Gas Migration Mechanism Research Articles

Influence of Aspect Ratio of Migration Space on Gas Migration and Accumulation Mechanisms of Different Types of Gas Reservoirs

Influence of Aspect Ratio of Migration Space on Gas Migration and Accumulation Mechanisms of Different Types of Gas Reservoirs

Groundwater flow-controlled migration of dissolved microbial gas in the Eastern Hungarian Pannonian Basin

The formation of commercial microbial gas accumulations requires specific migration and accumulation conditions which can be effectively provided by groundwater flow systems. Namely, if microbial methane can be gathered and transported in aqueous solution over large lateral distances, great amounts of free gas can be exsolved where flows turn upward, causing a decrease in solubility. Based on this concept and the flow pattern of regional scale topography-driven groundwater flow systems, the present study built a simplistic 2D model in MS Excel including a methane solubility database with 32 000 data points. The model can calculate the required flow pathway length to saturate groundwater with methane, the amount of free gas that can be released in a potential accumulation zone and the necessary time interval for the whole process. Application of the model was demonstrated based on the geological and hydrogeological conditions of a study area in the Central Pannonian Basin (Hungary), focusing on the Hajdúszoboszló gas field. In light of the upscaled results of the 2D model, the known microbial gas accumulations of the study area could be charged according to the model. In addition, parameter sensitivity analysis also provided valuable insights into the complex mechanisms of microbial gas migration.

Mathematical Model of the Migration of the CO2-Multicomponent Gases in the Inorganic Nanopores of Shale

Nanopores in shale reservoirs refer to extremely small pores within the shale rock, categorised into inorganic and organic nanopores. Due to the differences in the hydrophilicity of the pore walls, the gas migration mechanisms vary significantly between inorganic and organic nanopores. By considering the impact of irreducible water and the variations in effective migration pathways caused by pore pressure and by superimposing the weights of different migration mechanisms, a mathematical model for the migration of CO2-multicomponent gases in inorganic nanopores of shale reservoirs has been established. The aim is to accurately clarify the migration laws of multi-component gases in shale inorganic nanopores. Additionally, this paper analyses the contributions of different migration mechanisms and studies the effects of various factors, such as pore pressure, pore size, component ratios, stress deformation, and water film thickness, on the apparent permeability of the multi-component gases in shale inorganic nanopores. The research results show that at high pressure and large pore size (pore pressure greater than 10 MPa, pore size greater than 4 nm), slippage flow dominates, while at low pressure and small pore size (pore pressure less than 10 MPa, pore size less than 4 nm), Knudsen diffusion dominates. With the increase of the stress deformation coefficient, the apparent permeability of gas gradually decreases. When the stress deformation coefficient is less than 0.05 MPa−1, the component ratio significantly impacts bulk apparent permeability. However, when the coefficient exceeds 0.05 MPa−1, this influence becomes negligible. The research results provide a theoretical basis and technical support for accurately predicting shale gas productivity, enhancing shale gas recovery, and improving CO2 storage efficiency.

Disaster-causing mechanisms of gas migration under loading and unloading conditions

Disaster-causing mechanisms of gas migration under loading and unloading conditions

The roles of different migration mechanisms in the transport of H2O–CH4 gas mixture in shales

Studying the roles of different migration mechanisms in shale matrix gas transport is beneficial for shale gas extraction and CO2 sequestration. The roles of different transport mechanisms in shale H2O–CH4 gas mixture transport were investigated considering their contributions to fluid flux. A permeability model coupling multiple gas migration mechanisms was developed by combining the pore network-related effective medium approximation, which was validated using process data from simultaneous H2O–CH4 transport in shales. The results showed that the gas transport mechanisms dominating methane and water vapor migration in shale differed. Slip flow and Knudsen diffusion contributed approximately 99% to methane migration. Water vapor surface diffusion as well as slip flow and Knudsen diffusion primarily contributed to the water vapor permeability before and after a time point, respectively. Water vapor and methane permeability generally decreased with time. The equilibrium water vapor and methane permeabilities ranged from 2.7699×10−22–12.312×10−22 m2 and 0.2640×10−21–4.9036×10−21 m2, respectively. The methane surface diffusion permeability increased slowly with time before approaching methane monolayer adsorption (before ∼100 s). An increase in relative humidity (approximately 300–3600 s) can promote the transport of water vapor. Water vapor and methane adsorption, especially the condensation of adsorbed water, could largely explain the decrease in shale gas permeability.

The effect of structural preservation conditions on pore structure of marine shale reservoir: a case study of the Wufeng-Longmaxi Formation shale, Southern Sichuan Basin, China

The marine shale within the Sichuan Basin constitutes China’s significant shale gas production, featuring old formation age, high degree of thermal evolution, multiple tectonic movements, and complex structural conditions. However, there are significant differences in the shale gas preservation conditions and reservoir quality in different areas, limiting future large-scale exploration and development. Pore structure significantly influences shale reservoir quality, gas content, and exploration of shale gas occurrence, migration, and enrichment mechanisms. The influence of structural-dominated preservation conditions on shale pore structures is essential to comprehend for effective shale gas exploitation. This study employs field-emission scanning electron microscopy in conjunction with other techniques (low-temperature N2 adsorption, low-temperature CO2 adsorption, and nuclear magnetic resonance) for detailed analyses of the pore structure across varied structural zones, revealing the influence of structural attributes, fault systems, depth of burial, and formation pressure on pore architecture, and examining the relationship between pore structure and shale gas preservation conditions. The results show that stable structural condition is conducive to the development and preservation of shale pores. Structural compression causes inorganic and organic pores to become narrow and elongated due to shrinkage, with a significant increase in microfractures. The porosity of shale with stable structural conditions exhibits markedly increased porosity compared to samples under structural compressions. Under conditions of similar TOC and mineral composition, the pore size distribution (PSD), pore volume (PV), and specific surface area (SSA) of shale after structural compression are significantly lower than those of samples with stable structural conditions. As the burial depth increases, the shale porosity shows a decreasing trend, but the decrease is limited. Burial depth significantly impacts the SSA and PV of high-TOC samples (3%–6%). As the burial depth increases, both SSA and PV show a significant decreasing trend. When the burial depth reaches 4000 m, SSA and PV tend to concentrate. The formation pressure coefficient is an important factor for the development and preservation of shale pores, and porosity is positively correlated with the formation pressure coefficient. Increased formation pressure coefficient indicates superior preservation conditions and enhanced pore development.

Preliminary studies on radon concentration in CO2-rich mineral waters of the Elbrus area (Caucasus, Russia)

The geological and geochemical conditions as well as radon concentrations of ten manifestations of СО2-rich waters in the Elbrus area are considered in this paper. Also, content of uranium and thorium were measured in waters and in host rocks. The data were compared with chemical and isotopic composition of associated gases. It is established that water bearing rocks are classified into rocks of low and normal radioactivity. The contents of uranium (<1.37 μg/L) and thorium (<0.3 μg/L) in the water are low. Measurements of radon concentrations in water and gas flux of mineral springs in the Elbrus area indicate that most of them are weakly radioactive (6–33 Bq/L, 25-350 Bq/m3, respectively). The CO2-rich mineral waters of the Tyrnyauz exhibit the highest level of radioactivity (33 Bq/L) within the Elbrus area. Isotopical gas studies have shown that within the Elbrus region there is a connection with the deep horizons of the Earth's crust, from which various fluids (water and gases) can rise to the surface. Although Rn values are low and cannot be used for medical purposes, natural objects with similar characteristics can be used to observe the state of the geological environment (e.g. earthquakes) and as natural laboratories to establish the mechanisms of deep gas migration to the surface.

Molecular dynamics simulation of micro mechanisms of gas and water migration and diffusion in shale reservoirs

Clarifying the fluid transport mechanism in nanoporous shale media for shale gas extraction is crucial. In this study, the microscopic mechanism of gas and water migration and diffusion in the nanopores of shale clay minerals is obtained through molecular dynamics simulation. Because of the different compositions and structures of different clay minerals in shale, the adsorption heats of different clay mineral surfaces with CH4 and H2O molecules are different. As the pressure increases, the adsorption heat of the clay mineral surfaces with CH4 and H2O molecules increases. At the same temperature and pressure, the adsorption heat of the clay mineral surfaces with H2O molecules is greater than that with CH4 molecules, and the high-energy adsorption sites on the surface of clay minerals are mainly occupied by H2O molecules, leading to H2O molecules significantly aggregating on the shale clay mineral surfaces, while CH4 molecules are dispersed in an disordered manner in shale clay minerals. The higher the pressure or the lower the temperature is in shale formations, the higher is the degree of aggregation of H2O molecules on the surface of clay minerals. The migration speed and fluidity of gas and water increase with the increase in temperature and decrease with the increase in pressure. At the same temperature and pressure, the migration speed of CH4 molecules is greater than that of H2O molecules. This is because H2O molecules are polar and have stronger intermolecular forces compared to CH4 molecules, resulting in weaker mobility of H2O molecules. The diffusion coefficient of CH4 molecules in kaolinite is the highest, followed by that in illite, and that in montmorillonite is the lowest. When the diffusion coefficient of H2O molecules in shale clay minerals is lower, the corresponding diffusion coefficient of CH4 molecules is higher.

Faults and gas chimneys jointly dominate the gas hydrate accumulation in the Shenhu Area, northern South China Sea

Gas hydrates possess significant potential as an energy resource and exert a notable influence on global climate change. The Shenhu Area is one of the globally recognized focal points for gas hydrate research, and additional investigation is required to fully comprehend its gas migration mechanism. By utilizing the most recent core-log-seismic data and gas geochemical data, a comprehensive analysis was conducted to determine the influence of gas migration pathways on gas hydrate accumulation in the study area. This study investigated the various types of gas migration pathways, employing integrated geological models that incorporate faults and gas chimneys to understand their respective contributions to the accumulation of gas hydrates. Based on these findings and drilling constraints, a three-gas combined production model was subsequently proposed. Thermogenic gas, secondary microbial gas, and in situ microbial gas are all potential sources of the gas responsible for hydrate formation. Thermogenic gas plays a significant role in the gas hydrate system, as evidenced by distinct features of late-mature thermogenic gas observed in gas samples extracted from hydrates in Well W18. In the study area, the primary conduits for gas migration encompass deep faults, branch faults, and gas chimneys. Among these, deep faults act as the most crucial pathways of thermogenic gas migration. The integration of geological models that incorporating deep faults and gas chimneys has profoundly impacted the accumulation of gas hydrates in the Shenhu Area, consequently influencing the distribution of shallow gas and gas hydrate. Furthermore, the proposed three-gas combined production model, which involves the simultaneous extraction of deep gas reservoirs, shallow gas reservoirs, and gas hydrates, holds significant implications for exploring and developing deep-water natural gas resources. However, its successful implementation necessitates interdisciplinary collaboration among scientists.

Pore connectivity and microfracture characteristics of Longmaxi shale in the Fuling gas field: Insights from mercury intrusion capillary pressure analysis

Assessing the connectivity of pores and the characteristics of microfractures is essential for comprehending gas migration mechanisms and effectively implementing development strategies in deep shale gas plays. The Fuling gas field, a prominent shale gas reservoir in China, holds promise for deep shale development. This requires a better understanding of variations in pore connectivity among shale samples from different depths of the Fuling gas field. To achieve this goal, this study employs innovative methodologies based on mercury intrusion capillary pressure (MICP), a recognized technique for evaluating material pore structures. In this research, we conducted MICP tests on shale samples of varying sizes to examine variations in pore structure and quantify inaccessible pores. Additionally, repeated MICP tests were performed on the same sample to investigate the spatial distribution of residual mercury and distinguish different pore types associated with flow conduits and storage spaces. Furthermore, a directional MICP experiment was carried out to quantitatively characterize the degree of pore development and anisotropy of microfractures by considering the direction of mercury intruding the sample (parallel or perpendicular to lamination). Our findings revealed that deep shale samples exhibited a higher proportion of inaccessible pores compared to mid-deep ones. Regarding accessible pores, a greater proportion of connected pores was observed in the deep shale samples, while the mid-deep shale samples exhibited a lower proportion. The results from the directional MICP experiment indicated that samples with anisotropy in mercury intrusion volume possessed finer and denser microfractures. However, no evidence was found to correlate the ratio of dead-end and connected pores with sample anisotropy. Overall, this study presents a novel approach for experimentally evaluating pore connectivity and microfracture characteristics in shale reservoirs using MICP. The outcomes contribute to enhancing our understanding of shale gas migration mechanisms, particularly in deep ultra-low-permeability reservoirs, and hold significant implications for future developments in the field.

A coupled fluid-mechanical interaction model for controlled gas migration mechanism by dilatancy effect in saturated bentonite

A gas breakthrough in saturated bentonite is relevant to the safety of high-level radioactive waste repositories. The study of gas transport mechanisms in saturated bentonite is very important for the safety assessment of repositories. This paper proposed a coupled fluid-mechanical interaction model for predicting and simulating the path of gas transport and gas breakthrough in saturated Gaomiaozi bentonite. The model considered the effect of deformation and damage of bentonite on its permeability and introduced pore pressure into the deformation equation of bentonite. The damage coefficient was also introduced into the permeability evolution equation by combining the Mohr–Coulomb criterion, the maximum tensile stress criterion and the damage evolution. In addition, considering the heterogeneity of the soil, the Weibull distribution function was introduced to assign differential values to material parameters of the cells in the model. The numerical simulation of the bentonite stress field and seepage field was realized by the joint MATLAB and COMSOL secondary development, and the evolution law of the pore path in bentonite was explored under a flexible boundary. The gas breakthrough pressure and permeability pressures were calculated at various gas injection from a gas injection experiment into bentonite with flexible boundaries. Finally, the rationality and applicability of the model were verified by comparing the numerically calculated gas breakthrough pressure and permeability with experimental values.

Generation, accumulation, and distribution of Upper Paleozoic tight sandstone gas in the northeastern margin of the Ordos Basin

Tight sandstone gas in the Upper Paleozoic strata of the northeastern margin of the Ordos Basin exhibits remarkable exploration potential. The objective of this study is to characterize the generation, accumulation, and distribution of Upper Paleozoic tight sandstone gas in the Linxing–Shenfu gas fields located on the northeastern margin of the Ordos Basin. This study uses the following approaches: (1) porosimetry and permeability testing; (2) stable carbon isotope analysis; (3) organic geochemical examination; (4) gold tube pyrolysis experiments; (5) basin modeling; (6) fluid inclusion analysis; and (7) drilling and seismic data analysis. Reservoirs are classified based on the distance between the sandstone traps and source rocks: in Type I reservoirs, the sandstone traps are interbedded with the source rocks; in Type II reservoirs, the sandstone traps immediately overlie the source rocks; and in Type III reservoirs, the sandstone traps are located far from the source rocks. The three types of gas-bearing reservoirs in the Linxing–Shenfu gas fields formed between 190 Ma and 96 Ma (Early Jurassic to Late Cretaceous), and gas accumulation in the three types of reservoirs occurred during the same period. A pressure differential was created between the source rocks and neighboring sandstones as a response to gas generation, and this was the primary mechanism of gas migration and accumulation in the Type I and Type II reservoirs. The initiation and growth of the vertical faults enabled the gas to migrate from the deep source rocks along faults, eventually accumulating in the Type III reservoirs; additionally, the gas migration and accumulation were primarily driven by buoyancy. Type I reservoirs with high gas saturation was mainly distributed in regions with gas expulsion intensities greater than 7 × 108 m3/km2 and located away from the faults. Type II reservoirs with high gas saturation were predominantly distributed in regions with gas expulsion intensities greater than 10 × 108 m3/km2. Type III reservoirs with gas production capacity primarily distributed in conventional sandstone units with high reservoir quality near the faults.

Tight sandstone gas accumulation mechanisms and sweet spot prediction, Triassic Xujiahe Formation, Sichuan Basin, China

The prediction of continental tight sandstone gas sweet spots is an obstacle during tight sandstone gas exploration. In this work, the classic physical fluid charging experimental equipment is improved, the combination of the gas migration and accumulation process with the pore network numerical simulation method is investigated, and application of the permeability/porosity ratio is proposed to predict the gas saturation and sweet spots of continental formations. The results show that (1) as the charging pressure increases, the permeability of the reservoir increases because more narrow pore throats are displaced in the percolation process; and (2) based on pore network numerical simulation and theoretical analysis, the natural gas migration and accumulation mechanisms are revealed. The gas saturation of tight sandstone rock is controlled by the gas charging pressure and dynamic percolation characteristics. (3) The ratio of permeability/porosity and fluid charging pressure is proposed to predict the gas saturation of the formation. The ratio is verified in a pilot and proven to be applicable and practical. This work highlights the tight sandstone gas migration and accumulation mechanisms and narrows the gap among microscale physical experiments, numerical simulation research, and field applications.

A model for describing advective and diffusive gas transport through initially saturated bentonite with consideration of temperature

Understanding the gas migration mechanism in an initially saturated bentonite is of great importance for evaluation of the long-term performance of a deep geological repository for disposal of high-level radioactive wastes. Previous researches usually neglect the gas diffusion process, but adopted the advective gas flow theory (visco-capillary flow or dilatant gas flow) to describe the gas transport in the initially water-saturated bentonite clays. In this study, gas migration process was regarded as results of a combination of diffusive transport of dissolved gas through pore water and advective flow along the gas pathways formed in the bentonite matrix. Meanwhile, dilation of the gas flow pathways with variation of the external stress and gas pressure during the advective flow was also considered. Based on this concept, a new model was proposed to describe the advective and diffusive gas transports with consideration of temperature. Validation was conducted using the gas injection tests performed under different boundary conditions in literature. Comparisons show that the diffusion and advection-controlled process provided by the proposed model can effectively simulate gas transport in the compacted GMZ bentonite. For gas migration under lower injection pressures, the contributions of advective gas flow to the gas flux are comparable to that of diffusion. With the increase of the injection pressure, the advective gas fluxes would gradually dominate over the diffusive transport. Meanwhile, predicted degree of saturation of the compacted bentonite is maintained at above 99% during the whole gas migration process, indicating that advective movement of gas was highly discrete in initially saturated bentonite.

Shale gas transport model considering gas adsorption and desorption

The gas migration mechanism and apparent permeability in shale reservoirs are significantly different from those in conventional gas reservoirs, which is mainly caused by nanoscale phenomena and organic matter as gas storage and supply media. However, in shale reservoirs, gas flow behavior plays an important role in well performance, so it is necessary to develop a new apparent permeability model considering gas transport mechanism. Therefore, in order to study the change of matrix permeability under different pressures in the development process of shale adsorption layer, combined with previous studies, considering the seepage mechanism of stress sensitivity, real gas effect and adsorption, a new model of apparent permeability is created by combining multiple gas transmission mechanisms. The sensitivity analysis of the new model is carried out by changing the corresponding parameters, and the corresponding conclusions are drawn. In this work, the established model can accurately calculate the apparent permeability of viscous flow, Knudsen diffusion and desorption, which makes us have a more accurate understanding of the transmission mechanism of shale gas and contributes to the efficient and sustainable development of shale gas.

CT-based 3D pore-fracture network analysis of volcanic reservoirs of Lower Cretaceous Yingcheng formation in southern Songliao Basin, China: Impact on natural gas migration

Volcanic rocks as an important part of basins are found in almost all basins in the world. Although volcanic rock reservoirs are denser and more heterogeneous than conventional reservoirs, they still have good storage capacity in the deeper hydrocarbon-bearing basins. Due to the presence of a large number of irregular and random pores, throats and fractures in volcanic reservoirs at micron-nano scale, its permeability and reservoir capacity are complex. A series of experiments show an abnormal negative correlation between porosity, permeability and natural gas diffusion coefficient in volcanic reservoir rocks of the Yingcheng Formation in the southern Songliao Basin. The dacite sample from CS105 well in Changling Fault Depression and the andesite sample from DS9 well in Dehui Fault Depression with this negative correlation phenomenon were selected, the micron-nano CT scanning technology and Avizo software are applied to quantitively evaluate the 3D pore-fracture network system of the volcanic reservoir samples at multiscale. By comparing the statistical results from CT scanning of the two samples, the dacite sample of CS105 well with higher diffusion coefficient has higher porosity, more connected pores, larger average pore volume and higher average coordination numbers, in addition, the micro-fractures in the CS105 sample promotes the development of secondary pore-throats through later dissolution and further improve the physical properties of volcanic rock reservoirs. In the meantime, the gas seepage simulation results also prove the positive correlation between porosity, permeability and diffusion coefficient of volcanic rock reservoir. The micron-nano CT scanning can more accurately evaluate the physical properties of volcanic rock reservoirs with a large number of pore-throats and fractures at the micron-nano scale, providing theoretical basis for the study of natural gas migration mechanism in highly heterogeneous tight volcanic rock reservoirs.

Study on Gas Migration Mechanism and Multi-Borehole Spacing Optimization in Coal under Negative Pressure Extraction

In order to study the gas migration in gas-bearing coal, and reasonably arrange gas drainage boreholes to improve the efficiency of gas drainage, a gas-solid coupling model is established based on the pore-fracture dual medium porous model. The solid deformation of coal body, gas seepage and diffusion, and gas adsorption and desorption are considered in this model. The COMSOL software is used to simulate the gas change in the coal matrix and coal fracture under single borehole extraction. We analyze the effective extraction range and study the migration mechanism of gas between coal fracture and borehole, coal matrix and coal fracture, and coal matrix. The effective extraction area of multi-borehole negative pressure gas extraction varies with extraction time and borehole spacing. At 140 d, the effective extraction radius is r = 1.3 m, and the spacing of boreholes is 233 r=1.5 m, 2 r=2.6 m,4 m,5 m,and 6 m, respectively. The influence of the equilateral triangle shape of three boreholes on the gas extraction effect is studied. The simulation results show that when three boreholes are extracted for 140 days under different borehole spacing, different gas extraction effects will be affected by a superposition effect. Considering the change in gas pressure, the effect of gas extraction in the effective extraction area, and the safety and cost performance of gas extraction, it is concluded that the optimal hole spacing is 5 m around 140 d. This study aims to provide reference for underground gas drilling layout and reasonable hole spacing.

Research progress on isotopic fractionation in the process of shale gas/coalbed methane migration

The research progress of isotopic fractionation in the process of shale gas/coalbed methane migration has been reviewed from three aspects: characteristics and influencing factors, mechanism and quantitative characterization model, and geological application. It is found that the isotopic fractionation during the complete production of shale gas/coalbed methane shows a four-stage characteristic of “stable-lighter-heavier-lighter again”, which is related to the complex gas migration modes in the pores of shale/coal. The gas migration mechanisms in shale/coal include seepage, diffusion, and adsorption/desorption. Among them, seepage driven by pressure difference does not induce isotopic fractionation, while diffusion and adsorption/desorption lead to significant isotope fractionation. The existing characterization models of isotopic fractionation include diffusion fractionation model, diffusion-adsorption/desorption coupled model, and multi-scale and multi-mechanism coupled model. Results of model calculations show that the isotopic fractionation during natural gas migration is mainly controlled by pore structure, adsorption capacity, and initial/boundary conditions of the reservoir rock. So far, the isotope fractionation model has been successfully used to evaluate critical parameters, such as gas-in-place content and ratio of adsorbed/free gas in shale/coal etc. Furthermore, it has shown promising application potential in production status identification and decline trend prediction of gas well. Future research should focus on: (1) the co-evolution of carbon and hydrogen isotopes of different components during natural gas migration, (2) the characterization of isotopic fractionation during the whole process of gas generation-expulsion-migration-accumulation-dispersion, and (3) quantitative characterization of isotopic fractionation during natural gas migration in complex pore-fracture systems and its application.

Research on the gas migration trend and mechanism of the transition flow regime in coal based on MRT-LBM simulation

In order to reveal the process and mechanism of gas flow in a low-permeability coal seam, a new multiple-relaxation-time lattice Boltzmann method (MRT-LBM) model of gas migration in coal micro/nanopores based on Langmuir monolayer adsorption theory, the slip boundary scheme and Bosanquet effective viscosity was established. The software MATLAB was used to carry out the simulation study of uniform pore gas flow based on the MRT-LBM model, and the results were compared and verified with the porous anodic alumina membrane gas flow experimental results. On this basis, the gas flow in coal pores with different micro/nanopore sizes considering adsorption was simulated. The results show that the dimensionless permeability coefficient increases with decreasing pore size under the same pressure, which reflects the subsequent enhancement of the microboundary constraint effect and reveals that the pore system becomes the main controlling factor of coal seam permeability within the coal matrix in the middle and late stages of coal seam gas extraction, while the role of the microboundary constraint effect needs to be considered. The gas adsorption layer weakens the pore gas flow capacity, but for pores with a radius greater than 16 nm, the apparent change in permeability caused by the adsorption layer is less than 5%, and the adsorption effect can be ignored. N2, CH4, and CO2 enter the transition flow regime under different pressure conditions; with gas extraction, the gas pressure decreases, and the difference in the gas flow characteristics of the three gases increases.

Study on Pore Structure of Shale Reservoir by Low Temperature Nitrogen Adsorption Method

Shale gas is one of the most actively explored unconventional sources of natural gas. There are several types of pores in the shale reservoir, and their structural characteristics are complex. Evaluating the characteristics of the shale pore structure is the basis for understanding the shale reservoir performance and the oil and gas migration mechanism. In this paper, the micro- and nanopores of shale samples from the Yanchang Formation, Hunan basin, are studied by applying the low-temperature nitrogen adsorption method. Several pore structure parameters such as the specific surface area, the pore volume, and the pore size distribution of the analyzed samples are calculated. The predominant characteristics of nanopores that influence hydrocarbon accumulation and control the development of pores are discussed. The results indicate that the shale reservoir rocks from the study area are mainly mesoporous with an average pore size distribution between 2 and 50 nm. A small number of micropores (less than 2 nm) and macropores (more than 50 nm) are also present in the pore network. The development of micropores and mesopores in the shale samples is associated with organic matter content, while the development of macropores is linked to clay minerals content. Total organic carbon (TOC) content is the key element to control the nanopore volume and the specific surface area of the reservoir rocks from the study area. At the same time, the organic matter-enriched pore network also provides extensive space for the accumulation of shale gas.