Seepage-stress Coupling Research Articles

Experimental Study on Grouting Seepage Characteristics of Single-Fractured Rock Masses with Different Inclination Angles under Three-Dimensional Stress

In underground engineering, the fracture dip angle and three-dimensional stress have great influence on the permeability and mechanical properties of fractured rock mass. Firstly, single-fracture samples with similar three-dimensional fractal dimensions and different dip angles were selected by using CT scanning technology. Then, the grouting material seepage tests under different dip angles and different three-dimensional stresses were carried out by self-developed single-fracture stress-seepage coupling true triaxial test system. The experimental data of stress-strain curve, grouting pressure, and grouting material flow rate of single-fracture specimen under different fracture dip angles and three-dimensional stress were obtained. The results show that the three-dimensional fractal dimension can be used as an index to measure the influence of fracture factors on grouting seepage test. The third principal stress σ 3 had a greater influence on the fracture width than the first principal stress σ 1 . The larger the fracture inclination, the lower the strength of the single-fracture specimen. As the third principal stress σ 3 increases, the grouting pressure increased while the grouting flow decreased. With the increase of the fracture inclination angle, the influence of σ 1 on the hydraulic conductivity became larger, while the influence of σ 3 on the hydraulic conductivity became smaller. Moreover, the expression of hydraulic conductivity of single-fracture specimen with different dip angles under three-dimensional stress was obtained by nonlinear fitting of hydraulic conductivity data.

A 2D hybrid NMM-UPM method for waterflooding processes modelling considering reservoir fracturing

This study presents a methodology to integrate two simulation codes, i.e., cohesive element-based numerical manifold method (Co-NMM) and unified pipe network method (UPM), for 2D coupled two-phase seepage-stress analysis of waterflooding processes in fractured rock masses. The Co-NMM is an extension of the numerical manifold method (NMM) and designed for efficiently addressing continuity-discontinuity problems of rock especially complex multiple-fracture propagation, while the UPM is a relatively novel approach for underground seepage analysis with single−/multiphase fluid flow in both fractured and fractured porous reservoirs. To link the Co-NMM and UPM properly, a two-phase seepage-stress coupling model is proposed for considering interactions between the fractured rock mass and two-phase fluids. The mechanical behaviour of the fractured rock mass responds to the variation in skeletal effective stress induced by the average two-phase fluid pressure in both the rock matrix and fractures, including deformation of the rock matrix, change in fracture aperture and the initiation and propagation of fractures. Accordingly, the induced changes in hydraulic behaviours such as the porosity, permeability and capillary pressure are captured by nonlinear stress-dependent coupling functions. Furthermore, an implicit sequential solution is adopted to execute the two solvers, consisting of two iteration loops, i.e., the two-phase flow solving loop and seepage-stress coupling solving loop. The relative performance of the developed 2D hybrid NMM-UPM method is illustrated for four cases related to the waterflooding process in porous and fractured porous mediums.

Fracture Propagation Modes of Lower Cambrian Shale Filled with Different Quartz Contents under Seepage-Stress Coupling

The content and spatial distribution of brittle minerals, such as quartz, are important factors in determining the fracture initiation mechanism induced by hydraulic fracturing in shale reservoirs. To further research the impact of quartz content in shales of the Lower Cambrian Niutitang Formation in northern Guizhou on the fracture expansion of its reservoir, 7 groups of randomly filling shale models with different quartz contents were established using rock failure process analysis (RFPA2D-flow) code for numerical test studies under seepage-stress coupling, and 5 samples were also subjected to uniaxial compression tests using the INSTRON 1346 electrohydraulic servo-controlled material testing machine (200T). The results show that the average growth rate of the compressive strength and the fracture proportion for a quartz content of 50% to 65% are 4.22 and 1.15 times higher than those for 35% to 50%, respectively. Fractures sprout, expand, and breakdown in the shale matrix or at the junctions of the shale matrix and quartz grains. The mechanical properties and pattern of the fracture extension of the shale in the physical tests are similar to those in the numerical tests, indicating the reliability of the numerical simulations. The fractal dimension curves at different stress levels are divided into three stages: flattening, increasing, and surging, and the fractal dimension value for a quartz content of 50%~65% at a 100% stress level is 1.02 times higher than that for 35%~50%. The high degree of natural fracture development in high quartz content formations in shale gas reservoirs is of some reference value for logging data. The research results provide a reference value for the content and spatial distribution of brittle minerals for the initiation mechanism and fracture propagation of hydraulic fracturing in shale reservoirs.

Investigation on the mechanical behavior, permeability and failure modes of limestone rock under stress-seepage coupling

Limestone rock widely exists in various rock engineering, its mechanical behavior and permeability evolution have a great influence on the safety and stability of related engineering projects. To study the mechanical behavior, permeability and failure modes of limestone rock under stress-seepage coupling, triaxial compression coupled with permeability tests were carried out. Experimental results reveal that the failure of limestone still conforms to the Mohr-Coulomb criterion under stress-seepage coupling, and effective confining pressure σ3eff can indicate the effect of gas pressure PG and confining pressure σ3 on the deformation parameters of limestone samples. Peak stress σP and residual stress σR increase with the enhancement of confining pressure and decrease with the ascent of gas pressure. Permeability evolution of limestone under different gas pressures and confining pressures is generally similar, and the trend of initial permeability kI, peak permeability kP and residual permeability kR decreasing with effective confining pressure σ3eff can be expressed by the exponential function. The increase of confining pressure and decrease of gas pressure will retard the acoustic emission (AE) response and inhibit the propagation of microcracks. More importantly, limestone samples always show composite failure modes dominated by tensile cracks under low confining pressure, while under high confining pressure, the limestone samples show shear failure modes. The difference is limestone samples are composed of tensile microcracks on the microscale under stress-seepage coupling, and the proportion of shear microcracks grows as gas pressure PG and confining pressure σ3 increases. This research can provide a basic reference for relevant engineering applications.

Hydraulic Fracturing Mechanism of Rock Mass under Stress-Damage-Seepage Coupling Effect

According to the damage evolution model of rock mass under stress-seepage coupling effect, the representative element theory is employed to describe the change law of rock mesostructure. Based on the theory of elasticity and Weibull distribution, the statistical damage constitutive model of rock mass and the finite element numerical algorithm are established, by adopting the COMSOL Multiphysics numerical software and MATLAB program. Besides, the validity of the statistical damage constitutive model of rock mass is verified by the triaxial compression test. Besides, the hydraulic fracturing processes of rock mass under equal and unequal in situ stresses are numerically simulated, and the mechanical behavior of rock mass during hydraulic fracturing in complex underground environment is also studied. Under the condition of equal in situ stress, the stress distribution of surrounding rock of circular hole is annular, which is similar to the elastic stress distribution of surrounding rock. Under the condition of unequal in situ stress, the stress distribution tends to be circular with the increase of lateral pressure coefficient, and the stress distribution along the diagonal decreases. The simulation results are in good agreement with the theoretical results, which indicates that the damage mechanical model and the numerical model have correlation and certain accuracy. By analyzing the size and direction of horizontal in situ stress, the shape and extension direction of cracks are judged, which provides an important theoretical basis for water inrush prediction and engineering protection.

Tightness Analysis of Anhydrite Mine-Out Used for Underground Crude Oil Storage Considering Seepage–Stress Coupling: A Case Study

Good tightness is one of the necessary conditions for an anhydrite mine-out space to be used for underground crude oil storage. Combining engineering geological surveys, laboratory tests, and numerical simulations, this paper quantitatively analyses the tightness of the Anhui Hengtai anhydrite mine-out under different working conditions and discusses its suitability for underground crude oil storage. Triaxial compression tests of anhydrite rock considering seepage were carried out to study its hydraulic and mechanical characteristics during failure. The permeability coefficient–strain empirical formula of anhydrite was constructed. A numerical simulation was carried out on the basis of the finite difference method to study the tightness of the rock surrounding the mine-out after excavation and under 0.2 MPa, 0.6 MPa, and 1.0 MPa storage pressures, respectively. The results showed that the permeability coefficient increased with increasing storage pressure, and the growth rate gradually slowed after the storage pressure reached a certain value. Overall, the mine-out had good tightness, and the permeability coefficient of the surrounding rock was small. This study demonstrated the feasibility of using the anhydrite mine-out for crude oil storage and provides a scientific basis for the site selection of underground crude oil storage.

Experimental Study on Coal Permeability and Damage Evolution Under the Seepage-Stress Coupling

In order to study the damage evolution law of coal under the seepage-stress coupling, this paper adopted the rock servo triaxial test system to conduct permeability test for full stress-strain process on 10 sets of coal specimens using steady-state method under different confining pressure and osmotic pressure. The results show that coal permeability has a small fluctuation before the stress peak, and the permeability increases substantially after the peak. The relationship between the plastic damage variable of coal and the equivalent plastic strain is proposed. The research will provide a theoretical basis for hydraulic fracturing gas drainage reservoir.

Permeability evolution characteristics and microanalysis of reactive powder concrete of drilling shaft lining under stress-seepage coupling

This study aimed to solve the problem of drilling shaft support under deep water-rich and weakly cemented stratum conditions. Thus, ultra-high-performance reactive powder concrete (RPC) is proposed as the wall construction material for the drilling shaft lining. The mechanical behavior, permeability, and microscopic characteristics of RPC were tested and analyzed. First, RPC samples were prepared by standard and autoclave curing with strength grades of C110 and C140, respectively, and their quasi-static mechanical properties were tested and compared. Stress-seepage coupling tests were then conducted under different confining and osmotic pressures to analyze the mechanical deformation, permeability evolution, and strength and deformation characteristics. The results showed that autoclave curing significantly improved the strength grade and early strength growth of RPC. Additionally, RPC generally exhibited low permeability characteristics while its permeability evolution law corresponded to the stress–strain relationship under triaxial loading. The permeability was observed to undergo three stages: a gradual decrease, steady development, and rapid increase. Finally, mercury intrusion porosimetry, X-ray diffraction, and scanning electron microscope microscopy analyses were used to determine the mechanism of improving macro mechanical properties and impermeability of RPC by the autoclave curing method based on pore structure characteristics and microstructure formation aspects.

Study on strength characteristics of prefabricated single fracture rock with different dip angles under seepage stress coupling

Study on strength characteristics of prefabricated single fracture rock with different dip angles under seepage stress coupling

Numerical simulation of fracture failure characteristics of rock mass with multiple parallel fractures under seepage stress coupling

Numerical simulation of fracture failure characteristics of rock mass with multiple parallel fractures under seepage stress coupling

Analysis of the influence of groundwater seepage on the deformation of deep foundation pit with suspended impervious curtain

Taking the deep foundation pit of a metro station with rich water environment as the research background, a three-dimensional model of seepage-stress coupling is established. Numerical simulation is used to study the deformation process and influencing factors of deep foundation pit dewatering and excavation of the station, and the influence of dfferent pre-dewatering conditions and different depths of the water-stop curtain on the deformation of the foundation pit is analyzed. The results show that during the dewatering and excavation of foundation pits, seepage-stress has an obvious coupling effect in dewatering and excavation. The stress concentration position of retaining structure and the maximum displacement of lateral displacement are about 0.5–0.6 excavation depth of pile body. With the depth of precipitation increases, the seepage effect in the soil layer is obviously enhanced, which is not conducive to the stability of the foundation pit.

Force and deformation response analysis of dual structure slope excavation and support

Aiming at the problem of fuzzy selection basis of dual-structure slope simulation calculation parameters and the complicated force and deformation of the slope body in the supporting process. The study relies on the treatment project of the right side slope of the K5 + 220-K5 + 770 section of the TJ1A section of the Jiangwen Expressway. The slope is a gravel soil-weathered bedrock dual structure slope, which is referred to as Hetangba slope in the study. Use Midas/GTS NX to build a trial calculation model. A new method for checking simulation parameters of deep displacement monitoring data is proposed. The superposition calculation method of pore water pressure is proposed to realize the slope stress-seepage coupling. The analysis method of slope excavation and support process is put forward. The analysis shows that the most dangerous working condition in the slope excavation and support process is the second-level excavation. When there is a local displacement and plastic strain concentration area on the slope, the reference significance of the extreme value and the safety factor is reduced. Compared with the application of pore water pressure and no pore water pressure, there is no obvious change in the position of the extreme value of horizontal displacement and plastic strain in working conditions 2–8, but the extreme value changes are different. The slope safety factor of working conditions 2–8 is reduced. The continuous distribution zone of plastic strain inside the slope disappeared in condition 7. There is a very small unstable area on the top of the third-level slope of working conditions 9–11. The research results have great theoretical guiding significance for the measurement and control simulation and early warning analysis of the dual structure slope construction process.

True triaxial experimental study on fluid flow in single fracture with different dip angles under three-dimensional stress at different depths

The shape of ore body in deep underground engineering is complex, and its fracture development degree is affected by water abundance, fracture zone, fault and other factors. With the deepening of depth, the vertical and horizontal stress changes significantly, which has a great impact on the fracture opening. Therefore, the fracture dip angle and its three-dimensional stress state are the main factors affecting the fluid flow of fractures in a large area of ore body and its hanging and footwall rock mass. In this study, firstly, the three-dimensional fractal dimension of a single-fracture specimen was obtained by using CT (Computer Tomography) scanning technology and the single-fracture specimen with a similar three-dimensional fractal dimension was selected for testing. Then, the self-developed single fracture stress seepage coupling true triaxial test system is applied to test the seepage of single fracture samples with six different fracture dip angles under different stress states of underground depth. The experimental results show that the fluid flow and strain of fractured rock mass are comprehensively affected by the fracture dip angle and the stress state of the underground depth, and the influence degree of the fracture dip angle changes with the deepening of the underground depth. The above conclusions can be mutually verified in the aspects of stress-strain, water pressure and permeability coefficient. Furthermore, the relationship between the minimum injectable water pressure (MIWP) of fractured rock mass with different dip angles and underground depth is obtained, the expression of seepage coefficient of single fracture specimen with different dip angles and underground depth under three-dimensional stress is summarized, and the relationship between eh and em of single fracture specimen with different fracture dip angles is determined. This study provides a basic reference for the design scheme of hydraulic conductivity of fractured zone or fractured rock mass with different underground depths.

Research on Influencing Factors of Effective Gas Extraction Radius in Coal Mine Based on Multiple Linear Regression

In order to analyze the influence degree of different factors on gas extraction radius, on the basis of coal seam gas flow theory and elastic mechanics, considering the influence of coal seepage rate change on gas flow, the COMSOL Multiphysics numerical simulation software is secondarily developed by using the stress-seepage coupling mathematical model of coal and rock containing gas. Taking coal seam No.2 of Juji Coal Mine as an example, the effective extraction radius under different extraction negative pressure, extraction borehole diameter, extraction time, and initial permeability of coal seam are simulated, respectively. Then, the orthogonal test method is used to design different calculation schemes, and the effective extraction radius is used as the judgment index. The simulation software is used to calculate the effective extraction radius corresponding to different influencing factors. SPSS statistical software is used to analyze the parameters. The multivariate linear regression equation of the influencing factors of the effective extraction radius is obtained, and the multivariate linear regression model is established. The results show that the negative pressure of extraction has little effect on the effective extraction radius; the effective extraction radius gradually expands with the increase of extraction time and extraction aperture, but it has certain timeliness. The initial permeability of the coal seam has a great influence on the effective extraction radius. The influence degree of different influencing factors from large to small is the initial permeability of coal seam, extraction time, borehole diameter, and extraction negative pressure. Therefore, the key to improving the effectiveness of gas extraction is to increase the initial permeability of the coal seam before preextraction, ensure sufficient extraction time, appropriately increase the borehole diameter, and improve the negative pressure of extraction. Finally, the gas flow rate is used for a field test, and the measured results are basically consistent with the simulation results, which proves the correctness of the mathematical model of gas-containing coal and the feasibility of using numerical simulation methods to study the effective extraction radius of coal seams.

Numerical Simulation of Fracture Failure Characteristics of Rock-Mass with Multiple Nonparallel Fractures Under Seepage Stress Coupling

Numerical Simulation of Fracture Failure Characteristics of Rock-Mass with Multiple Nonparallel Fractures Under Seepage Stress Coupling

Experimental Investigations on Mechanical Properties of Consolidated Samples for Collapse Columns under Seepage-Stress Coupling Effects

Abstract Triaxial seepage tests were conducted on the consolidated collapse column specimens to investigate their mechanical properties under seepage-stress coupling effects using a triaxial multifield coupled mechanical test system for rocks. The effects of initial consolidation pressures and specimen components on mechanical properties and seepage characteristics of consolidated collapse column specimens were analyzed. Test results showed the following: (1) The stress–strain curves of consolidated collapse column specimens could be classified in three stages, namely, compaction stage, linear deformation stage, and creep-like deformation stage, while the permeability during loading showed an obvious four-stage evolution of gradual decrease, stable development, rapid increase, and slow decrease. (2) Under same sample components, the permeability characteristics of consolidated collapse column specimens showed an obvious initial consolidation pressure effect. The initial consolidation pressure changed the distribution of pores and fractures in the specimens, leading to a decreased peak permeability as the initial consolidation pressure increased. (3) At the initial stage of loading, the permeability of consolidated specimens was mainly affected by the initial consolidation pressure, and the corresponding permeability decreased with the increase of the consolidation pressure. When the consolidated specimens were gradually compacted, the main factor influencing the permeability changed to the specimen components. The peak permeability of consolidated specimens comprising grey mudstone and conglomerate was the largest, while the fuchsia mudstone would reduce the specimen permeability, and the peak permeability decreased with the fuchsia mudstone components.

An extended numerical manifold method for two-phase seepage–stress coupling process modelling in fractured porous medium

To investigate the two-phase seepage–stress coupling process in fractured porous medium, this study extends the cohesive element-based numerical manifold method (Co-NMM) by incorporating a two-phase seepage–stress coupling model considering the effect of matrix-fracture interface on the two-phase flow and fracture propagation induced by the two-phase seepage pressure. The proposed two-phase flow solving framework implicitly calculates the fluid pressure and saturation of the two-phase flow based on a two-phase unified pipe network method. Furthermore, to more realistically model the hydraulic behaviour of two-phase flow in fractured porous medium, a matrix-fracture interface condition called the extended capillary pressure condition is incorporated into the two-phase flow solving framework to capture the interactions among fluid flow in the fractures and matrix. Due to the inheritance of the Co-NMM, one key advantage of the extended method is the simulation of complex multi-fracture propagation caused by the two-phase seepage–stress coupling. The two-phase flow solving framework is first validated by reproducing the water displacing oil in a single fracture and the gas displacing water in a single-fractured porous medium against analytical and numerical solutions, respectively. The two-phase seepage–stress coupling procedure is then verified by performing a one-dimensional consolidation problem of soil column, in which comparisons between the numerical and analytical results regarding the pore pressure and compression displacement are presented. Finally, with the extended method, CO2-enhanced oil recovery in fractured reservoir is preliminarily studied by considering the effect of gas injection rate and capillary pressure on the evolution of two-phase flow and fracture propagation. The results elucidate that high CO2 injection rate can lead to fracture propagation in the reservoir, and both capillary pressure and fractures have a significant effect on the CO2 distribution.

Permeability Evolution Characteristics and Microanalysis of Reactive Powder Concrete of Drilling Shaft Lining Under Stress-Seepage Coupling

Permeability Evolution Characteristics and Microanalysis of Reactive Powder Concrete of Drilling Shaft Lining Under Stress-Seepage Coupling

Experimental investigation on the evolution of damage and seepage characteristics for red sandstone under thermal–mechanical coupling conditions

Rock masses in underground space usually experience the coupling of high-temperature field, stress field and seepage field, which gives them complex mechanical behavior and permeability characteristics. To study the mechanical properties and permeability characteristics of red sandstone under different temperature environments, a seepage test under high temperature and triaxial compression is carried out based on the RLW-2000 multi-field coupling tester. The results show that the plastic flow of red sandstone at the stress peak under the same temperature is more obvious with the increase of confining pressure. In addition, as the confining pressure gradient increases, the permeability decreases and the trend becomes slower. And the higher the operating temperature, the easier to produce seepage channels inside the rock sample. The development of fissures is rapidly developed under the effect of temperature, so the seepage channels are widened and increased, and the permeability is greatly increased. The constitutive model of rock statistical damage considering the interaction of high temperature and osmotic pressure was constructed based on the experimental data and combining theoretical methods to reveal the characteristics of permeability evolution induced by thermal damage of rocks. The research results can be used as a reference for monitoring rock stability during geological engineering projects involving thermal–seepage–stress coupling conditions.

Experimental Study on Seepage Characteristics of Fractured Rock Mass under Different Stress Conditions

In order to obtain the mechanical behavior and permeability characteristics of coal under the coupling action of stress and seepage, permeability tests under different confining pressures in the process of deformation and destruction of briquette coal were carried out using the electrohydraulic servo system of rock mechanics. The stress-strain and permeability evolution curves of briquette coal during the whole deformation process were obtained. The mechanical behavior and permeability coefficient evolution response characteristics of briquette coal under stress-seepage coupling are well reflected. Research shows that stress-axial strain curve and the stress-circumferential strain curve have the same change trend, the hoop strain and axial strain effect on the permeability variation law of basic consistent, and the permeability coefficient with the increase of confining pressure and decreases, and the higher the confining pressure, the lower the permeability coefficient, the confining pressure increases rate under the same conditions, and the permeability coefficient corresponding to high confining pressure is far less than that corresponding to low confining pressure. The confining pressure influences the permeability of the briquette by affecting its dilatancy behavior. With the increase of the confining pressure, the permeability of the sample decreases, and the permeability coefficient decreases with the increase of the confining pressure at the initial stage, showing a logarithmic function. After failure, briquette samples show a power function change rule, and the greater the confining pressure is, the more obvious the permeability coefficient decreases.