Dynamic Permeability Evolution and Characterization of Cleat-Hydraulic Fractures in Deep Coal Under Stress-Hydrochemical Coupling

The cleat-hydraulic fracture system in deep coal is the main medium controlling the seepage capacity of coal, but under the combined action of high in situ stress and retained fracturing fluid, the permeability of the fracture system is significantly affected by stress and hydrochemical condition. Clarifying the dynamic seepage evolution of coal fractures under stress-hydrochemical coupling is therefore of great significance for optimizing the production schedule and achieving efficient development of deep coalbed methane (CBM) wells. In this study, the No. 8 coal of the Benxi Formation in the Ordos Basin was taken as the research object. Eight standard cylindrical plugs (Φ25 mm × 50 mm) were tested, including four intact samples representing the natural cleat system and four pre-fractured (self-propped) samples representing the cleat-hydraulic fracture system. Overburden helium permeability experiments were conducted at a pore pressure of 1.38 MPa under stepwise confining pressures of 3.45–24.14 MPa along three stress paths (first loading, secondary loading and stepwise unloading). The hydrochemical treatment is realized by soaking the samples in water for 24 h followed by air-drying for 12 h, after which the permeability tests were repeated. Finally, a dynamic permeability model was established on the basis of the experimental results, and the permeability characterization parameters of coal fractures under different conditions were determined. The results show that the permeability of all samples decreases exponentially with increasing confining pressure, and that the fracture type and cleat connectivity dominate the sample permeability. A single directly through-going fracture can increase the initial permeability of intact coal by 5–10 times and markedly reduce its decay rate. A sample with highly developed and well-connected cleats has an initial permeability as high as 32.36 × 10−3 μm2, i.e., 8–30 times higher than other pre-fractured samples. Secondary loading reduces the permeability at the same confining pressure by 20–40%, and after stress recovery the permeability only rebounds to 35–60% of the first-loading level, indicating that a large part of the fracture closure is irreversible plastic deformation. Hydrochemical treatment can improve fracture connectivity and enhance permeability, and the effect is jointly controlled by the degree of cleat development and mineral filling characteristics. For a sample with well-developed cleats filled with calcite, the permeability after treatment increases to 2.0–3.7 times the pre-treatment value and the stress sensitivity decreases markedly, indicating that dissolution of the filling calcite reopens and interconnects the originally cemented and sealed cleats and natural fractures, forming more continuous and closure-resistant seepage channels. By fitting the experimental data with the permeability model, the distribution of the fracture stress-sensitivity coefficient under different conditions was obtained. The main limitations of this study are that each test condition was performed on a single specimen without replicate tests, that the water chemistry and dissolved-ion concentrations during soaking were not measured, and that the model parameters were calibrated under specific loading conditions, so caution is required when extrapolating them to reservoir conditions. This study clarifies the dynamic permeability evolution of deep coal fractures and the parameters for predicting the permeability of coal fracture systems, providing an important reference for hydraulic fracturing and the optimized design of production schedules in gas wells.