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To explore the effect of bedding and initial confining pressure on the energy evolution characteristics of shale during the unloading process, samples were drilled with different bedding angles, unloading tests were conducted under different initial confining pressures, and the mechanical and energy evolution characteristics of shale during the unloading process were analyzed. The results show that the stress–strain curve of the unloading test can be divided into the linear elasticity stage, the stable crack growth stage, the accelerated crack growth stage, and the post-failure stage. Critical confining pressure can show the relative strength of the rock samples. The elastic modulus and Poisson's ratio increase with an increase in axial preset load. The elastic modulus increases with the bedding angle, and the effect of the bedding angle on the Poisson's ratio is insignificant. The energy evolution of the unloading test can be divided into three stages: energy accumulation, energy dissipation, and energy release. The larger the axial preset load, the higher the critical confining pressure, the higher the elastic modulus, and the higher the Poisson's ratio. The total energy, elastic energy, and dissipation energy all increase with the increase in the initial confining pressure, and the correlation is high. Confining pressure enhances the ability of the shale sample to store elastic energy and improves the ability of the shale sample to resist internal crack propagation. The total energy, elastic energy, and dissipated energy of the samples in the failure point decrease first and then increase with the increase in the bedding angle. The maximum value can be obtained when the bedding angle is 0°. The elastic energy and dissipated energy of shale are highly heterogeneous due to bedding, and the effects of bedding should be taken into account when exploring the law of rock deformation and failure from an energy perspective.

Abstract Background Non-pharmaceutical measures and travel restrictions have halted the spread of coronavirus disease 2019 (COVID-19) and influenza. Nonetheless, with COVID-19 restrictions lifted, an unanticipated outbreak of the influenza B/Victoria virus in late 2021 and another influenza H3N2 outbreak in mid-2022 occurred in Guangdong, southern China. The mechanism underlying this phenomenon remains unknown. To better prepare for potential influenza outbreaks during COVID-19 pandemic, we studied the molecular epidemiology and phylogenetics of influenza A(H3N2) and B/Victoria that circulated during the COVID-19 pandemic in this region. Methods From January 1, 2018 to December 31, 2022, we collected throat swabs from 173,401 patients in Guangdong who had acute respiratory tract infections. Influenza viruses in the samples were tested using reverse transcription-polymerase chain reaction, followed by subtype identification and sequencing of hemagglutinin (HA) and neuraminidase (NA) genes. Phylogenetic and genetic diversity analyses were performed on both genes from 403 samples. A rigorous molecular clock was aligned with the phylogenetic tree to measure the rate of viral evolution and the root-to-tip distance within strains in different years was assessed using regression curve models to determine the correlation. Results During the early period of COVID-19 control, various influenza viruses were nearly undetectable in respiratory specimens. When control measures were relaxed in January 2020, the influenza infection rate peaked at 4.94% (39/789) in December 2021, with the influenza B/Victoria accounting for 87.18% (34/39) of the total influenza cases. Six months later, the influenza infection rate again increased and peaked at 11.34% (255/2248) in June 2022; influenza A/H3N2 accounted for 94.51% (241/255) of the total influenza cases in autumn 2022. The diverse geographic distribution of HA genes of B/Victoria and A/H3N2 had drastically reduced, and most strains originated from China. The rate of B/Victoria HA evolution (3.11 × 10−3, P

Abstract The water-shale interaction affect the shale structure, leading to wellbore instability and increasing drilling costs. The extent of structural changes within the shale can be determined non-destructively by analyzing its acoustic characteristics. Experiments were conducted to investigate the acoustic properties of shale from the Yanchang Formation in the Ordos Basin before and after exposure to brines of varying types, soaking times, and salinities. The study investigated the effects of brine type, soaking time, and salinity on shale’s acoustic properties, including changes in acoustic wave propagation speed, P/S wave velocity ratio, and both time-domain and frequency-domain amplitudes. The results indicate that although the type of brine has a limited impact on the water-shale interaction, KCl exhibits a significant inhibitory effect. However, the soaking time and the brine salinity have a significant impact on the acoustic properties of shale. As the soaking time increases, the decrease in wave velocity increases, the P/S wave velocity ratio increases, and the decrease in time-domain amplitude increases. The amplitude of the main frequency in the frequency domain signal also decreases with the increase of reaction time, which is consistent with the analysis results of the time domain signal. As the salinity of brine increases, the decrease in wave velocity decreases, the P/S wave velocity ratio decreases, and the decrease in time-domain amplitude decreases. The amplitude of the main frequency in the frequency domain signal also decreases with the increase of brine salinity, which is consistent with the analysis results of the time domain signal. This work establishes the relationship between water-shale interaction and acoustic characteristics, which can quantitatively evaluate the degree of interaction between water and shale without damaging shale. Furthermore, this research provides new insights and guidance for predicting drilling collapse cycles and optimizing drilling fluid compositions.

Abstract During the drilling process, the issue of drilling fluid loss can lead to changes in the mechanical properties of the formation, thereby altering the stress environment of nearby faults. In order to assess the risk of fault activation during drilling operations, the Ordos M area shale was selected as the research object. Mechanical experiments were conducted on rock samples immersed in water-based drilling fluid with a pressure differential of 2 MPa and a temperature of 50 °C. The changes in the mechanical properties of the shale before and after immersion in drilling fluid were determined. Based on the experimental results, combined with the spring combination model and fault activation theory, a quantitative evaluation of fault activation risk was conducted. The findings revealed that the shale in this region has a high clay content, demonstrating a certain level of water sensitivity. The presence of micro-pores and micro-fractures is well-developed, increasing the interaction probability between drilling fluids and clay minerals. After immersion in drilling fluid, there was a varied decline in all mechanical strengths of the shale. The elastic modulus is positively correlated with the shear strength and Coulomb stress of the fault plane. The Poisson’s ratio is positively correlated with the shear strength and negatively correlated with the Coulomb stress. The greater the internal friction and cohesion, the higher the shear strength of the fault plane, and the larger the friction coefficient, the smaller the Coulomb stress, resulting in a more stable fault.

BackgroundAccurate detection of influenza virus in clinical samples requires correct execution of all aspects of the detection test. If the viral load in a sample is below the detection limit, a false negative result may be obtained. To overcome this issue, we developed a modified transport medium (MTM) for clinical sample transportation to increase viral detection sensitivity.MethodWe first validated the MTM using laboratory-stocked influenza A viruses (IAVs: H1N1, H3N2, H7N3, H9N2) and influenza B viruses (IBVs: Yamagata, Victoria). We also tested clinical samples. A total of 110 patients were enrolled and a pair of samples were collected to determine the sensitivity of real-time polymerase chain reaction (RT-PCR) following MTM treatment.ResultAfter 24 h culturing in MTM, the viral loads were increased, represented by a 10-fold increase in detection sensitivity for H1N1, H9N2, and IBVs, a 100-fold increase for H3N2, and a 1,000-fold increase for H7N3. We further tested the effects of MTM on 19 IAV and 11 IBV stored clinical samples. The RT-PCR results showed that the positive detection rate of IAV samples increased from 63.16% (12/19) without MTM culturing to 78.95% (15/19) after 48 h culturing, and finally 89.47% (17/19) after 72 h culturing. MTM treatment of IBV clinical samples also increased the positive detection rate from 36.36% (4/11, 0 h) to 63.64% (7/11, 48 h) to 72.73% (8/11, 72 h). For clinical samples detected by RT-PCR, MTM outperformed other transport mediums in terms of viral detection rate (11.81% increase, P=0.007).ConclusionOur results demonstrated that the use of MTM for clinical applications can increase detection sensitivity, thus facilitating the accurate diagnosis of influenza infection.

Abstract Rock mechanical property is significant for shale gas development and exploitation. Shale compressive strength, tensile strength, elastic deformation and so on, are necessary parameters for drilling, completion and fracturing work in shale formation. Among all these shale mechanical parameters, brittleness is a tricky and significant rock property, which has been widely used to hydraulic fracturing design. Currently, although so many works have been conducted to investigate shale brittleness, there is no precise definition of brittleness. In particular, there is no consensus on which method is the most reliable for shale brittleness evaluation. It is vital to figure out how to evaluate shale brittleness in a reliable method. Thus, this paper presents an experimental study on shale mechanical properties, analyzing mechanical features in stress strain curve, relation between mineral content and strength, mechanical parameters at varying confined stress. Based on shale mechanical characteristics and its brittle exhibition, stress strain curve from triaxial compression test is divided into 3 stages, namely, elastic stage, plastic stage and post peak stage. In combined with brittle characteristics in 3 stages of axial and radial stress–strain curves, a new brittleness index has been established for assessing shale brittleness. In order to prove the applicability of new brittleness index, its result is compared with shale failure sample after triaxial test and existing brittleness indexes based on mineral content, elastic deformation, energy, stress and strain, showing a good consistency and proving its practicability. Based on this brittleness index, influence factors of shale brittleness have been discussed. It is shown that elastic module is the most important factor of shale brittleness. Bedding plane makes shale brittleness have strong anisotropy. Brittleness is not only relied on its structure and mineral (like bedding plane, silicate and clay mineral content), but is also highly affected by external stress. Large confined pressure is able to impair shale brittleness. Outcome in this study can offer theoretical guidance for shale exploitation.

According to the transversely isotropic theory and weak plane criterion, and considering the mechanical damages due to stress unloading and hydration during drilling, a shale wellbore stability model with the influence of stress unloading and hydration was established using triaxial test and shear test. Then, factors influencing the wellbore stability in shale were analyzed. The results indicate that stress unloading occurs during drilling in shale. The larger the confining pressure and axial stress, the more remarkable weakening of shale strength caused by stress unloading . The stress unloading range is positively correlated with the weakening degree of shale strength. Shale with a higher development degree of bedding is more prone to damage along bedding. In this case, during stress unloading, the synergistic effect of weak structural plane and stress unloading happens, leading to a higher weakening degree of shale strength and poorer mechanical stability, which brings a higher risk of wellbore instability. Fluid tends to invade shale through bedding, promoting the shale hydration. Hydration also can weaken shale mechanical stability, causing the decline of wellbore stability. Influence of stress unloading on collapse pressure of shale mainly occurs at the early stage of drilling, while the influence of hydration on wellbore stability mainly happens at the late stage of drilling. Bedding, stress unloading and hydration jointly affect the wellbore stability in shale. The presented shale wellbore stability model with the influence of stress unloading and hydration considers the influences of the three factors. Field application demonstrates that the prediction results of the model agree with the actual drilling results, verifying the reliability of the model.

This study establishes a three-dimensional cohesive model of multi-cluster hydraulic fracturing using finite element method (FEM). It fully considers the interaction between the interlayer and the reservoir and analyzes the key factors influencing fracture propagation. The results show that during the initial stage of hydraulic fracturing, the width of the edge fracture is greater than that of the mid fracture, while the situation is reversed for the fracture length. A larger cluster spacing leads to less interaction between fractures, while a greater number of clusters increases the interaction between fractures. With an increase in displacement, the lost fracturing fluid entering the formation enhances the interaction between fractures. An increase in elastic modulus results in a decrease in the width and height of edge fractures but an increase in their length, with little impact on mid fractures. As Poisson’s ratio increases, there is little change in the fracture morphology of edge fractures, while the width and height of mid fractures increase significantly. With an increase in permeability, the influx of fracturing fluid into the interlayer decreases, leading to a reduction in the interaction between fractures. Finally, the study analyzes and discusses the impact of these parameters on the SRV (stimulated reservoir volume) in both the reservoir and the interlayer. These findings provide new insights for hydraulic fracturing and contribute to improving its productivity.

The nonhomogeneity of conglomerate in terms of organization and the complexity of fracture extension make the design and effective implementation of fracturing in conglomerate reservoirs challenging. Considering the limitations of physical experiments and two-dimensional (2D) numerical modeling, this paper adopts the continuum-discontinue element method (CDEM) to carry out numerical simulation of three-dimensional (3D) conglomerate fracturing considering pore-fracture seepage. By introducing multiple parameters to quantify the correlation between fracture geometry, fracture complexity, and damage mode, the evolution mechanism of fracture morphology under the influence of multiple factors is systematically investigated. The results show that the numerical simulation experiments can control the variables well, but the random distribution of gravel leads to the unpredictability of fracture extension, and the concluding patterns obtained still show large fluctuations. The high permeability of the gravel promotes the development of gravel-penetrating fractures but has less effect on the overall morphology of the fractures. High-strength gravel promotes the development of branching fractures in the initiation phase, which acts as a barrier to expanding fractures, and the most complex fracture development occurs when the gravel strength is approximately 4–5 times that of the matrix. In the weakly cemented state, fracture development around the gravel contributes to the shear failure of the conglomerate, but the strength of the cemented interface has no obvious control on the overall fracture morphology. The correlation between gravel content and conglomerate damage mode is significant, with the highest degree of fracture complexity occurring when the gravel content is approximately 30%. Stress differential is the most significant controlling factor affecting fracture morphology, followed by minimum principal stress. When the stress difference reaches 8 MPa, the fracture morphology begins to stabilize, and too high a stress difference will cause the phenomenon that the fracture stops expanding, affecting the fracturing effect. A high level of minimum stress promotes tensile failure in conglomerate, and the scale and complexity of fracture decrease. High injection displacement promotes branch fracture development and reduces the effect of in situ stress on fracture extension, and too high a displacement leads to inhibition of main fracture development.

BackgroundAs China amends its “zero COVID” strategy, a sudden increase in the number of infections may overwhelm medical resources and its impact has not been quantified. Specific mitigation strategies are needed to minimize disruption to the healthcare system and to prepare for the next possible epidemic in advance.MethodWe develop a stochastic compartmental model to project the burden on the medical system (that is, the number of fever clinic visits and admission beds) of China after adjustment to COVID-19 policy, which considers the epidemiological characteristics of the Omicron variant, age composition of the population, and vaccine effectiveness against infection and severe COVD-19. We also estimate the effect of four-dose vaccinations (heterologous and homologous), antipyretic drug supply, non-pharmacological interventions (NPIs), and triage treatment on mitigating the domestic infection peak.ResultAs to the impact on the medical system, this epidemic is projected to result in 398.02 million fever clinic visits and 16.58 million hospitalizations, and the disruption period on the healthcare system is 18 and 30 days, respectively. Antipyretic drug supply and booster vaccination could reduce the burden on emergency visits and hospitalization, respectively, while neither of them could not reduce to the current capacity. The synergy of several different strategies suggests that increasing the heterologous booster vaccination rate for older adult to over 90% is a key measure to alleviate the bed burden for respiratory diseases on the basis of expanded healthcare resource allocation.ConclusionThe Omicron epidemic followed the adjustment to COVID-19 policy overloading many local health systems across the country at the end of 2022. The combined effect of vaccination, antipyretic drug supply, triage treatment, and PHSMs could prevent overwhelming medical resources.

Wettability is one of the important physical properties of reservoir rock surfaces, and it is a key factor affecting capillary force, relative permeability, bound water saturation and fluid micro-distribution. Based on the molecular simulation method, this paper made a study of the wetting behavior of a graphite surface (organic surface) modified by the oxygen-containing functional groups and the distribution characteristics of the methane-water system in organic slit pores. The results showed that the interaction energy between the water molecules and the surface decreased and the wetting contact angle of the organic matter surface increased with the increase in the oxygen-containing functional groups; with the increase in the temperature, the interaction energy between the organic matter surface and the water molecules increased, and the wetting contact angle decreased; in the graphite slit pore model with symmetrical C/O ratio, water molecules were symmetrically distributed near the wall of oxygen-containing functionalized graphite, and with the decreasing in the C/O ratio, the relative concentration of water molecules increased and the diffusion coefficient decreased, while methane molecules were clustered in the center of the pore. In the graphite slit pore model with asymmetric C/O ratio, the water molecules were asymmetrically distributed near the wall of the oxygenated functionalized graphite, while the methane molecules were clustered in the center of the pore, where the side with a low C/O ratio had strong hydrophilicity on the wall and a high relative concentration of water molecules, while the side with a high C/O ratio had strong hydrophobicity on the wall and a low relative concentration of water molecules. The research findings were extremely significant to make a study of the influences of shale reservoir characteristics.

The acoustic wave propagation characteristics of carbonate rocks have important application value. The systematic analysis of the effects of fluid type, fluid pressure, temperature and confining pressure on acoustic wave velocity and spectrum characteristics of carbonate rocks still needs to be strengthened. Carbonate rocks with pores and fractures mined from the Dengying Formation in the Hechuan and Tongnan areas, Sichuan Basin, were selected to carry out acoustic transmission experiments under different conditions. The effects of confining pressure, pore pressure, pressure difference, temperature and fluid category on the acoustic velocity of carbonate rocks and the dominant frequency characteristics of transmitted acoustic waves were analyzed. The results show that the sensitivity of acoustic velocity to pressure change in saturated formation water is lower than that in saturated nitrogen. In the process of pressure change, the variation amplitude of acoustic velocity is positively correlated with the porosity of rock samples. Within the experimental temperature range, with increasing temperature, the P-wave velocity and S-wave velocity of the saturated formation water and nitrogen samples decreases lightly. When the differential stress is low, the acoustic velocity corresponding to the way of changing the differential stress by changing the pore pressure is greater than that by changing the confining pressure, and the conclusion is opposite when the differential stress is high. Under the same differential stress condition, the acoustic velocity is more sensitive to changes in the confining pressure than the changes of pore pressure. The included angle and slope difference of the two acoustic velocity-differential stress curves corresponding to constant confining pressure and constant pore pressure can qualitatively reflect the relative size of the dynamic Biot effective stress coefficient of carbonate rock samples. With increasing differential stress, the contribution of pore pressure to effective stress decreases gradually. With increasing pore pressure, the dominant frequency amplitudes of the P-wave and S-wave gradually decrease. With increasing confining pressure, the dominant frequency amplitudes of the P-wave and S-wave gradually increase. With increasing temperature, the dominant frequency amplitudes of the P-wave and S-wave gradually increase. The research results are helpful to the theoretical research and engineering application of pore pressure predictionin carbonate formation based on logging acoustic information.

Rock mechanical properties play an important role in the exploration and development of shale oil reservoirs. To study the rock mechanical properties continuously distributed along the longitudinal direction of the formation, physical and mechanical property data of shales from the Permian Lucaogou Formation of the Junggar Basin were gathered through experimental tests. The regression analysis method was applied to obtain relationships between physical properties and rock mechanical properties. Based on this, new empirical relationships between rock mechanical properties were established. The results show that the uniaxial compressive strength (UCS) ranged from 48.40 to 147.86 MPa, the Young’s modulus (Es) was between 3.02 and 20.63 GPa, the Poisson’s ratio (νs) ranged from 0.13 to 0.36, the cohesive force (C) ranged from 14.65 to 34.60 MPa, and the internal friction angle (φ) was between 27.61 and 46.94°. The rock mechanical properties were more sensitive to the P-wave interval transit time (Δtc) and bulk density (DEN). Among them, the UCS was more sensitive to Δtc, while the C, Es, and νs were more sensitive to Δtc/DEN. For UCS and Es, an exponential function correlation is more reliable than linear expression and power function, whereas for C and νs, power function and linear expression were adopted for higher accuracy, respectively. Compared with the empirical equations presented in the literature, the empirical equations established in the paper are more accurate and reliable, making them applicable to the Permian Lucaogou Formation shale oil reservoirs in the Jimusar Sag of the Junggar Basin.

In this paper, the pore structure characteristics of shale samples from the Lower Silurian Longmaxi Formation in South of Sichuan Basin of China were investigated by total organic carbon (TOC) content determination, X-ray diffraction (XRD), scanning electron microscope (SEM), low pressure nitrogen adsorption (LPNA) and high pressure mercury injection (HPMI). The fractal dimension of shale samples was calculated based on Frenkel-Halsey-Hill (FHH) model and thermodynamic relation model. The results showed that the major mineral compositions of shales were quartz and clay content. Organic pores, intergranular pores, intragranular pores, microfractures were widely developed in the shale samples, of which organic pores were the most developed. The pore morphology was mainly ink bottle-shaped pores and slit-shaped pores; the pore size distribution of shale samples was complex with multiple distribution peaks, the pore size between 3 and 40 nm occupied the most of storage space. The fractal dimension Dn1 of pores between 2 nm and 10 nm was 2.7177–2.7933, while fractal dimension Dn2 of pores between 10 nm and 50 nm was 2.2439–2.5468. The fractal dimension Dr of macropores calculated by the thermodynamic model was 2.6401–2.7025.

The acoustic response characteristics of shales were investigated by the acoustic transmission experiment, which is the basis of solving geological and engineering problems using the seismic or logging information during the process of the exploration and development of shale gas reservoirs. Based on the theory of acoustic wave and the background of acoustic transmission experiment, the initial condition, vibration source condition, boundary condition and stability condition were constructed, and the numerical simulation of acoustic transmission experiment of shales were completed through Matlab programming. The results show that under the same bedding angle, the acoustic time and attenuation coefficient of shales shown positive correlation with the bedding density; whereas under the same bedding density, the variation laws of the acoustic time and the attenuation coefficient of shales were more complex with the change of the bedding angle, that is, the acoustic time and attenuation coefficient of shales increased first, then decreased and then increased again with the increase of the bedding angle.

The random distribution of gravels makes the conglomerate reservoir highly heterogeneous. A stress concentration occurs at the gravel-matrix interfaces owing to the embedded gravel and affects the local mechanical response significantly, making it difficult to control and predict hydraulic fracture (HF) propagation. The mechanism of HF propagation in conglomerate reservoirs remains unclear; thus, it is difficult to effectively design and treat hydraulic fracturing. Based on the global pore-pressure cohesive zone element (GPPCZ) model method, a two-dimensional (2D) fracture propagation model with flow-stress-damage (FSD) coupling was established to investigate HF nucleation, propagation, and coalescence in conglomerate reservoirs. This model was experimentally verified, and fractal theory was introduced to quantify the complexity of fracture morphology. The microscale interactions of the gravel, matrix, and interface have been taken into consideration during simulating HF propagation accurately in macroscale. The influence of the mechanical properties of gravel, matrix, matrix-gravel interface, and reservoir stress distribution state, on HF morphology (HF length, stimulated reservoir square, and HF complexity morphology), was investigated. Finally, the main factors affecting fracture propagation were analyzed. It was revealed that the difference between the mechanical properties of the gravel and the matrix in the conglomerate rock will affect the geometry of HF to varying degrees. The local behavior of fracture propagation is obviously dominated by the elastic modulus, tensile strength, and the strength for the matrix-gravel interface. However, the propagation of HF at the whole scale is mainly dominated by the horizontal stress state, including the minimum horizontal stress and horizontal stress difference. In addition, the difference in horizontal stress significantly affects the fracturing patterns (deflection, bifurcation, and penetration) when HF encounters gravel. In this study, a simulation method of HF propagation in conglomerate reservoirs is introduced, and the results provide theoretical support for the prediction of HF propagation morphology and plan design of hydraulic fracturing in conglomerate reservoirs.

Abstract The horizontal well‐staged fracturing technology has been widely used in many oilfields to enhance well production by creating fracture network. Accurate prediction of fracture initiation is significant for successful hydraulic fracturing operation. However, hydraulic fracture initiation in fractured reservoir is highly tricky due to its rich natural fractures and artificial fractures previously created during staged fracturing. These fractures are thought to be major factors in conducting efficient staged fracturing in fractured reservoir. Therefore, in this paper, induced stress from artificial fractures has been analyzed by using two‐dimensional analytical model. Besides, based on the assumption that rock in the formation is composed by rock matrix and fracture plane, considering tensile and shear failure type, failure criterion with multiple natural fracture planes has been developed. By combining impacts from natural fractures and artificial fractures, we establish an analytical model to calculate initial fracture pressure for horizontal staged fracturing in fractured reservoir. These results demonstrate that induced stress is associated with artificial fractures’ location, height, and number. This induced stress leads to increment of initial fracture pressure, restricting fracture initiation. Due to natural fractures, there are three types of initiation, which are tensile failure along rock matrix, tensile failure along natural fracture, and shear failure along natural fracture plane. When rock failure is related to natural fracture, initial fracture pressure decreases and is variable with natural fracture plane occurrence. Especially, with large number of natural fractures, only one initiation type exists, which is tensile failure along natural fracture. Additionally, for horizontal borehole, influencing extent of fracture planes is associated with wellbore trajectory. The case study proves the applicability of this analytical model, meaning it can be used to guide horizontal staged fracturing in oilfield.

Abstract The molecular simulation method was used to investigate the adsorption behaviors of methane on illite. The effects of several factors on methane adsorption, free gas amounts, and the proportions of absorbed gas in the illite nanopores, including the pore size, temperature, and water content, are discussed. The results obtained show that methane adsorption in illite nanopores is due mainly to van der Waals adsorption. With an increase in the pressure or of the pore size, the free gas amounts of methane increase, whereas the free gas amounts decrease with increasing temperature or water content. With increasing pressures or decreasing pore sizes, the methane adsorption capacity of the illite pores increases. When the pressure or pore size increases, the proportion of the adsorbed gas in the pores decreases. As the temperature increases, the methane adsorption capacity of the illite pores decreases, and the proportion of adsorbed gas in the illite pores decreases slightly. The adsorbed phase density decreases with increasing pore size and temperature, whereas the adsorbed phase density increases with increasing pressure. The electrostatic forces and hydrogen bonds have a positive effect on water adsorption in the illite nanopores, while the van der Waals forces have the opposite effect, which causes the water molecules in the illite pores to exist in the form of aggregates. The water molecules occupy the areas near the pore walls in a directional manner and occupy the adsorption space and the adsorption sites of methane, resulting in a decrease in the methane adsorption capacity and a slight reduction in the proportion of adsorbed gas.

Conglomerate reservoir is an important part of unconventional oil and gas resources, which has great developmental potential. However, its sedimentary environment and structural background are complex, and its cementation types, gravel volume fraction and shape are quite different, which leads to its strong heterogeneity. When developing a conglomerate reservoir, it is extremely difficult to drill because of its strong heterogeneity. It is difficult to obtain the mechanical properties and laws of the conglomerate through physical experiments, which further restricts the development process of conglomerate reservoirs. In order to study its failure law, a three-dimensional numerical model of a conglomerate is built based on the discrete element method, and the effects of cementation strength and gravel characteristics on the mechanical properties of the conglomerate are emphatically studied. The results show that the elastic modulus and uniaxial compressive strength of the conglomerate decrease obviously with the decrease in cementation strength. With the increase in cementation strength, the normal contact force of the conglomerate model increases significantly, the distribution of normal contact force changes from cylinder to sphere, and the heterogeneity of the conglomerate decreases. There is a threshold value for the influence of cementation strength on mechanical properties of the conglomerate, and when the threshold is exceeded, the mechanical properties of the conglomerate no longer change obviously. With the increase in gravel content, the uniaxial compressive strength of the conglomerate decreases at first and then increases, the phenomenon of penetrating gravels and bypassing gravels increases, and the single diagonal crack changes into diagonal cross cracks; the cementation strength and gravel content of gravel jointly affect the mechanical properties and fracture morphology of the conglomerate, and its stress–strain relationship is the external macroscopic expression of normal contact force of internal particles.

Hydraulic fracturing is a necessary technique for shale gas exploitation. In order to have efficient stimulation treatment, a complex fracture network has to be developed, whereas with rich bedding planes and natural fractures, the mechanism of forming a fracture network is not fully understood and it is so tricky to predict propagation and initiation of hydraulic fracture. Therefore, in this paper, considering the strong anisotropy of shale reservoir, numerical simulation has been conducted to analyze fracture propagation and initiation on the basis of finite element and damage mechanics. Simulation results indicate that hydraulic fracture is not merely controlled by in situ stress due to strong anisotropy in shale. With plenty of bedding planes, hydraulic fracture tends to have initiation and propagation along the bedding plane. In particular, this influence becomes stronger with low strength and high development density of bedding planes. Additionally, in combination with natural fracture and bedding plane, the initiation point is usually on a natural fracture plane, causing relatively small breakdown pressure. In the process of fracture propagation, hydraulic fracture connects with natural fractures and bedding planes, forming dendritic bifurcation and more complicated paths. Numerical simulation proves that bedding plane and natural fracture are vital factors of hydraulic fracture. Compared to natural fracture, the bedding plane has a stronger impact on hydraulic fracture propagation. For the initiation of hydraulic fracture, natural fracture is the major effecting factor. The outcome of this study is able to offer theoretical guidance for hydraulic fracturing in shale.

The complexity of hydraulic fractures (HF) significantly affects the success of reservoir reconstruction. The existence of a bedding plane (BP) in shale impacts the extension of a fracture. For shale reservoirs, in order to investigate the interaction mechanisms of HF and BPs under the action of coupled stress-flow, we simulate the processes of hydraulic fracturing under different conditions, such as the stress difference, permeability coefficients, BP angles, BP spacing, and BP mechanical properties using the rock failure process analysis code (RFPA2D-Flow). Simulation results showed that HF spread outward around the borehole, while the permeability coefficient is uniformly distributed at the model without a BP or stress difference. The HF of the formation without a BP presented a pinnate distribution pattern, and the main direction of the extension is affected by both the ground stress and the permeability coefficient. When there is no stress difference in the model, the fracture extends along the direction of the larger permeability coefficient. In this study, the in situ stress has a greater influence on the extension direction of the main fracture when using the model with stress differences of 6 MPa. As the BP angle increases, the propagation of fractures gradually deviates from the BP direction. The initiation pressure and total breakdown pressure of the models at low permeability coefficients are higher than those under high permeability coefficients. In addition, the initiation pressure and total breakdown pressure of the models are also different. The larger the BP spacing, the higher the compressive strength of the BP, and a larger reduction ratio (the ratio of the strength parameters of the BP to the strength parameters of the matrix) leads to a smaller impact of the BP on fracture initiation and propagation. The elastic modulus has no effect on the failure mode of the model. When HF make contact with the BP, they tend to extend along the BP. Under the same in situ stress condition, the presence of a BP makes the morphology of HF more complex during the process of propagation, which makes it easier to achieve the purpose of stimulated reservoir volume (SRV) fracturing and increased production.

Shale reservoirs are characterized by low permeability and natural fractures. In the process of reservoir development, the working fluid enters the reservoir. This may result in the formation of new fractures or expansion of natural fractures. When shale reservoirs are exploited, the fluid pressure in the fracture or pore is reduced. This destroys the stress balance of the reservoir, produces stress sensitivity damage, and reduces the reservoir permeability. Organic-rich shale from the Yanchang Formation, Chang 7 Member of the Ordos Basin, was selected for core flow experiment with helium. The effects of the type of brine, salinity, and soaking time on the stress sensitivity of an organic-rich shale reservoir were investigated. The acoustic characteristics were also investigated to study the effect of interactions between water and shale on stress sensitivity. The experimental results demonstrate that the interactions of water and shale increase the permeability of shale and reduce its stress sensitivity. Furthermore, when the permeability of the shale is excessively low, the stress sensitivity is high. In the acoustic studies, a higher attenuation coefficient of the acoustic wave corresponds to a larger variation in the shale structure and thus a larger permeability of the shale and smaller stress sensitivity coefficient. Whereas there is no apparent effect of the salt water type on the stress sensitivity, higher salinity levels cause higher stress sensitivity. After reacting with 15000 mg/L brine, the stress sensitivity coefficient of shale did not decrease significantly compared with that before action, all of which were above 0.97. However, after reacting with distilled water or 5000 mg/L brine, the stress sensitivity coefficient of shale decreased significantly, and all of them decreased to less than 0.9. Longer water exposures, corresponding to an increased duration of water-shale interactions, result in higher impacts on the stress sensitivity of shale. After 6 hours of shale-brine interaction, the stress sensitivity coefficient of shale is as high as 0.93, while after 48 hours of shale-brine interaction, the stress sensitivity coefficient of shale is reduced to 0.88. This study provides a highly effective reference with regard to the influence of the working fluid on the reservoir during drilling operations and the study of reservoir characteristics after fracturing.

At underground fluid injection sites with natural faults, understanding how to avoid the subsequent fault reactivation and induced seismicity plays a crucial role in the success of subsurface anthropogenic activities. In this work, with the objective of avoiding risky faults in site selection in the Shengli Oilfield, we investigated the faults that are usually encountered in the target demonstration zone; based on the geophysical observations of fault structures, we designed different fault tectonic scenarios to investigate the different penetration patterns of faults. We used the finite element-based numerical method to assess the influence of the effective lateral and vertical reservoir transmissivity in each fault penetration pattern. Our results indicate that when a permeable fault intersects into the target reservoir, it presents both barrier effect to reservoir transmissivity within the target reservoir and hydraulic connection between reservoirs. The effective lateral reservoir transmissivity is dominated by the barrier effect of the fault, and the effective vertical reservoir transmissivity is dominated by the hydraulic connection between reservoirs. Relatively impermeable faults with less contact with the target aquifer make higher effective lateral reservoir transmissivity and lower effective vertical reservoir transmissivity, which would mitigate the risk of caprock failure and the magnitude of the induced seismicity.

As a serious problem in drilling operation, wellbore instability restricts efficient development of shale gas. The interaction between the drilling fluid and shale with hydration swelling property would have impact on the generation and propagation mechanism of cracks in shale formation, leading to wellbore instability. In order to investigate the influence of the hydration swelling on the crack propagation, mineral components and physicochemical properties of shale from the Lower Silurian Longmaxi Formation (LF) were investigated by using the XRD analysis, cation exchange capabilities (CEC) analysis, and SEM observation, and we researched the hydration mechanism of LF shale. Results show that quartz and clay mineral are dominated in mineral composition, and illite content averaged 67% in clay mineral. Meanwhile, CEC of the LF shale are 94.4 mmol/kg. The process of water intruding inside shale along microcracks was able to be observed through high power microscope, meanwhile, the hydration swelling stress would concentrate at the crack tip. The microcracks would propagate, bifurcate and connect with each other, with increase of water immersing time, and it would ultimately develop into macro-fracture. Moreover, the macrocracks extend and coalesce along the bedding, resulting in the rock failure into blocks. Hydration swelling is one of the major causes that lead to wellbore instability of the LF shale, and therefore improving sealing capacity and inhibition of drilling fluid system is an effective measure to stabilize a borehole.

Because of high cost and pollution of oil-based drilling fluid, the water-based drilling fluid is increasingly used now. However, bedding planes and micro-cracks are rich in shale formation. When water-based drilling fluid contacts formation rock, it causes the propagation of crack and invasion of drilling fluid, which decrease shale strength and cause wellbore instability. In this paper, we analyzed influence of water-based drilling fluid on shale strength and failure mode by mechanics experiment. Based on those experimental results, considering the effect of bedding plane and drilling time, we established modeling of wellbore stability for shale formation. The result from this model indicates that in certain azimuth of horizontal well, collapsing pressure increases dramatically due to shale failure along with bedding plane. In drilling operation, those azimuths are supposed to be avoided. This model is applicable for predication of collapsing pressure in shale formation and offers reference for choosing suitable mud weight.

Pore structure has been widely observed to affect the seismic wave velocity of rocks. Although taking lab measurements on 1.0-inch core plugs is popular, it is not representative of the fractured-vuggy carbonates because many fractures and vugs are on a scale up to several hundred microns (and greater) and are spatially heterogeneous. To overcome this shortage, we carried out the lab measurements on full-diameter cores (about 6.5–7.5 cm in diameter). The micro-CT (micro computed tomography) scanning technique is used to characterize the pore space of the carbonates and image processing methods are applied to filter the noise and enhance the responses of the fractures so that the constructed pore spaces are reliable. The wave velocities of P- and S-waves are determined then and the effects of the pore structure on the velocity are analyzed. The results show that the proposed image processing method is effective in constructing and quantitatively characterizing the pore space of the full-diameter fractured-vuggy carbonates. The porosity of all the collected tight carbonate samples is less than 4%. Fractures and vugs are well-developed and the spatial distributions of them are heterogeneous causing, even the samples having similar porosity, the pore structure characteristics of the samples being significantly different. The pores and vugs mainly contribute to the porosity of the samples and the fractures contribute to the change in the wave velocities more than pores and vugs.

Hydraulic fracture dimension is one of the key parameters affecting stimulated porous media. In actual fracturing, plentiful uncertain parameters increase the difficulty of fracture dimension prediction, resulting in the difficulty in the monitoring of reservoir productivity. In this paper, we established a three-dimensional model to analyze the key factors on the stimulated reservoir volume (SRV), with the response surface method (RSM). Considering the rock properties and fracturing parameters, we established a multivariate quadratic prediction equation. Simulation results show that the interactions of injection rate (Q), Young’s modulus (E) and permeability coefficient (K), and Poisson’s ratio (μ) play a relatively significant role on SRV. The reservoir with a high Young’s modulus typically generates high pressure, leading to longer fractures and larger SRV. SRV reaches the maximum value when E1 and E2 are high. SRV is negatively correlated with K1. Moreover, maintaining a high injection rate in this layered formation with high E1 and E2, relatively low K1, and μ1 at about 0.25 would be beneficial to form a larger SRV. These results offer new perceptions on the optimization of SRV, helping to improve the productivity in hydraulic fracturing.

We mainly focus on the Permian, Lower Cambrian, Lower Silurian, and Upper Ordovician Formation; the fractal dimensions of marine shales in southern China were calculated using the FHH fractal model based on the low-pressure nitrogen adsorption analysis. The results show that the marine shales in southern China have the dual fractal characteristics. The fractal dimension D1 at low relative pressure represents the pore surface fractal characteristics, whereas the fractal dimension D2 at higher relative pressure describes the pore structure fractal characteristics. The fractal dimensions D1 range from 2.0918 to 2.718 with a mean value of 2.4762, and the fractal dimensions D2 range from 2.5842 to 2.9399 with a mean value of 2.8015. There are positive relationships between fractal dimension D1 and specific surface area and total pore volume, whereas the fractal dimensions D2 have negative correlation with average pore size. The larger the value of the fractal dimension D1 is, the rougher the pore surface is, which could provide more adsorption sites, leading to higher adsorption capacity for gas. The larger the value of the fractal dimension D2 is, the more complicated the pore structure is, resulting in the lower flow capacity for gas.

To obtain the influence of anisotropy and energy evolution characteristics on wellbore stability, the acoustic and mechanical anisotropy characteristics of shales are studied through various experiments, including scanning electron microscopy, ultrasonic pulse transmission, and uniaxial compression experiments, with the Longmaxi Formation shale in the southern area of the Sichuan Basin as the research object. The energy evolution characteristics of the Longmaxi Formation shale under different bedding angles are analyzed. The influence of anisotropy on the wellbore stability of shale formation is discussed on this basis. The results show that the acoustic and mechanical parameters, failure mode, and energy evolution characteristics of shale have significant anisotropy. Furthermore, the P-wave and S-wave time differences decrease with an increase in bedding angle. The compressive strength and Poisson's ratio decrease first and then increase with an increase in bedding angle. Meanwhile, the elastic modulus gradually increases with an increase in bedding angle. Rock samples with different bedding angles show diverse failure modes in mechanical tests, including splitting, shear, and shear-splitting failure. The total energy and elastic energy decrease first and then increase with an increase in bedding angle. Finally, the formation anisotropy affects the wellbore stability: the higher the formation anisotropy, the more vulnerable is the wellbore to instability. [ABSTRACT FROM AUTHOR]