Longmaxi Shale Research Articles

Brittleness index prediction using modified random forest based on particle swarm optimization of Upper Ordovician Wufeng to Lower Silurian Longmaxi shale gas reservoir in the Weiyuan Shale Gas Field, Sichuan Basin, China

The right placement of fractures helps to enhance gas production in shale gas reservoirs. One parameter that helps to determine the target layers to place hydraulic fractures is the Brittlenex index (BI). However, no universal and appropriate methods can be used to compute BI, with all established correlations being used under different conditions. This paper uses machine learning (ML) methods to predict the BI of Upper Ordovician Wufeng to Lower Silurian Longmaxi formation in the Weiyuan shale gas field, Sichuan Basin, China. Random forest based on particle swarm optimization (PSO-RF) was utilized for the first time to predict BI due to its ability to capture nonlinear relationships between many variables in the dataset, thus giving more accurate results than other models. Collected secondary data from the WY1 well were used for training, whereas WY2 well data were used for testing. The results revealed that PSO-RF outperformed Extreme gradient boosting (XGBoost), Light gradient boosting machine (LightGBM), and K-nearest neighbor (KNN) in predicting BI with high accuracy and minimum errors during training and testing. PSO-RF coefficient of determination (R2), root mean square error (RMSE), and mean absolute errors (MAE) after training and testing were 0.9934 and 0.9533,4.6327 and 15.5308,2.0974 and 5.3896, respectively. In addition, the best-developed PSO-RF model was used to predict BI in WY3 and WY4 wells for model results validation; it was found that the model predicted the BI with high accuracy. This confirms that the developed model can be used to predict the BI of new development wells without depending on laboratory measurements, which are expensive and time-consuming to compute; thus, the developed model can be adopted as an alternative technique to determine the sweet spot for hydraulic fracturing in shale gas reservoirs to enhance gas production.

3D Integrated Model-Driven Work Flow Developed for Shale Hydraulic Fracturing

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 212406, “3D Integrated G&G Model-Driven Mitigation Work Flow on Screenout, Frac Hits, and Casing Deformation in Ultradeep Shale Hydraulic Fracturing,” by Jianfa Wu and Bo Zeng, PetroChina, and Lipeng Wang, SLB, et al. The paper has not been peer reviewed. _ The Longmaxi shale gas play in China is unique because of multiple tectonic deformations in its geological history. While its hydraulic fracturing design has matured after a decade-long evolution, the success of every well cannot be ensured without considering heterogeneity. To address these challenges, a multidisciplinary team was formed to work on a pad; through the team’s efforts, designed proppant volume achieved zero casing deformations, fewer screenouts, and weaker fracture hits. The key to effective mitigation was continuous 3D geological and geomechanical (G&G) modeling through ongoing iterations with field data. Background The Longmaxi shale play in the Sichuan Basin is approximately 20 000 km2 and is buried between 2000 and 4500 m. This play has more than 10 trillion m3 of gas reserves and is made up of shallow and deep reservoirs. Currently, the operator has moved to the deeper Luzhou and Chongqing blocks buried at a depth greater than 3500 m. Nearly 200 horizontal wells per year are planned in the next 5 years. A fracture hit with tens of MPa increase was observed. Four casing failures were observed on 32 of the wells in this block, which made the unstimulated length more than 200 m and cost an extra $300,000 for interventions in each problematic well. While the risk of screenout, fracture hit, and casing deformation is common, risk levels vary for different reservoirs. No work flow is available that could be developed to identify and reduce these hazards. In this study, ultradeep shale gas at Pad X, buried almost 3900 m in the Luzhou Block, was selected to recognize and mitigate these risks (Fig. 1), where four wells were positioned 300 m apart. The lateral length of X-1 is roughly 1000 m, while the rest of the wells are roughly 1900 m in length. 3D G&G Modeling By combining seismic interpretation, seismic inversion, petrophysical analysis, and 1D geomechanical modeling while adhering to the Longmaxi-Wufeng Shale modeling technique used in neighboring blocks, the authors established a G&G model with dimensions of 40×40×0.5 m for the Luzhou Block. The pad model was refined to 8×8×0.5 m by using well logs to better characterize the variations in stage level. Natural Fractures. 3D surface seismic data was the only reliable source available to predict natural fractures in the shale and design the fracturing job, though the uncertainty was unavoidable. To display fault-/fracture-system characteristics, seismic attributes, including variance, ant tracking, and curvature were examined.

The water footprint of hydraulic fracturing for shale gas extraction in China

The rapid expansion of shale gas extraction worldwide has raised significant concerns about its impact on water resources. China is expected to undergo a shale revolution following the U.S. Most of the information on water footprint of shale gas exploration and hydraulic fracturing has been focused on the U.S. Here, we addressed this knowledge gap by establishing a comprehensive database of shale gas extraction in China, utilizing operational data from over 90 % of shale gas wells across the country. We present systematic analysis of water usage and flowback and produced water (FP water) production from all the major shale gas fields in China. Between 2012 and 2022, a total of 2740 shale gas wells were hydraulically fractured in China, primarily located in Sichuan and Chongqing Province. About 113 million m3 water was used for hydraulic fracturing, resulting in a cumulative shale gas production of 116 billion m3. As of 2022, the annual water use for hydraulic fracturing exceeded 20 million m3, and the annual FP water production reached 8.56 million m3. Notably, 80 % ~ 90 % of the FP water has been reused for hydraulic fracturing since 2020, accounting for 29 % to 35 % of the annual water usage for hydraulic fracturing. Water use per well in China varies primarily between 21,730 m3 to 61,070 m3 per well, and water use per horizontal length ranges primarily between 20 m3/m and 35 m3/m. The average ultimate FP water production per well in China was estimated to be 22,460 m3. The water use intensity (WUI) for shale gas extraction in China mainly ranges from 7 to 25.4 L/GJ, which is significantly higher than that of the U.S. This disparity is largely due to the lower Estimated Ultimate Recovery (EUR) of shale gas wells in China. Despite the considerable water consumption during the hydraulic fracturing process, shale gas has a relatively low water footprint compared to other conventional energy resources in China. The Produced water intensity (PWI) for shale gas extraction in China ranges from 3.9 to 7.3 L/GJ, which is consistent with the previously reported PWI values for shale gas extraction in the U.S. This study predicts water usage and FP production spanning the period 2023 to 2050 under two scenarios to assess the potential impact of shale gas extraction on water resources in the Longmaxi shale region in Sichuan Basin. The first scenario assumed a constant drilling rate, while the second assumed a yearly 10 % increase in drilling rate. With an assumed FP water reuse rate of 85 % for hydraulic fracturing, the estimated annual freshwater consumption for the two scenarios is 10.4 million m3 and 163 million m3, respectively. This accounts for only 0.28‱ and 4.4‱ of the total annual surface water resources in Sichuan and Chongqing Province. Our findings suggest that freshwater usage for hydraulic fracturing in humid Southern China is small relative to available surface water resources. However, prospective large-scale shale gas extraction in other arid and semi-arid regions may enhance the regional water scarcity. It is necessary to develop new hydraulic fracturing technologies that can use saline groundwater or other types of marginal water, and explore alternative management and treatment strategies for FP water.

Nano-Scale Pore Structure Characterization and Its Controlling Factors in Wufeng and Longmaxi Shale in the Zigong Area, Southwest Sichuan Basin

The nano-scale pore systems in shale reservoirs control shale gas transportation and aggregation, which is of great significance for the resource evaluation of shale oil and gas and the selection of a “sweet spot”. Taking twelve marine shale samples from the Wufeng–Longmaxi Formation in the Zigong area, southwest Sichuan Basin, as the research target, we carried out a series of experiments, including total organic carbon (TOC) analysis, X-ray diffraction (XRD), gas adsorption (CO2 + N2), and mercury intrusion porosimetry (MIP), to study the full-scale pore structure characterization and controlling factors of pore volume and specific surface area. The results presented the following findings. (1) Marine shale samples from the target area are rich in organic matter, with an average TOC value of 3.86%; additionally, the mineral composition was dominated by quartz and clay minerals, with average contents of 44.1% and 31.4%, respectively. (2) The full-scale pore size distribution curves of pore volume developed multimodally, with the main peaks at 0.5 nm–2 nm, 3 nm–6 nm, and 700 nm–2.2 um; moreover, the full-scale pore size distribution curves of a specific surface area developed unimodally, with the main peak ranging from 0.5 nm to 1.2 nm. (3) Pore volume was mainly contributed by mesopores and macropores, with an average contribution of 46.66% and 42.42%, respectively, while the contribution of micropores was only 10.91%. The specific surface area was mainly contributed by micropores and mesopores, with an average contribution of 64.63% and 29.22%, respectively, whereas the contribution of micropores was only 6.15%. (4) The TOC content mainly controlled the pore volume and specific surface area of micropores and mesopores, while the clay and feldspar content generally controlled the pore volume and specific surface area of macropores. Additionally, the quartz content had an inhibitory effect on the development of all pore types. These results will help researchers understand the laws of gas accumulation and migration.

Shale Gas Reservoir Pore Pressure Prediction: A Case Study of the Wufeng–Longmaxi Formations in Sichuan Basin, Southwest China

Pore pressure prediction is critical for shale gas reservoir characterization and simulation. The Wufeng–Longmaxi shale, in the southeastern margin of the Sichuan Basin, is identified as a complex reservoir affected by overpressure generation mechanisms and variability in lithification. Thus, standard methods need to be adapted to consistently evaluate pore pressure in this basin. Based on wireline logs, formation pressure tests, and geological data, this study applied the Eaton–Yale approach, which extends the theoretical basis of Eaton and Bowers methods to reservoir geological conditions and basin history. The method was developed by integrating petrophysical properties, rock physics interpretations, and geology information. The essential steps include (1) a multi-mineral analysis to determine mineral and fluid volumes; (2) a determination of the normal pressure trend line and extending it to overpressured sections; (3) predicting pore pressure using the basic Eaton approach and identifying overpressured zones; (4) correcting compressional velocity using lithology logs and a rock physics model; (5) determining the Biot Alpha coefficient and vertical-effective stress and estimating the new pore pressure values using the Eaton–Yale method. Overpressure zones were corrected, and reservoir pore pressure varied between 30.354 and 34.959 MPa in the wells. These research results can provide a basis for building reservoir simulation models, identifying reservoir boundaries, and predicting relative permeability.

Micropore Structure of Deep Shales from the Wufeng–Longmaxi Formations, Southern Sichuan Basin, China: Insight into the Vertical Heterogeneity and Controlling Factors

The microscopic pore throat structure of shale reservoir rocks directly affects the reservoir seepage capacity. The occurrence and flow channels of shale gas are mainly micron–nanometer pore throats. Therefore, to clarify the microstructural characteristics and influencing factors of the deep organic-rich shales, a study is conducted on the marine shale from the Upper Silurian to Lower Ordovician Wufeng–Longmaxi Formation in the southern Sichuan Basin. Petrographic lithofacies division is carried out in combination with petro-mineralogical characteristics, and a high-resolution scanning electron microscope, low-temperature nitrogen and low-temperature carbon dioxide adsorption, and micron-computed tomography are used to characterize the mineral composition and pore structure qualitatively and quantitatively, upon which the influencing factors of the microstructure are further analyzed. The results show that with the increase in burial depth, the total organic carbon content and siliceous mineral content decrease in the Wufeng formation to Long-11 subsection deep shale, while clay mineral content increases, which corresponds to the change in sedimentary environment from anoxic to oxidizing environment. Unexpectedly, the total pore volume of deep shale does not decrease with the increase in burial depth but increases first and then decreases. Using total organic carbon (TOC), siliceous mineral content showed a good correlation with total pore volume and specific surface area, with correlation coefficients greater than 0.7, confirming the predominant role of these two factors in controlling the pore structure of deep shales. This is mainly because the Longmaxi shale is already in the late diagenetic stage, and organic matter pores are generated in large quantities. Clay minerals have a negative correlation with the total pore volume of shale, and the correlation coefficient is 0.7591. It could be that clay minerals are much more flexible and are easily deformed to block the pores under compaction. In addition, the longitudinal heterogeneity of the deep shale reservoir structure in southern Sichuan is also controlled by the thermal effect of the Emei mantle plume on hydrocarbon generation of organic matter and the development of natural microfractures promoted by multistage tectonic movement. Overall, the complex microstructure in the deep shales of the Longmaxi Formation in the southern Sichuan Basin is jointly controlled by multiple effects, and the results of this research provide strong support for the benefit development of deep shale gas in southern Sichuan Basin.

Fracture Toughness Testing of Brittle Laminated Geomaterials Using Hollow Double-Wing Slotted Specimens.

The fracture toughness of shale is a key parameter guiding hydraulic fracturing design and optimization. The hollow double-wing slotted (HDWS) specimen is a typical specimen configuration for measuring the mode I fracture toughness of rock. The calibration of the shape factor (f) is the basis for accurately obtaining the fracture toughness of rocks. In this study, the influences of crack length, hole size, and the anisotropy of elastic parameters on f for specimens with three typical bedding orientations-arrester (A), divider (D), and short-transverse (ST) orientations-are systematically investigated using finite element software. The numerical simulation results support the following findings. The mode I f increases monotonically with an increase in hole size. The influence of crack length on f varies depending on hole sizes. Under different bedding orientations, significant anisotropy in f was observed. In addition, the degree of anisotropy in Young's modulus has a major impact on f, which is related to the bedding orientation of the specimen. The apparent shear modulus ratio has relatively little influence on f. As the hole size and crack length increase, the influence of the anisotropy of elastic parameters on f increases. Based on numerical calculations, hydraulic fracturing experiments were conducted on HDWS specimens of Longmaxi shale with three bedding orientations, and the results showed that the peak pressure and fracture toughness of the samples in the ST direction were the lowest, while those in the A direction were the highest.

Elasticity and Characteristic Stress Thresholds of Shale under Deep In Situ Geological Conditions.

The mechanical properties of shale are generally influenced by in situ geological conditions. However, the understanding of the effects of in situ geological conditions on the mechanical properties of shale is still immature. To address this problem, this paper provides insight into the elasticity and characteristic stress thresholds (i.e., the crack closure stress σcc, crack initiation stress σci, and crack damage stress σcd) of shales with differently oriented bedding planes under deep in situ geological conditions. To accurately determine the elastic parameters and crack closure and initiation thresholds, a new method-i.e., the bidirectional iterative approximation (BIA) method-which iteratively approaches the upper and lower limit stresses of the linear elastic stress-strain regime, was proposed. Several triaxial compression experiments were performed on Longmaxi shale samples under coupled in situ stress and temperature conditions reflecting depths of 2000 and 4000 m in the study area. The results showed that the peak deviatoric stress (σp) of shale samples with the same bedding plane orientation increases as depth increases from 2000 m to 4000 m. In addition, the elastic modulus of the shale studied is more influenced by bedding plane orientation than by burial depth. However, the Poisson's ratios of the studied shale samples are very similar, indicating that for the studied depth conditions, the Poisson's ratio is not influenced by the geological conditions and bedding plane orientation. For the shale samples with the two typical bedding plane orientations tested (i.e., perpendicular and parallel to the axial loading direction) under 2000 and 4000 m geological conditions, the ratio of crack closure stress to peak deviatoric stress (σcc/σp) ranges from 24.83% to 25.16%, and the ratio of crack initiation stress to peak deviatoric stress (σci/σp) ranges from 34.78% to 38.23%, indicating that the σcc/σp and σci/σp ratios do not change much, and are less affected by the bedding plane orientation and depth conditions studied. Furthermore, as the in situ depth increases from 2000 m to 4000 m, the increase in σcd is significantly greater than that of σcc and σci, indicating that σcd is more sensitive to changes in depth, and that the increase in depth has an obvious inhibitory effect on crack extension. The expected experimental results will provide the background for further constitutive modeling and numerical analysis of the shale gas reservoirs.

Pore connectivity and microfracture characteristics of Longmaxi shale in the Fuling gas field: Insights from mercury intrusion capillary pressure analysis

Assessing the connectivity of pores and the characteristics of microfractures is essential for comprehending gas migration mechanisms and effectively implementing development strategies in deep shale gas plays. The Fuling gas field, a prominent shale gas reservoir in China, holds promise for deep shale development. This requires a better understanding of variations in pore connectivity among shale samples from different depths of the Fuling gas field. To achieve this goal, this study employs innovative methodologies based on mercury intrusion capillary pressure (MICP), a recognized technique for evaluating material pore structures. In this research, we conducted MICP tests on shale samples of varying sizes to examine variations in pore structure and quantify inaccessible pores. Additionally, repeated MICP tests were performed on the same sample to investigate the spatial distribution of residual mercury and distinguish different pore types associated with flow conduits and storage spaces. Furthermore, a directional MICP experiment was carried out to quantitatively characterize the degree of pore development and anisotropy of microfractures by considering the direction of mercury intruding the sample (parallel or perpendicular to lamination). Our findings revealed that deep shale samples exhibited a higher proportion of inaccessible pores compared to mid-deep ones. Regarding accessible pores, a greater proportion of connected pores was observed in the deep shale samples, while the mid-deep shale samples exhibited a lower proportion. The results from the directional MICP experiment indicated that samples with anisotropy in mercury intrusion volume possessed finer and denser microfractures. However, no evidence was found to correlate the ratio of dead-end and connected pores with sample anisotropy. Overall, this study presents a novel approach for experimentally evaluating pore connectivity and microfracture characteristics in shale reservoirs using MICP. The outcomes contribute to enhancing our understanding of shale gas migration mechanisms, particularly in deep ultra-low-permeability reservoirs, and hold significant implications for future developments in the field.

Coupling grid nanoindentation and surface chemical analysis to infer the mechanical properties of shale mineral phases

Shale is a low-permeability, multi-phase, and multi-scale composite material with intrinsic heterogeneity in micro-texture and mineralogical composition. In this paper, relationships between the microscale texture and constituent phases of shale were investigated using SEM (scanning electron microscopy)/BSE (backscatter electron imaging)-EDS (energy-dispersive x-ray spectroscopy) methods, together with grid nanoindentation experiments. The mechanical properties of the constituent phases of the carbonate-rich Longmaxi shale samples were extracted based on the spatial distribution of mineral phases and the indentation interaction volume. We analyze the effects of particle size on the interpreted mechanical properties and establish a characteristic length scale based on a probabilistic analysis. The identified characteristic length for extracting the mechanical properties of constituent mineral phases from the grid nanoindentation technique is about 5.8–11.7 μm, i.e., up to 10 times greater than that proposed in prior research. A multi-scale mechanical model was established with considerations of a self-consistent scheme for granular morphology to link the microscopic characteristics with the multi-scale mechanical properties of shales. The modeling results show that the stiffness and the strength of the homogenized nano-porous illite/quartz aggregates in Longmaxi shale can be assessed from the nano-scale mechanical properties of the mineral phases and the nano-scale porosity. This study paves the way to accessing the mechanical properties of the constituent phases of composite materials based on the properties of their building blocks and provides extensive insights into their complex mechanical behavior, representing a major step towards developing reliable multi-scale models for engineering applications.

Estimating hardness and Young's modulus of shale using drill cuttings: Implications for hydraulic fracturing in shale gas reservoir development

Shale formations are widely prevalent in the oil and gas industry as potential hydrocarbon reservoirs. The hardness and Young's modulus of shale significantly impact the success of hydraulic fracturing operations in shale gas reservoir development. The unavailability of standard-sized core samples (length to diameter ratio ∼2.5) makes it challenging to determine the mechanical properties of shale in the laboratory. This study investigates techniques for estimating hardness and Young's modulus using drill cuttings. The microhardness was measured on shale cuttings of ∼1 mm in size using a micro indenter. Young's modulus was measured at the macro-scale using a uniaxial compression test on a 25 mm diameter core sample. The relationship between microhardness and Young's modulus established for Longmaxi (China) and Ice-Box (Western Canada) shales was applied to another shale (e.g., Sembar Shale of Pakistan), for which no core samples were available. The measured mineralogy of the studied shales provides a common base for the comparison of microhardness and Young's modulus using isotropic and anisotropic theoretical models. A positive correlation was observed between Young's modulus and microhardness for the three shales tested in this study. Although the scale of measurement/assessment is different, the theoretical models show an insignificant impact of the scale range (micro (mm) to macro (cm)) on Young's modulus, microhardness, and brittleness.

Effects of Mineral Composition and Lamina on Mechanical Properties and Fractures of the Wufeng–Longmaxi Shale in the Luzhou Area of the Southern Sichuan Basin

Effects of Mineral Composition and Lamina on Mechanical Properties and Fractures of the Wufeng–Longmaxi Shale in the Luzhou Area of the Southern Sichuan Basin

Upscaling the creep behavior of clay-rich and quartz-rich shales from nanoindentation measurements: Application to the Wufeng-Longmaxi shale, China

Creep of shale controls the stability of fault gouge in sedimentary rocks, aseismic deformation in the upper crust, and several engineering operations, such as proppant embedment, reservoir subsidence, and wellbore stability during unconventional shale gas exploitation. However, measuring the creep properties of shale is challenging due to the multi-scale structure of this rock. Here, our goal is to assess how creep parameters measured at the core sample scale depend on those measured at the nanoscale in shales. We selected three clay-rich and quartz-rich shale samples and conducted a series of experiments on shale samples at two scales: nanoindentation creep measurements on thin sections, and uniaxial creep tests on core samples. The creep parameters, including creep displacement (Δh), indentation creep (CIT), creep strain rate sensitivity (m), contact creep modulus (C), and viscoelastic parameters (E1, E2, η1, and η2) of the Burgers' model, were calculated by fitting the nanoindentation creep data of each single-phase component of shale matrix. From these data, we upscale the creep parameters (E1, E2, and C) of shale matrix to those of the bulk shale. Upscaled E2 and C values based on the Mori-Tanaka scheme are found to be relatively close to those measured in previous microindentation study on Longmaxi shale and are consistent with our measurements in uniaxial creep tests on core samples of clay-rich shale and quartz-rich shale. The upscaling value of the elastic parameters, E1, is almost twice larger than the results from uniaxial creep test. We interpret this difference as due to low elastic stiffness along grain contacts of the shale that nanoindentation measurements underestimate. Our results show a consistency of some creep parameters upscaled from nanoindentation data and those measured at a macroscopic scale on core samples.

Experimental Investigation of Stress Sensitivity of Elastic Wave Velocities for Anisotropic Shale in Wufeng–Longmaxi Formation

The shale of the Wufeng–Longmaxi formation in the Sichuan Basin is the preferred layer for shale gas exploration in China, and its petrophysical characteristics are the key to geological and engineering sweet spot prediction. However, the characteristics and impact mechanisms of its acoustic wave velocity and elastic anisotropy are currently unclear. In this paper, the Wufeng–Longmaxi shale is taken as the research object, and the P-wave and S-wave velocities of the samples are tested under the loading and unloading processes of confining pressure. The stress sensitivity variations in parameters such as wave velocity, wave velocity ratio, and anisotropy are discussed. P-wave and S-wave anisotropy parameters are correlated under different pressure conditions. X-ray diffraction, casting thin sections, scanning electron microscopy, micron CT scanning, and other analytical techniques are used to explore the mechanisms of stress sensitivity of elastic parameters. The research results indicate that: (1) the acoustic velocities of samples from different angles are V90° > V45° > V0°, and there is a positive correlation between the wave velocity and the confining pressure. After unloading the confining pressure, irreversible plastic deformation occurs due to the closure of some microfractures in the rock core, causing the wave velocity to be higher than the initial value. (2) The stress sensitivity coefficient of the P-wave (The mean is 3.00 m·s−1·MPa−1) is higher than that of the S-wave (the mean is 1.23 m·s−1·MPa−1), and the stress sensitivity coefficient of the compacted stage (the mean is 3.02 m·s−1·MPa−1) is higher than that of the elastic stage (the mean is 1.21 m·s−1·MPa−1). (3) The anisotropy of the P-wave and S-wave is negatively correlated with the confining pressure. When the confining pressure is loaded to 65 MPa, the change rate of the P-wave anisotropy coefficient is 23%, and its stress sensitivity is higher than that of S-wave anisotropy coefficient (the change rate is 13.7%). After unloading the confining pressure, the degree of anisotropy is reduced due to the closure of some microfractures. The empirical formula of P-wave and S-wave anisotropy parameters under different pressures is established through linear regression, which can provide a reference for mutual predictions. (4) The variation in wave velocity anisotropy with stress can be divided into stress and material anisotropy, which are related to the directional arrangement of microfractures and clay minerals, respectively. The quantitative characterization of shale anisotropy can be realized by evaluating the development degree of reservoir fractures and mineral components, providing a reference for logging interpretations, sweet spot prediction, and fracturing construction of shale gas reservoirs.

Experimental and numerical investigation on the fracture behavior of deep anisotropic shale reservoir under in-situ temperature

Deep resource extraction is of great importance in advancing shale gas development. To obtain the fracture behavior of deep anisotropic shale reservoirs under in-situ temperatures, a series of fracture experiment on the Longmaxi shale specimens with 5 bedding orientations after constructing in-situ temperatures of 25–200 °C was conducted, as well as corresponding extended finite element method (XFEM) simulation by ABAQUS. The results reveal that increasing in-situ temperature decreases the fracture toughness but increases the fracture toughness anisotropy. Bounded by 100 °C, the fracture toughness varying versus bedding orientation shows two categories. With increasing in-situ temperature, the acoustic emission (AE) activity of specimens with bedding orientations of 45° and 60° develops at lower load levels, showing rapid AE hit development starting at the 20% and 60% peak load at 200 °C. Fracture path in specimens with bedding orientations of 0°, 30° and 90° is roughly not impacted by increasing in-situ temperature, but that in specimens with bedding orientations of 45° and 60° is impacted obviously. XFEM models match well with the experimental observations and give deeper analyses on the interaction between the fracture propagation and bedding plane. Finally, potential applications of elevated in-situ temperatures on deep shale gas extraction were proposed.

A New Method for Predicting the Gas Content of Low-Resistivity Shale: A Case Study of Longmaxi Shale in Southern Sichuan Basin, China

Low-resistivity shales are widely developed in the Sichuan Basin. The production of low-resistivity shale gas reservoirs ranges from high to low to none. The existing methods for gas-content prediction cannot accurately predict the gas content of low-resistivity shale. This increases the risk of shale-gas exploration. To prove that the random forest algorithm has apparent advantages in predicting the gas content of low-resistivity shale and reducing the risks associated with shale-gas exploration and development, three prediction methods were selected in this paper to compare their effects. The first method is known as the grey-correlation multiple linear regression method. Low-resistivity shale-gas content logging series were optimized using the grey-correlation approach, and then the low-resistivity shale-gas-content prediction model was established using the multiple linear regression method. The second method we selected was the resistivity method. The improved water-saturation model was used to predict the water saturation of low-resistivity shale, and then the gas content of low-resistivity shale was predicted based on the free-gas content and the adsorbed-gas-content model. The random forest algorithm was the third method we selected. Fourteen logging series were used as input data and the measured gas content was used as supervised data to train the model and to apply the trained model to the gas-content prediction. The findings demonstrated that the grey-correlation multiple regression method had poor accuracy in predicting gas content in low-resistivity shale; The resistivity method accurately predicted water saturation, and the predicted gas content was higher than the actual gas content. Because the random forest algorithm accurately predicted low-resistivity shale-gas content, its use in the Sichuan Basin was advantageous. The selection of a low-resistivity shale-gas-content prediction model was guided by the research findings.

Unusual 13C depletion in the central carbon of propane in the gases of Longmaxi shale in the Sichuan Basin

Abiotically synthesized hydrocarbons are significant in the fields of astrobiology, biosphere, energy, environment, and geology. Abiotic hydrocarbons have never been unambiguously proven to occur in common sediments partly because identifying these components in crustal settings is challenging as they share similar characteristics with hydrocarbons of biotic origin. In this study, we measured intramolecular carbon isotopes of 10 propane samples from Longmaxi shale in the Sichuan Basin, China, and found distinct 13C-depletion in the central position carbon with its δ13C decreased by −8.0 to −7.0‰ (Vienna Pee Dee Belemnite) with respect to terminal carbons. The δ13C value of central carbon (H3C–#CH2-CH3) in propane is equal to the second methyl (H3C–#CH3) in ethane possibly resulting from surface catalytic polymerization reactions of methane. Our novel findings indicate that the abiotic synthesis of C2+ hydrocarbons by the surface catalyzed polymerization of methane occurs in commercial gas reservoirs and suggest that this process may be more common than previously thought in methane-rich scenarios on Earth and other planets.

Experimental and DEM Simulation Study on the Mechanical Characteristic and Strain Energy Evolution of Longmaxi Shale under a Confining Pressure Unloading Path

Drilling vertical and horizontal wellbores in the shale reservoir may trigger the in-situ stress release around the wellbore walls and change the original stress equilibrium state, leading the wellbores to instability. This stress change in the wellbore corresponds to the stress paths of confining pressure unloading and axial stress loading under laboratory conditions. In this paper, according to the conventional triaxial compression test results, laboratory experiments and DEM simulations by PFC2D were conducted to deeply study the strength, failure, strain energy evolution, and micro-crack damage mechanism of shale specimens under confining pressure unloading conditions. The shale specimens at different bedding inclinations were tested under different initial axial stress levels and confining pressure unloading rates, with fixed initial unloading confining pressure. This research revealed that confining pressure unloading induces greater plastic deformation, more micro-crack damage and strain energy dissipation, and a more complex failure pattern. The strain energy dissipation and dilatation under confining pressure unloading conditions are mainly induced by the generation and accumulation of tensile cracks. Moreover, the unloading rate has a significant effect on the mechanical properties, and the high unloading rate enhances the failure strength and induces more strain energy dissipation and micro tensile cracks. For the wellbore drilling in shale formations, when the buried depth and vertical stress are fixed, the lower the lateral stress is, the easier it is to form tensile failure around the wellbore wall in the drilling process, and the more induced fractures will be generated in the formation around the wellbore.

Origin of carbonate minerals and impacts on reservoir quality of the Wufeng and Longmaxi Shale, Sichuan Basin

The Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin were studied to understand the genesis and diagenetic evolution of carbonate minerals and their effects on reservoir quality. The results of geochemical and petrological analyses show that calcite grains have a negative Ce anomaly indicating they formed in the oxidizing environment of seawater. The high carbonate mineral contents in the margin of basin indicate that calcite grains and cores of dolomite grains appear largely to be of detrital origin. The rhombic rims of dolomite grains and dolomite concretions with the δ13C of –15.46‰ and the enrichment of middle rare earth elements were formed during the sulfate-driven anaerobic oxidation of methane. The calcite in radiolarian were related to the microbial sulfate reduction for the abundant anhedral pyrites and δ13C value of –11.34‰. Calcite veins precipitated in the deep burial stage with homogenization temperature of the inclusions ranging from 146.70 °C to 182.90 °C. The pores in shale are mainly organic matter pores with pore size mainly in the range of 1–20 nm in diameter. Carbonate minerals influence the development of pores through offering storage space for organic matter. When calcite contents ranging from 10% to 20%, calcite grains and cement as rigid framework can preserve primary pores. Subsequently, the thermal cracking of liquid petroleum in primary pores will form organic matter pores. The radiolarian were mostly partially filled with calcite, which combining with microcrystalline quartz preserved a high storage capacity.

Characteristics of Pyrite in Shale and Its Application in Shale Gas Enrichment Evaluation─a Case Study of Longmaxi Shales in the Xiang’exi Depression, China

Characteristics of Pyrite in Shale and Its Application in Shale Gas Enrichment Evaluation─a Case Study of Longmaxi Shales in the Xiang’exi Depression, China