Seepage Experiments Research Articles

Permeability evolution of sandstone caprock transverse fractures during intermittent CO2 injection in coal-bearing strata

The caprock of coal-bearing strata plays a critical role in CO2 geological storage, with the presence of fractures posing a heightened risk of CO2 leakage. The cyclic effects of CO2 injection and in situ stress influence the permeability of caprock fractures. However, the combined impact of CO2 and in situ stress on fracture permeability remains uncertain. This study conducted cyclic seepage experiments under varying amplitude stresses on fractured sandstone samples soaked in ScCO2 for different times (0, 15, 30, and 60 days). The microstructural characteristics of the fractured sandstone surfaces were analyzed using scanning electron microscopy and x-ray diffraction. The experimental results indicated that soaking in ScCO2 reduces sandstone fracture permeability, but the extent of this reduction is nonlinearly related to the soaking time. During the stress cycling process, due to the effect of plastic deformation, the permeability of sandstone fractures decreases with increasing cyclic amplitude and remains relatively constant with decreasing cyclic amplitude. At the same cyclic amplitude, the permeability of sandstone fractures initially increases and then decreases with prolonged soaking time. The impact of ScCO2 and stress cycling on the permeability of sandstone fractures is the result of a series of combined chemical–mechanical effects. The combined effects of chemical dissolution and mechanical degradation significantly influence the permeability of sandstone fractures, and this impact is notably time-dependent. During short-term soaking, geochemically induced changes in the surface structure of fractures cause fluctuations in permeability, while in long-term soaking, the combined chemical–mechanical effects promote a reduction in fracture permeability.

Pore-Scale Mechanism Analysis of Enhanced Oil Recovery by Horizontal Well, Dissolver, Nitrogen, and Steam Combined Flooding in Reducer Systems with Different Viscosities for Heavy Oil Thermal Recovery

Horizontal well, dissolver, nitrogen, and steam (HDNS) combined flooding is mainly applied to shallow and thin heavy oil reservoirs to enhance oil recovery. Due to the lack of pore-scale mechanism studies, it is impossible to clarify the oil displacement mechanism of each slug in the process combination and the influence of their interaction on enhanced oil recovery (EOR). Therefore, in this study, HDNS combined flooding technology was simulated in a two-dimensional visualization microscopic model, and three viscosity reducer systems and multi-cycle combined flooding processes were considered. In combination with an emulsification and viscosity reduction experiment, two-dimensional microscopic multiphase seepage experiments were carried out to compare the dynamic seepage law and microscopic occurrence state of multiphase fluids in different systems. The results showed that the ability of three viscosity reducers to improve viscosity reduction efficiency in HDNS combined flooding was A > B > C, and their contributions to the recovery reached 65%, 41%, and 30%, respectively. In the system where a high viscosity reduction efficiency was shown by the viscosity reducer, the enhancements of both sweeping efficiency and displacement efficiency were primarily influenced by the viscosity reducer flooding. Steam flooding collaborated to improve displacement efficiency. The thermal insulation characteristics of N2 flooding may not provide a gain effect. In the system where a low viscosity reduction efficiency was shown by the viscosity reducer, the steam flooding was more important, contributing to 57% of the sweeping efficiency. Nitrogen was helpful for expanding the sweep area of the subsequent steam and viscosity reducer, and the gain effect of the thermal insulation steam chamber significantly improved the displacement efficiency of the subsequent steam flooding by 25%. The interaction of each slug in HDNS combined flooding resulted in the additive effect of increasing production. In actual production, it is necessary to optimize the process and screen the viscosity reducer according to the actual conditions of the reservoir and the characteristics of different viscosity reducers.

Theoretical study on nonlinear seepage mechanism in fractal dendritic fracture network of low permeability coal with water injection

Coal seam water injection technology is adopted by many mines as an effective means of dust reduction in coal mines. There is a threshold pressure gradient phenomenon in the process of water injection in low permeability coal seam, which makes the flow of pressure water in the fracture structure of coal body present nonlinear seepage characteristics. To reveal the theoretical relationship between the structural parameters of coal and the nonlinear seepage characteristics, firstly, the Bingham fluid constitutive equation is used to describe the non-Newtonian behavior in low-permeability coal. Combined with the fractal tree-like bifurcation fracture network model, a mathematical analytical model of threshold pressure gradient is established. Secondly, the model was verified by high-pressure water invasion and radial seepage experiments, and the sensitivity of the model was analyzed. The results show that the error between the theoretical calculation value and the experimental measurement value is between 8.65 % and 42.4 %, which verifies the validity of the model. The above research results can provide a theoretical basis for improving the water injection effect of low permeability coal seam.

Ecological and geological security risk assessment of underground space development in cold and arid canyon cities-Taking Ping'an District, Haidong City, Qinghai Province, as an example.

The ecological and geological problems caused by the rise of groundwater level due to the development of underground space in cold and arid canyon cities are particularly typical. Reasonably assessing the ecological and geological security risks of utilizing underground space is conducive to reducing the occurrence of ecological and geological problems during the construction and operation of underground engineering projects. Taking Ping'an District of Haidong City as an example, the topography and geomorphology of the research area were investigated in the field, and the distribution of topography and geomorphology in the research area was understood; through geological drilling and geotechnical engineering testing, the distribution of different strata in the research area was obtained; through pumping and seepage experiments, the recharge, runoff, and discharge relationship between surface water and groundwater in the research area and the water abundance of different strata are obtained, and the causes and mechanisms of geological safety risks in forest and grassland ecosystems, farmland ecosystems, and human settlements ecosystems were analyzed based on literature. Corresponding ecological geological safety risk assessment index systems and methods were established, and the ecological geological safety risks before and after the development of underground rail transit projects along both banks of the Huangshui River in the study area were evaluated. PRACTITIONER POINTS: The development of underground space in canyon type cities can easily lead to ecological and geological problems. Taking Haidong City, Qinghai Province, as an example, this study investigates the causes of ecological and geological problems caused by the development of underground spaces in canyon type cities. An ecological geological security risk assessment index system and method for canyon-type cities were established. An evaluation was conducted on the ecological and geological safety risks before and after the development of the underground rail transit projects on both sides of the research area.

Migration Characteristics Analysis of Heavy Metal in Oil Shale Based on In-situ Core Seepage Experiments

Migration Characteristics Analysis of Heavy Metal in Oil Shale Based on In-situ Core Seepage Experiments

Fractal study on the nonlinear seepage mechanism during low-permeability coal water injection

In the initial stage of coal seam water injection, due to the high density and low permeability of coal bodies, an obvious startup pressure gradient is observed in relation to water seepage; this phenomenon leads to low-velocity nonlinear seepage. In this paper, we study the nonlinear seepage law and the main influencing parameters of the water injection process. First, based on the startup pressure gradient, the nonlinear seepage equation, and the fractal theory, we formulated a nonlinear seepage model of coal seam water injection that considered the fractal characteristics of a complex coal structure. Subsequently, we carried out coal seam water injection and gas radial seepage experiments under a high overburden pressure, obtaining the startup pressure gradient according to the seepage characteristics and the changes of dynamic parameters. Then, the dynamic parameters of water injection, the structural parameters of the coal samples, the physical parameters, and the fractal dimension were substituted into the theoretical model to obtain the theoretically calculated value. Finally, through comparative analysis of theoretical and experimental startup pressure gradient calculation results, it is found that with the increase in the overburden pressure, the permeability of coal and the connectivity effect are reduced, while the fracture tortuosity and the startup pressure gradient increase. Moreover, coal seam permeability does not seem to be the single decisive factor for the nonlinear startup pressure gradient.

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.

Effects of Clay Content on Non-Linear Seepage Behaviors in the Sand–Clay Porous Media Based on Low-Field Nuclear Magnetic Resonance

Clay is widely encountered in nature and directly influences seepage behaviors, exerting a crucial impact on engineering applications. Under low hydraulic gradients, seepage behaviors have been observed to deviate from Darcy’s law, displaying a non-linear trend. However, the impacts of clay content on non-linear seepage behavior and its pore-scale mechanisms to date remain unclear. In this study, constant-head seepage experiments were conducted in sand–clay porous media under various hydraulic gradients. Low-field nuclear magnetic resonance (LF-NMR) technology was utilized to monitor the bound-water and free-water contents of sand–clay porous media under different seepage states. The results show a threshold hydraulic gradient (i0) below which there is no flow, and a critical hydraulic gradient (icr) below which the relationship between the hydraulic gradient (i) and seepage velocity (v) is non-linear. Both hydraulic gradients increased with clay content. Moreover, the transformation between bound water and free water was observed during the seepage-state evolution (no flow to pre-Darcy or pre-Darcy to Darcy). As the hydraulic gradient reached the i0, the pore water pressure gradually overcame the adsorption force of the bound-water film, reducing the thickness of the bound-water film, and causing non-linear seepage behavior. When i0 < i < icr, the enlarging hydraulic gradient triggers the thinning of bound water and enhances the fluidity of pore water. Moreover, the increasing clay content augments the bound-water content required for the seepage state’s change.

Experimental Study of Non-Darcian Flow Characteristics in Low-Permeability Coal Pillar Dams.

The safe operation of underground reservoirs and environmental protection heavily rely on the water flow through coal pillar dams in coal mines. Meanwhile, research on the flow characteristics in coal pillar dams has been limited due to their low hydraulic conductivity. To address this gap, this study assembled a novel seepage experimental device and conducted a series of carefully designed seepage experiments to examine the characteristics of low-permeability in coal pillar dams. The experiments aim to explore the relationship between water flux and hydraulic gradient, considering varying core lengths and immersion times. Flow parameters were determined by fitting observed flux-gradient curves with predictions from both Darcy and non-Darcian laws. Several significant results were obtained. First, a noticeable non-linear relationship between water flux and hydraulic gradient was observed, particularly evident at low flow velocities. Second, the non-Darcy laws effectively interpreted the experimental data, with threshold pressure gradients ranging 13.60 to 58.64 for different core lengths. Third, the study established that water immersion significantly affects the flow characteristics of coal pillar dams, resulting in an increased hydraulic conductivity and flow velocity. These findings carry significant implications for the design of coal pillar dams within underground coal mine reservoirs, providing insights for constructing more stable structures and ensuring environmental protection.

Experimental study on gas slippage effect in coal at different temperatures based on nuclear magnetic resonance

Coalbed methane (CBM) can be recovered more effectively through heat injection mining. The slippage effect in coal becomes more significant as CBM is exploited, essential for maintaining consistent production of CBM wells. By “slippage effect,” we mean that gas molecules exhibiting non–zero velocity near the wall surface of coal pores, leading to higher gas permeability than liquid permeability. To explore the slippage effect and its control mechanism during heat injection mining, methane seepage experiments with constant effective stress were conducted under five different temperatures and seven pore pressures. An analysis was conducted on the mechanisms by which pore pressure and temperature affect gas slippage effect. Furthermore, the correlation between coal pore structure and the gas slippage effect was revealed using the nuclear magnetic resonance (NMR) technique. The results appear that, first, as pore pressure increases, the slippage effect is suppressed due to the influence of the average molecular free path, despite the positive impact of adsorption–induced matrix expansion. Second, as temperature rises, the average molecular free path and thermal expansion have a positive impact on gas slippage, while matrix shrinkage caused by desorption has a negative impact. The positive impact is stronger, resulting in a continuous enhancement of the slippage effect. Finally, a new slippage factor calculation based on NMR T2 distribution of micropores and transitional pores (<100 nm, T2 < 8.33 ms) was provided, and the method was verified by slippage factor fitted by permeability experimental data. The heat injection mining of CBM can be theoretically guided by the research results.

Clogging caused by coupled grain migration and compaction effect during groundwater recharge for unconsolidated sandstone reservoir in groundwater-source heat pump

In unconsolidated sandstone reservoirs, presence of numerous movable grains and a complex grain size composition necessitates a clear understanding of the physical clogging process for effective groundwater recharge in groundwater-source heat pump systems. To investigate this, a series of seepage experiments was conducted under in situ stress conditions using unconsolidated sandstone samples with varying grain compositions. The clogging phenomenon arises from the combined effects of grain migration and compaction, wherein the migration of both original and secondary crushed fine-grain particles blocks the seepage channels. Notably, grain composition influences the migration and transport properties of the grains. For samples composed of smaller grains, the apparent permeability demonstrates a transition from stability to decrease. In contrast, samples with larger grains experience a skip at the stability stage and directly enter the decrease stage, with a minor exception of a slight increase observed. Furthermore, a unique failure mode characterized by diameter shrinkage in the upper part of the sample is observed due to the combined effects of grain migration and in situ stress-induced compaction. These testing results contribute to a better understanding of the clogging mechanism caused by the coupled effects of grain migration and compaction during groundwater recharge in unconsolidated sandstone reservoirs used in groundwater-source heat pump systems.

Influence of Hydraulic Conditions on Seepage Characteristics of Loose Sandstone

Abstract To investigate the impact of hydraulic conditions on the seepage characteristics of loose sandstone, this study employed optimized methods to prepare loose sandstone samples. Subsequently, seepage experiments were conducted under different injection pressures, flow rates, and flow volumes. The permeability, porosity, particle size distribution, and other parameters of the rock samples were obtained. By analyzing the response of seepage characteristics to pore and particle size characteristics, the influence of different hydraulic conditions on the seepage characteristics of loose sandstone was explored. The results indicated that improvements in the parameters of hydraulic conditions had different effects on various rock samples. For rock samples with developed seepage channels, increasing the value of each hydraulic condition parameter could expand the channels and discharge particles, and improve permeability. For rock samples with a larger number of small pores, increasing each hydraulic condition parameter caused particles to crack under pressure, drove particles to block holes, and thus reduced permeability. In this experiment, the permeability parameter had a significant positive response to the proportion of pores larger than 0.1 µm and a significant negative response to the proportion of particles smaller than 150 µm.

Study on the seepage characteristics and influencing factors of low permeability reservoir based on microscopic seepage experiments in real sandstones-taking Dongsheng gas field in Hangjinqi area as an example

Taking the dense sandstone gas reservoirs of Shibang Group and Shanxi Group in Dongsheng area of Ordos Basin as an example, based on the characterization of the dense sandstone reservoirs in the study area in terms of petrological features, pore structure and diagenesis, we carried out real sandstone microscopic seepage experiments to analyze the gas-water seepage law and fluid storage state of different types of reservoirs, and systematically evaluated the efficiency of gas-water repulsion from the dense sandstone reservoirs and the microscopic influencing factors. The results show that the study area has strong non-homogeneity and different degrees of pore density, so a variety of fluid transportation channels are formed, and different transportation channels represent different types of evacuation. When the residual pore space and dense sandstone microporous space exist at the same time, gas and water are mainly transported along the residual pore space, and the microporous space of the dense particles almost does not enter; gas and water transport is very much affected by permeability, and the transport resistance will increase with the decrease of permeability; the pressure has a certain effect on the efficiency of the replacement, but it can only improve the efficiency of the replacement to a certain extent; the fluids in the process of rock transport, there will be varying degrees of bubbles stuck off In the process of fluid transport in the rock, there will be different degrees of bubble jamming phenomenon, resulting in the transport needs to overcome a large capillary resistance, which greatly increases the replacement pressure; for the low-permeability sandstones in Hangjinqi area, the good homogeneity of the pore structure of the conductor does not mean that the transport effect is good, in the area of the conductor pore space is strongly developed, the gas-water transport will form an advantageous channel, and its transport effect is better.

A new model for coal gas seepage based on fracture-pore fractal structure characteristics

Coal reservoir is a typical double pore medium, and the magnitude of its permeability is controlled by the parameters of the fracture pore structure. However, the relationship between fracture-pore parameters and permeability is not yet clear. Based on the fractal geometry theory, the fracture-pore morphology of coal body is characterized by tree bifurcation network and tortuous capillary bundle. This study also established a coal gas seepage flow model based on the fracture-pore fractal structure characteristics. The model includes the influence of coal structure parameters, different levels of original coal seam stress, and gas pressure on the gas seepage characteristics. Each parameter in the model has definite physical meaning and does not contain any empirical constants. Finally, the applicability of the model is evaluated by gas radial seepage experiment. It is shown that the gas permeability in the model is a function of fractal dimensions and structural parameters, and based on the sensitivity analysis of each parameter in the model, it is found that the tree diameter ratio β and length ratio γ of the bifurcation network have a significant impact on the permeability. When β increases from 0.1 to 0.9, the model permeability increases by 12 orders of magnitude. However, changes in the characteristic parameters of the pore structure have little effect on gas permeability. For dual porous media such as coal, the permeability of the fracture network dominates the overall permeability of the model, and small changes in the geometric structure of the fracture bifurcation network can lead to significant changes in permeability. The construction of this theoretical model provides a reliable basis for further improving coal-rock porous medium structure seepage models, as well as to better characterize the effect of gas drainage.

Interaction between Groundwater and Rock Fractures under Stress and Seepage Based on Extractive Water Resource Utilisation

In the fragile mining areas of western China, the contradiction between coal mining and water resource protection coexists prominently. Facing this issue, it is necessary to understand the dynamic relationship between mining-induced shear stress and groundwater seepage, as well as the fracture morphology of the overlying strata. In this study, the seepage characteristics of rough fractures within the rock mass are thoroughly investigated, with an emphasis on studying the effects of shear stress and seepage erosion. This research is carried out through seepage experiments and 3D fracture morphology scanning, and the research results are mainly as follows: (a) Rocks with higher shear strengths show less fluctuation in changes in fracture morphology and seepage velocity. (b) The permeability of sandstone and concrete fractures is inversely proportional to the shear stress. The layering of coal greatly limits seepage through the fractures and the permeability of coal is about one-tenth of the permeability of sandstone and concrete under the same conditions. (c) The mechanism of erosion damage to the rock by water-force coupling is the result of the shear and extrusion generated by rock fractures and the transport of rock particles. Changes in fracture erosion and seepage characteristics brought about by the damage will mutually promote and intensify the erosive effect of water force. (d) The degree of change in fracture morphology (fracture damage) under the same conditions is coal > concrete > sandstone. There are differences in the response of different types of rock to the shear stress–seepage erosion, but all show obvious changes in fracture damage and seepage characteristics.

Gas transport model in pore heterogeneous bedded salt rock: Implications for tightness evaluation of salt cavern gas storage

To address the gas flow behaviors in bedded salt rock, an apparent permeability model considering Knudsen diffusion, pore surface diffusion, and pore heterogeneity was presented. The evolutions of gas permeability and gas flow behaviors with pressure were hinted by a gas seepage experiment carried out for model verification. The results show that the new apparent permeability model could accurately invert the seepage behaviors in the cores of salt rock and interlayer, with an average error of 2.39% and 1.22% for helium and nitrogen, respectively. When the pore pressure is 0.5 MPa, the permeability contribution of Knudsen diffusion accounts for 82.35% of the apparent permeability. When the pore pressure is higher than 1.24 MPa, the permeability of Knudsen diffusion is lower than that of viscous flow. Once the gas pressure is higher than 2 MPa, the permeability contribution of Knudsen effect is basically equal to 0. A special focus was given to the different gases in salt cavern gas storage, which indicated that the leakage ratio of hydrogen is the highest, followed by methane, and helium is the lowest, with 7.27%, 5.76%, and 4.58%, respectively during the 30 years operation period. This work could add further insights into the gas flow behaviors in the pore heterogeneous salt rock and be employed for tightness evaluation of salt cavern gas storage and pillar width design of adjacent salt caverns.

Experimental study on the synergic effects of surfactants and nanoparticles on the seepage of water injected into coal seams

To study the synergic effects of surfactants and nanoparticles on the seepage of water injected into coal seams, we prepared three chip models with complex cracks using glass etching technology based on the real conformation of cracks in coal mass and set up a visual experimental platform for microscopic seepage. Based on these models, we carried out a series of seepage experiments on gas expelling with different solutions and investigated the effects of surfactants, nanoparticles, and flow velocities on seepage characteristics. The results showed that all tested anionic, non-ionic, and cationic surfactant solutions could reduce flow resistance, showing a downward trend with the rise of the mass fractions of these surfactants. The combination of nanoparticles with these surfactant solutions further reduced flow resistance, showing the best drag reduction effects with hydrophobic nanoparticles at all tested flow velocities. Moreover, all three tested surfactants could displace gas and reduce gas residue in the crack networks. The higher the mass fraction of the surfactant solution is, the more significant the gas displacement effect. Furthermore, the gas displacement effect of the surfactant solution is positively proportional to the flow velocity. The higher the flow velocity is, the more significant the gas displacement effect.

Study of fracturing fluid re-discharge based on percolation experiments and sampling tests – An example of Fuling shale gas Jiangdong block, China

Abstract Shale gas development requires the use of hydraulic fracturing, and the relationship between fracturing fluid drainage and production is not clear. Therefore, it is necessary to adopt the method of core experiment combined with engineering validation to achieve the description of the seepage-absorption-return mechanism of shale and to optimize the selection of fracturing fluids and the testing work system in engineering. In this study, a “seepage experiment → sampling test → engineering validation” working procedure is proposed, and it is found that seepage occurs only on the surface of the fracture where the liquid medium intrudes into the fracture and that the amount of water absorbed is directly proportional to the area of seepage; the rate of return is inversely proportional to the production rate in the same secondary tectonic unit; and the absorption rate per unit area of four types of cores with the same surface area is directly proportional to the yield of the fractured shale in the same medium. Under the premise of the same medium, the water absorption per unit area of the four types of cores varies with the rate of change with time, but the general trend is the same. Under the premise of different secondary tectonic units, when the time of good closure is similar, the correlation between the return rate and the test production is weak.

Influence Mechanism of Gas Pressure on Multiscale Dynamic Apparent Diffusion-Permeability of Coalbed Methane.

The permeability and diffusion coefficient of coal show multiscale characteristics due to the influence of multiscale pore sizes. The gas pressure will continuously decrease during the coalbed methane (CBM) extraction. However, there are contradictory perceptions in the effect of gas pressure on the diffusion coefficient and permeability. Therefore, it is essential to clarify the influence mechanism of gas pressure on multiscale diffusion-seepage. Diffusion-seepage experiments are carried out using particle coal and cylindrical coal without stress loading. Meanwhile, seepage experiments measured by the steady-state method are conducted under stress loading. The results show that the apparent diffusion coefficient is dynamically attenuated with time in the experiments of particle and cylindrical coal. A new model of multiscale dynamic apparent diffusion is proposed. The mechanism of gas flow in multiscale pores is elucidated. The multiscale pores determine the attenuation of the diffusivity and permeability of coal. The initial apparent permeability decreases and then increases with the increase of gas pressure, which is caused by the effect of gas pressure stretching and multiscale flow regime. Three patterns of permeability with gas pressure, monotonically increasing, monotonically decreasing, and U-shaped changes, will occur.

Non-linear seepage law and a characterization model of heavy oil-in-water emulsion in porous media

Stable emulsions with different continuous-phase viscosities, dispersed-phase volume fractions, and dispersed droplet sizes were prepared. Seepage experiments using heavy oil-in-water (O/W) emulsion were then conducted in sand packs to analyze the effects of different injection rates, permeabilities, and emulsion characteristics on the seepage law. Finally, an evaluation method for the seepage law was established, and a regression model that considered the factors influencing the non-linear seepage laws was developed using the 1stop software to quantitatively reveal the seepage mechanism. The matching relationship between droplet size and permeability determined the seepage behavior of the O/W emulsion. Emulsions with droplet sizes matched with permeability behaved as a pseudoplastic fluid, with a non-linear flow. With an increase in the dispersed-phase volume fraction, the emulsions displayed a more non-linear behavior due to the accumulated retention and disordered plugging of dispersed droplets. With an increase in the injection rate, the emulsion with a low continuous-phase viscosity had more significant non-linear characteristics than the emulsion with a high continuous-phase viscosity. The correlation of the characterization model was relatively strong under the experimental conditions, with a correlation coefficient of 93.28%. Therefore, the characterization model accurately predicted the effects of the non-linear seepage law for all factors studied.