Spontaneous removal behavior of water phase trapping damage in tight sandstone gas reservoirs

Water phase trapping of tight gas reservoir caused by its high capillary pressure prevents gas reservoirs from being detected, precisely evaluated and economically developed. The present physical and chemical methods for removal of water phase trapping is seldom applied because that they might cause other damages. The experiments of N2 displacement to stimulate formation water tight sandstone core samples with the permeability of less than 0.1×10-3μm2, (0.1-0.3)×10-3μm2, and more than 0.3×10-3μm2 are conducted under the conditions of in situ stress and constant flow pressure difference or increased pressure differences to recognize the water flowback behavior in tight sandstone. The results indict that water saturation of core samples decreases gradually, and the lower the permeability of cores is, the lower the decrease rate of water saturation will be, and that the decrease rate of water saturation of core with the permeability of (0.1-0.3)×10-3μm2 displaced by increasing pressure difference is lower than that displaced by constant pressure difference within 150 hours, but the water flowback rate of core displaced by increasing pressure difference is higher. The analysis of results show that water phase trapping is severe. Pore throat structure, permeability and pressure gradient are the most important factors. The process of water flowback from cores includes two effects, water displaced by gas and water evaporation due to gas flow. If the pressure difference of water flowback increases properly, it will accelerate water evaporation effect when gas can flow through different sizes of pore throats.