]

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Heat-soak and quick freezing could deteriorate the texture of vegetables. In this work, it was found that calcium chloride-sodium alginate (SA) could improve the appearance, brittleness, and chewiness of processed Heracleum moellendorffii (HM), a kind of popular nutritious wild vegetable in China. The effect resulted from the increase of calcium content in the vegetable, which was closely related to the ratio of calcium chloride to SA, the concentration of texture retaining agent, and soaking time significantly (P < 0.01). The best way to maintain the texture was to soak HM in 4 g/L of calcium chloride-SA (mass ratio 1 : 2) at 50°C for 30 minutes. The calcium content was increased to 71.56 mg/100g, and the brittleness and chewiness were 4630 gf and 2583.33 gf, respectively. The microstructure found that calcium could adhere to an inherent position on the cell membrane and protected the sample from cell damage and chloroplast spilling from the cell during thawing and quick freezing. The results showed that calcium chloride-SA treatment may be a promising method to improve the texture of vegetables during quick-frozen storage. [ABSTRACT FROM AUTHOR]

Rockbursts represent hazardous dynamic disasters for underground coal mines and other underground rock engineering projects. Some bursting liability indices are put forward and applied to identify the likelihood of rock burst occurrence. The classification criteria of the bursting liability indices are proved to be reasonable for coals, but they are still immature for non-coal rocks. Thus, it is uncertain that it is reasonable to use the classification criteria of coal for evaluating the bursting liability of non-coal rocks. Hence, in this study, a large amount of data, such as the basic mechanical parameters, i.e., Poisson's ratio μ, elastic modulus E, uniaxial compressive strength σc, and uniaxial tensile strength σt, and the bursting liability indices, i.e., elastic strain energy index WET, bursting energy index Wcf, dynamic fracture duration time DT, and brittleness index B, of different coals and non-coal rocks were collected in China. Then, the differences of mechanical parameters and rockburst tendency indices between coal and non-coal rocks were studied systematically, and apart from the Poisson's ratio μ, the other three basic mechanical parameters of coal and non-coal rocks have great differences in data distribution and concentration scope, which proved that the non-coal rocks cannot share the same index system and classification criteria of coals. In addition, the evaluation results of a single index for rock bursting liability of rocks were directly compared in pairs, and the inconsistency rate for coals is about 42–68%. It is necessary to build a comprehensive evaluation method to evaluate the bursting liability of rocks. At last, the modified rockburst tendency classification criteria for non-coal rocks were put forward. It is reasonable to use the classification criteria of the WET and Wcf to classify the bursting liability of non-coal rocks, while it is unreasonable to use that of the DT and σc. It has been concluded that the index B are more suitable for non-coal rocks, and a new index, named strength decrease rate (SDR), was proposed to determine the bursting liability, which is the ratio of uniaxial compressive strength σc to duration of dynamic fracture DT. [ABSTRACT FROM AUTHOR]

Mesoparameters of rock materials are the main factors affecting the macromechanical properties of dangerous rock slopes. Based on the principle of particle flow and synthetic rock mass technology (SRM), the influence of mesoparameters on macromechanical properties is investigated by calibrating mesoparameters of rock materials at depth for a rock sequence in Beichuan Qiang Autonomous County, Sichuan Province, China. By combining these parameters with conventional and dynamic cycle triaxial tests, sensitivity analysis of rock β-parameters was completed. As a result, the reliability of mesoparameters in the simulation of dangerous rocks is strengthened, providing a basis to examine the failure mechanism of earthquake dangerous rocks in this region. Results indicate that, in the triaxial test, sandstone failed in tension, and brittleness gradually weakened as confining pressure increased. Mudstone recorded shear failure, and the characteristic value of brittle attenuation showed a V-shaped change with increasing confining pressure. Under cyclic loading, cracks had a degrading effect on the damping ration (β) and the damping coefficient (C) of sandstone. Mudstone recorded relatively low β and low brittleness whilst sandstone had high β and high brittleness. In rock materials, β n is more sensitive than β s in mechanical properties. When the value of the β n -parameter was between 0.2 and 0.3 and the value of the β s -parameter was between 0.2 and 0.6, rock brittleness was more stable, and the reflected macroscopic mechanical properties were the most authentic. By using a deepened mesoparameter trial adjustment method, the failure mode of the Particle Flow Code (PFC) dangerous rock model near provincial highway 205, simulated under conditions for the Wenchuan earthquake, indicated a tensile fracture-horizontal slip failure. The simulated failure mode was consistent with that of real dangerous rocks, with the failure trend being concentrated between the first and the third layer of the rock mass. [ABSTRACT FROM AUTHOR]

It is important to predict the capacity related parameters of reservoir such as brittleness, total organic carbon (TOC), in-situ stress, maturity of organic carbon (Ro), and buried depth in shale gas exploration and exploitation. Among these parameters, brittleness and TOC are especially important for shale gas exploitation. The complex resistivity measurement and analysis show that the rich organic shale in southern China features low resistivity and high chargeability. These characteristics are a geophysical prerequisite for predicting the sweet spot characteristic parameters of shale, such as TOC and brittleness, with rapid developing of high resolution controlled source electromagnetic (CSEM) method prospecting which can give resistivity and chargeability distribution of underground. Based on the rich organic shale composition analysis and complex resistivity measurements in southern China, this paper analysed the presence of pyrite as an associated mineral in organic-rich shale, and examined its relationship to resistivity, chargeability, TOC content and brittleness, and the quantitative relationship between the models has been established. Therefore, this study provides a firm experimental foundation for predicting parameters of shale sweet spots with CSEM prospecting. [ABSTRACT FROM AUTHOR]

The lower Silurian Longmaxi Formation hosts a highly productive shale gas play in the Zhaotong region of southern China. According to core observation, X-ray diffraction analyses, and scanning electron microscopy (SEM) observations, the shale comprises primarily quartz, carbonate minerals, and clay minerals, with minor amounts of plagioclase, K-feldspar, and pyrite. The clay mineral content ranges from 15.0% to 46.1%, with an average of 29.3% in the Zhaotong region. Organic geochemical analyses show that the Longmaxi Formation has good potential for shale gas resources by calculating total organic carbon, vitrinite reflectance, and gas content. Scanning electron microscope images demonstrate that reservoir pore types in the Longmaxi shale include organic pores, interparticle pores, intercrystalline pores, intraparticle pores, and fractures. Reservoir distribution is controlled by lithofacies, mineral composition, and geochemical factors. In addition, we investigated the relationships between reservoir parameters and production from 15 individual wells in the Zhaotong region by correlation coefficients. As a result, the brittleness index, total organic carbon (TOC), porosity, and gas content were used to define high-quality reservoirs in the Longmaxi shale. Based on these criteria, we mapped the thickness and distribution of high-quality reservoirs in the Longmaxi Formation and selected highlighted several key sites for future exploration and development. [ABSTRACT FROM AUTHOR]

BACKGROUND: ‘High-quality layer’ is the focus and hotspot of shale gas exploration and development. Many explorations and important developments have been studied by researchers at home and abroad. However, the threshold of selection parameters, qualitative-quantitative system classification evaluation and template establishment need to be optimized.OBJECTIVES: To identify the ‘high-quality layer’ and optimize areas of marine shale gas with the commercial value in the Lower Paleozoic area of southern China.METHODS: The Ordovician Wufeng Formation—Lower Silurian Longmaxi Formation and Cambrian Niutitang Formation were the research objects. The identification factors of shale gas enrichment and high-yield ‘high quality layer’, and the shale gas area with commercial exploration value in the Lower Paleozoic marine facies in southern China were analyzed.RESULTS: Three qualitative parameters including sedimentary facies, tectonic preservation, lithologic combination, and eight quantitative parameters including gas content, pressure coefficient, organic carbon content, organic matter maturity, porosity, brittleness, effective thickness, buried depth were optimized. A discriminating principle of qualitative parameters to delineate favorable areas, and quantitative parameters to identify the ‘high-quality layer’ was proposed. On the basis of this, the parameter acquisition methods and thresholds were analyzed. A ‘high-quality layer’ quantitative identification template for marine shale gas of the Wufeng—Longmaxi Formation and the Niutitang Formation was established in southern China. This method was applied to the Zheng’an—Wuchuan area of Guizhou Province. Seven favorable shale gas areas and one ‘high-quality layer’ of shale gas(the Wufeng—Longmaxi Formation of Anchang syncline) were identified, which successfully verified by later drilling.CONCLUSIONS: The proposed method has certain reference significance for the identification of ‘high-quality layer’ of marine shale gas in southern China. [ABSTRACT FROM AUTHOR]

At present, shale gas plays a significant role in hydrocarbon reservoirs. Hydraulic fracturing is generally employed in the exploration and exploitation of shale gas. Economic and efficient hydraulic fracturing, known as volume fracturing, is largely associated with formation characteristics, including anisotropy and brittleness in rocks. Further research on the mechanical properties of rocks, particularly the anisotropy and brittleness behavior of shale, using hydraulic fracturing would be of practical significance. In this study, shale specimens were collected from an outcrop of the lower Silurian Longmaxi formation at Sichuan Basin in southwestern China, which is the most significant exploration area for unconventional gas in China. To better understand the size, type, and shape of brittle minerals, the matrix type (and rock texture), mineral composition, and microstructure of the shale matrix were tested through X-ray diffraction analysis and scanning electron microscopy. Furthermore, the anisotropic behavior of shale specimens, including strength, deformation, and failure behaviors, was tested and analyzed under conventional triaxial compression. In addition, the brittleness characteristics of shale specimens at different bedding inclinations under different confining pressures were analyzed based on the stress–strain curve characteristic and energy balance. Different brittleness indices, including a new one proposed in this study, were used to evaluate the brittleness of shale. The impacts of anisotropy and confining pressure on brittleness were discussed in detail. When compared with other brittleness indices, the proposed brittleness index demonstrates improved effectiveness and reflects the impact of confining pressure on brittleness significantly well. The relationship between brittleness and the failure mode was revealed using the new brittleness index, and the decreasing order of brittleness was concluded as follows: tensile splitting along bedding plane mode > tensile splitting through bedding plane mode > shear along bedding plane mode > shear through bedding plane mode. [ABSTRACT FROM AUTHOR]

Fracability is a property that indicates how easy reservoir rocks can be fractured in hydraulic fracturing operations. It is a key parameter for fracturing design and evaluation. In order to utilize continuous logging data to predict fracability, synchronous tests of dynamic and static mechanical parameters of rocks under different confining pressures were conducted on 13 tight sandstone samples derived from the central Junggar Basin, China. A modified formula between dynamic and static mechanical parameters was established. Fracability of the tight reservoir in the Junggar Basin was then evaluated based on brittleness index, fracture toughness, and fracability index. The effectiveness of fracturing was analyzed combined with the oil testing curve after hydraulic fracturing. The results show that: (1) The distribution of oil-bearing formations in the studied area coincides well with stratum of higher fracability index. (2) The critical fracability index is determined to be 0.3, three formations are selected as fracturing candidates, and a thin mudstone interbed is identified in the oil-bearing formation. (3) Well testing curve verifies the reliability of the fracability evaluation method and the accuracy of the modified formula between dynamic and static mechanical parameters. This study provides useful information for improving fracturing operations of tight oil and gas reservoirs. [ABSTRACT FROM AUTHOR]

China is the largest producer and exporter of Lentinus edodes in the world. To improve its market value, twin-screw extrusion and expansion machine has been developed to further process the Lentinus edodes. To resolve the problems of blockage, low prematurity and missing operation parameters of this machine in producing Lentinus edodes pufferent, we presented an improved twin-screw extrusion and expansion machine in this paper to lower puffer ability, improve water absorption and reduce its hardness. The equipment consists of a frame, a mixing device, a feeding device, a tempering device, a heating device, an extruding and bulking device and a rotary cutting device. It is equipped with a cooling system, a lubricating system, a transmission system, a control system and a pressure detection system. Design and determination of the anti-blocking device for the feed mixing device, the pre-maturing and tempering device, and the structural parameters of the twin screw were conducted. In order to investigate the influence of operation parameters on swelling performance of the product, the Box-benhnken combined test method was used to study the working parameters of the double screw extruder. The effects of screw speed, extrusion temperature and water content on the swelling rate, hardness, brittleness and water absorption of the expanded product were analyzed, and the determinants were optimized. The results showed that, based on the level of the significance, 1) factors affecting the expansion rate were ranked in material water content > expansion temperature > screw speed, 2) factors affecting hardness were ranked in material moisture content > screw speed > swelling temperature, 3) factors affecting brittleness were ranked in material moisture content > screw speed, 4) factors affecting water absorption were ranked in material moisture content > swelling temperature > screw speed. The optimal operating parameters were: screw speed 167.23 r/min, swelling temperature 151.68℃, material moisture content 16.83%. Under these conditions, the swelling rate, hardness, brittleness and water absorption of the product were increased by 4.04%, 18.61 N, -8.46 mm/cm2 and 313.86% respectively, compared to the traditional machine. Practical application of the optimized parameters revealed that the error between the optimized productivity and real productivity was less than 4%, with the maximum productivity being 165 kg/h. Compared with the existing equipment in the laboratory at School of Mechanical Engineering in Yangzhou University, the designed machine increased the swelling rate, brittleness, water adsorption and brittleness by 25.00%, 40.55% and 62.35% respectively, while reducing the hardness by 48.21%. Therefore, the design of this machine can provide relatively mature technical equipment for the development of mushroom swelling products. [ABSTRACT FROM AUTHOR]

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Copyright of Journal of China University of Petroleum is the property of China University of Petroleum and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In order to study the effects of high temperature and pre-existing cracks on the failure mode of rock, granitic rocks from Eastern China with three pre-fabricated cracks parallel to each other oriented at 30° and 45° and subjected to different temperatures are tested. Changes in the strength and failure modes of the rock are studied extensively. Various factors that affect the failure modes and their transitions are analyzed by measuring the mineral composition, chrominance, mass, thermal conductivity and P-wave velocity of the material. The results show that pre-existing cracks result in changes in the failure mode of plate-shaped granite samples from intermediate splitting to Y-shaped shear fractures. Under the combined action of pre-existing parallel cracks and high temperature (≥300 °C), the failure mode changes from a Y-shape to a stair-step shape. The angles of the crack selected for testing (30° and 45°) have no significant effect on the failure mode of the samples. The change in the failure mode of the rock caused by parallel cracks and high temperature is mainly due to stress concentration around the cracks (especially at the tip of the cracks). Moreover, high temperature causes thermal damage on the rock which changes the brittleness and plasticity of the material. Damage to this rock due to an increase in temperature is mainly caused by the evaporation of water inside the rock and the growth in crack (especially at temperatures above 400 °C, when the rate of the volume expansion of the quartz increases rapidly), which can be measured by the changes in the thermal conductivity, P-wave velocity, and rates of mass loss and quartz expansion with temperature. • The combined effect of pre-existing cracks and high temperature on granite is studied. • The temperature threshold of the transition of failure model of granite is about 300 °C. • The changes in rock properties versus high temperature and pre-existing cracks were studied. [ABSTRACT FROM AUTHOR]

Coal brittleness is one of the important indexes to characterize the post-peak characteristics of coal, which is closely related to the efficiency of underground coal mining and the degree of coal disaster. In this paper, the compression characteristics of coal are studied by laboratory experiment, which is characterized by relatively long nonlinear elastic stage, smaller plastic stage and stress drops in post-peak stage. On this basis, the existing brittleness evaluation methods are summarized, and the applicability of 11 indexes of 6 physical parameters to the quantitative evaluation methods of coal brittleness are analyzed and discussed. The results show that: (1) for the coal is a complexity heterogeneous medium with hole and fracture, there are different degrees of limitation on the methods of coal brittleness quantitative evaluation,which can be used to roughly estimate the brittleness of coal, but it can not accurately characterize its brittle characteristics. (2) Based on the whole process stress–strain curve, the brittleness degree index B5 is more suitable for the brittleness characterization of coal by quantitatively considering the relative size and absolute rate of post-peak stress drop. [ABSTRACT FROM AUTHOR]

Abstract The inter-salt shale in the Qianjiang formation of Jianghan Basin in China are characterized by multiple bedding planes, low rock strength, complicated mineral compositions and high heterogeneity of rock mechanics. The brittleness evaluation of the inter-salt shale has not been investigated. Core samples of the inter-salt shale from the fourth member of the Qianjiang Formation were tested by using CT scan, X-ray diffraction, tri-axial rock test at the in-situ condition. An integrated investigation was conducted on the brittleness evaluation of inter-salt shale from the perspective of core characteristics, brittle mineral components, elastic parameters and the complete stress–strain characteristics of rocks. The results indicate that the inter-salt shale exist with bedding planes and glauberite, which leads to high anisotropy and heterogeneity. The argillaceous dolomite is a good candidate for fracturing due to high brittle mineral content. The elastic parameters of the inter-salt shale at the in-situ conditions are quite different with that of the gas shale because of the difference in the mineral composition, the bedding plane structure and the fluid type stored in the pore space. The brittleness index of the inter-salt shale can be calculated through a modified Rickman model. The transformation model of the dynamic-static mechanic parameters has been built based on the logging data and the laboratory data in order to obtain the brittleness index profile along the wellbore. The post-peak stress-strain relations indicates the rock has high residual strength at confined stress state, which explains that the rock is damaged through shear failure along the bedding plane and fails to generate complex fractures. This study would provide theoretical basis for fracability evaluation model used in the inter-salt shale oil reservoir. Highlights • The elastic parameters of the inter-salt oil shale are quite different with that of the gas shale. • The mechanical brittleness index profile along the wellbore of the shale oil reservoir can be obtained. • The inter-salt oil shale is semi-brittle at the high level of confining pressure based on the full stress-strain curve theory. [ABSTRACT FROM AUTHOR]

The safe and efficient development of ultra-deep shale gas deeper than 4500 m is closely related to the comprehensive evaluation of the mechanical behavior, brittleness and anisotropy characteristics of reservoir rocks under in-situ stress conditions. However, the above-mentioned information about ultra-deep shale is rarely reported. This study aims to fill this gap by systematically investigating the mechanical and anisotropic characteristics of cores, with 0° and 90° bedding orientations, from different layers in the vertical depth of 4900–4930 m, under confining pressure of 110 MPa. Furthermore, a new brittleness index with three parts related to brittle failure, i.e., the energy storage performance in the pre-peak stage, the performance of elastic energy used to maintain the post-peak failure, and the severity of the energy released during the post-peak stage, all of which are physically meaningful, was proposed. Subsequently, the index was validated and applied to the brittleness evaluation of ultra-deep reservoir rocks. The brittleness was evaluated in the following descending order: black shale of Longmaxi Formation ＞ gray-black shale of Wufeng Formation ＞ limestone of Baota Formation. Overall, the Wufeng- Longmaxi Formation shale still exhibits brittle failure under such high confining pressure, while the Baota Formation limestone exhibits quasi-ductile failure, which is consistent with the phenomenological behavior of cores. Further, various experimental techniques have been employed to study the brittle and quasi-ductile failure mechanisms of reservoir rocks. The anisotropy evaluation results show that the anisotropy degree of the mechanical and brittleness parameters corresponding to the deep formation is significantly lower than that of the shallow formation. The anisotropy degree of strength and deformation parameters (Young's modulus, Poisson's ratio) is closely related to the content of quartz and clay minerals, respectively. The above results are expected to provide data support and theoretical guidance for the development of deep and ultra-deep shale gas in the southern Sichuan region of China. • The mechanical characteristics of ultra-deep reservoir rocks in the vertical depth range of 4900–4933 m are reported. • The influence of the bedding effect on the mechanical behavior of reservoir rocks under high confining pressures is studied. • A new three-part brittleness index in which each part has a clear physical meaning related to brittle failure is proposed. • The brittle and quasi-ductile failure mechanisms of reservoir rocks under high confining pressures are revealed. [ABSTRACT FROM AUTHOR]

The deep continental shale of the Permian Lucaogou Formation in the eastern Junggar Basin is abundant in oil resources, characterized by extensive development and various types of natural fractures. Based on core observations, image log interpretation, and thin section analyses, this study identified fracture types and analyzed the development characteristics of different types in deep continental shales. Moreover, the main controlling factors affecting fracture development and distribution were discussed by integrating statistical and experimental analysis. Results show that natural fractures in deep continental shale oil reservoirs can be grouped as non-bed-parallel and bed-parallel fractures based on their relationship with bedding. Non-bed-parallel fractures are subdivided into subvertical fractures (≥80°) (translayer and intralayer fractures) and moderately dipping fractures (<80°), and bed-parallel fractures are further divided into bedding fractures and stylolites. Since silty and dolomitic rocks appear widely as interlayers and increase the rock brittleness, the deep continental shales are more prone to developing non-bed-parallel fractures. Moreover, intralayer fractures with high dip angles are more developed within these thinner mechanical units and commonly terminate at the mechanical interface, while the translayer fractures are usually less developed. Deep continental shales with higher TOC and argillaceous content are more conducive to forming bed-parallel fractures because of their stronger plasticity, more densely developed laminar structure, and easier formation of abnormally high pressure during the hydrocarbon generation and evolution. In addition, due to the frequent interaction of different types of laminae and the stronger compaction and pressure solution produced by the larger burial depth, the bed-parallel fractures, especially the stylolites, are more developed as a prominent feature in deep continental shales. • Silty and dolomitic interlayers increase the shale brittleness and are conducive to developing intralayer fractures. • The deep continental shales with higher TOC and argillaceous content are more conducive to forming bed-parallel fractures. • Since the frequent interaction of laminae and the stronger compaction and pressure solution, stylolites are more developed. [ABSTRACT FROM AUTHOR]

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Lower Silurian Longmaxi formation (LSL) shale is widely and continuously distributed in the northeastern Sichuan Basin and based on structural analogies with the gas producing LSL formation in the southeastern Sichuan Basin, has significant potential for shale gas exploration. However, limited research has been performed to evaluate the shale gas potential in this region. Samples from a recently completed exploratory well (Well-WQ2) in the northeastern Sichuan Basin indicate that the LSL shale has a vertical property sequence that closely resembles the vertical property sequences in wells in the gas-producing sections of the southeastern Sichuan Basin. The continuous sampling and analyses ofWell-WQ2 have allowed a detailed investigation of the vertical variations in lithofacies, mineral characteristics, pore structures and organic geochemical characteristics. The Longmaxi formation was divided into two third-order sequences (SQ1 and SQ2) based on systematic core observations and well logging analyses. Both SQ1 and SQ2 include a transgressive system tract (TST) and a high-stand system tract (HST). The lithofacies exhibit an upward decrease in the organic content. From SQ1 to SQ2, the quartz content, in situ graptolite content, total organic carbon (TOC) content and brittleness index decrease, but the clay mineral content increases. The LSL shale sections from depths of 1204 to 1214 m and from 1271 to 1282 m possess well-developed fractures and high permeability. Additionally, the average porosity and permeability in SQ1 are higher than those in SQ2. In addition, the positive correlation between the TOC and quartz contents of the assayed samples suggests that much of the quartz is of biogenic origin. Changes in the sedimentary and diagenetic environments during deposition are two key factors that contribute to the observed vertical heterogeneity of the Longmaxi formation. In conclusion, the shale sections of the lower part of the SQ1, like their analogs in the southeastern Sichuan Basin, are the most favorable targets for shale gas production in the northeastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

Continental shale is abundant with unconventional oil and gas and has considerable potential for exploitation. However, due to differences in the deposition environment, continental shale presents unique characteristics in terms of mineralogy and mechanical properties compared to marine shale. Furthermore, the mechanical influence of fracturing fluid immersion must be considered because of the dual effects on the productive shale system caused by fracturing fluids remaining in the formation. In this study, tri-axial compression tests were performed on Jurassic continental shale specimens collected from an exploratory well (2778–2816 m deep) in the Sichuan Basin of China before and after exposure to fracturing fluids. Meanwhile, two quantitative indicators, the roughness R , which characterizes the widely distributed yield deformation, and the complexity F , which evaluates the complex multi-fractures, were used to distinguish the different brittle-plastic characteristics of specimens before and after fracturing fluid immersion. The results showed that the immersion of fracturing fluids weakened the mechanical properties of continental shale. Two types of fracturing fluid immersion effects were distinguished by the statistics of the failure characteristics of the different layers: lubrication-dominated and softening-dominated. Unlike conventional smooth tri-axial stress-strain curves, many of the post-peak curves were in a 'step-like' shape, corresponding to two sources: one part came from the formation of multi-fractures (brittle characteristics), and the other part came from the softening deformation of main failure surface (plastic characteristics). A new brittleness index, BI new , was proposed considering failure characteristics with different brittleness effects, which could effectively distinguish the brittleness difference before and after exposure to fracturing fluids. • Mechanical variations of continental shale along longitudinal profile were studied. • Different effects of fracturing fluids were distinguished by failure characteristic. • Different sources of additional energy dissipation were recognized. • New brittleness index was proposed incorporating different failure characteristics. [ABSTRACT FROM AUTHOR]

Shale brittleness is the key parameter to optimize reservoir fracturing modification and improve productivity. However, conventional methods based on rock mechanics and mineralogical parameters are limited by the lack of shear wave velocity and mineral composition data. Machine learning algorithms have been applied in the field of geophysical exploration, but the prediction accuracy of existing algorithms needs to be improved. In this study, a new hybrid model, an integration of the Sparrow Search Algorithm (SSA) and the Extreme Learning Machine (ELM), named SSA-ELM, is proposed for predicting the brittleness index. ELM is performed to create the original brittleness model, meanwhile, the SSA algorithm is utilized to automatically explore the hyperparameters of the model. Additionally, we also use 12 other favorite machine learning algorithms to validate the superiority of the proposed model combined with traditional logs and XRD-derived brittleness. 82 mineral brittleness databases from 5 exploration wells in the Songliao Basin, China were established. The simulation results of two Test wells indicate that the SSA-ELM model has the most accurate predictions and excellent generalization capability. The brittle profiles established based on the SSA-ELM model can effectively guide the fracturing exploration of shale oil reservoirs. • A novel hybrid machine learning method for brittleness index prediction. • SSA automatically searches for the hyperparameters of the ELM model. • The proposed model provides for reservoir fracture brittleness evaluation. [ABSTRACT FROM AUTHOR]

The Lower Cambrian organic-rich marine shale, which is a significant source of China’s shale gas, is widely distributed in southern China. An integrated characterization of the Niutitang shale is provided in this study in terms of organic geochemistry, mineralogy, pore characterization, methane sorption capacity, rock mechanical properties, fractures and gas content based on samples from three wells. The results indicate that the Lower Cambrian Niutitang shale is thermally over-mature and has rich shale gas resources, with a total organic carbon content (TOC) between 0.51% and 10.49% and a high quartz content between 35.3% and 78.5%. Compared to the major gas-producing shales in the U.S. and China, most of the organic matter (OM)-hosted pores in the Niutitang shale are generally smaller than 5 nm, significantly affecting the methane sorption capacity. The inter-particle and intra-particle pores and fractures are the primary sources of storage space, especially for free gas. For samples with TOC values less than 6.5%, TOC is positively correlated with the total porosity, total pore volume, brittleness (Young’s modulus), core fracture density, free gas content and Langmuir pressure; however, for samples with TOC values greater than 6.5%, the positive correlations become negative. These characteristics are due to the ductility and low hydrocarbon generation potential of organic matter in high thermal maturity shales that are vulnerable to compaction. Thus, TOC has a significant impact on the macroscopic (e.g., brittleness) and microscopic (e.g., pore structure and sorption capacity) properties of shale reservoirs, potentially controlling the enrichment and productivity of shale gas. These results can be used to optimize drilling and fracturing stimulation intervals during shale gas exploration and development. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is an effective stimulation method for the economic development of tight-gas reservoirs in which extremely low matrix permeability requires complex fracture networks. Petrophysical/mechanical experiments and XRD/SEM analyses demonstrate that volcanic sedimentary rock is characterized by developed natural fracture, strong brittleness, stress sensitivity, AE activity, weak anisotropy, and fluid sensitivity. Fracability index is often utilized as a key parameter to evaluate the ability to generate fracture networks. In this study, a new systematic experimental approach and a new mathematical model are established for the comprehensive evaluation of the fracability of tight-gas formations. These two methods integrate natural fracture, stress sensitivity, rock anisotropic nature, AE activity and crack density. The results indicate that the fracability index (FI) and the calculated crack density (CRD) are positively linearly correlated. The method is successfully applied to evaluate the fracability of the Yingtai gas field in northeast China. [ABSTRACT FROM AUTHOR]

Fracture morphology of shales represents the main feature related to their brittleness, and plays an important role in affecting the complexity of fracture network in shale reservoirs. However, no brittleness index model based on fracture morphology has been developed so far. In the present research, a detailed analysis on morphological characteristics of shale fractures under triaxial compression has been performed, focusing on the Longmaxi Formation shale in the Weiyuan area, China. As a next step, the fractal theory is used to establish a method to characterize fracture complexity. As a next step, the correlation between rock mechanics parameters, mineral composition, stress-strain curve, dilatability, and shale fracture complexity is analyzed, determining the most influencing factors affecting brittleness, whose weight is analyzed by applying the gray correlation method. Furthermore, a novel brittleness evaluation method based on shale fracture morphology is established. Results show that Young's modulus, peak strain, and dilatancy angle of shales are characterized by the highest correlation values with fracture morphology (0.4745, 0.6302, and 0.579, respectively), while Poisson's ratio, mineral composition, peak, and residual stress, and strain have little correlation. The calculation and analysis of a shale horizontal well using the brittleness index calculation method in this paper shows that the established method has a good correlation with the number of events of MS after fracturing, the reconstruction volume, which provides technical support for the optimization of fracturing sweet spot in shale horizontal wells, and the evaluation of shale reservoir compressibility. • We proposed a method of using fractal geometry and rock fracture angle to characterize the rock fracture shape. • We deduced the calculation model of rock fracture energy. • We took the gray correlation method to determine the weight coefficients of Young's modulus, dilatancy angle and peak strain in the model. [ABSTRACT FROM AUTHOR]

Thin interbedded tight sandstone of Shanxi Formation in Ordos basin, China, contains mudstone interlayers and laminas universally, and exhibits ultra-low matrix permeability, low brittleness, and areal heterogeneity. The stimulation effectiveness of this formation depends largely on whether multiple hydraulic fractures (HFs) can initiate and penetrate into the multiple thin sandstone layers upward and downward. To clarify the multi-fracture growth behavior of such formation, multi-cluster fracturing experiment in the horizontal well was performed on specimens prepared from the sandstone-mudstone outcrops of Shanxi Formation based on a true triaxial fracturing simulation system. The influences of interlayer stress difference, mudstone thickness, fluid viscosity, pumping rate and the number of clusters were mainly analyzed through post-fracturing specimen splitting and pressure curve analysis. Results show that the difference of rock mechanical parameters between sandstone and mudstone is relatively large. When interlayer stress difference is less than 3 MPa, the mudstone layer cannot function as a barrier to restrain the vertical growth of HFs, and the essential mechanism causing HF height containment is mainly the opening of laminas or/and interfaces. By contrast, when the interlayer stress difference is equal to or greater than 3 MPa and the dimensionless thickness of mudstone is more than 0.4, the HF height tends to be restrained. Increasing the fluid viscosity or/and pumping rate can reduce filtration, and facilitates HFs penetrating through the laminas or interfaces and consequently propagating into adjacent layers. It has been observed that when fluid viscosity reaches 100 mPa s (cross-linked gel) and pumping rate is 100 mL/min, HFs can pass through the overlaying mudstones. However, uniform initiation of all clusters is difficult to achieve due to the low brittleness of sandstone and heterogeneity along the horizontal wellbore. The experimental results prove that temporary plugging within the wellbore using the plugging agent of appropriate particle size is necessary to create multiple HFs in such formation. • Multi-fracture nonuniform initiation and vertical propagation behavior in thin interbedded tight sandstone was studied. • Influences of interlayer stress difference, mudstone thickness, fluid viscosity, flowrate and cluster numbers were analyzed. • Nonuniform initiation and height containment is universally observed due to heterogeneity and low brittleness of the rock. [ABSTRACT FROM AUTHOR]

Quartz is an important component of shale and has a significant influence on shale physical properties. However, the specific effects that different quartz types exert on shale porosity evolution and brittleness remain unclear. A series of experiments including petrographic observations, geochemical analyses, and triaxial compress experiments were conducted on shale samples from the Wufeng-Longmaxi Formations in southern China. The Wufeng-Longmaxi samples have been classified into siliceous shale, argillaceous shale, and mixed shale. Several quartz types were identified in the shale samples, including extrabasinal detrital quartz and authigenic quartz (microcrystalline quartz, quartz overgrowths, and bioclastic quartz). Based on the different origins, the authigenic quartz was divided into biogenic quartz derived from the dissolution of siliceous organisms and S–I quartz derived from the conversion of smectite to illite. Bioclastic quartz, most microcrystalline quartz, and most quartz overgrowths are biogenic quartz. A small number of quartz overgrowths and microcrystalline quartz are S–I quartz. The quartz content of different genesis processes was quantitatively calculated using major and trace elements data collected via XRF. Quartz types in siliceous shale samples are dominated by biogenic quartz (avg. 49%). Extrabasinal detrital quartz is the primary quartz type in argillaceous shale samples (avg. 68%) and mixed shale samples (avg. 50%). Porosity reduction mechanisms in most of the Wufeng-Longmaxi shale samples are primarily compaction-dominated. Biogenic quartz acts as a pore protector rather than destroyer, while S–I quartz derived from the conversion of smectite to illite acts as a pore destroyer rather than protector. The brittleness index (BI) of siliceous shale samples is the highest of all the shale samples since the abundant biogenic microcrystalline quartz particles dispersed within the shale matrix connect rigid grains to form an interconnected force framework. • The authigenic quartz in the Wufeng-Longmaxi shale samples was divided into biogenic quartz and S–I quartz. • Biogenic quartz acts as a pore protector rather than destroyer, but S–I quartz acts as a pore destroyer rather than protector. • Siliceous shale samples are featured by good brittleness, since the abundant microcrystalline quartz connect rigid grains to form a force framework. [ABSTRACT FROM AUTHOR]

Triangle zones, generally found in foreland fold-and-thrust belts, serve as favorable objects of petroleum exploration. Taking the Dabashan foreland belt as an example, we studied the formation and development of triangle zones, and investigated the effect of décollements and the mechanical contrast of lithology by employing the method of physical modeling. Four experimental models were conducted in the work. The results showed that 'sand wedges' grew episodically, recorded by deformational length, height and slope angle. The height versus shortening rate presented an S-shape curve, and uplifting occurred successively in the direction of the foreland belt. During the formation of the triangle zone, layer-parallel shortening took place at the outset; deformation decoupling then occurred between the upper and lower brittle layers, divided by a middle-embedded silicone polymers layer. The upper brittle layers deformed mainly by folding, while the lower sand layers by thrusting. As shortening continued, the geometry of a triangle zone was altered. We consider that the triangle zone in the Dabashan foreland belt was modified from an early one based on available seismic profiles and the experimental results. In addition, décollements and mechanical contrast impose significant influence on structural development, which can directly give rise to structural discrepancies. More décollements and obvious mechanical contrast between brittle layers can promote the coupling between the upper and lower brittle layers. Basal décollement controls the whole deformation and decreases the slope angle of the wedge, while roof décollement determines whether a triangle zone can be formed. [ABSTRACT FROM AUTHOR]

Abstract: Uniaxial compression experiments were carried out for the marble samples (located in the eastern ground of China) with different pre-existing flaws in non-overlapping geometry by the rock mechanics servo-controlled testing system. Based on the experimental results of complete axial stress–axial strain curves, the effect of flaw geometry on the strength and deformation behavior of marble samples is made a detailed analysis. Compared with the intact marble sample, the marble samples with different pre-existing flaws show the localization deformation failure. The uniaxial compressive strength (UCS), elastic modulus and peak axial strain of marble samples with pre-existing flaws are all lower than that of intact marble sample, and the reduction extent is closely related to the geometry of pre-existing flaws. The crack coalescence were observed and characterized from internal tips of different pre-existing flaws in brittle marble sample. Eight different crack types were identified based on their geometry and crack propagation mechanism (tensile, shear and compressive) for two pre-existing flaws, which can be used to analyze the failure mode and cracking process of marble sample containing different flaws in uniaxial compression. In the end, the influence of the crack coalescence on the strength and deformation failure behavior of brittle marble sample is analyzed under uniaxial compression. The present research provides increased understanding of the fundamental nature of rock failure under uniaxial compression. [Copyright &y& Elsevier]

Currently, Luzhou in the Sichuan Basin is a focal point for shale-gas exploration and development in China. However, a lack of detailed research on the mineral composition of the Wufeng Formation-Longmaxi Formation (WF-LF) shale is hindering the extraction of deep-buried shale gas in the Luzhou shale play. Herein, a field emission scanning electron microscope (FESEM) equipped with the Advanced Mineral Identification and Characterization System (AMICS) software was employed to analyze the mineral composition of the WF-LF shale from six wells in Luzhou. Quartz was the dominant mineral type, (16.9–87.21%, average 51.33%), followed by illite, calcite, dolomite, and pyrite. Our study revealed that (1) quartz content showed a moderate positive correlation with the total organic carbon (TOC) content, indicating that the quartz found in the shale is mostly of biological origin; and (2) the sum content of siliceous minerals and carbonaceous minerals was moderately positively correlated with the brittleness index (BRIT) in well SS1H2-7 and in the well group of RS8 and RS5, indicating that the siliceous minerals and carbonaceous minerals had an active effect on reservoir compressibility. Finally, according to the mineralogical features of each sublayer, we identified four types of reservoirs to determine their scope for exploration. [ABSTRACT FROM AUTHOR]

The lamellation fractures in marine shales are important to the accumulation and preservation of shale gas, whose characteristics and controlling factors are still unclear. This paper focus on the characteristics descriptions and controlling factor analyses based on core observations, thin sections, and scanning electron microscope (SEM) experiments. The marine shales of Paleozoic Wufeng Formation and Longmaxi Formation in the Fuling Area of Eastern Sichuan Basin are taken as an instance. The lamellation fractures are occurring along the direction of the lamellations, partially of them curved, bifurcated, and converged. The apertures of fractures typically range from 1 to 500 μm. Only a few (<1%) lamellation fractures are filled with quartz, calcite, pyrite, and bitumen. After a series of analyses, developments of lamellation fractures are controlled by total organic carbon (TOC), brittle minerals, laminae, and pyrite. (1) The organic matters control the formation and evolution of lamellation fractures by generating gas and resulting in overpressure. An increase in the TOC content of 1% can increase the lamellation fracture density by 0.26 cm-1. (2) Brittle minerals, especially authigenic quartz, is also an important controlling factor to the development of lamellation fractures. When BI increases by 10%, the lamellation fracture density increases by 19%. (3) Besides, siliceous and carbonate laminae in the shale are more suitable for lamellation fractures to develop. When the laminae density is smaller than 4 cm−1, it is easier to develop lamellation fractures with higher laminae density. However, too densely-distributed laminae will suppress the formation of lamellation fractures. (4) Higher pyrite content is good for the development of lamellation fractures. Lamellation fracture density will increase by 9%, with pyrite content increased by 1%. For the shale at the bottom of the Wufeng Formation - Longmaxi Formation, lamellation fractures develop extensively due to the high TOC and pyrite content, great brittleness, and moderate laminae density. The density of lamellation fractures decreases from the bottom to the top. • Quantitative characterize about development characteristics of the lamellation fractures. • The lamellation fractures is controlled by brittle mineral, TOC, laminae and pyrite. • TOC is the main one that affects the development of lamellation fractures. • Clarify vertical distribution characteristic of lamellation fractures in the Wufeng-longmaxi Fm. [ABSTRACT FROM AUTHOR]

The concept of state vector stems from statistical physics, where it is usually used to describe activity patterns of a physical field in its manner of coarsegrain. In this paper, we propose an approach by which the state vector was applied to describe quantitatively the damage evolution of the brittle heterogeneous systems, and some interesting results are presented, i.e., prior to the macro-fracture of rock specimens and occurrence of a strong earthquake, evolutions of the four relevant scalars time series derived from the state vectors changed anomalously. As retrospective studies, some prominent large earthquakes occurred in the Chinese Mainland (e.g., the M 7.4 Haicheng earthquake on February 4, 1975, and the M 7.8 Tangshan earthquake on July 28, 1976, etc) were investigated. Results show considerable promise that the time-dependent state vectors could serve as a kind of precursor to predict earthquakes. [ABSTRACT FROM AUTHOR]

Shales with a laminated structure typically exhibit inherent anisotropy and strong brittleness, resulting in complex failure patterns and fracture evolution during hydraulic fracturing. To investigate the fracturing mechanism and layer effect of Longmaxi shale from southwest China, a novel testing setup with a high-efficiency sealing method was developed for laboratory hydraulic fracturing, based on which a series of fracturing tests was conducted on shale samples under different confining pressures. The anisotropic mechanical behaviour due to the shale lamination effect was considered by drilling samples at seven different inclinations with reference to the bedding plane. The results showed apparent brittle failure characteristics and anisotropic phenomena both in strength and failure patterns. Three main failure patterns can be identified as split, split-bending and slip along the bedding plane. The corresponding fracture roughness was further analyzed by laser scanning, which shows that the slip mode with shale layer activation can provide a relatively smooth channel. Of particular interest is that anomalously high breakdown pressure, which fluctuates in the samples at different layer orientations and cannot be explained by conventional models, was also observed in our fracturing tests. Inspired by the nonlocal treatment of line stress criterion, an average tensile stress-based model by introducing an equivalent characteristic length was then proposed for interpreting such unusual breakdown pressures, and the inherent mechanism behind hydraulic fracturing was discussed. Comparison analysis showed that the proposed criterion not only predicts the testing data quite well, but can also be degenerated to conventional models in specific cases. The knowledge achieved in this study may provide practical implications for perforation design and breakdown pressure evaluation in field hydraulic fracturing operations. [ABSTRACT FROM AUTHOR]

The salt rock formations in China all contain a certain amount of impurities. It is necessary to study the mechanical performances of salt rock that contains impurities. However, to obtain the salt rock samples that have different types and contents of impurities is difficult. Therefore, man-made salt rock samples containing impurities (sodium sulfate salt rock- SSR and gypsum salt rock- GSR) were prepared to replace the natural salt rock for study. The results show that the density of man-made salt rock was slightly lower than natural ones. And the compactness of SSR samples was suitable while GSR samples were differed slightly, which might be related to the particle size and water absorption of the impurities. Laboratory compression tests were conducted to investigate the influence of impurities on the mechanical performances of the salt rock. The strengths of both SSR and GSR samples decreased with the increase in impurity content. The peak strength of the SSR samples fell straightly, while that of the GSR samples show a parabola-like dropping trend. The elastic modulus of SSR samples beavered enhanced effect and was increased with the increasing content of impurity. And the elastic modulus of GSR samples behaved a parabola-trend decreasing with the impurity content increased. On the evaluation of brittleness characteristics, when the impurity content was at a low level, it had almost no significant effect on the sample's whole ductility. Only at the higher impurity content, the brittleness was increased dramatically, resulting in a rapid rupture of the sample after the peak strength. At last, some discussions about the geological properties of salt mine pointed out the direction of future research. This study revealed the influence of impurities on the mechanical characteristics of salt rocks and could provide a reference for the stability evaluation of natural gas storage caverns in salt formations. The graphical abstract is the preparation of the man-made salt rock with different content of halite and curves of peak strength versus impurity content for SSR and GSR samples. sodium sulfate salt rock (SSR) and gypsum salt rock (GSR) are two main impure salt rock in the bedded salt rock of China. To reveal the influence of the type and content of impurities (sodium sulfate and gypsum) on the mechanical properties of salt rock. These two types of man-made samples are prepared with different content of impurities. The mechanical behaviors of these two types are conducted to reveal the influence of the type and content of the impurities. This study is a basic reference for the safety assessment of the wall rock of gas storage cavern in salt rock formation of China. Image 1 • A method to compress man-made salt rock samples is provided. • The strength of SSR and GSR samples both decreases with impurity content. • The man-made salt rock samples have similar properties as natural ones when IC <25%. • The selection of salt strata for gas storage should avoid high content of impurities. [ABSTRACT FROM AUTHOR]

• A comprehensive literature review was conducted on CO 2 -EOR in tight oil reservoirs. • Specialized technologies are needed in the exploitation of China's tight reservoirs. • The effect of CO 2 diffusion is relatively exaggerated in experimental results. • CO 2 -crude oil interaction in nanopores may lead to an oil recovery increment. • CO 2 -water-minerals interaction influences the geomechanical properties of rock. Primary oil recovery remains less than 10% in tight oil reservoirs, even after expensive multistage horizontal well hydraulic fracturing stimulation. Substantial experiments and simulation works have been performed to investigate CO 2 enhanced oil recovery (CO 2 -EOR) potential in tight reservoirs; however, some results conflict with each other. The objectives of this paper are to fully understand the CO 2 -EOR mechanisms and to figure out the difference between tight oil exploitation in North America and China through a comprehensive literature review. It is shown that compared with Bakken and Eagle Ford formation, China's tight oil reservoirs feature higher mud content and oil viscosity while they have a lower brittleness index of rock and formation pressure coefficient, leading to confined stimulated reservoir volume and further limited CO 2 -oil contact. The effect of CO 2 molecular diffusion is relatively exaggerated in experimental results, which can be attributed to the dual restrictions of exposure time and oil-CO 2 area in field scale. Numerical simulation works show that the shifted phase properties in nanopores lead to an oil recovery increment. The development of nano-scale chips withholding high pressure/temperature may advance the experimental study on the nanopore confinement effect. CO 2 -fluid-rock minerals interaction might be more complex due to the large specific surface area of nanopores in tight formations. The geomechanics coupling effect cannot be ignored when examining the CO 2 -EOR performance in tight reservoirs. And a comprehensive simulation study coupling with technical and economic feasibility is highly recommended before running a field test of CO 2 -EOR. [ABSTRACT FROM AUTHOR]

To reveal the effect of mineral composition on fracture propagation of sandstones, a series of experiments were performed on quartz arenite and sublitharenite. Two fracture propagation mechanisms were identified in sandstones: intragranular and intergranular fractures. As determined by the mechanical properties of minerals, grain fracturing occurred in a majority of detrital quartz and a small amount of quartz cement, feldspars and carbonate cement, exhibiting typical brittle behavior and forming intragranular fractures. Due to the residual space between brittle detrital grains, which act as "flaws" in the sandstones that could initiate a stress concentration, the intergranular fractures occurred only between brittle grains. As cement are situated at the contact locations between detrital grains, mechanical properties and the occurrence of cement played an important role in intergranular fracture propagation. No intergranular fractures occurred between detrital quartz cemented by clay or carbonate, while quartz overgrowths did not hinder the propagation of intergranular fractures. • Minerals show specific mechanical behaviors under compaction or uniaxial compression. • Fractures propagate within brittle grains or along brittle grain boundaries. • Grain fracturing predominantly occurs in detrital quartz grains. • Quartz overgrowths do not hinder the propagation of intergranular fractures. • Sandstones rich in quartz and poor in clay are highly brittle. [ABSTRACT FROM AUTHOR]