Stress Sensitivity Of Permeability Research Articles

Experimental Study on The Influence of Effective Stress on Stress Sensitivity of Coal Permeability

In order to study the influence of effective stress on the sensitivity of coal permeability stress, three groups of low permeability dense coal samples were selected in the range of confining pressure of 2 MPa ~ 4 MPa. Under the influence of single factor of effective stress, the cross-contrast seepage experiment of coal samples under different effective stress conditions was carried out by using steady seepage method, and the variation characteristics of effective stress sensitivity of coal samples were systematically analyzed by using permeability damage rate and permeability curvature. The results show that: (1) under various confining pressures, with the increase of effective stress, the damage rate of coal permeability decreases according to the law of negative exponential function, and the greater the effective stress, the greater the damage rate of coal permeability; (2) The effective stress is constant, the smaller the confining pressure is, the greater the curvature of permeability is, the greater the influence of effective stress on coal seam permeability is, and the stronger the sensitivity is.

Experimental and Numerical Validation of an Effective Stress-Sensitive Permeability Model Under Hydromechanical Interactions

Experimental and Numerical Validation of an Effective Stress-Sensitive Permeability Model Under Hydromechanical Interactions

Stress sensitivity of permeability in high-permeability sandstone sealed with microbially-induced calcium carbonate precipitation

Microbially induced carbonate precipitation (MICP) catalyzed by S. pasteurii has attracted considerable attention as a bio-cement that can both strengthen and seal geomaterials. We investigate the stress sensitivity of permeability reduction for the initially high-permeability Berea sandstone (initial permeability ∼110 mD) under various durations of MICP-grouting treatment. The results indicate that after 2, 4, 6, 8 and 10 cycles of MICP-grouting, the permeabilities decrease incrementally by 87.9%, 60.9%, 38.8%, 17.3%, and then 5.4% compared to the pre-grouting condition. With increasing the duration of MICP-grouting, the sensitivity of permeability to changes in stress gradually decreases and becomes less hysteretic. This stress sensitivity of permeability is well represented by a power-law relationship with the coefficients representing three contrasting phases: an initial slow reduction, followed by a rapid drop, culminating in an asymptotic response. This variation behavior is closely related to the movement and dislocation of the quartz framework, which is controlled by the intergranular bio-cementation strength. Imaging by scanning electron microscopy (SEM) reveals the evolution of the stress sensitivity to permeability associated with the evolving microstructures after MICP-grouting. The initial precipitates of CaCO3 are dispersed on the surfaces of the quartz framework and occupy the pore space, which is initially limited in controlling and reducing the displacement between particles. As the precipitates continuously accumulate, the intergranular slot-shaped pore spaces are initially bonded by bio-CaCO3, with the bonding strength progressively enhanced with the expanding volume of bio-cementation. At this stage, the intergranular movement and dislocation caused by compaction are reduced, and the stress sensitivity of the permeability is significantly reduced. As these slot-shaped pore spaces are progressively filled by the bio-cement, the movement and dislocation caused by compaction become negligible and thus the stress sensitivity of permeability is minimized.

Pressure and rate distribution performance of a multiple-fractured well with multi-wing fracture in low-permeability gas reservoirs

Abstract In this work, a new mathematical model of a fractured well considering multiple factors (permeability stress sensitivity, multiple well interference, and multiple fracture interference) is established to simulate wellbore pressure performance and rate distribution in tight gas reservoirs. The new fracture discrete coupling mathematical model is established. The wellbore pressure solution can be obtained by pressure drop superposition and Stehfest numerical inversion. Seven flow stages are observed according to the characteristics of the pressure derivative curve. The influence of several significant parameters, including rate ratio, fracture half-length, well spacing, and stress sensitivity are discussed. Based on the developed model, we demonstrated a field case to verify model accuracy. This work provides new supplementary knowledge to improve pressure data interpretation for multi-well groups in tight gas reservoirs.

Well-Testing Model for Dual-Porosity Reservoir considering Stress-Sensitivity and Elastic Outer Boundary Condition

Stress sensitivity and the elastic outer boundary (EOB) condition have a great effect on the analysis of the characteristics of the fluid flow in a reservoir. When researchers analyzed the characteristics of the fluid flow, they have considered the stress sensitivity and the EOB condition separately but have not considered them simultaneously. Therefore, errors are inevitable during the analysis of well testing. The main object of this work is to present a well-testing model for stress-sensitivity dual-porosity reservoir (DPR) with EOB to improve the accuracy of the analysis of well-testing data. To this end, in this paper, we established a well-testing model for the DPR, considering the stress sensitivity and the EOB simultaneously, and presented its semianalytical solution. On the basis of the consideration of the EOB condition and stress sensitivity of permeability (SSP), a seepage model for the DPR with the EOB is built using the continuity equation, motion equation, state equation, and interporosity flow equation between matrix and fracture, which considers the stress sensitivity, wellbore storage, and skin. To solve this model, a nonlinear partial differential equation is changed into a linear form of a partial differential equation by introducing an effective well radius and applying Pedrosa’s transformation and perturbation transformation. Applying the Laplace transformation, an analytical solution in the Laplace space is obtained, and curves of pressure and pressure derivative (PPD) are drawn by numerically inverting them. The model is verified by comparing it with the EOB without consideration of SSP and using case data. The sensitivity of parameters on the curves of PPD is analyzed. This work may be significant for evaluating more accurately the parameters of wells and reservoirs using well testing.

Influence of key geological factors on fluid production behaviors in marine natural gas hydrate reservoirs: A case study

Accurate geological model of hydrate reservoir is the prerequisite of obtaining the credible prediction results. Not only stress sensitivity and anisotropy of permeability but also non-uniformity in hydrate saturation and permeability distribution are key geological factors influencing fluid production behaviors. However, they have been overlooked to some extent in previous numerical simulations. To highlight the significance of these factors, this study develops a robust production model at site AT1 based on geological data and laboratory tests. Then we systematically investigate the effects of the aforementioned factors on production performance. The simulation results exhibit that all the factors have notable impacts on fluid production behaviors, especially the non-uniform hydrate saturation and permeability distributions. Ignoring the permeability stress sensitivity and adopting the homogeneous simplification of hydrate saturation may overestimate gas and water production. Besides, various characterization methods of reservoir permeability can give rise to the significantly different production behaviors for heterogeneous hydrate reservoirs. The stronger the reservoir permeability anisotropy, the better the production performance. In order to achieve the reliable predictions, the geological model built should properly incorporate these factors during numerical modeling. These findings provide valuable guidance for constructing robust models and optimizing production strategies for natural gas hydrate reservoirs.

A numerical pressure transient model of fractured well with complex fractures of tight gas reservoirs considering gas-water two phase by EDFM

Tight gas reservoirs considering gas-water two phase is common and show obvious permeability stress sensitivity and non-Darcy seepage characteristic. However, conventional analytical and semi-analytical solving method cannot simulate wellbore pressure response of tight gas reservoirs considering gas-water two phase. Numerical model can overcome the disadvantage of analytical and semi-analytical method and achieve wellbore pressure response simulation of fractured well with complex fracture. Therefore, a gas-water two phase mathematical model of the fractured well is established and solved by embedded discrete-fracture method (EDFM) and finite volume method. Transmissibility corrections is improved on the basis of the framework of Matlab Reservoir Simulation Toolbox (MRST). Result shows that the seven flow regimes can be found according to the pressure drop derivative curves characteristic. When permeability stress sensitivity and non-Darcy is considered, pressure and derivative curve of middle-late stage shows obvious ‘upwarp’ and non-Darcy make reservoirs pressure uneven distribution. Induced fractures parameters have obvious influence on pressure and derivative curves during early and middle flow regime. In addition, fracture intersection and neighbor well production style has obvious influence on pressure curve.

Stress-sensitivity analysis of geological confined pores with ultrasonics

Ultrasonics could be a promising technology to modify the geological formations for geo-energy productions, carbon geological sequestrations, monitoring of crack formation with stress and volcanic activity prediction. However, immature understanding of the post-ultrasonic geological formations seriously restricts the further applications in practices. This paper initially focuses on the in-situ geological porous media of low-permeability sandstones, analysing their stress sensitivity and relevant influential factors post-ultrasonics. Basically, the original and treated in-situ cores with and without ultrasonics were characterized through a series of experimental approaches, including permeability stress sensitivity (PSS), high-pressure mercury intrusion (HPMI), inline nuclear magnetic resonance (NMR), X-ray diffraction tests (XRD), triaxial stress test (TS) and scanning electron microscope (SEM) to explore the stress-induced rock compression mechanisms and the associated effects of pore restructures and mineral re-compositions. The experimental results reveal that the ultrasonics cause desorption of clay minerals at relatively low pressures (<5 MPa), which enlarges the pore throat and increases the permeability by 2.4 times. As the pressure increasing to 5 MPa, the porosity decreases from 8.79% to 6.13% because the plastic deformations of rock become irreversible without the cementing support (i.e., clay minerals). A further pressure increases from 5 to 25 MPa results in more porosity reduction from 6.13% to 5.77% through the rigid compressions. Overall, under the same pore pressure, the core porosity with ultrasonic treatments is usually smaller than the original ones, which thus indicates ultrasonics to some extent augment the stress sensitivity of core with compression. Hence, it is suggested to control pore pressure if the ultrasonics are introduced for geo-energy productions and carbon geological sequestrations.

Microscopic Analysis of Natural Fracture Properties in Organic-Rich Continental Shale Oil Reservoirs: A Case Study from the Lower Jurassic in the Sichuan Basin, China

Natural fractures are of paramount importance in storing carbon in shale oil reservoirs, where ultra-low porosity and permeability necessitate their essentiality for enhanced oil recovery. Therefore, comprehensively clarifying the characteristics of natural fractures in shale oil reservoirs is imperative. This paper focuses on investigating the microscopic features of natural fractures in organic-rich continental shale oil reservoirs that are commonly found in the Lower Jurassic strata of the Sichuan Basin, employing them as a representative example. Multiple methods were utilized, including mechanical testing, Kaiser testing, multi-scale CT scanning (at 2 mm, 25 mm, and 100 mm scales), and a numerical simulation of fluid seepage in fracture models. The results revealed that the in situ stress of the target seam displays the characteristic of σH &gt; σv &gt; σh, with σv and σh being particularly similar. The relatively high lateral stress coefficient (ranging from 1.020 to 1.037) indicates that the horizontal stresses are higher than the average level. Although the 2 mm CT scan provides a more detailed view of fractures and connected pores, it primarily exhibited more pore information due to the high resolution, which may not fully unveil additional information about the fractures. Thus, the 25 mm shale core is a better option for studying natural fractures. The tortuosity of the different fractures indicated that the morphology of larger fractures is more likely to remain stable, while small-scale fractures tend to exhibit diverse shapes. The simulations demonstrated that the stress sensitivity of fracture permeability is approximately comparable across different fracture scales. Therefore, our research can enhance the understanding of the properties of natural fractures, facilitate predicting favorable areas for shale oil exploration, and aid in evaluating the carbon storage potential of shale oil reservoirs.

Failure Characteristics of the Water-Resisting Coal Pillar under Stress-Seepage Coupling and Determination of Reasonable Coal Pillar Width

Groundwater inrush hazard has always been a great threat to the construction of vertical shafts in coal mines. Generally, the failure of the water-resisting coal pillar under coupled stress-seepage conditions around the vertical shaft is the main reason for the generation of the water inrush channel. In order to understand the mechanical behaviors of the water-resisting coal pillar, the strength of typical coal affected by the size and water content was investigated, and the stress sensitivity of permeability was investigated by a stress-seepage coupling test. The stress field and deformation of the water-resisting coal pillar were investigated by numerical simulation, the stability of the water-resisting coal pillars with different widths was evaluated, and the reasonable width of the coal pillars under coupled stress-seepage condition was determined. Results show that the water content and coal pillar width have a great influence on the mechanical characteristics of coal samples. Under the conditions of lower water content and larger coal sample width, the coal sample presents higher strength, smaller axial deformation, smaller permeability and porosity, and weak sensitivity to stress. The simulation results show that the boundary of the main roadway at the end of the coal pillar is dominated by tensile stress, and fractures can significantly contribute to the destruction of coal pillars. With the increase in the width of the water-resisting coal pillar, the internal damage variable, maximum tensile stress, porosity, and average water flow velocity of the coal pillar decrease, which reduces the risk of water inrush and improves the safety of the water-resisting coal pillar. An evaluation model of the reasonable width of the water-resisting coal pillar under the stress-seepage coupling was proposed, and the model was verified by the shear slip law and experimental results. This study provides theoretical and experimental guidance for the risk management of groundwater inrush disaster during the construction of vertical shafts in coal mines.

Porosity, permeability and rock mechanics of Lower Silurian Longmaxi Formation deep shale under temperature-pressure coupling in the Sichuan Basin, SW China

To investigate the porosity, permeability and rock mechanics of deep shale under temperature-pressure coupling, we selected the core samples of deep shale from the Lower Silurian Longmaxi Formation in the Weirong and Yongchuan areas of the Sichuan Basin for porosity and permeability experiments and a triaxial compression and sound wave integration experiment at the maximum temperature and pressure of 120 °C and 70 MPa. The results show that the microscopic porosity and permeability change and the macroscopic rock deformation are mutually constrained, both showing the trend of steep and then gentle variation. At the maximum temperature and pressure, the porosity reduces by 34%–71%, and the permeability decreases by 85%–97%. With the rising temperature and pressure, deep shale undergoes plastic deformation in which organic pores and clay mineral pores are compressed and microfractures are closed, and elastic deformation in which brittle mineral pores and rock skeleton particles are compacted. Compared with previous experiments under high confining pressure and normal temperature, the experiment under high temperature and high pressure coupling reveals the effect of high temperature on stress sensitivity of porosity and permeability. High temperature can increase the plasticity of the rock, intensify the compression of pores due to high confining pressure, and induce thermal stress between the rock skeleton particles, allowing the reopening of shale bedding or the creation of new fractures along weak planes such as bedding, which inhibits the decrease of permeability with the increase of temperature and confining pressure. Compared with the triaxial mechanical experiment at normal temperature, the triaxial compression experiment at high temperature and high pressure demonstrates that the compressive strength and peak strain of deep shale increase significantly due to the coupling of temperature and pressure. The compressive strength is up to 435 MPa and the peak strain exceeds 2%, indicating that high temperature is not conducive to fracture initiation and expansion by increasing rock plasticity. Lithofacies and mineral composition have great impacts on the porosity, permeability and rock mechanics of deep shale. Shales with different lithologies are different in the difficulty and extent of brittle failure. The stress-strain characteristics of rocks under actual geological conditions are key support to the optimization of reservoir stimulation program.

The Anisotropy Permeability Evolution and Deformation Behavior of Raw Coal under Cyclic Loading Conditions

As an anisotropic porous media, coal’s intrinsic permeability is sensitive to stress variably in different directions. Likewise, the degree of damage sustained by the internal solid skeletal structure under external stresses is anisotropic. Four parameters are introduced: permeability stress-based loss rate (PEL) and permeability strain-based loss rate (PSL) highlight the stress sensitivity of the intrinsic permeability and deformation characteristics of coal’s internal structure during the loading process; stress-based irrecoverable permeability coefficient (EIP) and strain-based irrecoverable permeability coefficient (SIP) describe the loss of intrinsic permeability and irreversible deformation features in the whole load cycle. Stress sensitivity of the vertical intrinsic permeability is greater than that of the horizontal, and the ability of the vertical internal structure to resist deformation induced by an external load is weaker. During the cyclic loading process for the same coal sample, the stress sensitivity of the latter loading cycle is significantly less than the former.

A PERMEABILITY HYSTERESIS MODEL FOR FRACTAL POROUS MEDIA BASED ON ELASTIC-STRUCTURAL DEFORMATION OF CAPILLARY CROSS SECTION

Permeability hysteresis under cyclic pressure loading and unloading has received a lot of attention in both science and engineering. But most of the existing model is only for a one-time pressure cycle. Therefore, a permeability hysteresis model is established based on the theory of elastic-structural deformation of capillary cross section and the fractal theory of porous media. Both the triangular and quadrilateral structures are considered. The stress sensitivity of structural deformation decreases with the increase in the cycle. The porosity hysteresis can also be predicted by the proposed model. Compared with experimental data with different permeability hysteresis, the prediction of the proposed model is consistent with the experimental results. Compared with other models, the proposed model has a smaller average error and a better agreement with experimental data. The proposed model can predict the permeability hysteresis under not only a one-time pressure cycle like the existing model but also multiple pressure cycles. The influence of parameters shows that the decrease in Young’s modulus and Poisson’s ratio of the solid cluster increases the permeability stress sensitivity but does not influence the permeability hysteresis. The increase in the proportion of quadrilateral structure and stress sensitivity of structural deformation increases the permeability hysteresis and stress sensitivity at the same time, while the capillary fractal dimension, tortuosity fractal dimension, and the decay rate of stress sensitivity of structural deformation during the cycles show the opposite. The proposed model has a significant meaning in underground resource mining and the study of permeability hysteresis mechanism.

A Semi-Analytical Model of an Off-Center Multi-Wing Fractured Well in a Low-Permeability Gas Reservoir

Summary Since there are several hydraulic fractures around a wellbore after a large-scale hydraulic fracturing and the well is not in the center of the reservoir, no corresponding semianalytical model for wellbore pressure analysis has been proposed. To bridge this gap, this paper aims to present a semianalytical model of the off-center multiwing fractured well. With consideration of permeability stress sensitivity, the reservoir model and hydraulic fracture model are established, respectively. The coupling approach of the reservoir model and hydraulic fracture model is used to obtain the wellbore pressure solution. Meanwhile, the off-center multiwing fractured well is verified with a numerical solution. The seven flow regimes can be distinguished according to the characteristics of the pressure derivative curve. Furthermore, the effect of different fracture distributions on wellbore pressure and the derivative curve is discussed and analyzed. Assuming that the fracture wing number is equal to the average length of all fracture wings, the wellbore pressure is lowest before the radial flow regime when the fracture wing has a uniform distribution around the angle and all fracture wings are equal in length. Besides, the influence of other important parameters (fracture wing number, off-center distance, etc.) is discussed. According to the analysis, we conclude that fracture wing number has a significant influence on the pressure and derivative curves before the radial flow regime. The off-center distance has no influence on the pressure and derivative curve before the radial flow regime, but it has an obvious influence on arc boundary reaction time. Finally, the advantages of the semianalytical solution are fast calculation speed and high calculation accuracy (especially in the early flow regime).

Pore Structure Characteristics and Permeability Stress Sensitivity of Jurassic Continental Shale of Dongyuemiao Member of Ziliujing Formation, Fuxing Area, Eastern Sichuan Basin

A shale condensate gas reservoir with a high clay content and a high formation pressure was found in the Jurassic shale of the Dongyuemiao Member in the Fuxing area of the eastern Sichuan Basin. Reservoir characteristics and formation pressure have a significant influence on optimal development. The present study investigated the continental shale of the Dongyuemiao Member in Well F. The petrological properties, physical properties, and pore structure of the Dongyuemiao Member were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), N2 adsorption, and mercury intrusion porosimetry (MIP). The permeability stress sensitivity characteristics of the shale reservoirs are discussed based on the change in shale porosity and permeability under overburden pressure. The tested shale samples yielded total organic carbon (TOC) and S1 + S2 values ranging mainly from 1.0 wt.% to 1.5 wt.% and from 0.39 to 2.28 mg/g, respectively, which was in the high maturity stage of the thermal evolution of organic matter (OM). The shales of the Dongyuemiao Member were found to contain high average clay mineral contents (more than 50%) of calcite and quartz, as well as albite, pyrite, dolomite, and halite. The main developments were identified as silica-rich argillaceous shale lithofacies, argillaceous shale lithofacies, and mixed argillaceous shale lithofacies. The pores were found to mainly be plate-like and flake-like interlayer pores of clay minerals and OM pores with various shapes. The pore size was mainly concentrated below 110 nm, and the pore volume increment increased in flakes with pore diameter. The average porosity and permeability of shale were found to be 4.827% and 0.243 mD, respectively. Clay minerals and quartz are beneficial for improving the porosity and permeability of reservoirs, while carbonate minerals have the opposite effect. The permeability of the shale showed a negative exponential change with increasing effective stress under overburden pressure. When the effective confining pressure was greater than 20 MPa, the decline rate of the shale permeability decreased with increases in the effective stress. The higher the clay mineral and TOC content, the stronger the stress sensitivity of shale permeability. The higher the carbonate mineral content, the weaker the stress sensitivity of shale permeability. The porosity sensitivity exponent indicates that matrix pores and micro-fractures are both developed in the Dongyuemiao Member, and the development of internal fractures is the main factor in the strong stress sensitivity of the shale permeability in the study area.

Study on Permeability Stress-Sensitivity in Seepage-Geomechanical Coupling of Fractured Deep Tight Sandstone Gas Reservoirs

Accurately predicting the characteristics and influencing factors of permeability stress-sensitivity contributes to improving gas production in gas reservoirs. In this paper, the effects of effective stress on the permeability of fractured deep tight sandstone reservoirs were studied by laboratory tests. With the experimental results, a coupled seepage-geomechanical model for fractured deep tight sandstone gas reservoirs was constructed. The influences of pore pressure and geo-stress on permeability characteristics and gas production were studied by numerical simulation. The results indicate: (1) When the effective stress increases from 0 to 65 MPa, the permeability of the natural sample with fractures decreases by 81.28%, and the permeability of the intact core sample decreases by 54.67%. (2) When the pore pressure decreases from 120 to 85 MPa, the three-dimensional effective stress increases. The largest increase of the effective stress was along the vertical direction, which increased by 11~19 MPa. In addition, the permeability of the fractured zone and the intact rock along the vertical direction decreased by about 40% and 16%, respectively. (3) The mean square error between the historical gas production results and the results by simulation was 2.22 when considering the permeability stress-sensitivity, and 4.01 without considering the permeability stress-sensitivity. The proposed coupled seepage-geomechanical model with permeability stress-sensitivity proved to be more accurate in historical gas production comparison and prediction. This study provides a reliable optimization scheme for the development of fractured deep tight sandstone gas reservoirs.

Study on flow model of multi-stage fracturing horizontal well in stress-denpendent dual medium reservoir

Often with abundant of natural fracture, Carbonate reservoirs are characterized with the stress sensitive and dual media. Mostly, its flow model solved by numerical method. In this paper, the semi-analytical solution for this problem is presented: firstly, the point source function considering stress sensitivity in infinite dual medium reservoir is obtained in Laplace space by Perturbation Transformation and Laplace Transformation; Secondly, the Laplace space solution of multi-stage fracturing horizontal well in infinite plate reservoir is obtained by Image Principle and Superposition Principle; Finally, the spacial solution of multi-stage fracturing horizontal well is obtained by Stehfest numerical inversion and perturbation inverse transformation. The calculation results show that the flow regime of multi-stage fracturing horizontal well can be divided into six stages: Ⅰ- linear flow, Ⅱ-first radial flow, Ⅲ-double radial flow, Ⅳ-radial flow of natural fracture system, Ⅴ-channeling flow regime and Ⅵ-radial flow of the whole system. The impact of stress sensitivity of formation permeability on linear flow is lower, and mainly affecting the last five flow regimes, and the dimensionless pressure drop derivative curve tends to rise in the later stage of development, showing the characteristics of closed boundary. In this paper, when the reservoir stress sensitivity is not considered, the calculation results will produce a large error, and the wrong well test interpretation will be obtained.

Digital rock analysis to estimate stress-sensitive rock permeabilities

Reservoir deformations due to stress changes may have a substantial impact on the effective flow properties of reservoir rocks, which may potentially impact productivity and injectivity performance. Absolute permeability has been a primary factor in measuring hydraulic conductivity of porous materials. In order to investigate the impact of the granular structure and the extent of intergranular cementation on stress-dependent absolute permeability, we present a workflow that utilizes limited knowledge about the rock sample in the form of micro-Computed Tomography images to simulate stress-induced deformations. This analysis is then followed by Lattice Boltzmann Methods to simulate fluid flow in the deformed medium. While previous works using digital rocks ignored the impact of intergranular cements, we should consider the impact of these cements on the stress sensitivity of absolute permeability of the rock. It is notable that changes in the rock properties like the nature of stress would be a continuous change, so measurements at few stress points may not be enough to describe the overall response in any arbitrary loadings. Despite empirical formulae that are limited to few measurements, we proposed a method based on the equation of states to estimate rock permeability through three independent strain invariants. Digital rock physics, finite element analysis, and Lattice Boltzmann Methods were integrated to obtain a quick estimation of the rock permeability using micro-CT images without the need for repetitive laboratory experiments. Our results show that absolute permeability decreases significantly with increasing first and third strain invariants while it slightly increases with the second strain invariant. The stress sensitivity of absolute permeability was found to be negatively correlated with Young’s modulus of the intergranular cement. We observed that incorporating more intergranular cement will make the absolute permeability more sensitive to stress changes.

Effective stress effect and slippage effect of gas migration in deep coal reservoirs

The deep coal reservoir is in a high in-situ stress regime, and the drop in pore pressure during the coalbed methane extraction can induce significant slippage effect and effective stress change. Both of them play a competitive role in permeability evolution. In our research, coal seepage experiments under variable bulk stress and pore pressure paths are performed to investigate the slippage effect and effective stress effect during gas migration in deep coal seams. The results show that the gas flow pattern changes from effective stress–control to slippage effect–control during the increase of bulk stress. Because of the dual pore structure and rich clay, coal presents microscopically inhomogeneous assemblage in mechanical and seepage properties. It makes the effective stress coefficient (ESC) for permeability, κ, always greater than 1, up to 1.94–2.64. By substituting the elastic modulus of the bedding for that of the clay in the coal, the clay-particle model can be used to explain the κ > 1. The linear-Klinkenberg equation is no longer applicable to the correction of gas slippage effect in deep coal reservoir, but conforms to the double-slip-Klinkenberg equation. Slippage effect weakens the stress sensitivity of permeability. As the effective stress increases, the slippage coefficient b increases accordingly, and the slippage effect becomes more pronounced. The gas flow pattern in deep coal reservoir is basically in the region of slippage flow, and the slippage effect cannot be ignored in gas migration. Furthermore, the anisotropy of κ is weakened in deep coal reservoirs, and the intrinsic permeability of coal, k∞, has the strongest stress sensitivity to σ3.

Productivity prediction with consideration of stress-sensitive permeability in naturally fractured carbonate reservoir

The aim of this paper is to illustrate the impact of stress-sensitive permeability on productivity in naturally fracture carbonate reservoirs. First, stress sensitivity evaluation experiments were conducted on artificial cores with matrix, uncemented fracture, partially- cemented fracture and fully-cemented fracture by measuring their permeability in the process of increasing confining pressure. Then, a new mathematical model for productivity prediction in naturally fractured carbonate reservoir with consideration of stress-sensitive permeability is developed. Comparisons have been made between the new model prediction results and field measured data to verify the accuracy of the new model. Finally, based on the validated model, the detailed impact of permeability modulus on productivity is analysed. The experiment results show that the strength of stress sensitivity, evaluated by permeability modulus, of cores with uncemented fracture and partially-cemented fracture is generally an order of magnitude larger than that of cores with matrix and fully-cemented fracture. The results show the new model can be simplified to the classic model (Vogel equation) under the condition of the stress sensitivity being not taken into account, which means the class model is corresponding to a special case of this new model. Field application shows that the new model is reliable and correct. Moreover, with the help of the new model, it is found that the impact of stress-sensitive permeability on productivity increases with increasing permeability modulus and decreasing flow bottom hole pressure.