Wellbore stability model in shale formation under the synergistic effect of stress unloading-hydration

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

Effect of fatigue loading-confining stress unloading rate on marble mechanical behaviors: An insight into fracture evolution analyses

Rocks in underground works usually experience rather complex stress disturbance.For this,their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions.The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies.However,little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading(FL-CSU)conditions that influence rock failure.In this paper,we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography(CT)scanning technique and the GCTS RTR 2000 rock mechanics system.Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble.The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests,and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests.In FL-CSU tests,as the unloading rate increases,the dissipated energy grows and elastic energy reduces.CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension.It shows that the crack pattern after FL-CSU tests depends on the unloading rate,and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released.The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.

Shear sliding of rough-walled fracture surfaces under unloading normal stress

Through high-precision engraving,self-affine sandstone joint surfaces with various joint roughness coefficients(JRC=3.21e12.16)were replicated and the shear sliding tests under unloading normal stress were conducted regarding various initial normal stresses(1e7 MPa)and numbers of shearing cycles(1 e5).The peak shear stress of fractures decreased with shear cycles due to progressively smooth surface morphologies,while increased with both JRC and initial normal stress and could be verified using the nonlinear Barton-Bandis failure criterion.The joint friction angle of fractures exponentially increased by 62.22%e64.87%with JRC while decreased by 22.1%e24.85%with shearing cycles.After unloading normal stress,the sliding initiation time of fractures increased with both JRC and initial normal stress due to more tortuous fracture morphologies and enhanced shearing resistance capacity.The surface resistance index(SRI)of fractures decreased by 4.35%e32.02%with increasing shearing cycles due to a more significant reduction of sliding initiation shear stress than that for sliding initiation normal stress,but increased by a factor of 0.41e1.64 with JRC.After sliding initiation,the shear displacement of fractures showed an increase in power function.By defining a sliding rate threshold of 5105 m/s,transition from“quasi-static”to“dynamic”sliding of fractures was identified,and the increase of sliding acceleration steepened with JRC while slowed down with shearing cycles.The normal displacement experienced a slight increase before shear sliding due to deformation recovery as the unloading stress was unloaded,and then enhanced shear dilation after sliding initiation due to climbing effects of surface asperities.Dilation was positively related to the shear sliding velocity of fractures.Wear characteristics of the fracture surfaces after shearing failure were evaluated using binary calculation,indicating an increasing shear area ratio by 45.24%e91.02%with normal stress.

Experimental and Numerical Study on the Shear Strength and Strain Energy of Rock Under Constant Shear Stress and Unloading Normal Stress

Excavation and earth surface processes(e.g.,river incision)always induce the unloading of stress,which can cause the failure of rocks.To study the shear mechanical behavior of a rock sample under unloading normal stress conditions,a new stress path for direct shear tests was proposed to model the unloading of stress caused by excavation and other processes.The effects of the initial stresses(i.e.,the normal stress and shear stress before unloading)on the shear behavior and energy conversion were investigated using laboratory tests and numerical simulations.The shear strength of a rock under constant stress or under unloading normal stress conforms to the Mohr Coulomb criterion.As the initial normal stress increases,the cohesion decreases linearly and the tangent of the internal friction angle increases linearly.Compared with the results of the tests under constant normal stress,the cohesions of the rock samples under unloading normal stress are smaller and their internal friction angles are larger.A strength envelope surface can be used to describe the relationship between the initial stresses and the failure normal stress.Shear dilatancy can decrease the total energy of the direct shear test under constant normal stress or unloading normal stress,particularly when the stress levels(the initial stresses in the test under unloading normal stress or the normal stress in the test under constant normal stress)are high.The ratio of the dissipated energy to the total energy at the moment failure occurs decreases exponentially with increasing initial stresses.The direct shear test under constant normal stress can be considered to be a special case of a direct shear test under unloading normal stress with an unloading amount of zero.

Deep-seated rock fracture of valley slopes in China:A review

Deep-seated rock fractures(referred to as DSRF hereafter)in valley slopes are uncommon geological phenomena that challenge our previous understanding of slope unloading processes.These fractures weaken the strength and integrity of the rock mass,potentially forming unstable block boundaries with significant volume,thereby affecting the stability of slopes,chambers,and dam abutments.DSRF has emerged as a critical environmental and engineering geological issue that hinders large-scale projects in deep canyon areas.Despite the attention and practical treatment given to DSRF in engineering practice,theoretical research on this topic still lags behind the demands of engineering applications.To garner widespread attention and promote the resolution of DSRF-related problems,this review aims to redefine DSRF through comprehensive data collection and analysis,engineering geological analogies,and field investigations,and provide a summary and analysis of the research progress on DSRF,along with future research directions.The study defines DSRF as the intermittent tension cracks or relaxation zones within a slightly weathered or fresh,and intact or relatively intact rock mass distributed below the surface unloading zones of a deep canyon slope,and should be distinguished from"loose rock mass"and"deep-seated gravitational slope deformations".The article provides an overview of the development and distribution,rupture characteristics,and genesis mechanism of DSRF.It proposes that DSRF is formed based on the fluvial deviation-undercutting evolution mode,wherein the energy accumulated in the rock mass is violently released when the river further down cuts the slope after the rock mass has undergone cyclical loadingunloading.However,further research is necessary to establish a comprehensive database for DSRF,refine exploration techniques,understand evolutionary processes,develop engineering evaluation methods,and predict the distribution of DSRF.

Experimental study of coal flow characteristics under mining disturbance in China

With annually increased coal mining depth,gas extraction becomes more and more problematic.The gas extraction efect depends on coal seam permeability,which,in turn,is afected by many factors,including loading and unloading stresses and strains in the coal seam.Stresses induce internal cracks,resulting in cleats and gas emission channels,the coal seam permeability permanently changes accordingly.To clarify the stress-induced efects on coal seam permeability,this survey summarized the available approaches used to link the stress path and seepage law in the coal body seepage law,which can be classifed into two design methods:single load variation and combined feld mining method.The characterization methods used to observe the surface of coal samples and three-dimensional reconstruction include electron microscopy,CT scanning,and Nuclear Magnetic Resonance(NMR).According to the stress paths designed by the above two approaches,the seepage laws and similarities of three kinds of coal samples with the fractured structure were summarized in this paper.The following directions are recommended to study the seepage law of coal bodies with three kinds of fractured structures under stress.Firstly,the stress path of the experimental coal body should be designed by the combined feld mining method.The stressed environment of a deep coal seam is complicated,and the axial and confning pressures change simultaneously.Therefore,one cannot fully refect the real situation on-site by studying permeability evolution alone.Secondly,during the coal seam mining,the stressed state changes from time to time,and the development of coal seam fractures is afected by mining.When studying the stress efect on seepage of coal samples,the fractured structure of coal samples should be considered.Finally,the available structural characterization methods of coal samples can be combined with the 3D printing technology,which would produce artifcial samples with the fractured structure characteristics of natural coal.