Permeability evolution of fractured coal under proppant-embedded conditions

Deep coal reservoirs exist in high-temperature, high-pressure environments with low porosity and permeability. Hydraulic fracturing is the main permeability-enhancement method, and proppant embedding changes fracture morphology, flow, and compressibility. This study investigated temperature and effective stress effects on coal flow under proppant-embedded conditions. Comparative steady-state flow experiments were performed on unpropped and proppant-embedded coal samples. Temperature tests covered 308–348 K; gas pressure tests covered 0.5–1.5 MPa to examine effective stress effects on permeability and fracture compressibility. Hertz contact theory, thermal damage theory, and fracture compressibility theory were integrated to develop a fracture compressibility model and a permeability prediction model considering proppant embedding, validated against experimental data. Results show good agreement. Increasing temperature and external stress intensified fracture compression and reduced permeability. At 1.5 MPa external stress, increasing temperature from 308 to 348 K reduced permeability by 75.6% for unpropped coal but only 7.7% for proppant-embedded coal. At 0.5 MPa gas pressure, increasing external stress from 1.5 to 7.5 MPa reduced permeability by 96.2% for unpropped coal and only 11.3% for proppant-embedded coal. Proppants reduce temperature and stress sensitivity. Thermal damage sensitivity analysis showed coal thermal damage positively correlated with fracture compressibility, while proppant thermal damage was negatively correlated. These findings support fracturing optimization and efficient deep coalbed methane development.