Influence of Temperature and Stress on Water Injection Seepage Behavior in Coal Fracture Network

Abstract

Deep coal seams in China are generally characterized by high geothermal temperatures and high in situ stresses. Their coupled effects can alter fracture seepage conditions and promote gas-lock retention, thereby impairing water-injection seepage. However, the microscopic gas-lock evolution and the comparative seepage responses of different surfactants under coupled temperature–stress conditions remain insufficiently understood. Therefore, an analytical fracture-permeability model incorporating temperature–stress effects derived based on the fracture cubic law and deformation superposition, and a microscale visualization platform integrating temperature regulation, mechanical loading, fluid displacement, and real-time observation were developed using a fracture-network glass micromodel. Deionized water, sodium dodecyl sulfate (SDS), Triton X-100 (TX-100), and dodecyl dimethyl benzyl ammonium chloride (DDBAC) were employed as displacement fluids under different temperature, confining-pressure, and flow-rate conditions. The evolution of gas-lock proportion, gas-lock number, seepage pressure differential, and equivalent fracture permeability was quantified, and the temperature–confining pressure model was fitted to the experimental data. The results showed the following: (1) as the temperature increased from 25 °C to 40 °C and the confining pressure increased from 1 MPa to 3 MPa, the gas-lock proportions of deionized water and SDS increased by 41.7% and 38.9%, respectively, while their equivalent fracture permeabilities decreased by 59.3% and 54.1%; (2) in comparison, TX-100 and DDBAC showed smaller increases in gas-lock proportion of 13.2% and 15.6%, accompanied by permeability decreases of 22.5% and 25.8%, respectively; (3) under otherwise identical conditions, increasing the flow rate from 0.1 to 0.3 mL min−1 reduced the average gas-lock proportion by 17.4% and increased the equivalent fracture permeability by 23.9%, indicating enhanced liquid displacement and reduced gas-lock blockage; (4) TX-100 and DDBAC exhibited comparatively smaller variations in gas-lock retention and equivalent fracture permeability with increasing temperature and confining pressure, whereas SDS showed a stronger temperature-dependent response, which is associated with the combined effects of fluid properties, gas–liquid redistribution, and local interfacial trapping; and (5) the fitted models for the four injection media yielded coefficients of determination (R2) greater than 0.92 and mean absolute percentage errors (MAPE) below 8%, demonstrating good internal fitting consistency within the present dataset rather than independent predictive capability. Based on controlled microscale visualization experiments, this study clarifies the relative gas-lock and seepage responses of different injection media under the investigated temperature–confining pressure conditions and provides an experimental and theoretical basis for further studies on surfactant-enhanced water injection in fractured coal.