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The exploration of Hot Dry Rock (HDR) geothermal energy is essential to fulfill the energy demands of the increasing population. Investigating the physical and mechanical properties of heated rock under different cooling methods has significant implications for the exploitation of HDR. In this study, ultrasonic testing, uniaxial strength compression experiments, Brazilian splitting tests, nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) were conducted on heated granite after different cooling methods, including cooling in air, cooling in water, cooling in liquid nitrogen, and cycle cooling in liquid nitrogen. The results demonstrated that the density, P-wave velocity (Vp), uniaxial compressive strength (UCS), tensile strength (σt), and elastic modulus (E) of heated granite tend to decrease as the cooling rate increases. Notably, heated granite subjected to cyclic liquid nitrogen cooling exhibits a more pronounced decline in physical and mechanical properties and a higher degree of damage. Furthermore, the cooling treatments also lead to an increase in rock pore size and porosity. At a faster cooling rate, the fracture surfaces of the granite transition from smooth to rough, suggesting enhanced fracture propagation and complexity. These findings provide critical theoretical insights into optimizing stimulation performance strategies for HDR exploitation. [ABSTRACT FROM AUTHOR]

To master changes in rock mechanical properties and the impact of high temperature on rock-breaking efficiency, the rock mechanical properties of granite, limestone, and sandstone under different temperatures and single-tooth static-pressure experiments were studied. The results show that the compressive strength, shear strength, internal friction angle, and elastic modulus of granite and limestone initially increased and then decreased as the temperature rose. The experimental temperatures were 25 °C, 100 °C, 200 °C, 300 °C, 400 °C, and 500 °C. The mechanical properties of granite reached the maximum at 200 °C, while those of limestone reached the maximum at 100 °C. The compressive strength, shear strength, and internal friction angle of sandstone gradually diminished, while the elastic modulus gradually increased at the abovementioned five temperature points. Among the samples of granite, limestone, and sandstone, the crushing-specific work of conical teeth, wedge teeth, and scoop teeth was smallest when the temperature was 300 °C. Compared to the normal temperature, the load of conical teeth, wedge teeth, and scoop teeth was reduced by 32.1%, 28.4%, and 22.9%, respectively, when they were pressed into sandstone. At the same temperature, the conical tooth had the highest rock-breaking efficiency, followed by the wedge tooth; the scoop tooth had the lowest efficiency. Conical teeth can be used to select the tooth shapes of bits to improve drilling efficiency. Optimizing the tooth profile and conducting research on rock-breaking efficiency under different temperatures and rocks have an important role in bit design and can greatly improve drilling efficiency.

A comprehensive understanding of the mechanical characteristics of deep coalbed methane reservoir rocks (DCMRR) is crucial for the safe and efficient development of deep coalbed gas resources. In this study, the microstructural and mechanical features of the coal seam roof, floor, and the coal seam itself were analyzed through laboratory experiments. The impact mechanisms of drilling fluid and fracturing fluid hydration on the mechanical properties and failure behavior of coal seam rocks were investigated. The experimental results indicate that the main minerals in coal seams are clay and amorphous substances, with kaolinite being the predominant clay mineral component in coal seam rocks. The rock of the coal seam roof and floor exhibits strong elasticity and high compressive strength, while the rock in the coal seam section shows a lower compressive capacity with pronounced plastic deformation characteristics. The content of kaolinite shows a good correlation with the mechanical properties of DCMRR. As the kaolinite content increases, the strength of DCMRR gradually decreases, and deformability enhances. After immersion in drilling fluid and slickwater, the strength of coal seam rocks significantly decreases, leading to shear fracture zones and localized strong damage features after rock compression failure. The analysis of the mechanical properties of DCMRR suggests that the horizontal well trajectory should be close to the coal seam roof, and strong sealing agents should be added to drilling fluid to reduce the risk of wellbore collapse. Enhancing the hydration of slickwater is beneficial for the formation of a more complex fracture network in deep coalbed methane reservoir. [ABSTRACT FROM AUTHOR]

The exploration of Hot Dry Rock (HDR) geothermal energy is essential to fulfill the energy demands of the increasing population. Investigating the physical and mechanical properties of heated rock under different cooling methods has significant implications for the exploitation of HDR. In this study, ultrasonic testing, uniaxial strength compression experiments, Brazilian splitting tests, nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) were conducted on heated granite after different cooling methods, including cooling in air, cooling in water, cooling in liquid nitrogen, and cycle cooling in liquid nitrogen. The results demonstrated that the density, P-wave velocity (Vp), uniaxial compressive strength (UCS), tensile strength (σt), and elastic modulus (E) of heated granite tend to decrease as the cooling rate increases. Notably, heated granite subjected to cyclic liquid nitrogen cooling exhibits a more pronounced decline in physical and mechanical properties and a higher degree of damage. Furthermore, the cooling treatments also lead to an increase in rock pore size and porosity. At a faster cooling rate, the fracture surfaces of the granite transition from smooth to rough, suggesting enhanced fracture propagation and complexity. These findings provide critical theoretical insights into optimizing stimulation performance strategies for HDR exploitation.

Heterogeneity is an essential feature of rock, while most rock mechanics models only adopt conventional rock physical properties and ignore the influence of heterogeneity. Quantifying rock heterogeneity is of great significance for accurately predicting rock mechanical properties. This paper introduced homogeneous coefficient to quantify rock heterogeneity and designed a model for automatically evaluating rock homogeneous coefficients by combining computer tomography and image recognition technology. In addition, this paper optimized the model parameters by correlation analysis and sensitivity analysis to predict various rock mechanical properties. When using rock homogeneity coefficient as a model parameter, the predicted loss value of rock strength is 2.95%, which is 25.7% lower than that of the model without rock homogeneity coefficient. The predicted loss value of elastic modulus is reduced by 3.9%, while it is almost unchanged for Poisson’s ratio. Empirical equations are also proposed adopting optimized model parameters, providing reliable predicted results as accurate as artificial neural network model. The Pearson correlation coefficient for rock strength could reach 0.9537. In this paper, the mature technology combination in the computer field is applied to predict rock mechanical properties, which shows great application prospects. [ABSTRACT FROM AUTHOR]

Rock mechanical properties are complex but essential for reservoir stimulation. Both experimental and empirical equation method have inevitable defects, and artificial neural network model (ANN) shows great potential in rock mechanics. This paper established a preliminary rock database from the Keshen block in Tarim Basin and improved clustering algorithm to assess the similarity between wells, creating sub-databases and realizing data sharing among similar wells. 18 parameters are used to predict ten rock mechanical properties and corresponding model inputs are optimized by correlation and sensitivity analysis separately. The results show that more accurate predictions can be obtained using optimized sample data and model inputs. The average predicted loss value using all 31 wells (221 samples) was 7.28%, while the value was reduced to 5.4% after clustering analysis using seven wells (95 samples). The average loss value can be further reduced to 4.3% using the optimized model inputs. As for rock peak stress, the predicted loss value is 2.5%, the determination coefficient (R2) is 0.983, and the root mean square error (RMSE) is 0.03. By optimizing training method, improving training sample quality, and optimizing model inputs, the problem of overfitting can be alleviated and a more reliable ANN model can be obtained. Highlights: Defined homogeneity coefficient based on computed tomography and image recognition, which is important for predicting rock mechanical properties. A preliminary database was established based on the rock samples from 31 wells for the training of the artificial neural network model. Improved clustering algorithm and performed similarity analysis between wells, created sub-databases, and improved data quantity and quality. 18 parameters are used to predict 10 rock mechanical properties, and model inputs are optimized by correlation and sensitivity analysis. [ABSTRACT FROM AUTHOR]

Rocks may be continually unloaded and loaded until they break in subsurface engineering tasks such as tunnel excavation. To investigate the effect of pre-peak constant amplitude cyclic loading (PPCACL) on fatigue damage and rock fracture features, three-point bending (TPB) experiments were performed on granite samples of varying amplitudes subjected to pre-peak constant amplitude cyclic loading (PPCACL). The calculation technique of fracture toughness formula is innovated and changed in conjunction with theoretical analysis, based on experiments and original standards, and AE events in the fracturing process are monitored by AE technology. The results demonstrate that in the instance of PPCACL, the first cycle of PPCACL causes the most damage to the rock. When the test results are compared, the damage to the rock specimen increases as the amplitude increases. After PPCACL, the sample was monotone loaded by manipulating crack mouth opening displacement (CMOD) until it was destroyed. With increasing PPCACL amplitude, the fracture energy, tensile strength, fracture toughness, and monotone loading peak of granite samples all decreased. Meanwhile, increasing the PPCACL amplitude increased the brittleness of rock samples and flattened the fracture surface. [ABSTRACT FROM AUTHOR]

To reveal the mechanical properties of the rocks of the Fengcheng Formation, the rocks of the Lower Permian Fengcheng Formation in the Mahu Sag of Junggar Basin were taken for the laboratory mechanical tests to obtain mechanical behaviors of the rocks, the influence of mineral compositions on strength parameters of the rocks of the Fengcheng Formation was then discussed. The rocks of the Fengcheng Formation in the Mahu Sag have strong heterogeneity, which results in obvious differences of mechanical properties of rocks with different lithologies. The mechanical strength and elastic modulus of dolomitic rocks are higher, but the Poisson's ratio is lower. The form of rock fracturing is relatively simple. Under uniaxial conditions, rock samples have strong brittleness, which is characterized by tensile fracturing, while under high confining pressure, rock samples have weaker brittleness and enhanced ductility, which is mainly characterized by single shear fracturing. The compressive strength, tensile strength and fracture toughness of rocks decrease with the increase of siliceous mineral content, while increase with the increase of calcium mineral content.

Hydraulic fracturing is a popular method used in the petroleum industry to increase the well performance by improving the permeability of the reservoir. However, while there has been extensive research on the development of the length of the fracture, the fractured height has been frequently assumed to be equal to the reservoir thickness. The objective of this paper is to study the influence of formation rock characteristics and the impact of underground stress state on the development of the fracture height. To achieve this objective, a finite element model was built using a cohesive zone method to predict the development of fracture height in time and space. Different scenarios were then effectuated by varying the values of the primary formation variables which are the Young's modulus, the porosity, the Poisson ratio, the fracture energy, and the maximum horizontal stress of the reservoir and of the beddings. This research therefore covered principally uncontrolled factors which are the formation properties and stress state underground, which contribute mostly to the erroneous prediction in fracture height. The findings revealed that the fracture height was strongly influenced by the properties of the formation and of the adjacent layers. However, the influence levels are not the same for different kinds of properties. This study showed that the most influential mechanical property of the rock on the fracture height is the Young's modulus. Regarding the porosity, its effect on the fracture height is extremely small. However, it is worth noting that the porosity is still an important parameter in hydraulic fracturing because it can be used to estimate others parameters and to model the development of fracture geometry which are width, length, and height. Practical suggestions for real-life hydraulic fracturing jobs can be deduced from this study, in order to control the fracture height as accurately as possible, and to decrease financial cost by concentrating mostly on the high influential factors instead of doing all kinds of tests for other less influential mechanical properties of the rock. [ABSTRACT FROM AUTHOR]

The fracture network of a deep geothermal reservoir forms the place for heat exchange between injected fluid and rock mass with high temperature. The fracture resistance ability of reservoir rocks will affect the formation of fracture-network structure, heat exchange and transmission characteristics, and reservoir mechanical stability. However, there are few reports on the fracture toughness and trajectory prediction of geothermal reservoirs with different depths. In this paper, the modified maximum tangential stress criterion (MMTS) is analyzed. The results show that the experimental data are significantly different from the theoretical estimate of MMTS under the influence of different occurrence depths. It is found that the fracture process zone (FPZ) seriously affects the accuracy of predicting fracture initiation angle and mixed-mode (I+II) fracture toughness by MMTS. The FPZ value, considering the influence of different occurrence depths, is modified, and the accuracy of MMTS in predicting the fracture mechanical characteristics of granite is improved. In addition, the mechanical test results show that the Brazilian splitting strength (σt) of granite fluctuates increase with the increase in temperature. With the increase in deviatoric stress, the Brazilian splitting strength and the Brazilian splitting modulus of rock show a trend of first increasing, then decreasing, and then increasing. [ABSTRACT FROM AUTHOR]

Acidizing is one of the most used stimulation techniques in the petroleum industry. Several reports have been issued on the difficulties encountered during the stimulation operation of the Ahdeb oil field, particularly in the development of the Mishrif reservoir, including the following: (1) high injection pressures make it difficult to inject acid into the reservoir formation, and (2) only a few acid jobs have been effective in Ahdeb oil wells, while the bulk of the others has been unsuccessful. The significant failure rate of oil well stimulation in this deposit necessitates more investigations. Thus, we carried out this experimental study to systematically investigate the influence of acid treatment on the geomechanical properties of Mi4 formation of the Mishrif reservoir. The acid core-flood experiments were performed on seven core samples from the oil reservoir in central Iraq. The porosity, permeability, acoustic velocities, rock strength, and dynamic elastic parameters were computed before and after the acidizing treatment. To determine the optimal acid injection rate, different injection flow rates were used in the core-flooding experiments. The propagation of an acid-induced wormhole and its effect on the rock properties were analyzed and compared to that of intact rocks. Computed tomography (CT) scan and a 3D reconstruction technique were also conducted to establish the size and geometry of the generated wormhole. To analyze the influence of mineralogical variation and heterogeneity and confirm the consistency of the outcomes, acidizing experiments on different rock samples were conducted. The results demonstrate that for all the rock samples studied, the mechanical properties exhibit rock weakening post-acid treatment. The Young's modulus reduced by 26% to 37%, while the Poisson's ratio, the coefficient of lateral earth pressure at rest, and the material index increased by 13% to 20%, 23% to 32%, and 28% to 125%, respectively. The CT scan visually confirmed that the acid treatment effectively creates a pathway for fluid flow through the core. [ABSTRACT FROM AUTHOR]

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Copyright of Mining & Metallurgy (10057854) is the property of Beijing Research Institute of Mining & Metallurgy Technology Group 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.)

Major breakthroughs have been made in the exploration and development of shale gas in the Longmaxi Formation in the southern Sichuan Basin, China. The previous have adopted various methods to study the fracturability of shale, but the quantitative characterization of the comprehensive properties of shale during hydraulic fracturing is still difficult. For this reason, in this study, taking the Longmaxi Formation shale in the Changning and Luzhou Blocks in the southern Sichuan Basin as an example, the mineral composition, petrophysical properties, rock mechanical properties and in-situ stress of the shale were systematically studied using X-ray diffraction, pulsed porosity-permeability analysis, rock mechanics and in-situ stress tests. Furthermore, the brittle mineral content, elastic modulus, and in -situ stress parameters were calculated, and the Analytic Hierarchy Process (AHP) method was adopted to establish a comprehensive evaluation index of shale fracturability. The research results show that the shale of the Longmaxi Formation in southern Sichuan is dominated by mixed shale facies, and it has the characteristics of high content of brittle minerals, low porosity, large compressive strength and Young’s modulus, and small value of in-situ stress difference coefficient. Therefore, the Longmaxi Formation shale has good fracturing conditions. TOC and quartz contents have important effects on the fracturability of the Longmaxi shale. The analytic hierarchy process was adopted to determine the weight coefficients of the modulus brittleness index, mechanical brittleness index, in-situ stress difference coefficient, rock compressive strength and TOC content. Furthermore, a comprehensive evaluation index of fracturability was constructed. Calculations show that the comprehensive fracability indices of shale in the Changning and Luzhou Blocks are 0.49 and 0.59, respectively. Moreover, the quality of shale in the Luzhou Block is better than that in the Changning Block, which is related to the organic matter and quartz content and the microscopic pore structures inside the shale.

Comprehensive research on reservoir rock mechanics and in-situ stress properties combined with petrophysical experiments, logging models and numerical simulation is an important means to achieve efficient development of tight sandstone oil reservoirs. In this study, a large number of rock mechanics and acoustic experiments, full-wave train array acoustic wave tests, hydraulic fracturing data and three-dimensional finite element simulations were used to study the rock mechanical properties and in-situ stress characteristics of continental tight oil reservoirs in the Yanchang Formation. The results show that under uniaxial conditions, the tight sandstone samples mainly suffer from tensional ruptures. With the increase of confining pressure, the tight sandstone samples undergo obvious shear ruptures. When the confining pressure is loaded to 35 MPa, a typical vertical shear fracture will be formed. The hydraulic fracturing calculation results show that the in-situ stress state of the target layer satisfies σv (vertical principal stress)>σH (maximum horizontal principal stress)>σh (minimum horizontal principal stress). Based on the results of rock mechanics and acoustic tests, we have constructed the dynamic and static mechanical parameter conversion models of tight oil reservoirs and the logging interpretation model of current in-situ stress. Furthermore, the finite element method is used to simulate the three-dimensional structural stress field of the target layer. The simulations show that the horizontal principal stress distribution in the work area is consistent with the applied environmental stress. The σH of the target layer is mainly distributed in 32–50 MPa, and the σh is mainly distributed in 20–34 MPa. Both σH and σh are relatively high in the southern uplift of the work area; among them, σH is usually greater than 44 MPa, and σh is usually greater than 24 MPa. The northern part of the study area developed several grooved areas with relatively low stress values. The regions with high stress values are often distributed in bands, which may be related to the compression caused by the deformation of the strata. For shear stress, left-handed and right-handed regions usually alternate with each other. However, the extent of the left-handed area in the southern uplift area is larger than that of the right-handed area, indicating that the tight oil reservoirs in the study area are mainly affected by left-handed activities.

Perhaps the most critical challenge faced during drilling operations is related to the stability of the well. Additionally, drilling mud plays a crucial role in wellbore stability, as one of its main uses is to support the wellbore wall during the drilling operation. However, ignorance of the effects of drilling mud on the mechanical properties of rock formation can also lead to well failure. The stability of the wellbore is also influenced by pore pressure during the drilling process. The analysis of changes in rock poroelastic parameters after drilling mud saturation was found to be useful regarding the abovementioned issues. Therefore, the measurement of the dynamic Young’s modulus, Poisson’s ratio and Biot’s coefficient of sandstone samples was carried out to determine their trends of variations with confining pressure in different conditions such as dry, water and drilling mud filtrate saturation. The findings indicate that both the dynamic Young’s modulus and the Poisson’s ratio of the sandstone rock increased after saturation with water and drilling mud filtrate, while the Biot’s coefficient was reduced. Furthermore, the velocity of the P wave, the dynamic Young’s modulus and the dynamic Poisson’s ratio of the sandstone rock were proportional to the confining pressure, while the Biot’s coefficient were inversely proportional to the confining pressure. The results imply that effective stress calculation can be influenced by changes in poroelastic parameters established from geophysical measurements, and risk management of wellbore stability stability was increased. [ABSTRACT FROM AUTHOR]

The fracture network of a deep geothermal reservoir forms the place for heat exchange between injected fluid and rock mass with high temperature. The fracture resistance ability of reservoir rocks will affect the formation of fracture-network structure, heat exchange and transmission characteristics, and reservoir mechanical stability. However, there are few reports on the fracture toughness and trajectory prediction of geothermal reservoirs with different depths. In this paper, the modified maximum tangential stress criterion (MMTS) is analyzed. The results show that the experimental data are significantly different from the theoretical estimate of MMTS under the influence of different occurrence depths. It is found that the fracture process zone (FPZ) seriously affects the accuracy of predicting fracture initiation angle and mixed-mode (I+II) fracture toughness by MMTS. The FPZ value, considering the influence of different occurrence depths, is modified, and the accuracy of MMTS in predicting the fracture mechanical characteristics of granite is improved. In addition, the mechanical test results show that the Brazilian splitting strength (σt) of granite fluctuates increase with the increase in temperature. With the increase in deviatoric stress, the Brazilian splitting strength and the Brazilian splitting modulus of rock show a trend of first increasing, then decreasing, and then increasing.

Affected by multiple factors such as magma composition, eruption method, surrounding environment of volcanic crater, diagenesis, and later transformation, volcanic rocks have the characteristics of multiple rock types, complex lithology, rapid changes in lithofacies, and large differences in physical properties and pore structures. There are very few studies on the evaluation of volcanic rock mechanical properties and in-situ stress. In this paper, taking the Permian volcanic rock oil and gas reservoir in Xinjiang Oilfield as an example, we have conducted a systematic study on the rock mechanical properties of volcanic rock and the in-situ stress logging evaluation method, based on clarifying the type of volcanic rock. The study found that the volcanic rocks in the study area include volcanic lava, pyroclastic lava, pyroclastic and plutonic intrusive rocks. The porosity of volcanic rock reservoirs is mainly distributed between 0.1% and 15.9%, with an average of 3.3%. The permeability is mainly distributed from O.OlmD to 597mD, with an average value of 7.31mD. According to the rock mechanics experiment and logging interpretation, the dynamic and static mechanical parameter conversion model of volcanic rock is established. At the same time, the Terzaghi model, Newberry model, Huang model and Mohr-Coulomb model are selected to perform logging evaluation of the in-situ stress of the target formation. The applicability of the horizontal in-situ stress model is analyzed, and it is found that the trend of the isotropic model curve and the pore pressure curve are basically the same. This shows that the pore pressure is greatly affected, and it is difficult to reflect the true horizontal stress change trend of the formation. However, the curve of the Mohr-Coulomb-based in-situ stress model changes drastically, and it is greatly affected by the larger friction angle, and the curve is prone to distortion. Therefore, the Huang's model was finally selected to evaluate the horizontal ground stress. The average absolute error between the rock fracture pressure calculated in this study and the actual fracture pressure measured during fracturing construction is 1.65 MPa, and the average relative error is 4%, indicating that the log calculation results are reliable. [ABSTRACT FROM AUTHOR]

Rock anisotropy is an intrinsic property of natural rock mass, and layered rock has the most significant effect on the stress distribution and deformation of a rock mass. With the development of rock mechanics theory and constitutive theory, the study of rock anisotropy has become one of the focuses and hotspots in the field of rock mechanics. Based on the rock isotropic self-consistent theory, the rock anisotropy theory of different media such as triclinic anisotropy, monoclinic anisotropy, and orthorhombic anisotropy has been greatly developed. Rock mechanical property test, not only limited to the traditional single-axis, triaxial compression test, Brazilian split test and pure bending beam tensile test, and meso-structural characteristic test (XRD, ultrasonic test, etc.), is also widely used. With the development of numerical analysis theory and computer technology, numerical analysis methods such as finite difference method, discrete element method, finite element method, boundary element method, and interface element method have played an important role in the study of rock anisotropy characteristics. According to the assumptions and reasoning methods of anisotropic failure criterion, it can be classified into two categories: continuous failure criterion and discontinuous piecewise failure criterion. Continuous medium failure criterion can also be classified into mathematical and empirical ones. Because of the complexity of rock anisotropy, the research on yield criterion and constitutive relation of rock anisotropy is still in the stage of theoretical research, and it is seldom applied in actual rock engineering. The physics model of rock has an important influence on the study of the anisotropic characteristics of rock. Therefore, the establishment of an appropriate rock physics model has a direct impact on the accurate determination of rock anisotropy parameters and P-S wave velocity. Finally, the key and difficult points of anisotropic rock research and the direction of future research are discussed and analyzed to provide a reference for engineering practice and related problems. [ABSTRACT FROM AUTHOR]

A large set of continental shale reservoirs has been developed in the Xujiahe Formation of the western Sichuan Basin, China, which has great potential for shale gas exploration and development, but the current research level is still shallow. Based on the current situation, this paper combines X-ray diffraction, physical properties, rock mechanics and acoustic test experiments to comprehensively study the petrophysical properties of continental shale in the western Sichuan Basin, China. The results show that the main reservoir space of the study shale is organic pores, clay mineral intercrystalline pores, dissolution pores and microcracks. The static Young's modulus of the potential shale reservoir is large, and there is no obvious positive correlation between the dynamic and static Young's modulus of the non-reservoir shale. The Fp/Fs value of the shale is around 1.6. The shale Fp/Fs has a positive correlation with porosity and permeability. According to the relationship between longitudinal wave and shear wave velocity, the study shale is a clay-rich siliceous shale facies, which has similar petrophysical properties to the high-yield shale in the Fort and Barnett regions of the United States. The brittleness of the study shale is close to the brittle shale area and the calcareous shale area, indicating that the brittleness index of the study continental shale is higher, and its brittleness is slightly lower than that of the Barnett high- yield marine shale in the United States. [ABSTRACT FROM AUTHOR]

PetroChina’s Xinjiang oilfield has a large quantity of tight oil reserves and hydraulic fracturing technology has been widely used to achieve commercial production. Some parts of this tight glutenite formation are laumontite-rich and the actual productivity of the hydraulically fractured wells is less than expected. To figure out the ways that laumontite affects tight glutenite well productivity, comprehensive experimental and numerical simulation studies have been conducted to investigate the rock mechanical properties, fluid flow behaviors and the major controlling factor of productivity. Laboratory results indicate that the tight glutenite formation with higher laumontite content has higher initial porosity, permeability but lower yield strength and more severe stress sensitivity in both permeability and fracture conductivity. For laumontite-rich glutenite rocks, there are commonly three types of rock deformation during the loading process: elastic compression, shear dilation and shear enhanced compaction. Both elastic compression and shear enhanced compaction will cause the reduction on rock porosity and permeability. A fully coupled finite element model (FEM) considering stress-induced permeability evolution was introduced to simulate the production process. Permeability evolution models of three different deformation stages were presented, respectively. Simulation results showed that our model is in good agreements with the well testing data. The simulated oil production characteristics for permeability evolution coupled and uncoupled models were discussed. Results showed the strong stress-induced permeability reduction is the major factor that laumontite causing the low and quickly declining oil rates. Initial permeability has a positive effect on productivity and stress-induced fracture conductivity reduction has slight influence on productivity. The results of this paper indicate that the stress-induced permeability evolution in the oil production process must be considered to accurately evaluating reservoirs in the studied area.

• Acid stimulation can alter the geomechanical and petrophysical properties of carbonate rocks. • High dissolution rate of the injected acid with carbonate minerals was reported for various rock types. • Rocks deformation increases after acidizing resulting in reducing the rock surface hardness. • Mineralogy alteration after acidizing can reduce fracture conductivity, permeability, and overall productivity. • The effectiveness of the acidizing treatment is affected by acid system, injection rate and rock type. Acid treatment is a widely used stimulation technique in the petroleum industry. Matrix acidizing is regarded as an effective and efficient acidizing technique for carbonate formations that leads to increase the fracture propagation, repair formation damage, and increase the permeability of carbonate rocks. Generally, the injected acid dissolves into the rock minerals and generates wormholes that modify the rock structure and enhance hydrocarbon production. However, one of the key issues is the associated degradation in the mechanical properties of carbonate rocks caused by the generated wormholes, which may significantly reduce the elastic properties and hardness of rocks. There have been several experimental and simulation studies regarding the impact of acid wormholes on rock weakening in various carbonate rocks (chalk, limestone, and dolomite). However, considering the number of effective parameters which are associated with matrix acidizing, it is crucial to carefully monitor the acidizing procedure as well as perform a post-treatment evaluation of the targeted rocks. This work aims to review the fundamentals of matrix acidizing as a viable stimulation technique for carbonate rocks and provide deeper insight into the potential alterations in geo-mechanical properties. This review also evaluates a group of key parameters including acid type, injection rate, wellbore geometry, and rock type, and highlights the various analytical techniques used for acid stimulation evaluation in carbonate rocks. [ABSTRACT FROM AUTHOR]

Reactive transport is simulated on rock geometries to explore the variation of mechanical properties of porous media. A numerical framework is developed to model reactive transport in sandstones and carbonates. Fluid flow, solute transport and chemical reactions are simulated directly on micro-CT images. Porosity profiles along the flow direction are computed during the dissolution to describe the change in the pore structures. Stress load cases on porous media are simulated and maps of deformation in the rocks are compared. Uniform deformation is observed in high Péclet regimes. Young’s modulus and Poisson’s ratio are calculated separately for a range of Péclet and Damköhler regimes. The findings show that mechanical properties in low Péclet regimes are more sensitive to porosity variations. For the same reaction regime, carbonates present a lower decrease in Young’s modulus after reactions but a higher decline in Poisson's ratio in comparison with sandstones. This work reveals the strong dependency of mechanical properties on Péclet and Damköhler regimes in reactive transport. [ABSTRACT FROM AUTHOR]

In mountain geosystems, rockfalls are among the most effective sediment transfer processes, reflected in the regional-scale distribution of talus slopes. However, the understanding of the key controlling factors seems to decrease with increasing spatial scale, due to emergent and complex system behavior and not least to recent methodological shortcomings in rockfall modeling research. In this study, we aim (i) to develop a new approach to identify major regional-scale rockfall controls and (ii) to quantify the relative importance of these controls. Using a talus slope inventory in the Turtmann Valley (Swiss Alps), we applied for the first time the decision-tree based random forest algorithm (RF) in combination with a principal component logistic regression (PCLR) to evaluate the spatial distribution of rockfall activity. This study presents new insights into the discussion on whether periglacial rockfall events are controlled more by topo-climatic, cryospheric, paraglacial or/and rock mechanical properties. (i) Both models explain the spatial rockfall pattern very well, given the high areas under the Receiver Operating Characteristic (ROC) curves of > 0.83. Highest accuracy was obtained by the RF, correctly predicting 88% of the rockfall source areas. The RF appears to have a great potential in geomorphic research involving multicollinear data. (ii) The regional permafrost distribution, coupled to the bedrock curvature and valley topography, was detected to be the primary rockfall control. Rockfall source areas cluster within a low-radiation elevation belt (2900–3300 m a.s.l,) consistent with a permafrost probability of > 90%. The second most important factor is the time since deglaciation, reflected by the high abundance of rockfalls along recently deglaciated (< 100 years), north-facing slopes. However, our findings also indicate a strong rock mechanical control on the paraglacial rockfall activity, declining either exponentially or linearly since deglaciation. The study demonstrates the benefit of combined statistical approaches for predicting rockfall activity in deglaciated, permafrost-affected mountain valleys and highlights the complex interplay between rock mechanical, paraglacial and topo-climatic controls at the regional scale. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing operation success is critically dependent on pre-operation fracture geometry analysis. Meantime, hydraulic fracture opening determination is crucial because it deals with maintaining sufficient aperture and efficient communication pathways to accomplish proppant placement and avoid screen-outs or proppant bridging. It is well understood that, to efficiently estimate the hydraulic fracture opening, a thorough understanding of the impact of rock mechanical properties as well as operational parameters such as injection fluid viscosity and leak-off are essential. In this study, a three-dimensional model has been developed for parametric study of fracture stiffness, formation elastic modulus, formation Poisson's ratio, tensile strength of the rock, fracturing fluid viscosity and leak-off, and their influences on hydraulic fracture opening. Cohesive elements with traction-separation law have been applied to simulate the fracturing process in a fluid-solid coupling model. The maximum nominal stress criterion has been selected for initiation of damage in the cohesive elements. The results revealed that any change in influential rock mechanical properties as well as operational parameters would significantly lead to increasing or decreasing the hydraulic fracture opening. It was observed from the controllable HF parameters including fracturing fluid viscosity and leak-off, by increasing fracturing fluid viscosity from 10 to 10 Pa.s, fracture width has been moderately increased from 8.83 to 13.3 mm; on the other hand, by reducing leak-off coefficient as a polymer-dependent HF parameter from 5 × 10 to 5 × 10 m/kPa.s, fracture width has been gradually risen from 7.27 to 11.52 mm. In addition, all the uncontrollable rock mechanical parameters such as fracture stiffness, Young's modulus, Poisson's ratio, and tensile strength have steeply increased or decreased the hydraulic fracture opening and consequently, all of their analysis will be discussed in detail in this research. The results from this study can be applied to hydraulic fracturing jobs in different conditions of fracture stiffness, formation elastic modulus, formation Poisson's ratio, tensile strength of the rock, fracturing fluid viscosity, and leak-off. [ABSTRACT FROM AUTHOR]

In terms of the problem that deep coal resources are under the threats of high water pressure, it is necessary to further study the outburst mechanism of working face floor karst water from the view of studying varying patterns of rock mechanical parameters and confined water pressure distribution of floor rock under the mining influence in the process of mining. Through the theoretical research of fluid-solid coupling and rock seepage experiment, the changing rule of the rock porosity and the Young's modulus is analyzed, the changing law of the floor displacement is obtained by establishing the floor mechanic model, and the relation between elastic energy density and rock mass unit strain is established. Based on the numerical model of fluid-solid coupling analysis by the Comsol Multiphysics software, the changing law of floor displacement corresponds to the theoretical solution, the different variation laws of rock mass of different depths under the actions of ground pressure are acquired by simulation calculation, the distribution characteristics of pore water pressure are discussed, and the elastic energy density of the rock mass under the action of imbalance water pressure before and after mining is comparatively analyzed. Studies show that the distribution of pore water pressure is horizontally different before and after the stress peak line, and it mainly has four distribution patterns vertically. When mining above the confined water, the spatiotemporal position when and where the upper and lower boundaries of the rock mass in the lower water-resisting layer release energy at the same time is critical to the occurrence of water inrush. [ABSTRACT FROM AUTHOR]

Cryogenic fracturing is a type of thermal shocking in which a cold liquid or gas is injected into a hot formation to create fractures. Research has shown that like traditional hydraulic fracturing, cryogenic fracturing could improve oil/gas recovery from unconventional reservoirs. Research has also shown, though, that, unlike traditional hydraulic fracturing, which uses water-based fluids, cryogenic fracturing limits and can even heal damage that is near the wellbore. Previous studies on thermal shocking, however, have generally examined only a few parameters at a time. To provide a more complete overview of the process, this study examines the effects of thermal shocking with low-temperature nitrogen gas on the porosities, permeabilities, and rock mechanical properties of unconventional reservoirs. Three cycles of thermal shocking were applied to a core sample and an outcrop sample from an unconventional reservoir. Each sample was heated at 82 °C for 1 h, and then nitrogen at −18 °C was injected at 6.89 MPa for 5 min. The porosities and permeabilities of the cores and the velocities at which ultrasonic waves travelled through them were measured both before and after each thermal shock. The results strongly suggest that the thermal shocking produced cracks. The porosity increased by between 1.34% and 14.3%, the permeability increased by between 17.4% and 920%, and the average P-wave velocity decreased by up to 100 m/s. From the reduction in P-wave velocity, it was determined that the brittleness ratio increased by between 2 and 4 and the fracability index increased by between 0.2 and 0.8.

Understanding rock mechanical properties is essential to many engineering geology applications. Comprehensive rock mechanics testing, however, is often not practical due to limited available sample volume, time constraints, and cost. Therefore, an alternative means for estimating mechanical properties at relatively low cost is desirable. In this investigation, we evaluate microrebound measurement on cut rock surfaces as a substitute for rock mechanical testing. We first explore the influences of rock sample thickness and testing substrate (material upon which samples rest) on microrebound, then perform systematic microrebound analysis of a range of well-characterized rock types with associated rock-mechanics datasets. Our analyses show that microrebound measurements are not sensitive to sample thickness greater than approximately 5 cm. Microrebound appears to be insensitive to the substrate among tested substrates including 2-mm-thick foam rubber, three different thicknesses of polystyrene foam (1.1 cm, 2.2 cm, and 3.6 cm), and standard lab tabletop. Strongest relationships between microrebound and rock properties are for unconfined compressive strength, tensile strength, and cohesion. Moderately strong correlations were found for density, Young's modulus, and porosity. A weak correlation was found for Poisson's ratio, and a very weak correlation was found for friction angle. Microrebound data can be obtained quickly and in a non-destructive manner using cut samples collected from outcrop or core, and at relatively low cost compared to traditional rock mechanics tests. Microrebound measurement can substitute for rock mechanics measurements for a multitude of applications in engineering geology where rock property data are needed, but full rock mechanics testing is not practical. • Robust rock property data can be derived via systematic microrebound analyses. • Microrebound is insensitive to substrate and sample thickness for thickness > 5 cm. • Microrebound predicts unconfined compressive strength, tensile strength, & cohesion. • Microrebound exhibits positive correlations with density and Young's modulus. • Microrebound exhibits negative correlations with porosity and Poisson's ratio. [ABSTRACT FROM AUTHOR]

Evaluation of the gas shale mechanical properties is very important screening criteria for determining the potential intervals for hydraulic fracturing and as a result in gas shale sweet spot mapping. Young's modulus and Poisson's ratio are two controlling mechanical properties that dictate the brittleness of the gas shale layers. These parameters can be determined in the laboratory by testing the rock sample under different conditions (static method) or can be calculated using the well-logging data including sonic and density log data (dynamic method). This study investigates the importance of the shale composition and geochemical parameters on the Young's modulus and Poisson's ratio using log data. The data set of this study is coming from five different wells targeting the Kockatea Shale and Carynginia formation, two potential gas shale formations in the Perth Basin, Western Australia. The results show that converse to the common idea the effect of organic matter quantity and maturity on the rock mechanical properties of the gas shale reservoirs is not so much prominent, while the composition of the rock has an important effect on these properties. Considering the weight percentage of shale composition and organic matter quantity it could be concluded that effect of these parameters on rock mechanical properties is dependent on their weight contribution on the shale matrix. As well as effect of thermal maturity on the shale matrix and consequently on the rock mechanical properties of the shales is dependent on the organic matter content itself; therefore, obviously with a low organic matter content thermal maturity has no prominent effect on the brittleness as well. [ABSTRACT FROM AUTHOR]

Rock deformation parameters including Young's modulus, Poisson's ratio, shear modulus and bulk modulus ( E , υ , K and G , respectively) are important geomechanical properties that have many applications in wellbore stability studies. In this study, rock mechanical properties were predicted from well log data by using an improved Fuzzy Inference System (FIS) in the Kangan and Dalan supergiant gas reservoirs, South Pars Gas Field. Using the results of laboratory tests on 33 core samples (static value) and formation wave velocity outcomes (dynamic value) empirical correlations were developed between them. Static rock deformation parameters were calculated using the empirical correlations in all logged intervals. In the first stage of estimating rock deformation modules in un-cored wells, four individual fuzzy models were generated. The correlation coefficients between predicted and calculated E , υ , K and G in the test data were 0.89, 0.49, 0.94 and 0.82, respectively. In an effort to achieve further prediction accuracy, a larger fuzzy model which can simultaneously estimate all parameters was generated. The new model is actually an integration of four previous models in which the outputs of each individual fuzzy model besides well log data are considered as its inputs. Using such an improved fuzzy model the correlation coefficients between predicted and calculated K , E , υ and G data in the testing samples increased to 0.941, 0.993, 0.983 and 0.999, respectively. The results of this study show that there is a good agreement between real and fuzzy-logic derived rock deformation parameters. Fuzzy logic is successful in estimation of static mechanical properties from well logs in un-cored wells where laboratory experiments and shear wave velocity data are missing. [ABSTRACT FROM AUTHOR]

Highlights: [•] The prediction of UCS values of natural building stones was aimed. [•] 37 Different carbonate rocks were collected from different regions of Turkey. [•] Ultrasonic pulse velocity, Schmidt hammers hardness, Shore hardness values were used for UCS prediction. [•] The ANNs approach and conventional multivariate regression analysis were used. [•] The UCS values for carbonate rocks can be predicted successfully from ANNs models. [Copyright &y& Elsevier]

The deep shale gas reservoirs are characterized by great burial depth, high formation pressure, high temperature. During the hydraulic fracturing process, the fracturing fluids will inevitably exchange heat with the surrounding rock, causing the shale in the near-well zone to undergo temperature alternation, the physical and mechanical properties of shale reservoirs are significantly affected, which brings technical difficulties to fracturing optimization design. In this paper, the Longmaxi Formation shale is used to pay attention to the rock physical and mechanical characteristics of the shale undergo temperature alternation, the microstructure and damage and fracture characteristics of the shale undergo temperature alternation are analyzed. The results show that: (1)As the heat treatment temperature increases and the number of heating-cooling cycles increases, the permeability of the shale increases, the P-wave velocity decreases, the tensile strength (TS) decreases, and the brittleness index rises. (2) Heat treatment temperature and the number of heating-cooling cycles have little effect on the triaxial compressive strength (TCS) of shale. However, it has a significant effect on the failure mode of shale. The increase of temperature and the number of heating-cooling cycles will enhance the degree of fracture of the shale failure surface. (3) Based on industrial CT scanning and scanning electron microscopy (SEM) observations, the thermal cracks caused by heating-cooling cycles are mostly inter-granular cracks and inter-layer cracks, which unfold on the surface of the sample and extend to the interior. Heating-cooling cycles also lead to an increase in the diameter of organic pores. The study results can provide a certain theoretical basis for the optimization design of staged fracturing parameters for horizontal wells in the process of deep shale gas development. • Damage of high-temperature shale subjected to heating-cooling cycles was investigated. • Damage induced by multiple heat-cooling cycles becomes more significant than a single air cooling. • Heating-cooling cycles can lead to initiation and propagation of shale cracks. • The number of heating-cooling cycles affects the frequency and width of cracks. [ABSTRACT FROM AUTHOR]

Eagle Ford shale formations have great hydrocarbon potential to minimise the supply and demand problems of the overgrowing population. Whereas, due to reduced permeability and marginal porosity, it is very difficult to produce from these formations. Previously, hydraulic fracturing and acidizing techniques are used to produce the hydrocarbons from tight gas shale formations; however, due to technical difficulties and environmental problems have led to a ban on these techniques in many countries. Therefore, it is suggested to use water-less fracturing methods to avoid these problems. Recently, liquid nitrogen (LN 2) cooling have shown a great potential to mitigate technical and environmental flaws, which works on the principle of providing sudden thermal shocks in the pore matrix, thus inducing micro-cracks and enhancing the petro-physical potential. In this study, we have collected tight gas shale samples from the Eagle Ford shale's Upper Late Cretaceous geologic formation in the United States. Initially, these samples were exposed to LN 2 for various time intervals of 30, 60, and 90 min and were characterised based on their petro-physical, morphology, topography, and rock mechanical properties before and after LN 2 exposure. To accomplish that, various techniques were used such as core flooding, porosity determination, microstructural characterisation via field emission scanning electron spectroscopy (FESEM) and energy dispersive spectroscopy (EDS), topographic characterisation via atomic force microscopy (AFM), and rock mechanical characterisation via nano-indentation technique. It is evident from our results that LN 2 cooling has a substantive effect on petro-physical properties by increasing permeability from 12 nano-Darcy (nD) to 2.6 milli-Darcy (mD) and porosity from 2.32% to 4.46%. Similarly, topographic properties have shown an increase in surface roughness from 177 nm to 692 nm, and hardness (indentation moduli, GPa) was substantially reduced due to optimum exposure of LN 2 at 90 min. Further, thermal shocks of LN 2 cooling have induced vivid micro-cracks in the pore matrix up to 7 μm. In a nutshell, it is vital to augment the effects of LN 2 cooling on microstructural, petro-physical, topography, and rock mechanical properties for better understanding in the development of hydrocarbon potential in Eagle Ford shale formations. • Eagle Ford shale samples were acquired from upper late Cretaceous geologic formation in the United States. • Various time intervals (30, 60, and 90 min) were selected for liquid nitrogen (LN 2) cooling treatment. • Sudden thermal shocks induced by liquid nitrogen (LN 2) cooling were very effective in Eagle Ford shale development. • Topographical, rock mechanical, mineralogical, and petro-physical characteristics were argued before and after LN 2 cooling. [ABSTRACT FROM AUTHOR]

PetroChina's Xinjiang oilfield has a large quantity of tight oil reserves and hydraulic fracturing technology has been widely used to achieve commercial production. Some parts of this tight glutenite formation are laumontite-rich and the actual productivity of the hydraulically fractured wells is less than expected. To figure out the ways that laumontite affects tight glutenite well productivity, comprehensive experimental and numerical simulation studies have been conducted to investigate the rock mechanical properties, fluid flow behaviors and the major controlling factor of productivity. Laboratory results indicate that the tight glutenite formation with higher laumontite content has higher initial porosity, permeability but lower yield strength and more severe stress sensitivity in both permeability and fracture conductivity. For laumontite-rich glutenite rocks, there are commonly three types of rock deformation during the loading process: elastic compression, shear dilation and shear enhanced compaction. Both elastic compression and shear enhanced compaction will cause the reduction on rock porosity and permeability. A fully coupled finite element model (FEM) considering stress-induced permeability evolution was introduced to simulate the production process. Permeability evolution models of three different deformation stages were presented, respectively. Simulation results showed that our model is in good agreements with the well testing data. The simulated oil production characteristics for permeability evolution coupled and uncoupled models were discussed. Results showed the strong stress-induced permeability reduction is the major factor that laumontite causing the low and quickly declining oil rates. Initial permeability has a positive effect on productivity and stress-induced fracture conductivity reduction has slight influence on productivity. The results of this paper indicate that the stress-induced permeability evolution in the oil production process must be considered to accurately evaluating reservoirs in the studied area. [ABSTRACT FROM AUTHOR]

Comprehensive investigation and accurate models of geo-mechanical properties are crucial to maintain wellbore stability and optimize the hydraulic fracturing process. This review attempts to revisit the existing correlations/models and also critically summarizes the research gaps related to acoustic waves, elastic constants, and rock strength models in geomechanics. In addition, it highlights the present status of predictive models and features ranking for uniaxial compressive strength in reservoir geomechanics. It is found that rock strength models have a distinct relationship with sonic velocities and petrophysical properties of rocks. For instance, it also explains the novel implementation of machine learning-based hybrid connectionist model and variable ranking strategies to obtain accurate geomechanical properties using core and log data. It is anticipated that the proposed research approaches will enable researchers to estimate accurate rock mechanical properties and variable ranking for sanding potentiality and wellbore failure analysis in a cost-effective through a timely manner. • Existing correlations/models of sonic acoustic velocity and rock strength parameters are critically reviewed. • Data-driven models critically analyzed to find the research gaps in rock mechanical model. • Introduced novel implementation of LSSVM-CSA based hybrid model to predict geomecahnical properties. • A feature ranking strategy explained for formation evaluation and geomechanically studies. [ABSTRACT FROM AUTHOR]

Fractures have a significant effect on the mechanical properties of coal and rock, and fracture structure is complex. Combinined with indoor physical tests and corresponding numerical simulation tests, the relationships among rock mechanics parameters, cleat complexity, and wave velocity were analysed. Based on the conversion relationships between coal and rock cleat complexity and coal seam fissure complexity, four calculation models of rock mechanics parameters based on coal bed fracture complexity and wave velocity were established. The results indicate that coal with different cleat density, quantity, or angle has noticeable fractal characteristics, and cleat fractal dimension can be used to characterize the cleat complexity. The fractal dimension of deep lateral resistivity curve can characterize the fracture complexity of coal beds. There is a good linear positive correlation between cleat fractal dimension and fractal dimension of deep lateral resistivity logging curve. Velocity and cleat complexity have no significant correlation, and the attenuation coefficient increases with the increase in cleat complexity. Uniaxial compressive strength of coal and rock decrease with the increase in cleat complexity and is more sensitive to the change in cleat angle than to cleat density and cleat quantity changes. Young's modulus of coal and rock decrease with the increase in cleat complexity and is more sensitive to the change in cleat quantity than cleat density or angle changes. Poisson's ratio of coal increase with the increase in cleat complexity and is more sensitive to the change in cleat quantity than cleat density or angle changes. The tensile strength of coal and rock increases with the increase in cleat complexity and is more sensitive to the change of cleat quantity than cleat density and angle changes. Centralized distribution of cleats has more significant influence on the uniaxial compressive strength, Young's modulus, Poisson's ratio, and tensile strength of coal than an even distribution. The calculation models of rock mechanics parameters based on fracture complexity are suitable for the study area. The method of combining the fracture complexity of coal beds with acoustic wave velocity to calculate rock mechanical parameters are feasible. • Coal with different cleat distribution has noticeable fractal characteristics. • Rock mechanical parameters are significantly affected by cleat complexity. • Fracture complexity is feasible to calculate rock mechanical parameters. • Acoustic wave velocity is feasible to calculate rock mechanical parameters. [ABSTRACT FROM AUTHOR]

Cryogenic fracturing is a type of thermal shocking in which a cold liquid or gas is injected into a hot formation to create fractures. Research has shown that like traditional hydraulic fracturing, cryogenic fracturing could improve oil/gas recovery from unconventional reservoirs. Research has also shown, though, that, unlike traditional hydraulic fracturing, which uses water-based fluids, cryogenic fracturing limits and can even heal damage that is near the wellbore. Previous studies on thermal shocking, however, have generally examined only a few parameters at a time. To provide a more complete overview of the process, this study examines the effects of thermal shocking with low-temperature nitrogen gas on the porosities, permeabilities, and rock mechanical properties of unconventional reservoirs. Three cycles of thermal shocking were applied to a core sample and an outcrop sample from an unconventional reservoir. Each sample was heated at 82 °C for 1 h, and then nitrogen at −18 °C was injected at 6.89 MPa for 5 min. The porosities and permeabilities of the cores and the velocities at which ultrasonic waves travelled through them were measured both before and after each thermal shock. The results strongly suggest that the thermal shocking produced cracks. The porosity increased by between 1.34% and 14.3%, the permeability increased by between 17.4% and 920%, and the average P-wave velocity decreased by up to 100 m/s. From the reduction in P-wave velocity, it was determined that the brittleness ratio increased by between 2 and 4 and the fracability index increased by between 0.2 and 0.8. [ABSTRACT FROM AUTHOR]