Experimental investigation on the permeability evolution and mechanism of burnt rock under fluid–solid coupling conditions

Petroleum rock mechanics: An area worthy of focus in geo-energy research

A large number of experimental studies have demonstrated that the permeability and damage of rock are not constant but rather functionally dependent on stresses or stress-induced deformation. Neglecting the influence of damage and permeability evolution on rock mechanics and sealing properties can result in an overestimation of the safety and stability of underground engineering, leading to an incomplete assessment of the risks associated with surrounding rock failure. To address this, the damage and permeability evolution functions of rock under compression were derived through a combination of experimental results and theoretical analysis, unifying the relationship between porosity and permeability in both porous media flow and fractured flow. Based on this, a fluid–solid coupled seepage model considering rock damage and permeability evolution was proposed. More importantly, this model was utilized to investigate the behavior of deformation, damage, and permeability, as well as their coupled effects. The model’s validity was verified by comparing its numerical results with experimental data. The analysis results show that the evolution of permeability and porosity resulted from a competitive interaction between effective mean stress and stress-induced damage. When the effective mean stress was dominant, the permeability tended to decrease; otherwise, it followed an increasing trend. The damage evolution was primarily related to stress- and pressure-induced crack growth and irreversible deformation. Additionally, the influence of the seepage pressure on the strength, damage, and permeability of the investigated rock was evaluated. The model results reveal the damage and permeability evolution of the rock under compression, which has a certain guiding significance for the stability and safety analysis of rock in underground engineering.

This paper presents an experimental investigation on the mechanical behavior and permeability evolution of a typical porous limestone, the Anstrude limestone. Hydrostatic and triaxial compression tests are first performed under drained condition to study the basic mechanical behavior of the porous rock. Permeability measurement under both hydrostatic and triaxial compression is carried out for investigating effects of stress state on the permeability evolution along the axial direction of sample. The obtained results allow to identifying two basic plastic deformation mechanisms, the plastic shearing and pore collapse, and their effects on the permeability evolution. Under low confining pressures, the permeability diminution in the elastic phase is controlled by deviatoric stress. After the onset of plastic shearing, the deviatoric stress induces a plastic volumetric dilatation and a permeability increase. When the deviatoric stress reaches the peak strength or after the onset of shear bands, the permeability slightly decreases. Under high confining pressures, the deviatoric stress also induces a permeability diminution before the onset of plastic pore collapse. After the onset of pore collapse, the deviatoric stress leads to a plastic volumetric compaction and permeability decrease. When the deviatoric stress reaches the onset of plastic shearing, the two plastic mechanisms are in competition, the permeability continuously decreases but with a reduced rate. Finally, after the compaction–dilatation transition, the plastic shearing dominates the deformation process while the pore collapse still controls the permeability evolution.

A series of triaxial compression tests with permeability measurements was carried out under different confining pressure and pore pressure difference coupling conditions to investigate some mechanical properties and permeability evolution with damage of sandstone. It is found that the shapes of stress–strain curves, permeability evolution curves, and failure patterns are significantly affected by the confining pressure but are only slightly affected by the pore pressure difference. In addition, the corresponding numerical simulations of the experiments were then implemented based on the two-dimensional Realistic Failure Process Analysis-Flow (RFPA2D-Flow) code. In this simulator, the heterogeneity of rock is considered by assuming the material properties of the mesoscopic elements conform to a Weibull distribution and a statistical damage constitutive model based on elastic damage mechanics and the flow–stress–damage (FSD) coupling model. The numerical simulations reproduced the failure processes and failure patterns in detail, and the numerical results about permeability–strain qualitatively agree with the experimental results by assigning different parameters in the FSD model. Finally, the experimental results about relationship between permeability evolution and volumetric strain are discussed.

In order to investigate the mechanical properties and permeability characteristics of sandstone during damage evolution under hydromechanical condition, a series of coupled hydro-mechanical triaxial tests on sandstone specimens were conducted based on the Rock Top 50HT full-stress multi-field coupling triaxial test system. Variations in permeability as a function of confining pressure, seepage pressure gradient, and volumetric strain during damage evolution were obtained. The results show that: (1) When the confining pressure is constant and the specimen is gradually changed from a dry to a saturated state, the failure mode of sandstone changes from shear failure to single-slope shear failure. (2) There are four distinctive stages in the permeability evolution of sandstone: gradual decrease, steady development, gradual increase, and rapid growth. These stages correspond to the complete stress–strain curve under the respective working conditions. (3) Employing the Weibull distribution formula, this study investigates the evolution of fracture damage under varying working conditions and determines the permeability evolution relationships associated with damage variables. This exploration reveals an intrinsic link between permeability and damage variables. These findings enhance our understanding of the interplay between stress, deformation, permeability, and damage evolution in seepage-stress coupled sandstone. The results contribute valuable insights to the field of rock mechanics and hold implications for diverse geotechnical and engineering applications.

Experimental investigation of the effect of dissolution on sandstone permeability, porosity, and reactive surface area

The temporal permeability and damage evolutions of low-permeability sandstone cores during triaxial and long-term dissolution experiments were measured using a triaxial-flow system. Three triaxial experiments were performed on sandstone cores having initial permeability ranging from 78 × 10 − 18 m 2 to 120 × 10 − 18 m 2 . Two sets of long-term dissolution experiments were conducted on cracked sandstone cores. All dissolution experiments were performed at room temperature and using a 10 g/L H2SO4 and 0.2 g/L H2O2 input solution. Permeability evolution was determined using Darcy’s law. The cores experienced an average increase of 25% in permeability in the dissolution experiment and 900%~1500% increase at the end of the experiment. The dissolution was fairly homogeneous during the long-term experiments whether on the 1 mm scale or the 10 μm scale. The relationship between damage and permeability was speculated and its correlation coefficient has been proved to be close to 1. These results suggest that hydraulic fracturing works well in permeability increase in low-permeability sandstone reservoir.

The mechanical behavior and permeability evolution of saturated hard rock are significantly important to the stability and safety of super-high arch dams. A series of triaxial compression tests were conducted on amygdaloidal basalt under different pore pressures. Based on the experimental results, a micromechanical-based elastoplastic damage model is proposed for such saturated hard rock. The hydromechanical coupling is formulated using the thermodynamic framework for saturated media. Damage is induced by the growth of microcracks and the plastic deformation is related to the frictional sliding between rough crack surfaces. The thermodynamic forces associated with damage and plasticity are deduced with a special thermodynamic potential for saturated hard rocks. New plastic and damage criteria are proposed to describe the evolutions of the internal variables. The permeability evolution is estimated by the volumetric averaging of the local permeability in microcracks. The developed models were calibrated and validated by a series of triaxial compression tests on saturated hard rocks. The main mechanical behaviors and permeability evolution properties of saturated hard rock are well captured by the proposed model.

In this paper, we present an experimental investigation regarding the stress sensitivity of permeability in naturally fractured shale. Gas permeability tests were performed on the fractured cylindrical shale samples under loading and unloading conditions. Different hydrostatic stress and gas pressure levels were chosen to investigate the dependence of permeability on stress. The permeability of the fractured shale decreases with increasing hydrostatic stress, re-increases during unloading and is irreversible during loading and unloading processes. The gas pressure exhibits a significant effect on the permeability in comparison with the hydrostatic stress. Small gas pressure changes (e.g., 2 MPa) induce a comparable change in permeability with a large hydrostatic stress change (e.g., 40 MPa). The gas pressure gradient on the permeability will be discussed. The fracture aperture was estimated by recording the volume change during loading and shows that the aperture change is consistent with the permeability evolution during loading, which is more complicated at a higher hydrostatic stress value. The roughness of the fractured surface was also analyzed and will be discussed in combination with the permeability evolution.

Experimental Investigation on Permeability Evolution of Limestone Caprock under Coupled THM Processes

SummarySubsurface injection of fluids in a stress‐sensitive naturally fractured rock faces the problem of near‐wellbore fracture evolution and associated changes in rock properties. Numerical modeling of the changes in permeability and poroelastic properties of the near‐wellbore region is challenging due to the coupling between fracture dynamics and poromechanics across multiple length scales of fractures and the host rock. We present a numerical framework to model anisotropic and dynamic evolution in rock properties based on a coupled formulation of fluid flow, rock mechanics, and fracturing, where fracturing‐induced damage is used to update the rock properties. A generalized fixed‐stress method, which accounts for damage‐induced anisotropy in flow and deformation processes, is developed to sequentially solve the equations of flow, mechanics, and fracture evolution. We demonstrate the usefulness of our framework in quantifying the effects of injection rate variability, initial fracture distribution, and in‐situ stress state on the evolution in permeability, elastic stiffness, and the Biot parameters. Our framework does not require the computational mesh to conform to existing or future fractures, allows simultaneous growth of multiple randomly distributed fractures, and can be implemented relatively easily in existing coupled flow‐geomechanics simulators to extend them to model fracturing at the reservoir scale.

Low permeability sandstones are widely found in oil and gas reservoirs. To gain a better understanding of the mechanical behavior of low-permeability sandstones, the permeability evolution of sandstone is investigated experimentally under triaxial compression loading, especially with multiple loading/unloading cycles, using two groups of experimental tests. The one is the triaxial compression tests with and without loading/unloading cycles. The other one is gas permeability tests in triaxial compression. The test results show that the stress–strain curves exhibited obvious nonlinear behavior and softening characteristics. It was demonstrated that important strain hysteresis occurred during the loading and unloading test. The permeability evolution of sandstone exhibited three phases: slow decrease, slow increase and rapid increase. Meanwhile, the permeability evolution followed the evolution of damage. The permeability under different confining pressures was different when the damage variable was approximate to 0.1. As the confining pressure increases, the permeability and the damage variable tend to be constant. The total permeability change is 0.568 when the confining pressure is 30 MPa. Moreover, a deviatoric stress threshold 0.8σp\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\sigma }_{\\rm p}$$\\end{document} is observed in the process of permeability evolution. When the applied stress is less than the threshold, the influence of confining pressure doesn’t play a dominant role in the permeability evolution. When the applied stress is larger than the threshold, the permeability evolution depends on the combination of axial stress and confining pressure.

The permeability evolution of natural fractures during shearing is one of the critical issues in enhanced geothermal system (EGS). In this study, a series of numerical simulations of shear-flow tests under different normal stresses were carried out to investigate the permeability evolution and shear behavior of a natural fracture during shearing. All numerical simulations in this study were implemented based on a coupled hydro-mechanical pore network model (PNM) of fluid flow in discrete element method (DEM). Fluid grids in the simulations are continuously refreshed as the shear progresses. The results indicate that the fracture permeability positively correlates with the shear displacement when the normal stress is relatively large. The essence of permeability evolution is the variation of the local aperture distribution. Greater fracture roughness makes it easier to form high-permeability pathways. Besides, high normal stress amplifies the effect of fracture roughness on permeability evolution. These findings can contribute to a better understanding of the hydraulic-mechanical coupling effects in the fracture shearing process of EGS. INTRODUCTION Enhanced geothermal system (EGS), as a key technology for the extraction and utilization of geothermal energy from high-temperature rock masses in deep formations, has become a research and development hotspot in the field of earth energy and renewable energy (Huang et al., 2018). Many studies have shown that fluid injection in EGS may lead to induced seismicity (Hanano, 2004; Zang et al., 2014; Gaucher et al., 2015; Kim et al., 2018). EGS utilizes fluid to create a new fracture network in hot dry rock or to further expand the original natural fracture network, thereby creating a large number of fluid flow channels. This process involves a series of complex fluid-solid coupling effects and may cause shear slips of fractures or faults (Ishibashi et al., 2018). Fracture shear slip is often accompanied by the evolution of fracture permeability, and the evolution process of fracture permeability can reflect the frictional stability of fractures to a certain extent (Wu et al., 2017). It can be seen that the study of the permeability evolution process and its influence factors in the shearing process of natural fractures is of great significance for evaluating the safety and stability of EGS.

The shearing of pre-existing fractures plays an important role in the permeability enhancement of shale reservoirs during hydraulic fracturing or refracturing treatments. The process reactivates pre-existing fractures around a hydraulic fracture causing them to slip and dilate and can also cause fracture propagation in the shear and tensile modes creating secondary cracks resulting in increased permeability. However, laboratory data on fluid flow and fracture slip in reservoir rocks particularly shale rocks are rare, and the mechanisms of permeability evaluation with shear slip and dilation are still not well understood. In this work, we present the results of laboratory scale shear stimulation tests and numerical simulations to illustrate fracture permeability changes with fracture shear slip and complex network formation. Eagle Ford shale samples containing a natural fracture have been used to run triaxial shear tests and injection-induced shear tests. The multistage triaxial shear test has been performed to measure fracture mechanical properties including shear strength, friction angle, normal stiffness, and shear stiffness; the injection-induced shear test has been used to investigate fracture dilatant shear slip and the coupled permeability evolution. The multistage triaxial shear test show that this type of Eagle Ford fracture has a 37° friction angle, and average 1.39*106 psi/in. normal stiffness and 1.11*106 psi/in. shear stiffness. In the injection-induced shear test, we achieved 6 times increase in flow rate even with only a small induced shear sliding (<0.1 mm or <0.004 inch). Furthermore, permeability evolution during injection-driven shearing tends to linearly evolve with the shear slip and dilation. The irreversible behavior of shear slip was found to explain the permeability hysteresis during shear sliding. The relevant laboratory data has been used in numerical simulations to quantify the impact of shear slip along natural fractures during stimulation. This has been achieved using a newly developed complex fracture network model which robustly simulates hydraulic fracture propagation in a naturally fractured reservoir. The numerical results indicate that shear slip induced permeability enhancement in ultra-low permeability reservoirs is a critical component of stimulation particularly when most of the natural fractures are mechanically closed and may not be favorable for proppant placement.

PetroChina’s Xinjiang oilfield has a large quantity of tight oil reserves and hydraulic fracturing technology has been widely used to achieve commercial production. Some parts of this tight glutenite formation are laumontite-rich and the actual productivity of the hydraulically fractured wells is less than expected. To figure out the ways that laumontite affects tight glutenite well productivity, comprehensive experimental and numerical simulation studies have been conducted to investigate the rock mechanical properties, fluid flow behaviors and the major controlling factor of productivity. Laboratory results indicate that the tight glutenite formation with higher laumontite content has higher initial porosity, permeability but lower yield strength and more severe stress sensitivity in both permeability and fracture conductivity. For laumontite-rich glutenite rocks, there are commonly three types of rock deformation during the loading process: elastic compression, shear dilation and shear enhanced compaction. Both elastic compression and shear enhanced compaction will cause the reduction on rock porosity and permeability. A fully coupled finite element model (FEM) considering stress-induced permeability evolution was introduced to simulate the production process. Permeability evolution models of three different deformation stages were presented, respectively. Simulation results showed that our model is in good agreements with the well testing data. The simulated oil production characteristics for permeability evolution coupled and uncoupled models were discussed. Results showed the strong stress-induced permeability reduction is the major factor that laumontite causing the low and quickly declining oil rates. Initial permeability has a positive effect on productivity and stress-induced fracture conductivity reduction has slight influence on productivity. The results of this paper indicate that the stress-induced permeability evolution in the oil production process must be considered to accurately evaluating reservoirs in the studied area.