Permeability Evolution Research Articles

Time-dependent behavior and permeability evolution of limestone under hydro-mechanical coupling

Time-dependent behavior and permeability evolution of limestone under hydro-mechanical coupling

Experimental study on the impact of effective stress and cyclic loading on permeability of methane hydrate clayey sediments

Permeability evolution directly determine gas production and its efficiency of gas hydrates in the South China Sea. Due to existence of submarine overlying rocks and change of soil internal mechanics during the hydrate exploitation, permeability evolution is significantly affected. Therefore, in this paper, influence of effective stress on the porosity and permeability of sediment with different hydrate saturation was studied, and functional relationship between them was revealed. Effect of cyclic loading and unloading stress on the permeability recovery and damage degree was also discussed. The results show that when hydrate saturation increases, the relation of permeability and effective stress are power function rules, while porosity and effective stress are exponential function rules in clayey sediments. An empirical prediction equation for gas permeability of clay sediments with effective stress was presented, which takes into account hydrate saturation and porosity. In addition, during cyclic loading experiments, the results showed a linear increase in permeability during unloading. The permeability recovery rate and permeability damage rate are completely opposite with change of hydrate saturation. Permeability damage rate of kaolin is different from that of other two kinds of sediment. With the increase of cycle times, the proportion of plastic deformation of sediment decreases gradually.

Experimental study on mechanical behavior and permeability evolution of weakly cemented sandstone under unloading conditions

Experimental study on mechanical behavior and permeability evolution of weakly cemented sandstone under unloading conditions

Temperature-dependent evolution of permeability and pore structure of marble under a high-temperature thermo-hydro-mechanical coupling environment

Temperature-dependent evolution of permeability and pore structure of marble under a high-temperature thermo-hydro-mechanical coupling environment

Crustal permeability generated through microearthquakes is constrained by seismic moment

We link changes in crustal permeability to informative features of microearthquakes (MEQs) using two field hydraulic stimulation experiments where both MEQs and permeability evolution are recorded simultaneously. The Bidirectional Long Short-Term Memory (Bi-LSTM) model effectively predicts permeability evolution and ultimate permeability increase. Our findings confirm the form of key features linking the MEQs to permeability, offering mechanistically consistent interpretations of this association. Transfer learning correctly predicts permeability evolution of one experiment from a model trained on an alternate dataset and locale, which further reinforces the innate interdependency of permeability-to-seismicity. Models representing permeability evolution on reactivated fractures in both shear and tension suggest scaling relationships in which changes in permeability (Δk\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta k$$\\end{document}) are linearly related to the seismic moment (M\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M$$\\end{document}) of individual MEQs as Δk∝M\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta k\\propto M$$\\end{document}. This scaling relation rationalizes our observation of the permeability-to-seismicity linkage, contributes to its predictive robustness and accentuates its potential in characterizing crustal permeability evolution using MEQs.

Capillary‐Driven Backflow During Salt Precipitation in a Rough Fracture

AbstractSalt precipitation is a crucial process occurring during CO2 injection into saline aquifers. It significantly alters the porous space, leading to reduced permeability and impaired injectivity. While the dynamics of precipitation have been studied within porous media, our understanding of precipitation patterns and permeability evolution within rough fractures remains inadequate. Here, we conduct flow‐visualization experiments on salt precipitation, wherein dry air invades brine‐filled rough fractures under various flow rate conditions. Our observations reveal that the precipitation pattern shifts from ex situ precipitation to homogeneous form as the flow rate (capillary number Ca) increases. Through real‐time imaging of the salt precipitation process, we determine that ex situ precipitation is due to capillary‐driven backflow. This backflow phenomenon occurs when previously precipitated salt, acting as a hydrophilic porous medium, attracts the brine flow backward. As a result, precipitation occurs at a location different from the original site. We further show that the impact of capillary‐driven backflow is significant at low flow rates and is gradually suppressed as the flow rate increases. We provide a theoretical estimation for the critical Ca for the occurrence of capillary‐driven backflow. As Ca is smaller than this critical value, backflow‐precipitation positive feedback causes fracture voids to become completely clogged, thereby leading to a more substantial permeability reduction. In contrast, a homogeneous precipitation pattern tends to only partially clog the fracture voids, causing a relatively smaller permeability reduction. This study enhances our understanding of the role of capillary‐driven backflow in controlling salt precipitation and permeability reduction in fractures.

Mechanical-permeability characteristics of composite coal rock under different gas pressures and damage prediction model

Determining the influence of gas pressure on the mechanics, permeability, and energy evolution of gas-bearing composite coal is helpful to better understand the formation process and prevention measures of gasdynamic disasters. In this paper, true triaxial mechanical-permeability tests are carried out on the gas-bearing composite coal rock under different gas pressures, focusing on the influence of gas pressure on the mechanics, permeability, and energy response characteristics of the composite coal rock, and a damage constitutive model based on energy dissipation is established. The results show that increasing the gas pressure decreases the load bearing capacity, strain, pre-peak relative permeability, and deformation capacity of the sample. The greater the gas pressure is, the greater the relative permeability decreases and the greater the post-peak relative permeability increases. The gas pressure has a great influence on the energy of the sample. The elastic strain energy ratio (Ue/U) increases with the increase in gas pressure, and the dissipative energy ratio (Ud/U) decreases with the increase in gas pressure. The coal-rock composite constitutive model based on energy dissipation is in good agreement with the experimental curves.

On the role of poroelastic stressing and pore pressure diffusion in discrete fracture and fault system in triggering post-injection seismicity in enhanced geothermal systems

Injection-induced seismicity has become one of the most critical challenges for the widespread deployment of Enhanced Geothermal Systems (EGS). In particular, some EGS development projects have led to large, damaging earthquakes that unexpectedly occurred far off the stimulated reservoir region and, in particular, after stopping fluid injection. Yet, the causative mechanisms of these seismicity patterns remain highly elusive. Here, we identify a combination of mechanisms that could explain delayed seismicity in EGS sites by conducting fully-coupled hydromechanical simulations of the hydraulic stimulation of a naturally-fractured granitic reservoir. The model comprises a sparse network of long, variably-oriented fractures interacting with a nearby, critically-oriented fault. The results show that the presence of fractures introduces notable nonlinearities in the flow field and rock deformation and significantly expands the rock volume affected by fluid injection. First, the stimulated fracture network provides highly-permeable conduits for communicating elevated pore pressure over long distances. Second, the anisotropic expansion of fractures generates shear stress that is transmitted almost instantaneously across the reservoir. The pore pressure and stress perturbations can not only cause slip along fractures, inducing (micro)seismicity during injection, but also affect the stability of nearby faults, which may not necessarily be pressurized during injection. The transferred poroelastic stresses can increase or decrease the slip tendency along different fault segments. However, the fault may reactivate only after several months following injection when a progressive pore pressure diffusion modulated by the transient fault permeability evolution brings a critically-stressed fault segment to failure conditions. We also find that the spatiotemporal evolution of seismicity depends largely on the nearby fault orientation, hydromechanical properties, and hydraulic connection with the fracture network, as well as the initial state of stress. We conclude that accurate subsurface characterization and continuous monitoring during and after injection should allow for managing the risks posed by injection-induced seismicity and safely unlocking the immense potential for clean and sustainable geothermal energy.

Investigation on damage-permeability model of dual-porosity coal under thermal-mechanical coupling effect

Understanding the permeability characteristics of coal under mining disturbance is crucial for ensuring coal mine safety and efficient gas extraction. Coal is a dual porosity medium consisting of matrix and fracture, and there are some differences in mechanics and permeability between matrix pores and fracture. However, research on the damage and gas seepage characteristics of dual-porosity coal is limited. In this paper, a coupled damage-permeability model considering matrix pores and fractures in coal is established, incorporating the influences of effective stress, gas adsorption, thermal expansion, and thermal cracking. And the reliability of the newly developed model was verified using experimental data of temperature and pressure variations under different stress boundary conditions. The results indicate that during the elastic stage, the total permeability and fracture permeability of coal exhibit a trend of decreasing and then increasing with pore pressure, while matrix permeability gradually increases. In the full stress-strain stage, the overall damage-permeability and fracture damage-permeability of coal exhibit an “S"-shaped variation trend with axial strain, while matrix damage-permeability shows a slow increasing trend. To accurately characterize the evolution of gas seepage in coal, contributions from matrix pores and fractures must be comprehensively considered, and the effects of gas adsorption, effective stress, thermal cracking, and thermal expansion on permeability cannot be ignored. Furthermore, through parameter sensitivity analysis, it is found that during the elastic stage, coal permeability decreases with an increase in the adsorption sensitivity coefficient and thermal expansion coefficient. In the full stress-strain process, coal permeability increases with an increase in the damage-permeability coefficient. The results of this study are of great significance for predicting the evolution of coal permeability in the working face ahead under mining disturbance.

Rock mass permeability evolution during triaxial shearing based on three-dimensional distinct element model (3DEC) simulation

The evolution and change in jointed rock mass hydraulic conductivity (krm) during triaxial compression was investigated using the discrete element method code 3DEC bonded block model approach. The code’s fluid flow logic was used to simulate flow tests at various stages, from the initial confined state through to the post-peak (i.e., failed) state. A target percent of joint contacts were randomly “broken” to produce different initial fracture intensities. The rock mass fluid flow behaviour was compared with laboratory flow tests of thermally brecciated granular marble. A new joint constitutive model was developed to account for nonlinear, hysteretic load–displacement behaviour to accurately reproduce changes in joint aperture due to complex inter-block stress changes. The simulations provided quantifiable measurements of krm evolution with axial strain throughout the test for several rock mass quality conditions. The evolution of krm with loading was found to be nonlinear, with significant increases associated with yielding-induced dilation, followed by relatively minor changes in the post-peak loading range. These trends are similar to the laboratory tests conducted on intact rock and on rock joints. The findings will be useful for improving groundwater flow models for stability assessment of rock slopes, particularly where significant shearing is expected.

Evolution of Permeability and Gas Seepage in Deep Coal Under 3D Stress

Evolution of Permeability and Gas Seepage in Deep Coal Under 3D Stress

Phase diagram and permeability evolution for dissolving vertical fractures in a gravity field

Rock fractures always provide the main flow pathways in geological systems. When exposed to reactive flow, mineral dissolution causes different dissolution regimes and significantly impacts permeability change. Previous studies have examined dissolution regimes and permeability evolution in horizontal geometry, but dissolution dynamics and permeability changes of vertical fractures in a gravity field remain unexplored. Here, we conduct flow-dissolution experiments combined with a pore-scale modeling approach to investigate regime transition and permeability evolution in vertical fractures. We validate the modeling approach using experimental results and illustrate the importance of gravitational effects in dissolving vertical fractures. Our 3D numerical simulations reveal the regime transition, represented by a critical Richardson number Ri, from the forced convection regime to the buoyancy-driven convection regime as the importance of gravity increases. In the buoyancy-driven regime, gravitational effects promote the development of localized dissolution/flow channels. However, gravitational effects become negligible in the forced convection regime where wormholes and uniform patterns can be separated by the critical Pe number. Using the critical Ri and Pe numbers, we establish a phase diagram of dissolution regimes for vertical fractures in a gravity field, exhibiting good agreement with experiments. By correlating this phase diagram with permeability-aperture curves, we directly estimate the power law exponent of permeability evolution for each dissolution regime. This approach extends the classic phase diagram of dissolution regimes to consider gravitational effects on vertical fractures and provides a guide to directly estimate the parameters for permeability evolution, which is essential in reactive-transport modeling at the field scale.

Experimental Study on Permeability and Gas Production Characteristics of Montmorillonite Hydrate Sediments Considering the Effective Stress and Gas Slippage Effect

Summary Gas permeability in hydrate reservoirs is the decisive parameter in determining the gas production efficiency and gas production of hydrate. In the South China Sea (SCS), the gas flow in tight natural gas hydrate (NGH) silty clay reservoirs is significantly affected by the gas slippage effect and the effective stress (ES) of overlying rock. To improve the effectiveness of hydrate exploitation, it is necessary to understand the influence of gas slippage in hydrate reservoirs on the permeability evolution law. For this paper, the gas permeability characteristics and methane production of hydrate montmorillonite sediments were studied at different pore pressures and ESs. Experimental data revealed that the gas permeability of montmorillonite samples before methane hydrate (MH) formation is seriously affected by the Klinkenberg effect. The gas permeability of montmorillonite sediments before hydrate formation is generally smaller than that after hydrate formation, and the gas slippage effect in the sediments after hydrate formation is weaker than that before hydrate formation. With the change in ES, the intrinsic permeability of sediment has a power law relationship with the simple ES. The ES law coefficient n was determined using the response surface method to eliminate the influence of gas slip on gas permeability. As pore pressure decreases and MH decomposes, montmorillonite swelling seriously affects gas permeability. However, the gas slippage effect has a good compensation effect on the permeability of montmorillonite sediments after MH decomposition under low pore pressure. The multistage depressurization-producing process of MH in montmorillonite sediments is mainly 3 MPa depressurization-producing stage and 2 MPa depressurization-producing stage. In this paper, the influence mechanism of gas slippage effect of hydrate reservoir is studied, which is conducive to improving the prediction accuracy of gas content in the process of hydrate exploitation and exploring the best pressure reduction method to increase the gas production of hydrate in the process of exploitation.

An analytical transient coal permeability model: Sorption non-equilibrium index-based swelling switch

Long-term permeability experiments have indicated that sorption-induced swelling can switch from internal to bulk depending on the evolutive sorption status. However, this sorption swelling switch mechanism has not been considered in current analytical permeability models. This study introduces a normalized sorption non-equilibrium index (SNEI) to characterize the sorption status, quantify the dynamical variations of matrix swelling accumulation and internal swelling partition, and formulate the sorption swelling switch model. The incorporation of this index into the extended total effective stress concept leads to an analytical transient coal permeability model. Model results show that the sorption swelling switch itself results in the permeability switch under stress-constrained conditions, while the confined bulk swelling suppresses the permeability recovery to the continuous reduction under displacement-constrained conditions. Model verifications show that current experimental observations correspond to the early stages of the transient process, and they could be extended to the whole process with these models. This study demonstrates the importance of the sorption swelling switch in determining permeability evolution using simple boundary conditions. It provides new insights into experimentally revealing the sorption swelling switch in the future, and underscores the requirement of a rigorous model for complex coupled processes in large-scale coal seams.

Experimental study of cryogenic treatment of Karaganda coal samples

Karaganda Coal Basin bears the largest undeveloped reserve of coalbed methane (CBM) in Kazakhstan, which lacks water resources for implementing large-volume hydraulic fracturing. Cryogenic fracturing utilizing liquid nitrogen (LN2) has been trialled in fields and is a waterless fracturing technique under intensive research these days. This study aimed to evaluate the cryogenic treatment efficacy of Karaganda coal samples as well as to understand the coal permeability evolution during the thawing period. X-ray fluorescent spectrometry (XRF) and microscope imaging identified the compositional and structural heterogeneities of coal specimens mined from different interlayers. Acoustic emission test, permeability measurement, and microscope imaging comparatively characterized the dry coal structure alteration before and after immersion into LN2. Cryogenic treatment slowed down the S-wave velocity through coal specimens, enhanced permeability by over 65 % after temperature recovery as well as created new fractures, enlarged existing ones, and spalled coal particles. Dynamic permeability evolution against temperature rise during the thawing process has been successfully captured for the first time. Overall, the experimental measurements support that the LN2 cryogenic fracturing technique would be effective in stimulating coalbeds for CBM production in Karaganda Coal Basin.

A novel apparent permeability model for shale considering the influence of multiple transport mechanisms

Changes in pore pressure during the extraction of shale gas lead to dynamic alterations in the pore structure and permeability, making it challenging to gain a comprehensive understanding of the flow behaviors of shale gas. The pore structure of shale is complex, with a variety of storage modes and gas transport processes constrained by a number of factors. For instance, when gas flows through a transport channel with a finite length, it is imperative to take into account the flow loss caused by the bending of inlet and outlet streamlines, prior models typically neglect the impact of end effects, resulting in an exaggerated estimation of the shale permeability. Furthermore, a decrease in pore pressure corresponds to an increase in the Knudsen number, resulting in the breakdown of the continuity assumption of the Navier–Stokes equation, this signifies the gradual shift of the transport regimes from continuum flow to other transport regimes. The gas flow process is nonlinear due to the alternating impact of multicomponent transport mechanisms and various microscale effects. In this paper, we presented a novel apparent permeability model for shale that incorporates the impact of real gas effect, end effects, transport regimes, adsorption, and effective stress. First, we assumed the channel for shale gas transport to be circular pore and calculated the viscosity under the influence of a real gas effect as well as the corresponding Knudsen number. Subsequently, building upon the foundation of the slip model, we introduce the influence of the end effects to establish a bulk phase permeability for shale, further considering the impact of surface diffusion. Then, the pore radius was quantified under the influences of adsorption and effective stress. Using the intrinsic correlation between permeability and pore radius as a bridge, a shale apparent permeability model was further derived. The model encompasses various transport regimes and microscale effects, replicating the gas flow behaviors in shale. The new model was verified through comparison with published experimental data and other theoretical models, while analyzing the evolution of apparent permeability. Additionally, this paper discusses the influence of various factors, including end effects, pore radius, internal swelling coefficient, sorption-induced strain, and model-related parameters on the shale apparent permeability.

Influence of cooling path on solidification morphology and hot tearing susceptibility of an Al−Cu−Fe−Mg−Si alloy

AbstractIn this work, a numerical approach is applied to study the impact of local cooling conditions on the hot tearing in an aluminium‐copper alloy with iron, magnesium and silicon as impurities. At first, CALPHAD‐coupled multicomponent and multiphase‐field simulations were performed to elucidate the effect of cooling conditions on solidification morphology in the final, critical stage of solidification. Then, the evolution of the melt flow permeability is derived from the simulated three‐dimensional microstructures and discussed in the context of the Rappaz‐Drezet‐Gremaud criterion for hot tearing susceptibility. With increasing cooling rates, the microstructure becomes finer and the resulting permeability first increases and then saturates within the investigated range. It is shown that the Rappaz‐Drezet‐Gremaud hot‐tearing criterion in combination with microstructure simulations responds to different cooling conditions in contrast to a Scheil‐Gulliver based model or an evaluation of the average Kou‐index.

Micromechanism and mathematical model of stress sensitivity in tight reservoirs of binary granular medium

Research on reservoir rock stress sensitivity has traditionally focused on unary granular structures, neglecting the binary nature of real reservoirs, especially tight reservoirs. Understanding the stress-sensitive behavior and mathematical characterization of binary granular media remains a challenging task. In this study, we conducted online-NMR experiments to investigate the permeability and porosity evolution as well as stress-sensitive control mechanisms in tight sandy conglomerate samples. The results revealed stress sensitivity coefficients between 0.042 and 0.098 and permeability damage rates ranging from 65.6% to 90.9%, with an average pore compression coefficient of 0.0168–0.0208 MPa−1. Pore-scale compression occurred in three stages: filling, compression, and compaction, with matrix pores playing a dominant role in pore compression. The stress sensitivity of binary granular media was found to be influenced by the support structure and particle properties. High stress sensitivity was associated with small fine particle size, high fines content, high uniformity coefficient of particle size, high plastic deformation, and low Young's modulus. Matrix-supported samples exhibited a high irreversible permeability damage rate (average = 74.2%) and stress sensitivity coefficients (average = 0.089), with pore spaces more slit-like. In contrast, grain-supported samples showed low stress sensitivity coefficients (average = 0.021) at high stress stages. Based on the experiments, we developed a mathematical model for stress sensitivity in binary granular media, considering binary granular properties and nested interactions using Hertz contact deformation and Poiseuille theory. By describing the change in activity content of fines under stress, we characterized the non-stationary state of compressive deformation in the binary granular structure and classified the reservoir into three categories. The model was applied for production prediction using actual data from the Mahu reservoir in China, showing that the energy retention rates of support-dominated, fill-dominated, and matrix-controlled reservoirs should be higher than 70.1%, 88%, and 90.2%, respectively.

A STUDY OF THE THERMAL EVOLUTION OF PERMEABILITY AND POROSITY OF POROUS ROCKS BASED ON FRACTAL GEOMETRY THEORY

The temperature effect on the permeability of porous rocks continues to be a considerable controversy in the area of reservoirs since the thermal expansion of mineral grains exhibits complicated influence on pore geometries in them. To investigate the degree of effect of pore structures on the hydro-thermal coupling process, a study of the thermal evolution of permeability and porosity of porous rocks is performed based on fractal theory and on thermal as well as stress effects. This work can provide a general physical explanation on some arguments in this area. The proposed models for permeability and porosity can be associated with temperature and the pore-structural parameters as well as physical parameters of porous rocks, such as the initial porosity ([Formula: see text], the initial fractal dimension ([Formula: see text], the fractal dimension for tortuosity ([Formula: see text] and the thermal expansion coefficient of pore volume ([Formula: see text]. The validity of the proposed models for temperature-dependent permeability and temperature-dependent porosity is validated by comparing them with the available experimental results. The investigations are performed in detail considering the essential effects of pore-structural parameters and physical parameters of porous rock on the dimensionless temperature-dependent permeability and temperature-dependent porosity as well as the fractal dimensions for pore areas and tortuosity. It is found that the pore distribution scale range ratio ([Formula: see text], and pore thermal expansion coefficient ([Formula: see text] have significant effects on the dimensionless temperature-dependent permeability and temperature-dependent porosity of porous rock as well as the fractal dimensions for pore areas and tortuosity. The proposed models may provide a fundamental understanding of the coupled hydro-thermal process of rocks.

Shear-induced permeability anisotropy in liquefiable sands

In principle, numerical simulations of boundary value problems that involve fluid-soil interaction should account for the evolution of permeability due to soil deformation. For many applications of interest in geotechnical engineering, an accurate assessment of the permeability is key to an accurate prediction of settlements and pore water pressure changes. Finite element models rely on laboratory or field testing to characterise permeability; however, these methods cannot easily evaluate anisotropy or moderate variations of permeability. Current testing tools have a limited accuracy and a rigid experimental set-up, and are usually restricted to consider one flow direction. In this study, the influence of shearing on the intrinsic permeability and the anisotropy of permeability in medium-loose liquefiable sands is investigated. The discrete element method (DEM) was used to simulate monotonic undrained and drained triaxial test simulations on model soils comprising spherical particles. The particle positions were recorded at discrete strain levels and the data were taken as input into finite volume method (FVM) simulations which were used to evaluate intrinsic permeability in selected subsamples. In the FVM simulations, permeability was evaluated in the three orthogonal directions. The results indicate that shear deformation induces an anisotropy in permeability, in both drained and undrained triaxial conditions and this anisotropy increases with axial strain. Specifically, the results show an increase in permeability in the direction of the major principal stress, whereas a reduction permeability is observed in the orthogonal plane. Undrained simulations exhibit a jump in vertical permeability around the liquefaction onset; this can be attributed to the sudden loss of particle contacts.