Study on evaluation of microscopic water locks and macroscopic water seals in low-permeability sandstone gas reservoirs

The problem of water coning into the sandstone gas reservoirs has become one of the major concerns in terms of productivity and increased operating costs. However, the microscopic and macroscopic mechanism of water coning in the phenomenon remains unclear. To evaluate the microscopic water locks and macroscopic water seals in low-permeability sandstone gas reservoirs, the evaluation models of microscopic water locks and macroscopic water seals in low-permeability sandstone gas reservoirs were established based on the critical pressure gradient and water phase relative permeability of the water locking in the reservoir. The impact of production displacement pressure difference and stress sensitivity on the degree of water lock damage was analyzed in this study. The feasibilities of the models were verified through the water lock damage experiments in the sandstone, and the results showed that the model results were in good agreement with the experimental results. The water locking effect is the result of the reduction in gas phase relative permeability and absolute permeability caused by stress sensitivity, and the increasing back-pressure differential can reduce the water saturation in the sealed area. The stress sensitivity of the rocks can enhance the microscopic water locking effect in gas reservoirs, which leads to the increasing damage rate of the water locking permeability. Moreover, the macroscopic water sealing evaluation illustrates that natural gas needs to overcome adsorption force, capillary pressure, water phase gravity, static friction force, and frictional resistance along the flow path to generate flow when it is sealed. As the length of the flow path increases, the critical pressure differential for sealing increases in a positive correlation, while the critical pressure differential for sealing increases in a quadratic relationship when the flow velocity increases.