Coal Cleat Research Articles

Calculation of coal cleats spacing for methane degassing by image processing

Coal bed methane is one of the clean energy sources in the world. Methane molecules are confined inside the pores of coals and when the gas drainage wells are drilled into the coal seams, due to the resulted pressure difference. Coal cleats are spread all over the coal seam as face and butt cleats and play an essential role in the methane gas drainage operation from coal mines. Calculating the coal cleats spacing makes great help in modeling the amount of emitted gas followed by calculating the spacing of gas suction wells. Furthermore, the time required for gas drainage will be determined. Computer-based image processing technique has been utilized to identify the cleats spacing. In this study, three-dimensional computed tomography scan images were prepared for coal samples from Tabas Coal Mine in Iran. Computer-based image processing technology was conducted to determine the coal cleats. Their spacing was then calculated. Based on the results obtained using image processing with computer, the average distance between face cleats was 20 to 30 mm and the distance between butt cleats was 15 to 25 mm. The results of this study are used to modeling the amount of methane gas in the coal cleats. Based on this modeling, the amount of released gas and the distance between the wells can be designed.

Coal cleat network stimulation through a combination of acidizing and liquid nitrogen fracturing

Gas extraction from Coal Bed Methane has proved challenging mainly due to the low permeability of coals. This study investigated a combination of 2 stimulation techniques, Liquid Nitrogen (LN2) fracturing and acidizing, to increase pore structure and cleat network connectivity in coals. Microcomputed Tomography (μ-CT) scanning results suggested that a combination of those 2 applications, in either sequence of implementation, has substantial potential in recovering mineralized fractures and inducing new fractures; thus, leading to wide-open cleat networks. X-ray images show that if LN2 fracturing of coal is preceded by acidizing, the stimulation efficiency increases from more than double the initial value to over 700% of it, where a porosity of 12% can be achieved. Alternatively, the efficiency of acidizing in porosity enhancement can increase from 300% to 530% when a preceding application of LN2 freezing provides an open cleat network to facilitate acid access to remote areas. Acid diffusion and digital microscopy images also determined that in fractured coal, HCl concentrations of both 1% and 3% can result in desirable demineralization (13% porosity increment), whereas in the acidizing of original coals, 1% HCl might not be promising. Further, the latter experiments created a wide and connected cleat network after LN2 fracturing of efficiently acid-stimulated coal. Therefore, a combination of two enhanced recovery techniques, acidizing and LN2 fracturing, was used in this study to achieve a dramatically boosted cleat network in coals. Additionally, an analytical model was developed to predict the acid reaction rate with coal minerals; it had a good match with experimental results.

Study on Numerical Simulation of Gas–Water Two-Phase Micro-Seepage Considering Fluid–Solid Coupling in the Cleats of Coal Rocks

The increasing demand for natural gas energy will promote unconventional natural gas, such as coal seam gas and shale gas, to play a key role in future energy development. The mechanical properties of coal seams are weaker compared with conventional natural gas reservoirs. The fluid–solid coupling phenomenon exists widely at the pore scale and macro scale of coal seams, and runs through the whole process of coalbed gas exploitation. The objective of this study is to establish a microscale gas–water flow model for coalbed methane considering fluid structure coupling. Frist, this study used scanning electron microscopy (SEM) to obtain microscopic pore images of coal rocks. Then, we constructed a numerical model to simulate the movement of coalbed methane and water within the scale of coal cleats based on the Navier–Stokes equation, phase field method, and solid mechanics theory. Finally, we analyzed the effects of injection pressure and wettability on the microscopic two-phase seepage characteristics and displacement efficiency of coal. Our research shows that when the injection pressure is increased from 60 kPa to 120 kPa, the displacement completion time is shortened from 1.3 × 10−4 s to 7 × 10−5 s, and the time is doubled, resulting in a final gas saturation of 98%. The contact angle increases from 45° to 120°, and the final gas saturation increases from 0.871 to 0.992, an increase of 12.2%.

Experimental investigation on fracture growth for integrated hydraulic fracturing in multiple gas bearing formations

The key to extracting multiple natural gases from coalbed methane, shale and other tight gases through integrated hydraulic fracturing in coal measure strata (CMS) is determining whether hydraulic fracture can connect different gas-bearing zones. In this paper, several groups of combination samples comprised of artificial rocks were prepared. True triaxial hydraulic fracturing experiments were conducted to stimulate the hydraulic fracture propagation and penetration behavior. The effects of in-situ stress, injection rate, natural fracture and well type on fracture growth were studied. The main results were as follows: (1) Fracture geometries in CMS exhibited strong asymmetric, nonplanar extension and unilateral expansion features in the vertical plane. (2) Higher values of the vertical stress difference coefficient and injection rate led to larger vertical fracture extension and an increased likelihood of fracture connection to adjacent gas-bearing layers. (3) The behavior of the hydraulic fracture varied significantly between vertical and horizontal wells. In vertical wells, the main fracture presented a "+” shape and fully communicated natural coal cleats system, while in horizontal wells, fracture height growth was limited, resulting in fracture patterns that presented vertically as “H” or “T” shape due to the hydraulic fracture's tendency to turn direction when propagating to natural weak planes. The study found that the fracture vertical extension ability increased with rising vertical stress difference coefficient. (4) Natural fractures could induce fracture turning and branching, which improved fracture network complexity but also caused fracturing fluid loss and hydraulic energy dissipation. Therefore, choosing shale or coal layer with developed natural fractures as fracturing layer was unhelpful for integrated fracturing operation. Finally, higher fracturing fluid viscosity, injection rate and gradient sanding could enhance fracturing operation efficiency. These results could provide theoretical guidance for integrated fracturing operation in CMS.

Numerical simulation study on the effectiveness of temporary plugging and fracturing in deep coal seam to construct complex fracture network

The temporary plugging and fracturing (TPF) technique is an effective method for refracturing low-productive wells and obtaining complex fractures by fracturing in deep coal seams. In this work, the applicability of TPF technique and the growth law of diverting hydraulic fractures (HF) in high stress levels (HSL) were analyzed by numerical simulation, and feasible reservoir construction recommendations were proposed. To simulate the entire TPF process, a composite progressive modeling concept was introduced. The results indicated that the presence of coal cleats facilitated HF growth, particularly under low stress difference (LSD) and low stress level (LSL) conditions. During the TPF process, the initial diverting distance of HF increased from 2.26 m to 3.22 m as the stress difference (ΔS) decreased from 12 MPa to 2 MPa under HSL conditions. Additionally, the tortuosity of the HF increased from 24.67% to 65.94%, while deviation angle remained constant. Under LSL conditions, these three parameters showed a negatively correlation with ΔS (6 MPa–2 MPa), and their sensitivity to ΔS was greater than that observed under HSL conditions. Overall, the effectiveness of the TPF technique was weaker in deep seams compared to shallow ones. In deep seams with HSL conditions, a high stress difference (HSD) contributed to HF growth but lacked sufficient diverting ability, whereas the LSD condition showed the opposite trend. To enhance the effectiveness of the TPF technique, engineering measures such as higher liquid injection rate were necessary. Increasing the injection rate from 0.001 m3/s to 0.005 m3/s (ΔS = 10 MPa) improved the diverting deflection angle from 0° to 10.25° and the tortuosity from 30.62% to 35.48%. These improvements were more significant in LSD (ΔS = 6 MPa and 2 MPa), resulting in a ∼90 reduction in fracturing duration. The high-pressure fluid provided strong impetus for HF growth, and its alteration of the local stress field enhanced the induction of weak surface, thereby prompting the formation of complex fractures.

A three-dimensional feature extraction-based method for coal cleat characterization using X-ray μCT and its application to a Bowen Basin coal specimen

Cleats are the dominant micro-fracture network controlling the macro-mechanical behavior of coal. Improved understanding of the spatial characteristics of cleat networks is therefore important to the coal mining industry. Discrete fracture networks (DFNs) are increasingly used in engineering analyses to spatially model fractures at various scales. The reliability of coal DFNs largely depends on the confidence in the input cleat statistics. Estimates of these parameters can be made from image-based three-dimensional (3D) characterization of coal cleats using X-ray micro-computed tomography (μCT). One key step in this process, after cleat extraction, is the separation of individual cleats, without which the cleats are a connected network and statistics for different cleat sets cannot be measured. In this paper, a feature extraction-based image processing method is introduced to identify and separate distinct cleat groups from 3D X-ray μCT images. Kernels (filters) representing explicit cleat features of coal are built and cleat separation is successfully achieved by convolutional operations on 3D coal images. The new method is applied to a coal specimen with 80 mm in diameter and 100 mm in length acquired from an Anglo American Steelmaking Coal mine in the Bowen Basin, Queensland, Australia. It is demonstrated that the new method produces reliable cleat separation capable of defining individual cleats and preserving 3D topology after separation. Bedding-parallel fractures are also identified and separated, which has historically been challenging to delineate and rarely reported. A variety of cleat/fracture statistics is measured which not only can quantitatively characterize the cleat/fracture system but also can be used for DFN modeling. Finally, variability and heterogeneity with respect to the core axis are investigated. Significant heterogeneity is observed and suggests that the representative elementary volume (REV) of the cleat groups for engineering purposes may be a complex problem requiring careful consideration.

Three-dimensional cleat scale modelling of gas transport processes in deformable fractured coal reservoirs

To understand the flow processes in naturally fractured coal reservoirs, a 3D numerical model for coupled gas flow, adsorption and deformation at the scale of coal cleat and matrix blocks is presented in this study. A discrete fracture matrix (DFM) modelling approach has been adopted where flow patterns in fractures and matrices are described separately and explicitly. Different from previous studies in which constant diffusion coefficient, equilibrium adsorption and lumped deformation of matrix and fracture are assumed, in this study, adsorbed gases are treated as an independent phase and the mass exchange process between free phase and adsorbed phase is described using the Langmuir kinetic model. Different gas transport mechanisms in a porous coal matrix are considered for both phase gas transport. Particularly, an equivalent poroelastic continuum model is applied to represent deformation of fracture-matrix system, in which impacts of fracture deformation on the bulk matrix-fracture deformation is accounted for. The hybrid dimensional elements have been employed to discretize the governing equations where fractures are discretized using lower-dimensional interface elements. The accuracy of developed model is validated against experimental results collected from literatures. The simulation results indicate that the gas diffusion process in coal matrices is pressure dependent, surface diffusion of adsorbed gas can contribute to the bulk gas diffusion in coal matrices. Individual cleat initially exhibits a slight opening, followed by significant closure due to adsorption-induced swelling. Ignoring the effect of fracture on bulk deformation, the aperture change is overestimated.

Pore structure and flow properties of coal with continued bioconversion

Biogenic natural gas is estimated to be 10% of the total natural gas resource in the United States. Significant research advances have been made towards engineering ideal nutrients for optimizing microbial populations that enhance methane generation from coal in laboratory environments. To transition the technique from laboratory to field applications, it is essential to study the geomechanical and flow behavior of coal during the bioconversion process. This study presents the experimental and modeling studies used to estimate permeability changes of coal during continued bioconversion. Several important results are established in this study. First, the capacity of coal to generate methane from bioconversion is not affected by its size, powdered sample versus fractured core. Second, the coal cleat compressibility increases with bioconversion, while the absolute permeability decreases. Third, the rate of bioconversion depends directly on the cleat porosity of coal. Finally, we established that the permeability decreases because of irreducible water saturation before and after the bio-treatment, swelling of coal due to produced methane, CO2 and water, and generation of fines. We propose that irreducible water saturation and swelling would not adversely affect coal permeability in situ given that most US coals are water saturated to begin with. The overall results point to a promising prospect of field application of the technology.

Prospect Analysis of Paleocene Coalbed Methane: A Case Study of Hangu Formation, Trans-Indus Ranges, Pakistan

The methane trapped in the coal seams has emerged as an unconventional clean energy resource worldwide in this century. The proximate composition, ultimate content, cleat structure, porosity type, and pore structures are the debatable components for its trapping mechanism. Further, the brittle and ductile properties of minerals influence in the extraction of the methane from the coal seams. In this research, Coalbed methane prospect is demonstrated by analysing the Hangu Formation’s coal seam of Trans-Indus Ranges, Pakistan. This case study is helpful to find the occurrence, trapping ability, and methane extraction capacity in its cleat structures and pores. A number of samples were tested from the different coal mines in terms of these debatable components. The results indicate that the rank of studied coal is bituminous to subbituminous in which the carbon ratio, volatile matter, and sulfur contents are increasing with depth towards the south. The transitional connected face and butt cleats are partially filled with minerals and the intergranular, dissolved, and tissue pore types are also identified in it. It can be helpful for the occurrence, migration, and trapping of methane. Furthermore, the higher surface areas and cumulative pore volumes enhance the capacity of gas adsorption with depth in the study area. Moreover, the increasing brittle minerals in the coal composition towards the south can be helpful for the fracking of coal seams for economical gas extraction. It is suggested that this workflow can be implemented in any region with same coal rank and cleat types.

Influence of Coal Cleats on Conical Pick Cutting Process Based on PFC

With the rising mining mechanized level, the conical pick is widely used in mining machines. The structure and mechanical properties of rocks and minerals are important factors affecting the cutting characteristics of the conical pick. In this paper, how the coal cleats affect the cutting process of shearers was researched, which will be of great significance to mining technology design. After analyzing the cutting process of conical pick and the test of coal mechanical properties, a group of coal cutting models of conical pick was established based on the empirical cutting parameters, various cleat spacing, simulation of mechanical performance of coal specimens, profile of conical pick, and SRM (synthetic rock mass) method in PFC (Particle Flow Code). Then, the simulations of the cutting process were carried out. The simulation results show that lump coal chips begin to form and fall off after the conical pick cuts to 50°; the cleats near the conical pick grow to be more obvious fractures; the mean and maximum fluctuations of cutting force increase with the increase of face and butt cleat spacing; the lower the density of cleat is, the more the cracks are formed during the cutting process; the face cleat spacing has greater influence on the service life of conical pick than the butt cleat spacing. This study more vividly reflects the influence of cleats on coal cutting process of the conical pick and plays a guiding role in design of mining technology.

An improved capillary pressure model for coal seam gas reservoirs

Coal cleats are the main flow path for gas and water during coal seam gas (CSG) production, and the two-phase flow behaviour in coal cleats is sensitively affected by the capillary pressure. Cleat aperture is a key factor controlling the magnitude of capillary pressure. The deformation of coal cleats is significantly affected by the reservoir depressurization and desorption-induced matrix shrinkage, but there does not appear to be any existing capillary models explicitly considering this effect. In this work, an improved capillary pressure model is developed, the impact of the cleat aperture change on capillary pressure value and residual saturation is considered, which is a key contribution of this work compared to the existing models. The model is then extended to reservoir scale to understand the impact of cleat aperture change on capillary pressure and CSG production performance.The results show that the cleat aperture change can alter the capillary pressure and gas production rate considerably. Larger Young's modulus and Langmuir volumetric strain constant can lead to smaller capillary pressure and higher gas rate as the cleat aperture rise more. Higher Poisson's ratio delivers larger capillary pressure and smaller gas rate as it gives less cleat aperture increase. Influence of Langmuir pressure constant on capillary pressure and production rate transit varies along the production process due to its complex impact on the change of cleat aperture. For the two newly introduced parameters in improved model, the cleat geometry factor can reduce capillary pressure and raise the production rate, while a larger residual saturation fitting factor gives smaller capillary pressure and production rate. Sensitivity analyses are also conducted to evaluate the relative importance of various factors. The findings from this study can help better understanding the impact of cleat aperture change on capillary pressure during CSG production, and is expected to deliver more reliable production estimation.

Hydrogen Flooding of a Coal Core: Effect on Coal Swelling

AbstractHydrogen is a clean fuel which has the potential to drastically decarbonize the energy supply chain. However, hydrogen storage is currently a key challenge; one solution to this problem is hydrogen geo‐storage, with which very large quantities of H2 can be stored economically. Possible target formations are deep coal seams, and coal permeability is a key parameter which determines how fast H2 can be injected and withdrawn again. However, it is well known that gas injection into coal can lead to coal swelling, which drastically reduces permeability. We thus injected H2 gas into a coal core and measured dynamic permeability, while imaging the core via x‐ray micro‐tomography at reservoir conditions. Importantly, no changes in coal cleat morphology or permeability were observed. We conclude that H2 geo‐storage in deep coal seams is feasible from a fundamental petro‐physical perspective; this work thus aids in the large‐scale implementation of a hydrogen economy.

Gas-phase production equation for CBM reservoirs: Interaction between hydraulic fracturing and coal orthotropic feature

The efficient development of coalbed methane (CBM) reservoirs entails the accurate evaluation of production behavior, particularly for the gas phase performance. However, subjected to complexity arising from the coal inherent heterogeneous properties and dynamic gas-water two-phase flow, pursuit of high prediction accuracy remains challenging. Although massive excellent contributions have been devoted to addressing this issue, several key factors fail to be captured, and in-depth analysis is lacking, involving coal orthotropic feature, as well as associated interaction with hydraulic fractures. In light of the current knowledge status, the research has developed a robust gas-phase productivity equation for CBM wells, putting the emphasis on the coupling influence from coal orthotropic feature as well as hydraulic fracture. Also, gas-water two-phase flow mechanism is captured by incorporating the relationship between pressure and fluid saturation in coal cleat system. Moreover, traditional factors are considered as well, including gas desorption, stress dependence, matrix shrinkage. Numerical simulation is utilized to clarify the reliability of the proposed equation at the stable state, and desirable agreements can be achieved. Results show that (a) Augment effect, induced by enlarging intersection angle, on gas production rate is evident, and the calculation case suggests the optimized angle is 90°, requiring further investigations; (b) The inherent orthotropic feature acts the detrimental role for gas production performance, and fortunately hydraulic fracturing, treated with suitable intersection angle, is able to alleviate the influence; (c) Gas desorption ability lays the profound basis for evaluation of the CBM development, supposed to be selected as the prominent index for the favorable target development area.

Flow and heat transfer of nitrogen during liquid nitrogen fracturing in coalbed methane reservoirs

Liquid nitrogen (LN2) fracturing has the potential to induce complex fracture networks, avoid formation damage, and eliminate water consumption. However, the flow and heat transfer of nitrogen in fractures and its effect on the fracture generation during LN2 fracturing have not been studied, and the fracturing mechanism remains unclear. In this paper, the nitrogen flow during LN2 fracturing in a straight fracture was simulated. First, a 3D unsteady-state fluid flow and heat transfer model for LN2 fracturing in coalbed methane (CBM) reservoir was developed, which considered the phase transition of nitrogen, thermophysical properties variation of coal and the heat transfer between nitrogen and formation. This model was then validated against published analytical solutions. Subsequently, the model was applied to elucidate the phase distribution of nitrogen and its influence on fracture generation. Finally, the factors that affect the flow and heat transfer of nitrogen were analyzed. The results showed that the nitrogen at the fracture tip was in a supercritical state. Thermal stress had minor effects on the propagation of the main fracture. In addition, fracture aperture, injection velocity, reservoir temperature, injection fluid temperature, fracture propagation pressure and coal cleat porosity could affect the effectiveness of LN2 fracturing in a coal seam. The main findings of this study are the keys to the research of liquid nitrogen fracturing mechanisms in CBM reservoirs. • Nitrogen at the fracture tip is in supercritical state during the fracture propagation. • Thermal stress had minor effects on the propagation process of the main fracture. • LN2 injection parameters and coal cleat porosity could affect the heat transfer efficiency in LN2 fracturing in coal seam.

Coal cleat network evolution through liquid nitrogen freeze-thaw cycling

This study investigated the potential of liquid nitrogen (LN2) freeze–thaw process in coal fracturing, focusing on the effect of different freezing cycles. μ-Computed Tomography (μ-CT) images revealed a promising efficiency in cleat network evolution after three freezing cycles. The initial coal showed some fractures with the maximum opening of 15 μm, where the treated coal demonstrated several new fractures with the maximum opening of 10 μm, almost all of which were interconnected to the cleat network. The volume and length of the largest fracture more than doubled, 2.6 × 108 to 5.9 × 108 μm3 and 8 × 105 to 1.9 × 106 μm, due to interconnection with new fractures and isolated fractures. Connectivity analysis illustrated that the number of pores increased by 50% (92715 → 142650), where the number of interconnected pores almost doubled (42060 → 78905). The porosity of the coal also doubled from 0.6% to 1.2% based on μ-CT scan results. SEM along with μ-CT images highlighted more encouraging efficiency of second and third freezing cycles, particularly in terms of enhancing fractures interconnection. SEM images revealed the generation of a thoroughgoing fracture, which increased in aperture size through successive freeze–thaw cycles (17 μm to 48 μm) and extended a cleat network in the coal, particularly in latter cycles. Atomic Force Microscopy demonstrated an increase in the area roughness, which was in a direct relationship with freezing cycles. Mechanical properties analysis revealed more significant damage in the coal in the latter freezing cycles, being initially 3.49 GPa and decreasing to 2.81, 2.11 and 1.52 GPa through three freezing cycles. Finally, the permeability of the coal under 1000 kPa confining pressure increased from 0.035 mD to 0.18 mD, with larger increments in later cycles. Numerical permeability study ran in different directions, where resulted in the most promising enhancement in the Z direction (3.7 to 14 md, 277%).

A 3d Feature Extraction-Based Method for Coal Cleat Characterization Using X-Ray Μct and its Application to a Bowen Basin Coal Specimen

A 3d Feature Extraction-Based Method for Coal Cleat Characterization Using X-Ray Μct and its Application to a Bowen Basin Coal Specimen

Dissolution behaviour of different rank coals in L‐glutamic acid N, N‐diacetic acid chelating agent: Implications to enhance coalbed methane recovery by acid stimulation

AbstractAcid stimulation is a promising method in improving coal seam permeability by demineralization in coal cleats. However, using conventional acids such as HCl and mud acid (12 wt.% HCl + 3 wt.% HF) may cause clay breakage, migration, and precipitation, leading to formation damage. In this study, we investigated the effect of L‐glutamic acid N, N‐diacetic acid (GLDA) chelating agent treatment on the dissolution behaviour of lignite (Coal L) and anthracite (Coal A), collected from Er′Lian Basin and Qinshui Basin, China. Characteristics of the raw and acid treated coal samples, including mercury intrusion porosimetry (MIP) results as well as metal elements distribution analysis of reacted leachates, micromorphology, mineral composition, and mass change were taken into consideration. It was found that high‐rank Coal A is more suitable for chemical stimulation. For both Coal L and Coal A, 5 wt.% GLDA solution is preferred to avoid sticking reaction products. Total porosities of Coal L and Coal A were increased by 3.9 and 1.1 times, respectively. Based on the water chemistry analysis of the reacted solutions, we found that acidic condition is beneficial for clay minerals and carbonates dissolution, while alkaline condition is favourable for the dissolution of quartz. Besides, the dissolution of minerals can trigger the release of trace elements, which is a potential environmental risk that needs to be taken into consideration before its in‐situ application in coal fields.

On swelling stress–strain of coal and their interaction with external stress

Gas sorption and its induced volumetric strain can significantly influence gas flow in coal by controlling the change in the conductivity of coal cleats. A significant number of studies have been conducted to date to describe the coal swelling stress with sorbing gases and several theoretical models have been proposed to estimate swelling stresses. Despite these efforts, the accurate prediction of coal swelling stress remains a challenge hence a comprehensive theoretical and experimental investigation of swelling stress is required. This is particularly important when swelling stress of fractured coals develops under external loading. We, therefore, perform two sets of time-dependent diffusion and volumetric strain experiments under various stress conditions and gas pressures on coal specimens (a highly cleated and a dense sample) exposed to helium and carbon dioxide. These experiments are conducted to investigate i) the performance of commonly used theoretical models for swelling stress estimation, ii) the validity of thermodynamics swelling coupling coefficient defining the swelling stress and iii) the effect of external stress on coal volumetric strain response (hydromechanical versus swelling/shrinkage). Our analysis shows that the developed models based on non-equilibrium thermodynamics can predict both the evolution and the final swelling stress closely. We further show that the new definition of swelling coupling coefficient leads to greater performance of non-equilibrium thermodynamics-based models and further suggest a new experimental technique for characterizing this coupling coefficient. Finally, the results reveal that the effect of external stress on the hydromechanical volumetric strain of coal (during gas adsorption by unloading and desorption by loading) has a considerable hysteresis, especially at high pore pressures.

Relative permeability measurement of coal microchannels using advanced microchip technology

The gas–water flow in coal cleats is an important factor affecting coalbed methane production. However, difficulties are often encountered in the prediction of gas production. Limited understanding of two-phase flow behaviour in microchannels reduces prediction accuracy, often leading to overestimation. In this study, based on micro-computed tomography, advanced microchip technology is used to predict the optimal coal cleat in the polydimethylsiloxane chip. Gas and liquid injection experiments are carried out on the microchannels through the syringe pumps, as the two-phase flow morphology in the system can obtain flow information that cannot be provided by conventional laboratory measurements. The flow capacities of single-phase gas, water, and wetting liquid in the microchannels are compared, alongside determining the flow laws under different wettability and flow rates. The results show that in the single-phase fluid injection experiment, gas is more sensitive to the microchannel size effect than the liquid phase, and hydrophilic liquids flow readily. The gas–water relative permeability value in this study is significantly smaller than that obtained from conventional experiments, based on the steady-state method. This is a result of the gas–water forming a discontinuous plug flow in the microchannel, which produces higher resistance and reduces flow capacity. The occurrence of discontinuous flow regimes challenges the laminar flow assumption adopted in the steady-state method and suggests that conventional laboratory measurements of relative permeability measurements may misrepresent the physics of fluid flows in microchannels. The results also reveal that a hydrophilic environment and a suitable flow rate can increase the two-phase relative permeability. The results of this study enhance our understanding of and approach to core flooding measurement, with the differences in the relative permeability curves indicating that data uncertainty associated with the steady-state method is worthy of further research.

Characteristics of in situ stress and its influence on coal seam permeability in the Liupanshui Coalfield, Western Guizhou

AbstractThe current in situ stress regime is of great significance to the exploration and development of coalbed methane (CBM). In this study, the vertical stress (), maximum horizontal principal stress (), and minimum horizontal principal stress () in the Liupanshui Coalfield were studied. Variations of the maximum and minimum envelopes and trend lines of the lateral pressure coefficient () with depth were obtained, and the non‐monotonic decrease of permeability with burial depth was determined. On this basis, the effect of in situ stress on the coal reservoir permeability was evaluated. The results show that the average vertical stress gradient of the main coal seam in the Liupanshui Coalfield is 0.024 MPa/m. Generally, there are two stress regimes of > > (44.23%) and > > (46.15%); > > accounts for a smaller proportion (9.62%), and only occurs in relatively shallow coal seams (<622.85 m). In the range of the tested burial depths, for <600 m, > > is dominant; > > is dominant between 600 and 800 m; and between 800 and 980 m, the coal reservoir is affected by both > > and > > , representing a transition zone between the two stress states. With the increase of burial depth, the permeability of the coal reservoir shows a complex non‐monotonic decline, and the variation of the permeability of the coal reservoir differs under the influence of different stress regimes. Further analysis shows that under the background of stress regime transformation, permeability is mainly affected by the stress value, horizontal principal stress difference, stress regime, and coal cleat, indicating that in situ stress is the main controlling factor of coal reservoir permeability in the Liupanshui Coalfield.