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Investigations of the directional permeability evolution of intact and fractured coal are conducted under different simulated geological conditions. The effects of fracture geometry, water adsorption and stress conditions on the permeability evolution of coal as a function of stress are systematically studied. The results indicate that permeability anisotropy is more pronounced in fractured coal than in intact coal. The permeability order, i.e., the kfa > kbu > kbe relationship, is maintained following the introduction of macrofractures into coal. The fracture compressibility in the butt cleat plane flow direction is higher than that in the other two flow directions for both intact and fractured coal. The presence of water in coal can reduce the permeability by up to one order of magnitude, and a more significant permeability decrease is observed in coal specimens containing rough macrofractures. Permeability hysteresis for both intact and fractured coal is somewhat dependent on the stress condition. The hysteresis effect of coal is more significant under triaxial stress conditions and less pronounced under true-triaxial stress conditions.

It is important to understand the gas flow properties in coal and sandstone under different stress conditions for underground carbon dioxide storage and coal bed gas drainage in underground engineering. In an actual project, the rock layers are complex and varied, and the in situ stress presents a significant three-dimensional anisotropy (σ1 > σ2 ≠ σ3). Although a large amount of data on the permeability of coal have been reported previously, studies on the permeability of layered composite coal rock under true triaxial stress conditions are lacking. In this study, the permeability of layered composite coal rock under true triaxial stress conditions was measured by using a true triaxial apparatus. Under experimental stress conditions, the permeability always decreased as the principal stress increased. The thickness changes in coal seams and sandstone formations have major influences on the evolution of composite coal rock permeability. Furthermore, the normalized permeability increases with the thickness ratio of sandstone to coal. We analyzed the stress state of layered composite coal rock by considering the interaction of coal and sandstone under true triaxial stress conditions. Based on the anisotropy consideration of coal rock and the stress difference of composite coal rock masses, a permeability model for layered composite coal rock under true triaxial stress condition was proposed (TCP model: layered composite coal rock permeability model). It was found that this model can well reflect the permeability characteristics of composite coal rock under true triaxial stress conditions while simultaneously considering the effect of the three principal stresses and the mutual strain restraint effects of coal and sandstone.

Coal belongs to sedimentary rock with obvious bedding planes . The existence of bedding weakens the continuity and integrity of coal. Deformation and permeability anisotropy of coals, to a certain extent, is attributed to the bedding planes. We performed experiments of deformation and permeability evolution of coals under true triaxial stress conditions. The experimental results indicate that the volumetric strain and bedding jointly affect permeability. An analytical model for stress-strain relationship of coals containing beddings is developed to quantify the bedding effects under true triaxial stress conditions. In the model, the deformation and permeability of the bedding and non-bedding are separated through bedding ratio and bedding elastic modulus . The bedding ratio is defined as the ratio of the total length of bedding to the total length of coal normal to bedding planes, which is a parameter closely related to the porosity. The elastic modulus is divided into four parts: a bedding elastic modulus and a non-bedding elastic modulus in the direction normal to bedding planes, and two elastic moduli in the direction parallel to bedding planes. The four elastic moduli are calculated by the analytical model. The bedding elastic modulus is relatively small, which shows that the bedding effect cannot be ignored. The permeability is highly sensitive to the bedding of coal with low stress in the direction normal to the bedding. The bedding ratio always affects the volumetric strain. The permeability of the coal is divided into bedding and non-bedding permeability. The ratio of the bedding permeability to the total permeability is different due to different stresses in the direction normal to bedding planes, which is linking with the compression of bedding. A permeability model which can reflect the different effects of beddings and non-beddings to the total permeability is proposed and verified.

Hydraulic fracturing to enhance permeability of tight reservoirs is meaningful but difficult in constructing of complex fracture network. This study aimed to examine the effectiveness of tight porous reservoir stimulation in the context of hydraulic fracturing to increase the fracture complexity as much as possible. The simulation experiments were performed to reasonably establish the relation between the laboratory results and the field fracturing operations. Large size stratified and intact samples were casted to resist the size and boundary effects by mixing river sand and/or pulverized coal with concrete cement and freshwater in specific proportions. The samples with artificial slot orienting were casted to understand the fracture initiation, orientation, and deflection mechanisms considering the influence of caprock structure and re-fracturing without temporary plugging. The samples were tested under the scaled geostress conditions of corresponding in-situ stress with a true triaxial loading system, and the reasonable dimensionless groups derived from the literature. Consequently, this study proposed an asymmetric fracturing method based on the experimental results considering the scaling structure of overlying-reservoir-underlying in porous reservoirs. The local asymmetric fracturing is a type of reservoir stimulation method with small scale high-pressure fluids such as carbon dioxide，liquid nitrogen, water, etc., jet cutting to create slot and perforation for the aid of fracture initiation and orientation. Hydraulic fractures were influenced by the fracture-induced stress shadow and ratio of the maximum to minimum horizontal stress. The validation and comparative results showed that, compared with the re-fracturing, the asymmetric fracturing significantly increased the complexity of fracture network. Although the applicability of the asymmetric fracturing depending on the corresponding engineering pilots, the derived method could hopefully address the properties of the strata structure of the reservoirs efficiently. Therefore, it has a high potential for application in the fracturing of local underground coal seam and special-trap type oil gas reservoirs to construct fracture network.

Because of the low or ultra-low permeability of coal seams, hydraulic fracturing technology has been widely used to improve the permeability of coal seams in both ground and underground extraction. Recently, liquid carbon dioxide (L-CO2) as an environment-friendly fracturing fluid has drawn attention from researchers and engineers. In this study, a true tri-axial system was adopted to perform hydraulic and L-CO2 fracturing experiments on coal specimens from the South Sichuan Basin in China. The effects of geological and engineering factors (horizontal stress ratio, injection rate, and fracturing fluid) on induced fracture propagation were investigated in this study. The various geological and engineering factors can synthetically, rather than unilaterally, impact the fractures propagation to generate three types of stimulation mechanism. Based on our experimental results, we succeeded in approximately predicting the stimulation mechanism for induced fracturing under different conditions. Generally, the effect of pre-existing natural fractures on the fracture propagation was enhanced, whereas the effect of the geo-stress was reduced, when L-CO2 was adopted as the fracturing fluid.

Recently, the use of CO2 as a treatment fluid in the petroleum and gas industry as well as enhanced geothermal system projects has attracted the attention of many scholars. In this paper, hydrofracturing experiments were performed on shale and coal specimens employing CO2 and water as fracturing fluids, respectively. The differences of hydraulic and L-CO2 fracturing in the breakdown pressure and fracture features were investigated under different geological and engineering conditions. The experimental results indicate that preexisting flaws in reservoirs have a significant effect on the stimulation mechanism of the hydrofracturing treatment. The non-reactivation of preexisting flaws leads to tensile stimulation. In contrast, the reactivation of preexisting flaws might lead to shear stimulation under a high horizontal stress difference. The shear stimulation would impact fracture propagation, and can even induce the slip of slant fractures, that generates splay cracks at the fracture tips. Because shear slip requires a high horizontal stress difference, which reduces the conductivity of reservoirs. Comparing with hydraulic fracturing, the low viscosity of CO2 is beneficial for shear stimulation.

It is critical to understand the gas flow behavior in coal under a reservoir stress condition for coal bed methane production, underground coal mining, and CO2-sequestration in deep coal seams. With respect to coal seams, the in-situ stress is anisotropic and generally exists under true triaxial stress (σ1 > σ2 > σ3) conditions. Additionally, the flow channels determining the permeability of coal are also anisotropic. This dual anisotropy produces difficulties in replicating the gas transport characteristics of coal at the laboratory scale, and there is a paucity of relevant studies. In this study, we performed a series of permeability measurements using cubic anthracite coal samples and changing the principal stresses and flow directions under various true triaxial stress conditions. The coal permeability exhibited greater anisotropy in the vertical direction as a result of the presence of minerals in cleats across the bedding plane. After each principal stress compression at a differential stress of 20 MPa, the permeability in each direction decreased by an order of magnitude. With an increase in the intermediate stress parameter, the permeability values of two horizontal cleats experienced higher decreasing rates compared with the vertical bedding permeability. This increased the significance of the horizontal permeability anisotropy. With respect to the true triaxial stress condition with a higher horizontal principal stress (σH > σh > σv), a higher permeability reduction was observed during the principal stress loading period. The butt cleat plane was more sensitive to changes in the principal stress because of the lower connectivity of the flow channels induced by the closure of the face cleat that acted as a cross-linked pathway. The anisotropic permeability data measured under true triaxial stress conditions were well expressed by an exponential equation containing different mean cleat compressibility and stress terms. The cleat compressibility values in different directions were obtained by data fitting.

Although the production mechanism of CO2-enhanced coalbed methane (ECBM) recovery has been previously reported and corresponding models have been proposed, investigations on the effect of the injection pressure on the production efficiency considering the mass transfer between the coal fracture and matrix are lacking. In this study, laboratory core flooding experiments were conducted on the coal sample from South Sichuan Basin, China. Experimental results indicate that earlier breakthroughs and a higher initial CH4 flow rate and cumulative CH4 flow rate occurred under a higher injection pressure in the CO2-ECBM recovery. However, as CO2 was continuously injected, the CH4 flow rate and cumulative CH4 flow rate under a lower injection pressure exceeded those at the higher injection pressure. Furthermore, a dual porosity model was used to perform numerical simulations. The simulation results under four injection pressures matched the experimental results well. The simulation results indicate that the dispersion in fractures and the mass transfer rate between matrices and fractures significantly affect the CH4 flow rate. The injection pressure leads to changes in the experimental observations by affecting the dispersion and the mass transfer rate.

The permeability anisotropy of shales can be attributed largely to bedding planes. It is critical to study the bedding effect on permeability evolution for shales. We conducted an experiment on shale permeability anisotropy under true triaxial stress conditions. Even though the mean stress of shales was different, the permeability still exhibits obvious bedding dependent. In this study, we present a new permeability model considering bedding planes. The introduction of characteristic functions and parameters characterizes the transformation process from bedding to non-bedding during compression. Compared with the experimental data, we obtained a good fitting result. Owing to the increase in the resistance of the pressurized gas into the fracture system, Biot's coefficient α decreases with increasing mean stress. The porosity sensitivity exponent s parallel to the bedding planes are larger than those perpendicular to the bedding planes. In our opinion, owing to the smaller stiffness of the bedding plane, the relatively large amount of pore deformation is caused by the loading on the vertical bedding planes. Furthermore, we introduce η to characterize the effect of non-bedding and bedding on permeability. The permeability of shale or bedding is more sensitive at lower stress than at higher stress and the fluid flow through the non-bedding in shales cannot be ignored.