Naigen Tan

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 ($R^2$) 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.