Hydraulic fracturing is an important method to enhance permeability in oil and gas exploitation projects and weaken hard roofs of coal seams to reduce dynamic disasters, for example, rock burst. It is necessary to fully understand the mechanism of the initiation, propagation, and coalescence of hydraulic fracture network (HFN) caused by fluid flow in rock formations. In this study, a coupled hydro-mechanical model was built based on synthetic rock mass (SRM) method to investigate the effects of natural fracture (NF) density on HFN propagation. Firstly, the geometrical structures of NF obtained from borehole images at the field scale were applied to the model. Secondly, the micro-parameters of the proposed model were validated against the interaction between NF and hydraulic fracture (HF) in physical experiments. Finally, a series of numerical simulations were performed to study the mechanism of HFN propagation. In addition, confining pressure ratio (CPR) and injection rate were also taken into consideration. The results suggested that the increase of NF density drives the growth of stimulated reservoir volume (SRV), concentration area of injection pressure (CAIP), and the number of cracks caused by NF. The number of tensile cracks caused by rock matrix decrease gradually with the increase of NF density, and the number of shear cracks caused by rock matrix are almost immune to the change of NF density. The propagation orientation of HFN and the breakdown pressure in rock formations are mainly controlled by CPR. Different injection rates would result in a relatively big difference in the gradient of injection pressure, but this difference would be gradually narrowed with the increase of NF density. Natural fracture density is the key factor that influences the percentages of different crack types in HFN, regardless of the value of CPR and injection rate. The proposed model may help predict HFN propagation and optimize fracturing treatment designs in fractured rock formations.

Hydraulic fracturing is a key technology for the development of unconventional resources such as shale gas. Due to the existence of numerous bedding planes, shale reservoirs can be considered typical anisotropic materials. In anisotropic shale reservoirs, the complex hydraulic fracture network (HFN) formed by the interaction of hydraulic fracture (HF) and bedding plane (BP) is the key to fracturing treatment. In this paper, considering the anisotropic angle, stress state and injection rate, a series of hydraulic fracturing experiments were conducted to investigate the effect of anisotropic characteristics of shale reservoirs on HFN formation. The results showed that the breakdown pressure increased first and then decreased when the anisotropic angle changed at 0°–90°, while the circumferential displacement had the opposite trend with a small difference. When θ = 0°, fracturing efficiency of shale specimens was much higher than that under other operating conditions. When θ ≤ 15°, the bedding-plane mode is ubiquitous in all shale reservoirs. While θ ranged from 30°–45°, a comprehensive propagation pattern of bedding-plane and crossing is presented. When θ ≥ 60°, the HFN pattern changes from comprehensive mode to crossing mode. The propagation pattern obtained from physical experiments were verified by theoretical analysis. The closure proportion of the circumferential displacement was the highest when the propagation pattern was the bedding-plane mode (θ ≤ 15°), following by crossing. The closure proportion was minimum only when the bedding-plane and crossing mode were simultaneously presented in the HFN. The results can provide some basic data for the design in hydraulic fracturing of tight oil/gas reservoirs.

Comprehending the water absorption process inherent to coal, including the associated spatial distribution patterns of water, proves indispensable in the design and evaluation of coal pillar dams in underground water reservoirs. To better understand this process, a series of NMR (nuclear magnetic resonance) tests were carried out on cylindrically shaped coal samples immersed in water for varying durations, with the upper and lower surfaces of the samples sealed. A method involving image digital processing and finite element simulation was used to quantitatively characterise the water absorption process, as well as the spatial distribution of water in the samples. The results showed that NMR imaging colour brightness differences were positively correlated with water content and that the wetted ring gradually increased in width as the water immersion time increased. The expectation and sum of squared deviations of the pixel greyscale values of the NMR images, which were used to characterise the water saturation and spatial distribution of the coal samples, represented positive and negative exponential functions of the water immersion time, respectively. This indicated that the water saturation gradually increased and became more uniformly distributed. Furthermore, based on the set threshold value of the target variable rate of change, the limiting expectation of the pixel greyscale values was obtained, and the limiting water absorption time of the coal sample was predicted. The water diffusion equation was then used to characterise the water absorption process of the coal samples, and a water diffusion model was developed to accurately obtain the wet ring boundary data. A reasonable value of the diffusion coefficient was determined by comparing and correcting the results of the numerical simulation and physical experiments with full consideration of the non-homogeneity of the numerical model. This water diffusion model can better characterise the water transport phenomena in the macroscopic barrier zone of coal pillar dams. Finally, the application prospects in terms of practical engineering were investigated.

As the key load-bearing structure of a coal mine underground reservoir, the coal pillar dam body is subjected to the long-term coupling effect of overburden stress and mine water erosion. To investigate the coupled mechanical-seepage evolution mechanism of coal-rock under mining-induced stress, triaxial compression-seepage tests were conducted on coal samples taken from the coal pillar dam body of the underground reservoir in Chahasu Coal Mine. The effects of confining pressure and stress loading path on the mechanical deformation and permeability evolution of coal-rock were analyzed, a damage-permeability model incorporating fracture compaction, damage-induced cracking, and matrix deformation was established, and sensitivity analysis of key parameters was performed. Additionally, the model’s predictive accuracy was verified against experimental data, and its overall characterization capability was further assessed. The results show that: (1) The peak axial stress, peak deviatoric stress and elastic modulus of coal samples increase exponentially with increasing confining pressure. A distinct inflection point is observed in the stress–strain curve under the unloading confining pressure-axial loading (UCP-AL) and constant confining pressure-axial loading (CCP-AL) paths, with the stress increasing at a markedly higher rate before the inflection point than after it. (2) Under different stress loading paths, the permeability of the coal samples first decreases, then slowly increases, and then rapidly increases with increasing axial strain, and it shows higher sensitivity to deviatoric stress. The minimum permeability exhibits an exponentially decreasing trend with increasing confining pressure. (3) A damage-permeability model incorporating a Logistic-type damage variable was developed to continuously characterize the permeability evolution of coal specimens over the entire loading process from elastic deformation to macroscopic failure. Furthermore, the key model parameters (ω, ζ, and τ) are strongly correlated with confining pressure. (4) The single-stress-parameter model exhibits limitations in characterizing permeability evolution under triaxial stress conditions. To address this deficiency, a multi-parameter modified damage-permeability model was established, in which the effective total stress is used to describe the fracture compaction effect, while the effective deviatoric stress accounts for the damage-induced cracking effect. These laboratory-scale findings provide an experimental and theoretical basis for understanding the stress–damage–seepage evolution of coal. Further upscaling and calibration using field monitoring data (piezometric and drainage measurements from underground reservoir) is required before its application in seepage prediction and stability evaluation of in situ coal pillar dams.