Permeability Evolution Research Articles

Deformation characteristics on anisotropic consolidated methane hydrate clayey-silty sediments of the South China Sea under heat injection

Natural gas hydrate is a new energy source with huge reserves, and the carbon content of hydrates is twice the carbon content of all proven fossil fuels combined, but the natural gas hydrate exploitation involves solid-liquid-gas phases change, which could induce a potential geological and environmental risk. Accurate description of the deformation behavior of sediments during hydrate dissociation is the key to predict the long-term stability of hydrate reservoirs. Nowadays, related studies mainly focus on the deformation behavior of hydrate-bearing sediment without dissociation. In this study, the deformation behaviors of remolded cores during thermal dissociation under anisotropy stress states were studied. The results show that 1) hydrate dissociation can lead to sediment deformation, the deformation rate of the sediments during the process of hydrate dissociation is 3 orders of magnitude larger than that before the dissociation. 2) the sediments under larger effective confining pressure (burial depth) present less deformation. 3) the sediments under isotropic stress conditions exhibit smaller deformation than that under anisotropic stress conditions. 4) dynamic permeability evolution during hydrate dissociation process can lead to effective confining pressure changes of the sediment, which can exacerbate the deformation.

A Novel Numerical Method for Geothermal Reservoirs Embedded with Fracture Networks and Parameter Optimization for Power Generation

Geothermal recovery involves a coupled thermo-hydro-mechanical (THM) process in fractured rocks. A fluid transient equilibrium equation, considering thermal conduction, convection, and heat exchange, is established. The evolution of the reservoir permeability and the variance in the fracture aperture due to a change in the stress field are derived simultaneously. THM coupling is accomplished through iterative hydromechanical and thermo-hydro processes. To overcome the difficulty of geometric discretization, a three-dimensional THM coupler model embedded with discrete fracture networks, using a zero-thickness surface and line elements to simulate fractures and injection/production wells, is established to evaluate the geothermal production. The reliability of the method is verified by a case study. Then, this method is applied to evaluate the influence of the geometric topological characteristics of fracture networks and the fracture aperture on the reservoir temperature evolution and heat extraction effectiveness. The results show that the power generation efficiency and geothermal depletion rate are significantly affected by the injection–production pressure. Injection wells and production wells with pressures higher than the initial fluid pressure in the fractures can be used to significantly increase power generation, but the consumption of geothermal energy and loss of efficiency are significant and rapid. To achieve better benefits for the geothermal recovery system, an optimization algorithm based on simultaneous perturbation stochastic approximation (SPSA) is proposed; it takes the power generation efficiency as the objective function, and the corresponding program is developed using MATLAB to optimize the position and pressure values for each production well. The results show that the heat transfer for the entire EGS reservoir becomes more uniform after optimization, and the heat transfer efficiency is greatly improved.

Apparent Permeability Model of Coalbed Methane in Moist Coal: Coupling Gas Adsorption and Moisture Adsorption.

Under specific conditions, moisture in natural coal seams can be adsorbed in the pores of the coal matrix, reducing the amount of methane adsorption sites and the effective area of the transport channels. This makes the prediction and evaluation of permeability in CBM exploitation more challenging. In this paper, we developed an apparent permeability model of coalbed methane coupling viscous flow, Knudsen diffusion, and surface diffusion which considers the effects of adsorbed gas and moisture in the pores of the coal matrix on the permeability evolution. The predicted data of the present model are compared with those of other models, and the results show good agreement, verifying the accuracy of the model. The model was employed to study the apparent permeability evolution characteristics of coalbed methane under different pressure and pore size distribution conditions. The main findings are as follows: (1) moisture content increases with saturation, with a slower increase for smaller porosities and an accelerated non-linear increase for porosities greater than 0.1. (2) Gas adsorption in pores decreases permeability, further weakened by moisture adsorption under high pressure but negligible at pressures below 1 MPa. (3) Higher water saturation weakens gas transport capacity, especially with pore sizes smaller than 10 nm. (4) The non-Darcy effect weakens with higher initial porosity, and neglecting moisture adsorption may significantly deviate from actual values in modeling methane transport in coal seams. The present permeability model can capture the transport characteristics of CBM in moist coal seams more realistically and is more applicable for predicting and evaluating the gas transport performance under dynamic variations of pressure, pore size, and moisture. The results in this paper can explain the transport behavior of gas in moist, tight, porous media and also provide a foundation for coalbed methane permeability evaluation.

Permeability Damage Mechanism and Evolution of Gas-Bearing Coal Seams Induced by Drilling Fluid

Permeability Damage Mechanism and Evolution of Gas-Bearing Coal Seams Induced by Drilling Fluid

Strain evolution in coal seam exposed to injected gas with different species for enhanced CBM extraction: a numerical observation

The viscosity and density of different gases (CO2 and N2) vary with the gas species, composition and temperature, which may raise variant results of gas injection enhanced coalbed methane (ECBM) extraction. The fluid–structure interaction within the coal seam was established to study the evolution of coal strain in the process of ECBM extraction by injecting CO2 or N2. After verifying the equations governing the interaction via experimental tests, the ECBM extraction by injecting different gases was simulated. The characteristics of coal strain induced by gas sorption was comprehensively analyzed. Results show that N2 has strong fluidity in coal fractures, leading to wider influencing range of injected N2 than that of injected CO2. Due to the greater affinity of CO2 to coal, the effect of gas displacement and competitive sorption is more obvious, manifesting in more likely to migrate towards the coal matrix. Compared with regular extraction, the CH4 content at 180d in CO2-ECBM and N2-ECBM extraction has decreased by 24.3% and 13.8%, respectively. The effect of gas extraction is CO2-ECBM > N2-ECBM > regular extraction. The coal strain induced by gas sorption mainly depends on the proportion of adsorbed gas in the coal matrix. The permeability evolution is opposite to the coal strain induced by gas sorption. For CO2-ECBM, the proportion of CH4 decreases gradually caused by the competitive sorption with CO2 in matrix, and the coal strain increases. The influencing factors on the coal strain are injection pressure, initial permeability, water saturation and extraction pressure in order. While for N2-ECBM, the influencing factors on the coal strain are initial permeability, injection pressure, water saturation and extraction pressure in order.

A research on excavation compensation theory for large deformation disaster control and a review on the multiphysical–multiscale responses of salt rock for underground gas storage

A research on excavation compensation theory for large deformation disaster control and a review on the multiphysical–multiscale responses of salt rock for underground gas storage

Effect of Hydrate Spatial Distribution on Dynamic Permeability Evolution Using an Unstructured Hydrate-Bearing Network with Complex Pore Morphology and Anisotropy

Hydrate spatial distribution plays an important role in dynamic permeability evolution in the development of hydrate-bearing sediments. In this work, an irregular pore network model with complex pore throat cross sections and anisotropy is first generated. Afterward, the hydrate spatial distribution is classified into four patterns and assigned to the established network in the grain-coating mode from large pores to small ones gradually. After the validation, impacts of the hydrate spatial distribution on the effective pore throat radii distribution are investigated in terms of the hydrate-bearing pore network. Furthermore, dynamic permeability evolution in different hydrate spatial distribution modes is studied in detail. Results show that the hydrate spatial distribution has a significant effect on dynamic permeability evolution by influencing the effective size distribution of pore bodies and throats. This study provides a novel insight into dynamic permeability evolution at different hydrate spatial distributions, laying a solid foundation for the efficient development of hydrate-bearing deposits.

A novel permeability model for hydrate-bearing sediments integrating pore morphology evolution based on modified Kozeny-Carman equation

Permeability of hydrate-bearing sediments is the key parameter to determine the gas production performance, which is mainly affected by the pore morphology and hydrate saturation. Existing models of permeability could not well capture the dynamic characteristic of permeability in sediments with changing hydrate saturation and pore morphology. In this paper, a novel normalized permeability model using logistic function linked different hydrate pore morphology evolution was established by modifying tortuosity, surface area and pore shape factor simultaneously in Kozeny-Carman equation. It was verified by different published data and compared with other models. The introduced critical hydrate saturation and transitional intensity of hydrate pore morphology were optimized by genetic algorithm. Results indicated that this model was more powerful than existed models and better captured the permeability evolution in hydrate-bearing sediments at various conditions. The critical hydrate saturation and transitional intensity of hydrate pore morphology dominated the characteristics of permeability evolution. In addition, the effect of porosity would not be ignored especially below the critical hydrate saturation when hydrate pore morphology changed from pore filling to grain coating. The proposed model brings reliable predictions and mechanistic understandings of the permeability evolution in hydrate-bearing sediments, and builds fundamental theory for development of natural gas hydrate in safety and efficiency.

Evolution of permeability and pore structure of salt rock and its self-healing mechanism under coupled thermo-hydro-mechanical environment

The self-healing characteristic of salt rock under the coupled thermo-hydro-mechanical (THM) environment is important for the safety assessment of underground repositories. In order to study the evolution of permeability and pore structure of salt rock and its self-healing mechanism under the coupled THM environment, the salt rock is treated with self-designed high-temperature and high-pressure THM coupled equipment. The pore/fissure evolution of the specimens is observed by computerized tomography (MCT) and metallographic microscopy and compared with the characteristic of salt rock after heat treatment only. The results show that (1) the salt rock after high-temperature treatment only does not have healing properties from 25 to 700 °C, and the salt rock damage increases sharply when the temperature is higher than 500 °C; (2) under the coupled THM environment, the pressure and temperature make some pores and fissures of the salt rock appear to heal. It is not that the higher the pressure and temperature, the stronger the healing effect, but there is an overall complex competing effect of damage and healing. When the temperature is 25–400 °C, the damaging effect is higher than the healing effect, so the overall microscopic parameters increase; when the temperature is 400–500 °C, the healing effect in the competing relationship is greater than the damaging effect, and the overall microscopic parameters decrease; when the temperature is 500–700 °C, the damaging effect dominates, the overall microscopic parameters increase sharply, and the damage is severe. The healing effect is best in the temperature range of 400–500 °C.

Impact of supercritical CO2 exposure time on the porosity and permeability of dry and wet shale: The influence of chemo-mechanical coupling effects

CO2 storage in shale formation is a potential way to reduce CO2 emissions for carbon neutrality. The variation of permeability in shale triggered by CO2-shale interaction plays an important role in CO2 sequestration. The effect of supercritical CO2 (ScCO2) exposure times (0, 10, 20, 30, 40 days) on the porosity and permeability evolution of dry and wet shale samples was investigated herein. Results show that the porosity and permeability of both dry shale and wet shale increased first and then decreased over exposure time, and there is an inflection point. The porosity and permeability evolution of shale is controlled by the chemical-mechanical coupling effects. The dissolution of mineral induced by ScCO2 exposure increases the porosity and permeability of shale. The mechanical weakening induced by ScCO2 exposure enhances the pores compressibility of shale, makes the porosity and permeability decrease at stressed condition. In the early stage, the chemical effect is dominant, after the time of inflection point, the mechanical weakening effect is dominant. The inflection time of wet shale is earlier than that of dry shale, indicating that the chemical effect dominant stage in wet shale is shorter than that in dry shale. This can be attributed to that the water promoted the chemical reaction and enhanced the weakening of mechanical properties, then shortened the chemical corrosion dominated stage. Our findings suggest that ScCO2-shale interaction can contribute to the decrease in porosity and permeability of shale with the increase of ScCO2 exposure time, and hence adversely affecting the shale gas recovery and the continue injectivity of CO2. Thus, the time dependent chemical-mechanical coupling effects should be considered in evaluating the shale gas production and CO2 storage capacity.

Experimental study on seepage characteristics and stress sensitivity of desulfurization gypsum based concrete under triaxial stress

Converting industrial by-products into green building materials is critical to achieve a low-carbon circular economy. Permeability is an important characteristic of a material that can affect its various engineering applications. Therefore, this study investigated seepage characteristics and stress sensitivity of a green building material, i.e., modified desulfurization gypsum based concrete. Different initial load damage and dry-wet-freeze-thaw alternating cycle test groups were established by simulating the service environment. Permeability evolution of specimens under different stress states during the loading and unloading of confining pressure was obtained. The loading phase was divided into high sensitive, medium sensitive, and low sensitive areas according to the sensitive coefficient, where the corresponding permeability decreased rapidly, moderately, and slowly. The recovery degree of permeability increased gradually with the decrease in confining pressure in the unloading stage. However, the permeability value was generally lower than that in the loading stage due to the plastic deformation of pores in specimens. The higher the damage degree of initial load and the more the number of dry-wet-freeze-thaw alternating cycles, the greater the permeability loss rate of specimens. Combined with nuclear magnetic resonance results, it was found that environmental factors affect the development of pore structure of each size in specimens, resulting in different initial permeability and permeability recovery. The T2 relaxation time curve shifted to the left under confining pressure, indicating that pore size in specimens became smaller, and seepage channel became narrower, and the permeability value decreased.

The permeability of shale exposed to supercritical carbon dioxide

Permeability is a critical parameter of tight reservoir rocks and one of the important parameters for characterizing fluid flow and production from reservoirs. It determines the feasibility of its commercial development. SC-CO2 has been used in shale gas exploitation for efficient fracturing and the added benefit of CO2 geo-storage. And SC-CO2 plays an important role in permeability evolution of shale gas reservoirs. In this paper, Firstly, the permeability characteristics of shale under CO2 injection are discussed. The experimental results show that the relationship between permeability and gas pressure is not a single exponential relationship, but there is an obvious segmentation phenomenon, which is particularly obvious when it is close to the supercritical state, and the overall trend is first decreased and then increased. Subsequently, other specimens were selected for SC-CO2 immersion, and nitrogen was used to calibrate and compare shale permeability before and after treatment to assess changes in shale permeability after SC-CO2 treatment at pressures from 7.5 to 11.5 MPa and X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) were used to analyze the raw and CO2-treated shale particle sample, respectively. Results indicate the permeability increases significantly after SC-CO2 treated, and permeability growth is a linear function of SC-CO2 pressure. According to (XRD) analysis and (SEM) analysis, SC-CO2 not only can act as a solvent and dissolve carbonate minerals and clay minerals, but also can complex chemical reactions with mineral components in shale, Further dissolution of carbonate minerals and clay minerals, widened gas seepage channels and enhancing the permeability.

Evolutionary Model and Experimental Validation of Gas-Bearing Coal Permeability under Negative Pressure Conditions.

Coal bed methane drainage is the main approach to lower risks of coal seam while raising the efficiency in natural resource utilization. The negative pressure used for extraction in coal mines is largely determined empirically due to a lack of experimental research on how coal permeability changes under the combined influence of effective stress and negative pressure. This results in low gas extraction efficiency and concentration. In this paper, to study the effect law of complex stress and extraction on coal permeability during coal and gas co-mining, a test system was specially designed to determine the gas flow and coal permeability of coal samples under different stress paths and negative pressure conditions in the lab. The study analyzed the correlation between coal permeability, effective stress, and negative pressure and subsequently developed a permeability evolution model for gas-bearing coal under negative pressure conditions. The results showed that the permeability of coal increases with the increase in negative pressure and decreases with the increase in effective stress; the permeability of coal can be abruptly changed by changes in stress loading patterns; the established model of permeability evolution of gas-bearing coal can better reflect the correlation between permeability, effective stress, and negative pressure. The research outcomes offer a valuable theoretical foundation for the efficient extraction and utilization of methane in coal mines.

Numerical simulation on Geothermal extraction by radial well assisted hydraulic fracturing

The mid-deep geothermal energy is extracted from the fracture network created by hydraulic fracturing. Generally, the hydraulic fracture created in the deep thermal reservoir shows the planar shape but not the network shape, and this significantly impacts the geothermal extraction efficiency. In this study, a geothermal extraction technology by radial well assisted hydraulic fracturing, which connects hydraulic fractures to each other, was proposed. An operation technique of radial well-assisted hydraulic fracturing in geothermal reservoirs without well-developed NFs was established. The model considering evolution of fracture permeability was resolved by thermo-hydromechanical (THM) coupling numerical simulation. The results show that the radial wells should be placed vertically within a large range. The thermal production increases by 20%, about 0.25 MW as the radial well length increases from 50 m to 200 m. As many fractures as possible are needed in thermal exploitation, and the fracture number should be at least 7. The simulation demonstrates that the radial well-assisted hydraulic fracturing heat recovery method is feasible in simulation. In this thesis, a more accurate numerical simulation method of geothermal production capacity is proposed, which has engineering guiding significance for the actual development and prediction of geothermal energy.

Study on the Effect of Strain-Softening on Permeability and Gas Pressure in Surrounding Rocks of a Hydraulic Flushing Borehole.

Hydraulic flushing gas extraction technology is gradually being applied in coal mines because it can effectively improve the gas extraction efficiency. This study aimed to explore the effect of strain-softening on permeability and gas pressure, due to the deficiency of previous studies on hydraulic flushing that did not consider the strain-softening of surrounding rocks. First, we analyzed the essence of strain-softening based on previous studies and proved the existence of strain-softening in surrounding rocks of the hydraulic flushing borehole by a field example. Subsequently, we established gas extraction models for different borehole diameters with and without considering strain-softening through the permeability evolution equation, the gas migration equation, and the strain-softening equation. We found that strain-softening significantly effects the permeability and gas pressure of the surrounding rocks of the borehole: the larger the diameter, the greater the effect on strain-softening. Compared with strain-softening not considered, the permeability, the permeability increase range, and the gas extraction standard range in surrounding rocks of hydraulic flushing boreholes will increase under the same borehole diameter when strain-softening is considered. The borehole diameter increases from 0.113 to 0.7 m, and the increases in permeability are 592.76 × 10-12, 614.25 × 10-12, 615.13 × 10-12, and 655.29 × 10-12 m2, respectively, the increases in permeability increase radius are 0.050, 0.121, 0.193, and 0.255 m, respectively, and the increase in gas extraction standard range radius under the two indicators are 0.119, 0.124, 0.190, 0.253 m and 0.052, 0.102, 0.150, 0.191 m, respectively. The gas pressure distribution in surrounding rocks of the borehole will also be changed by considering strain-softening, resulting in a slowly increasing gas pressure zone near the borehole: the larger the diameter, the greater the zone. Our research results proved that it is necessary to consider the strain-softening of surrounding rocks when studying gas extraction through hydraulic flushing boreholes. Moreover, we provide a novel explanation for the mechanism of gas extraction through hydraulic flushing boreholes based on the study's results. The finding of this study can help better understand the mechanism of enhanced gas extraction through hydraulic flushing boreholes.

Studying the Favorable Zone for Pressure-Relief Gas Extraction by Combining Numerical Investigation and On-Site Application

Gas extraction in the pressure-relief area using the depressurization effect of seam mining is an effective way to solve coal-mine gas problems. In this paper, we performed numerical modelling of the 204 working face via FLAC 3D, by which the stress variation of the rock mass and associated permeability evolution during the working face advancement were investigated. Furthermore, the favorable gas extraction zone for pressure relief was proposed based on the gas extraction performance of the on-site boreholes. The results show that: (1) gas extraction varies with the evolution of the three zones of the overlying rock and the associated permeability variation, which becomes stable when the height of the three zones remains unchanged; (2) the favorable zone for gas extraction of the 204 working face ranges from 13 m to 19 m of Dv, in which the gas extraction of on-site boreholes are located, presents better performance than other areas; (3) the gas extraction volume of the optimized gas extraction scheme rises to 77.46 m3/min with a 77.26% gas extraction rate, eliminating gas overrun in the working face and upper corner and increasing the advancement speed to 6.4 m/d. The on-site verification confirms the numerical simulation scope, more accurately determining the favorable zone for gas extraction. This may provide a reliable approach for improving the pressure-relief gas extraction performance and reducing the associated engineering costs.

A 3D hybrid DFPM-DFM Model for gas production from fractured shale reservoirs

Hydraulic fracturing as a key technology has been applied to improve gas productivity from shale reservoirs. Both induced hydraulic fractures (HFs) and inherent natural fractures (NFs) delineate a shale reservoir into matrix and fracture domains. Accurate characterizations of matrix and fracture are critical in evaluating gas production. In this work, an in-house built 3D discrete fracture model (DFM) was proposed to explicitly represent the distribution of the NFs network. Gas flow in shale matrix containing multiscale pores in inorganic matters (IOM) and organic matters (OM) was represented by a dual-fractal permeability model (DFPM). Consequentially, a hybrid DFPM–DFM model was proposed to couple gas flow and geomechanics the model was solved by commercial software based on the finite element method. The proposed model was verified using field data from a shale reservoir. The model was further employed to investigate the impacts of fractally-distributed pore size and fracture attributes on the evolutions of permeability and the associated gas production. The results showed that the fractally distributed pore size of IOM accounts for the diverse profiles of permeability evolution that exhibits a downward trend before rebounding. The impacts of pore diameter and tortuosity index on cumulative gas production were quantitatively analysed. We also highlighted the remarkable effects of fracture patterns (fracture connectivity, aperture and number) on cumulative gas production. This work provides a framework to explore the multiscale structural heterogeneities on hydrocarbon recovery and shale deformation.

Experimental Study on Permeability Evolution of Bituminous Coal Under High Temperature and Volumetric Stress

Experimental Study on Permeability Evolution of Bituminous Coal Under High Temperature and Volumetric Stress

Real-time permeability evolution of limestone under high temperature and triaxial stresses

In deep underground geological disposal of nuclear waste, geothermal energy development, underground coal gasification, geothermal resources development and other underground high temperature projects, they are all closely related to the permeability and thermal expansion characteristics of rocks in high temperature high-pressure real-time state. In this experiment, the real-time evolution of permeability of limestone under high-temperature and triaxial stresses condition was studied using the HTHP rock mechanics triaxial test system and compared with the permeability of limestone after high temperature cooling treatment. The test results show that: the effect of temperature on limestone permeability facilitation under high temperature cooling condition is significant, and the permeability increases by 1–2 orders of magnitude relative to the initial permeability for every 100 °C increase in the preheat temperature after 300 °C. Under high-temperature and high-pressure in real time condition, the difference values of permeability at different temperature points relative to room temperature always do not more than one order of magnitude. During the heating process at 500 °C, the permeability curves of the triaxial pressures range from 15 to 35 MPa are very different. Under the pressure condition of 15 MPa, the permeability curve increases monotonically and rapidly with the increasing temperature; When the pressure reaches 25 MPa pressure, there is a peak value area in the permeability curve; the permeability decreases monotonically with temperature when the pressure up to 35 MPa. Compared with the only thermal effect, the thermal- mechanical coupling effect has a significant inhibitory effect on limestone permeability development, and the permeability disparity between the two types of heat treatment rocks become more and more significant with increasing temperature and hydrostatic pressure.

Experimental study on damage characteristics and permeability evolution of K1/J2 paleoweathered rocks under triaxial unloading-loading confining pressure path

Experimental study on damage characteristics and permeability evolution of K1/J2 paleoweathered rocks under triaxial unloading-loading confining pressure path