Mechanical Properties Of Shale Research Articles

Mechanical properties of shale during pyrolysis: Atomic force microscopy and nano-indentation study

Quantitative characterization of the geo-mechanical properties of shale and organic matter (OM) holds paramount significance in the assessment of shale gas reserves and the design of hydraulic fracturing. However, the mechanical evolution processes during shale hydrocarbon generation and its influencing factors have received limited attention. This study examines the changes in shale mechanical properties during pyrolysis at high temperatures (415–600 °C) and high pressure (50–125 MPa) for mature to over-mature stages. The nanoindentation and in-situ AFM-QNM analysis are utilized to characterize the changes in mechanical properties during evolution. Subsequently, gas adsorption, Fourier Transform infrared spectroscopy (FTIR), and laser Raman spectroscopy (Raman) are used to investigate the factors influencing the mechanical properties of shale and the associated OM, and establish a model for the evolution of the mechanical properties. The results demonstrate that with increasing maturity, the overall Young's modulus of the bulk shale gradually increases from 42.8 GPa to 58.4 GPa for the temperature increment from 415 °C to 600 °C. During the thermal maturation process, the mesopore structure and quartz content of the shale significantly influence its mechanical properties. Young's modulus of OM shows an S-shaped trend, with variations in the micromechanical properties of OM corresponding to stages of hydrocarbon generation. In particular, two peaks of Young's modulus increase are observed during the mature and over-mature stages. In the mature stage, the aromatization of the kerogen leads to substantial detachment of aliphatic side chains and oxygenated functional groups, resulting in a higher degree of aromatization. This reduces the kerogen spacing and consequently increases the Young's modulus. In the over-matured stage, the process of aromatics condensation leads to the orientation and arrangement of aromatic rings, reducing the number of key site vacancies and crystal defects, thereby forming large aromatic clusters and significantly increasing the graphite-like structure. This study will facilitate the analysis of shale matrix deformation mechanisms at the microscale, providing a fundamental theoretical and scientific basis for shale fracturing design, exploration, and development.

Short-term interactions between Longmaxi shale and carbon dioxide-based fracturing fluids

The shale is a dense quarried rock material containing abundant shale oil/gas. Extracting shale gas usually requires fracturing the rock formation to speed up gas desorption and migration. Supercritical carbon dioxide fracturing, as an environmentally friendly and waterless fracturing technique of reservoir stimulation, gains increasing recognition in commercial exploitation of shale oil and gas. However, there is insufficient understanding on the interaction between carbon dioxide-based fracturing fluids and shale, as well as its influence on the mechanical parameters of shale, which are pivotal for determining injection parameters and forming the fracture network. This study focuses on the injection of carbon dioxide-based fracturing fluids to enhance shale gas extraction and carbon dioxide sequestration. Simulation tests of injection were conducted to investigate the influence of different fracturing fluids (deionized water, gaseous/supercritical carbon dioxide, gaseous/supercritical carbon dioxide mixed with brine) injected into shale reservoirs on the microstructure and mechanical properties of shale after short-term physicochemical interaction. This article adopts research methods such as uniaxial compression and uniaxial tensile mechanical experiments based on acoustic emission monitoring and various physical characterizations such as scanning electron microscopy, X-ray diffraction, X-ray fluorescence spectrometer, etc. Results show that following treatment with different carbon dioxide-based fluids, the uniaxial tensile/compressive strength, elastic modulus, fractal dimension of pore structure, and the brittleness index of shale exhibit varying degrees of decrease compared to those of dry untreated shale samples. After interacting with different carbon dioxide-based fracturing fluids, the mechanical parameters and brittleness index of shale decrease more significantly than that of the others as the addition of brine. This study shows that the water in carbon dioxide-based fracturing fluids is a controlling factor affecting the elastic modulus of shale. Additionally, the ductility of the shale increases and the acoustic emission emitting lag due to the coupling effects of brine and carbon dioxide. The change law of the elemental and mineral composition of the shale is consistent with the mechanical strength; and the change of element content and mineral composition is the most significant. Compared to gaseous carbon dioxide, the supercritical carbon dioxide has a greater impact on mineral composition of the shale. The change of mechanical strength and microstructure evolution mechanism caused by the short-term interactions between the shale and fracturing fluids provide theoretical references and implications for the determination of injection parameters and permeability transformation of the Longmaxi shale reservoirs.

Effects of CO2-saturated brine imbibition on the mechanical and seepage characteristics of Longmaxi shale

Supercritical carbon dioxide (SC– CO2) serves as a promising anhydrous fracturing fluid that has the potential to simultaneously enhance shale gas recovery and accomplish CO2 sequestration within shale reservoirs. However, the hydromechanical properties of shale are considerably affected by CO2-brine-shale interaction. This study conducted shale imbibition experiments involving subcritical/supercritical CO2 saturated brine (Sub/SC-CO2+brine). Triaxial compressive strength tests were performed to evaluate the impact of Sub/SC-CO2+brine imbibition on the mechanical properties of shale, as well as the influence of SC-CO2+brine imbibition on shale permeability. The results indicate that, after SC-CO2 brine imbibition, noticeable macroscopic cracks are formed parallel to the bedding plane. The imbibition of Sub/SC-CO2+brine results in the reduction in axial stress and Young's modulus, while simultaneously increasing the axial strain of shales. Compared to Sub-CO2+brine, SC-CO2+brine exhibits a higher imbibition pressure and a more intense geochemical reaction with shale. Due to increased compressibility sensitivity, the averaged permeability after SC-CO2+brine imbibition shows a reduction of nearly 70 % compared to that before imbibition. The insights from this research are expected to provide theoretical support for shale gas recovery and CO2 geological sequestration.

Shale oil recovery by CO2 injection in Jiyang Depression, Bohai Bay Basin, East China

Laboratory experiments, numerical simulations and fracturing technology were combined to address the problems in shale oil recovery by CO2 injection. The laboratory experiments were conducted to investigate the displacement mechanisms of shale oil extraction by CO2 injection, and the influences of CO2 pre-pad on shale mechanical properties. Numerical simulations were performed about influences of CO2 pre-pad fracturing and puff-n-huff for energy replenishment on the recovery efficiency. The findings obtained were applied to the field tests of CO2 pre-pad fracturing and single well puff-n-huff. The results show that the efficiency of CO2 puff-n-huff is affected by micro- and nano-scale effect, kerogen, adsorbed oil and so on, and a longer soaking time in a reasonable range leads to a higher exploitation degree of shale oil. In the “injection + soaking” stage, the exploitation degree of heavy hydrocarbons is enhanced by CO2 through its effects of solubility-diffusion and mass-transfer. In the “huff” stage, crude oil in large pores is displaced by CO2 to surrounding larger pores or bedding fractures and finally flows to the production well. The injection of CO2 pre-pad is conducive to keeping the rock brittle and reducing the fracture breakdown pressure, and the CO2 is liable to filter along the bedding surface, thereby creating a more complex fracture. Increasing the volume of CO2 pre-pad can improve the energizing effect, and enhance the replenishment of formation energy. Moreover, the oil recovery is more enhanced by CO2 huff-n-puff with the lower shale matrix permeability, the lower formation pressure, and the larger heavy hydrocarbon content. The field tests demonstrate a good performance with the pressure maintained well after CO2 pre-pad fracturing, the formation energy replenished effectively after CO2 huff-n-puff in a single well, and the well productivity improved.

An investigation on the pyrolysis of low-maturity organic shale by different high temperature fluids heating

This study evaluates the potential of oil and gas generation from thermal pyrolysis of low-maturity organic rich shales by high temperature fluids injection. This method aims to convert the solid organic matter into hydrocarbons, thereby enhancing the quality of shale oil. The effectiveness of this technique hinges on its ability to convert retained heavy compounds and kerogen into light, moveable hydrocarbons, while also improving the oil recovery factor. Our experimental findings reveal that the decrease of total organic carbon content predominantly occurs between 400–550 °C or even higher. Different heating fluid media were tested for their pyrolysis effectiveness, among the heating media tested, CO2 and steam mixture proved the most productive. A comparison of CO2, steam and CO2-steam pyrolysis experiments were carried out. It is observed that the oil yield is the lowest in a pure CO2 atmosphere, while oil yield from the mixture of CO2-steam pyrolysis is the highest. In addition, the pore structure and mechanical properties of shales during pyrolysis process are examined.This investigation provides a theoretical basis for the large-scale upgrading of organic-rich shale formations, highlighting the critical influencing factors of thermal pyrolysis in optimizing shale oil recovery and quality.

Analysis of changes in shale mechanical properties and fault instability activation caused by drilling fluid invasion into formations

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.

Porosity and bedding controls on bio-induced carbonate precipitation and mechanical properties of shale and dolomitic rocks: EICP vs MICP

Biocementation is an emerging field within geotechnical engineering that focuses on harnessing microbiological activity to enhance the mechanical properties and behavior of rocks. It often relies on microbial-induced carbonate precipitation (MICP) or enzyme-induced carbonate precipitation (EICP) which utilizes biomineralization by promoting the generation of calcium carbonate (CaCO3) within the pores of geomaterials (rock and soil). However, there is still a lack of knowledge about the effect of porosity and bedding on biocementation in rocks from a mechanistic view. This experimental study investigated the impact of porosity and bedding orientations on the mechanical response of rocks due to biocementations, using two distinct biocementation strategies (MICP and EICP) and characteristically low porosity but interbedded rocks (shale) and more porous but non-bedded (dolostone) rocks. We first conducted biocementation treatments (MICP and EICP) of rock samples over a distinct period and temperature. Subsequently, the rock strength (uniaxial compressive strength, UCS) was measured. Finally, we analyzed the pre- and post-treatment changes in the rock samples to better understand the effect of MICP and EICP biocementations on the mechanical response of the rock samples. The results indicate that biocementations in dolostones can improve the rock mechanical integrity (EICP: +58% UCS; MICP: +25% UCS). In shales, biocementations can either slightly improve (EICP: +1% UCS) or weaken the rock mechanical integrity (MICP: −39% UCS). Further, results suggest that the major controlling mechanisms of biogeomechanical alterations due to MICP and EICP in rocks can be attributed to the inherent porosity, biocementation type, and bedding orientations, and in few cases the mechanisms can be swelling, osmotic suction, or pore pressurization. The findings in this study provide novel insights into the mechanical responses of rocks due to MICP and EICP biocementations.

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.