Mechanical Properties Of Shale Research Articles

Study on failure characteristics and evaluation index of aquifer shale based on energy evolution

AbstractThe presence of abundant clay components and microporous structure in shale results in its high hydrophilicity, making a water-rich environment inevitable in petroleum exploration projects. Therefore, it is crucial to consider the influence of bedding structure, moisture content, confining pressure, and their combined effects on the geomechanical properties of shale. This article aims to investigate the mechanical properties of deep shale under varying water content conditions, elucidate the failure mode and failure mechanism of shale in actual engineering scenarios, and explores the interplay between stress, structure, moisture content, and other factors on its mechanical properties. The evaluation of wellbore stability and fracture propagation effects is proposed based on laboratory experiments using triaxial stress and strain data, along with the application of energy evolution theory. The experimental procedures encompass an analysis of shale's microscopic components and structure, as well as anisotropic shale triaxial compression tests conducted under different moisture contents and confining pressures. The results demonstrate that shale exhibits dense pores in its microstructure and displays pronounced anisotropic characteristics in its macrostructure. The presence of water within these pores, combined with the in situ stress within the formation, significantly influences the mechanical properties of shale. This anisotropy decreases with increasing moisture content, but the mechanical performance still decreases. Under triaxial compression conditions, the increase in confining pressure to some extent enhances the anisotropy of shale's deformation characteristics, which is related to the failure modes of shale. However, the detrimental effect of moisture content on shale's mechanical properties still persists. In order to quantify the impact of these factors, this study utilizes the elastic modulus as an indicator of the coupling effect. It combines the triaxial strain curve obtained from laboratory tests and proposes an evaluation index for shale mechanical properties based on the energy evolution theory. This index is suitable for assessing wellbore stability (the stability index called SIr) and crack expansion (the brittleness index called BIr). The calculation results reveal that, during the wellbore drilling process, excavating parallel to the direction of shale bedding while maintaining low moisture content and high confining pressure yields a higher SIr value, indicating better wellbore stability. On the other hand, during reservoir fracturing, fracturing perpendicular to the shale bedding direction and maintaining low confining pressure and moisture content result in a smaller BIr value. This approach is more beneficial for the expansion of shale fracture network in engineering.

Experimental and damage model study of layered shale under different moisture contents

To investigate the mechanical properties and damage evolution law of layered shale under varying moisture contents, we conducted triaxial compression experiments on rock samples with different bedding angles and moisture levels. This study analyzed the variations in mechanical properties of layered shale under different conditions, and established a predicted model for elastic modulus based on different bedding angles and moisture content. Additionally, the damage constitutive model of layered shale was improved. The study revealed that shale’s mechanical properties display anisotropy, which is influenced by the bedding angles and moisture contents. The elastic modulus of the rock increases with the rise of bedding angle, exhibiting a ‘U’-shaped change. Conversely, the mechanical properties of rocks deteriorate, and their brittleness weakens with the increase in moisture content. When the confining pressure increased, the overall mechanical properties of shale were enhanced, and the influence of bedding on shale was weakened, but the deteriorating effect of water on rocks was hardly affected. Based on the above experiments, a predicted model of equivalent elastic modulus of shale considering the coupling effect of bedding and different moisture contents was proposed, which could effectively predict the elastic modulus of layered shale with different moisture content under different confining pressures. Furthermore, based on the predicted model of elastic modulus, an improved damage constitutive model of layered shale under triaxial loading was established, and the damage accumulation trend of layered shale was obtained, which showed an “S”-shaped change with strain. Under the coupling effect of bedding and different moisture contents, the damage of shale was advanced, but the accumulation rate of damage slowed down. With the increase of confining pressure, the influence of bedding and moisture content on the damage characteristics of shale decreased, and the damage curves under different conditions gradually tended to isotropy. The developed damage constitutive model for layered shale under different moisture contents provides theoretical support for the study of reservoir fracturing and wellbore stability.

Depositional environmental controls on mechanical stratigraphy of Barakar Shales in Rajmahal Basin, India

Understanding mechanical behaviour of shale is essential for efficient shale gas extraction, which can vary in different depositional settings. The impact of sedimentary environment on shale characteristics, such as mineralogical composition, total organic carbon content (TOC), and petrophysical properties, has been extensively researched. However, its influence on shale mechanical properties, especially in defining mechanical stratigraphy for targeting specific fracturing intervals, remains less explored. In this study, the influence of depositional environment on the mechanical properties of shale samples from the Rajmahal Basin is evaluated. Tensile strength of the samples was measured by the Brazilian splitting tensile strength and the brittleness index was calculated as a measure of mechanical properties. In addition, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray fluorescence spectroscopy (XRF), Rock-Eval 6, and X-ray diffraction (XRD) analysis were carried out to assess geochemical characteristics of the samples from different perspectives. The results revealed that such geochemical variations that are generally controlled by the depositional environment, would impact the mechanical properties of the samples. Based on major and trace elements proxies, the depositional environment was determined to be passive continental margin, with hot and humid paleoclimatic conditions and freshwater anoxic settings. Tensile strength and brittleness index of the shale samples was observed to vary between 0.93 and 4.12 MPa and 0.71 to 3.40, respectively, while samples with the TOC exceeding 15 wt% had a strong negative correlation with tensile strength, as reasonably expected, due to weakening impact of the sedimentary organic matter on the shale matrix. Tensile strength and brittleness index correlated positively with clay mineral content, particularly their type, but negatively with the quartz content. Furthermore, samples abundant in biogenic silica exhibited reduced brittleness compared to those with lithogenic silica. Nevertheless, the variation in mechanical properties with burial depth was not substantial, and the examination of stress-strain curves indicated an overall brittle nature of the layer where the samples were retrieved from. Overall, achieving more robust conclusions regarding mechanical stratigraphy within the studied section of the Rajmahal Basin, would necessitate additional vertical sampling.

Direct tensile damage process of calcite vein shale based on 3D CT reconstruction

AbstractFrom core observations of shales of the Niutitang formation in the northern part of Guizhou Province, China, calcite was often found to act as a natural fracture filler and affect the extension of fractures in hydraulic fracturing. Therefore, it is crucial to understand the tensile mechanical behavior of shales by calcite veins. In this paper, by computed tomography scanning of shales containing calcite veins of different dip angles from the Niutitang formation, a three‐dimensional numerical model reflecting the internal fine structure of the shale was constructed. Direct tensile numerical simulations were carried out to investigate the effect of calcite veins at different angles on the fine‐scale damage process and mechanical properties of shale. The experimental results show that the tensile capacity of the shale increases with the increase in calcite veins. Depending on the dip angle of the calcite vein, the damage pattern of the shale is divided into three types of damage: pull‐off damage along the calcite vein, jagged damage, and horizontal damage perpendicular to the loading method. At high dip angles, the shale damage is more intense and the fracture network more complex. The temporal and spatial characteristics of the acoustic emissions provide a good indication of the microscopic behavior of the shale specimens during the damage process. The box dimension method was used to calculate the fractal dimension of the shale specimens at the final damage, and it was found that the damage was more intense and the fracture network was more complex at high dip angles, and there was an angular threshold.

Assessing the effect of oriented structure characteristics of laminated shale on its mechanical behaviour with the aid of nano-indentation and FE-SEM techniques

Variations in the mechanical properties of oriented shale within the nano-to-micro scale are crucial to determine the deformation and fracture of shales. However, the relationships between the oriented structure characteristics of shale and its mechanical properties need further investigation. In this study, the Yan-Chang #7 shale in the Ordos Basin is selected to elucidate the micro-mechanical properties of different shale grains using nano-indentation experiments. Results show that pyrite grains exhibit the highest elastic modulus, followed by quartz, feldspar, calcite, dolomite, clay minerals, and organic matter. Additionally, the experiments demonstrate an abrupt shift at the contact edge of the grains due to high heterogeneity. By applying a modified Mori-Tanaka method, we upscale the micromechanical characteristics of grains to the matrix. The size, flatness of grains, and structure orientation entropy (SOE) of shale indicate a good correlation with the mechanical properties of shale or grains. A larger size, more flatness of grains, and lower SOE result in smaller values of the elastic modulus of shale. The layered structure of shale samples creates a weak surface between the layers, making them more susceptible to fracturing and deformation when subjected to stress.

The investigation on shale mechanical characteristics and brittleness evaluation

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.

Influences of shale microstructure on mechanical properties and bedding fractures distribution

The difference in microstructure leads to the diversity of shale mechanical properties and bedding fractures distribution patterns. In this paper, the microstructure and mechanical properties of Longmaxi marine shale and Qingshankou continental shale were studied by X-ray diffractometer (XRD), field emission scanning electron microscope (FE-SEM) with mineral analysis system, and nanoindentation. Additionally, the typical bedding layers area was properly stratified using Focused Ion Beam (FIB), and the effects of microstructure and mechanical properties on the distribution patterns of bedding fractures were analyzed. The results show that the Longmaxi marine shale sample contains more clay mineral grains, while the Qingshankou continental shale sample contains more hard brittle mineral grains such as feldspar. For Longmaxi marine shale sample, hard brittle minerals with grain sizes larger than 20 μm is 18.24% and those with grain sizes smaller than 20 μm is 16.22%. For Qingshankou continental shale sample, hard brittle minerals with grain sizes larger than 20 μm is 40.7% and those with grain sizes smaller than 20 μm is 11.82%. In comparison to the Qingshankou continental shale sample, the Longmaxi marine shale sample has a lower modulus, hardness, and heterogeneity. Laminated shales are formed by alternating coarse-grained and fine-grained layers during deposition. The average single-layer thickness of Longmaxi marine shale sample is greater than Qingshankou continental shale sample. The two types of shale have similar bedding fractures distribution patterns and fractures tend to occur in the transition zone from coarse-grained to fine-grained deposition. The orientation of the fracture is usually parallel to the bedding plane and detour occurs in the presence of hard brittle grains. The fracture distribution density of the Longmaxi marine shale sample is lower than that of the Qingshankou continental shale sample due to the strong heterogeneity of the Qingshankou continental shale. The current research provides guidelines for the effective development of shale reservoirs in various sedimentary environments.

Fracture energy evolution and fractal characteristics of shales with different quartz contents

Quartz is an important mineral component of shale. In order to explore the influence of quartz content on the mechanical properties of shale, this article conducted numerical simulation of the Lower Cambrian Niutitang Formation shale in Fenggang Block 3, Guizhou using RFPA software. The results show that the effect of quartz content on compressive strength is more significant than that on elastic modulus. The damage modes of shale with different quartz content are presented as 5 types, namely, oblique tension damage (21%, 27%), inverted N-shaped damage (33%), V-shaped damage (39%), inverted V-shaped damage (45%), and λ-shaped damage (51%). When the content of quartz is higher than a certain value, the microscopic damage of shale tends to be more complicated. The destruction acoustic emission energy and fractal dimension show significant difference with quartz content. The research results of this article will play a certain role in promoting shale gas exploitation. Highlights The influence of quartz content on the mechanical properties of shale is numerically simulated. The energy evolution law of shale destruction in the presence of quartz is analyzed. The complexity of shale destruction caused by quartz content is analyzed. It is proven by a similarity test that shale changes with different mechanical properties of quartz content.

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.

Assessment of inherent heterogeneity effect on continuous mechanical properties of shale via uniaxial compression and scratch test methods

Shale reservoirs have been a significant focus of hydrocarbon production over the past few decades, and the mechanical assessment of target shale reservoirs has been critical to successful field operations, especially in hydraulic fracturing and well completions. The Unconfined compressive strength (UCS) and Poisson's ratio (ν) are critical mechanical properties in shale reservoir assessment. The estimation and measurement of shale mechanical properties are often erroneous by not accounting for their heterogeneous and pre-existing features, which yield variability of shale mechanical properties along their lithostratigraphy. Thus, there is a need to investigate the degree of correlation and accuracy in multiscale mechanical evaluations of heterogeneous shales, and the correlation between such micromechanical and macromechanical measurements. This study investigated the impact of inherent heterogeneity on the measurement of continuous micromechanical and macromechanical properties of shale reservoirs using scratch test (ST) and uniaxial compression test (UCT) methods, and the degree of correlation (correlation coefficient, r) of measurements in shale was further assessed for the variability of their measured properties. Shale core samples from three distinct shale formations were utilized and studied, and the core samples were subjected to ST and UCT, respectively. The results from this study showed that despite inherent heterogeneous anomalies and natural fractures in the shale samples analyzed, there is a good degree of correlation (UCS: r ​= ​0.73; ν: r ​= ​0.89) in the micro- and macro-mechanical properties of shales using two independent experimental tests (ST and UCT). This study provides insights for improving the accuracy of mechanical evaluations and numerical modeling in shales with a high degree of heterogeneity and pre-existing natural fractures. The results indicate that when considering the structural complexity and heterogeneity of unconventional reservoirs such as shales, the ST method can provide a better continuous micromechanical assessment of shales. In contrast, the UCT can provide a better bulk macromechanical measurement of shales.

Experimental study on the static and dynamic elastic stiffness of rock samples from a shale oil reservoir

Shale reservoirs play essential roles in numerous geoengineering applications, including storing CO2 and radioactive waste and producing hydrocarbons. To fulfill these applications, the mechanical properties of shale are of great interest so that people can locate shales with preferred sealing or frackable characteristics. One can estimate the static stiffness related to the rock deformation from the dynamic stiffness derived from the wave velocity through some empirical correlations. However, a gap exists between dynamic and static stiffness in sedimentary rocks. Anisotropy further complicates the prediction of the static stiffness from the dynamic stiffness of shale. This paper investigates the link and contrast between the dynamic and static stiffness and the effects of anisotropy by performing triaxial tests on two shale samples cored in the vertical and horizontal directions from a shale oil reservoir in China. We analyze the effects of stress (strain) rate, stress (strain) amplitude, and the base level of deviatoric stress. The secant Young’s modulus is found to decrease with the increasing stress rate or stress amplitude. By introducing the nonlinear elastic theory, Young’s modulus can be expressed as a linear function of axial strain, where the slope is related to the stress sensitivity of the static stiffness, and the intercept is associated with the elastic property at the infinitesimal strain. The slopes and intercepts of linear functions are negatively correlated. Furthermore, six linear fittings from the two samples in the three tests are found to link with the strain amplitude, the base stress level, and anisotropy. This study provides insights into the prediction of static stiffness from dynamic one. With proper models of dynamic-static transition, seismic or wireline logging measurements can assist in solving engineering issues. People can better ensure the quality of the geomechanical model and reduce the cost of various engineering applications.

Hydration of Transitional Shale and Evolution of Physical Properties Constrained by Concurrent NMR and Acoustic Observations

Hydration state is a key factor affecting the pore structure and mechanical properties of shale that directly impacts the migration and then production of geofluids. We quantify the impact of the hydration state on physical properties through spontaneous imbibition experiments constrained by concurrent nuclear magnetic resonance (NMR) and acoustic observations as diagnostics of response. The long-duration experiments (24 days) are on samples from the Permian Shanxi Formation of the Ordos Basin, complemented by measurements of composition and physical properties. These shales primarily comprise quartz and clay minerals with intraparticle pores present within the clay minerals. The NMR T2 spectra indicate that mesopores (2–50 nm) comprise ∼80% of the total pore space. The imbibition rate decreases by a factor of 3 from 0.08 g/h to a stable rate of 0.03 g/h from the 1st to the 24th hour. This is accompanied by a 10% decrease in the deformation modulus from 44 to 40.3 GPa over 24 days. The reduction in the modulus is consistent with observations of the shedding and spalling of hydrophilic quartz particles from the unconstrained surface and the generation of fractures. Reduction in the imbibition rate is consistent with the swelling followed by squeezing of clay minerals and the corresponding reduction in pore diameters in the confined core of the sample, together with the reduction in saturation gradients with the progress of imbibition. Understanding the mechanisms and effects of hydration on the pore structure and mechanical properties is important in understanding the shale hydration process and defining diagnostic (acoustic and NMR) signatures that may be used in reservoir surveillance.

A Novel Upscaling Method for Evaluating Mechanical Properties of the Shale Oil Reservoir Based on Cluster Analysis and Nanoindentation

Abstract Shale formations as major unconventional energy resources are crucial in satisfying the global energy needs of the future. Via nanoindentation method and upscale method, the macromechanical parameters of shale, such as hardness, elastic modulus, are obtained. The conventional Mori–Tanaka upscale method only divides the data into three mineral classes and fails to fully incorporate micromechanical properties to reflect the macroscale properties of samples. The research measures micromechanical parameters of shale via nanoindentation and performs cluster analysis of nanoindentation measurements. The results of cluster analysis are then combined with the Mori–Tanaka upscale model to evaluate the macroscale mechanical property of shale. The elastic modulus, hardness, and fracture toughness are divided into five groups (clusters) via cluster analysis, with each representing a certain mineral composition. This research is of great significance for more reasonably and accurately characterizing shale mechanical properties, optimizing the recovery scheme, and improving the recovery efficiency of shale gas.

Weakening Effect of Water on Mechanical Properties of Brittle Shale under Triaxial Compression

Abstract The mechanical properties of rock materials are remarkably affected by water. To reveal the effect of water on the mechanical properties of shale, the conventional triaxial compression tests were implemented for shale with the five kinds of water saturation coefficient. According to the test results, the influence of the saturation coefficient on strength and deformation parameters was analyzed. Based on the fitting result of the Mohr-Coulomb criterion, the relationship between the strength parameters and the water saturation coefficient was analyzed. The softening effect of water on shale is mainly due to the decrease of cohesion and internal friction coefficient with increasing of water saturation coefficient. The strength criterion considering the influence of water weakening was established according to the function relationship between the fitting parameters and the water saturation coefficient. In addition, three kinds of brittleness indexes were selected to quantitatively analyze the influence of the saturation coefficient on shale brittleness. The results indicated that the brittleness indexes all decreased with the increase of the saturation coefficient, and the brittleness of shale is weakened after water absorption. The research results can provide a reference for the evaluation of saturated shale brittleness.

Effect of supercritical CO2-water-shale interaction on mechanical properties of shale and its implication for carbon sequestration

The mechanical behaviors of shale are important parameters influencing the CO2 storage security in shale reservoirs. Thus, understanding the mechanical properties alteration of shale induced by the injected CO2 is essential for evaluating the stability of shale reservoir rock and caprock. In this study, the impact of supercritical CO2(ScCO2)-water soak pressure and temperature (P = 0, 10, 15, 20 MPa, T = 308, 323, 338, 353 K) on the mechanical properties of shale were investigated. Mineral, pore and mechanical alterations in shale triggered by ScCO2-water soak were obtained by multiple characterization methods. The results indicated that ScCO2-water soak caused the decrease in clay and carbonate mineral contents, and the increase in macropore volume and porosity. Then, the average triaxial compressive strength and elastic modulus of shale decreased from 286.65 MPa to 156.90–247.20 MPa, and 13.62 GPa to 10.52–12.99 GPa, respectively, while Poisson's ratio v in shale increased from 0.1147 to 0.1179–0.1780. ScCO2-water soak induced mechanical weakening in shale is positively correlated with the soak pressure and negatively associated with soak temperature. The dissipated energy ratio decreased first and then increased during the stress loading process in all the tested shales, ScCO2-water soaked shale has a larger proportion of energy dissipation in each stage of stress loading. The results inferred that ScCO2-water soak caused an enhanced plastic deformation behavior in shale. The mechanical degradation in shale may increase the failure risk of reservoir rock or caprock, which should be concerned in CO2 geological storage engineering.

超临界CO<sub>2</sub>渗流的流-固-热多场耦合机理研究

<p indent="0mm">Clay swelling and dispersion easily occur when encountering water for shale formation, resulting in problems related to wellbore collapse when using water-based fluids, such as drilling fluid or poor fracturing stimulation when using water-based fluids as the fracturing fluid. Supercritical carbon dioxide (ScCO₂) can effectively protect the clay from hydration and avoid formation damage. Based on this, basic research studies have been performed by researchers regarding the exploitation of shale oil and gas resources using ScCO₂. The multi-physics coupling model describing the water seepage process cannot explain the ScCO₂ seepage because of a great variation in the physical properties of CO₂ with temperature and pressure. To construct multi-physics coupling models capable of reflecting the ScCO₂ seepage process when developing oil and gas resources using ScCO₂, the thermal-hydro-mechanical coupling models are established in this study based on the transport and thermodynamic properties of ScCO₂, combined with the law of effect of ScCO₂ dissolution on the mechanical properties of shale. The finite element method is used to study the distribution of formation temperature, pore pressure, and stress with time and location of ScCO₂ seepage, and the comparison of multi-physics fields is performed between ScCO₂ and water seepage. The results indicate that the compared with water seepage, the formation temperature variation of ScCO₂ seepage is greater, the pore pressure is lower, the stress difference near the wellbore in the direction of minimum <italic>in</italic>-<italic>situ</italic> stress is bigger, and the tangential stress in the direction of maximum <italic>in</italic>-<italic>situ</italic> stress is lower. This study can provide theoretical basis for wellbore stability analysis when using ScCO₂ as the drilling fluid and fracturing design and evaluation when using ScCO₂ as the fracturing fluid.

NANOINDENTATION EXPERIMENTS TO STUDY THE MICROMECHANICAL PROPERTIES OF STRATIFIED SHALE

The micromechanical properties of stratified shale are critical for understanding wellbore stability and hydraulic fracture extension patterns. Based on nanoindentation experiments and data analysis, the mechanical parameters of shale specimens from two laminar directions of the Jimsar Lucaogou Formation in Xinjiang are measured from a microscopic perspective in this paper. The mechanical parameters of multiple indentation points are obtained using grid indentation. The mineral composition of the samples is quantified using field emission scanning electron microscopy (SEM) and XRD ray diffraction. The results show that the shale mechanical parameters have a good linear relationship with each other at the nanoscale. However, the relationship between horizontal bedding and vertical bedding is quite different. The average elastic modulus, average hardness, and average fracture toughness of the parallel laminated specimens are 22.94 GPa, 1.04 GPa, and 1.19 MPa&amp;#183;m&lt;sup&gt;1/2&lt;/sup&gt;, respectively. The average elastic modulus, average hardness, and average fracture toughness of the vertical laminated specimens are 35.23 GPa, 3.47 GPa, and 1.32 MPa&amp;#183;m&lt;sup&gt;1/2&lt;/sup&gt;, respectively. The shale mechanical parameters in the vertical bedding direction are higher than those in the horizontal laminae. The mechanical parameters at the nanoscale are related to the orientation of the laminae. The shale is anisotropic at the nanoscale and the anisotropy of different mechanical parameters behaves differently. The microscopic elastic modulus, hardness, and fracture toughness of shale conform to Weibull distribution. The dispersion of hardness is the highest. The results can help analyze the relationship between shale mechanical properties and laminar orientation.

Study on rock mechanics characteristics of deep shale in Luzhou block and the influence on reservoir fracturing

AbstractThis paper aims to study the influence of deep formation conditions on the mechanical properties of shale, which is of great significance for the engineering application of efficient development of deep shale gas. In this study, a real‐time high‐temperature (25°C–170°C) triaxial compression experiment was first carried out on shale samples. Then, based on the analysis results of rock mineral components and discrete element numerical simulation technology, a thermo‐mechanical coupling model (TMCM) was constructed, and the accuracy of the numerical model was verified. Finally, based on the micromechanical parameters, a numerical fracturing mechanism model considering the influence of temperature, natural fracture density, and fracture length was established, and the influence of current reservoir conditions on hydraulic fracturing was discussed. The results show that confining pressure has a greater effect on rock mechanics than temperature. When the temperature exceeds 110°C, the plasticity of rock samples increases, and the fracture morphology becomes complex. In addition, the increase in temperature promotes the fracture of microcracks to a certain extent. The research results are expected to provide a sufficient theoretical basis for the development and utilization of deep shale gas resources.

Shale Formation Damage during Fracturing Fluid Imbibition and Flowback Process Considering Adsorbed Methane

Hydraulic fracturing of shale gas reservoirs is characterized by large fracturing fluid consumption, long working cycle and low flowback efficiency. Huge amounts of fracturing fluid retained in shale reservoirs for a long time would definitely cause formation damage and reduce the gas production efficiency. In this work, a pressure decay method was conducted in order to measure the amount of fracturing fluid imbibition and sample permeability under the conditions of formation temperature, pressure and adsorbed methane in real time. Experimental results show that (1) the mass of imbibed fracturing fluid per unit mass of shale sample is 0.00021–0.00439 g/g considering the in-situ pressure, temperature and adsorbed methane. (2) The imbibition and flowback behavior of fracturing fluid are affected by the imbibition or flowback pressure difference, pore structure, pore surface properties, mechanical properties of shale and mineral contents. (3) 0.01 mD and 0.001 mD are the critical initial permeability of shales, which could be used to determine the relationship between the formation damage degree and the flowback pressure difference. This work is beneficial for a real experimental evaluation of shale formation damage induced by fracturing fluid.

Investigation of the Temperature Dependence of the Chemical-Mechanical Properties of the Wufeng-Longmaxi Shale.

Shale rocks have been widely investigated to evaluate the productivity of oil/gas. The high temperature generated by the explosive fracturing to stimulate the gas reservoir has a significant impact on the chemical-mechanical properties of shale rocks. Pioneering works have been carried out at temperatures below 500 °C, but little has been done to quantify the correlation between the chemical and mechanical properties of shale at temperatures above 500 °C. Therefore, an experimental study on the effect of temperature on the chemical-mechanical properties of shale rocks is presented in this paper. The temperatures used in our experiments are between 0 and 800 °C. Results indicate that there exist strong chemical reactions leading to a big reduction in the sample's weight and mechanical strength for a temperature over 500 °C. Thermogravimetric analysis data demonstrates that the weight of shale powders has little change below 400 °C and largely decreases after 600 °C. It shows that the chemical reaction rate corresponding to shale compositions varies with temperature. X-ray diffraction and Fourier transform infrared are integrated to quantify the occurrence of contained reactions including the decomposition of kerogen, carbonates, and quartz transition. This can provide a temperature range for all possible reactions. Changes in the compositional information of shale samples have been proven to significantly influence the mechanical properties. A 25% decrease in dynamic Young's modulus emerges as the temperature approaches 700 °C. As the brittle minerals, for instance, carbonates, decrease with temperature, a brittle-ductile transition happens in shale. This work provides very meaningful results different from that at low temperatures to help people better understand the effects of high temperatures in many fields, such as explosive fracturing and radioactive waste disposal.