Impact of fluoride on seepage ability of tight sandstone gas

Aiming at the phenomena such as prone water-phase trapping damage and gas-phase permeability reduction in tight sandstone gas reservoirs, a study was conducted on impacts of the rock-surface absorbing quaternary ammonium salt fluoride on gas percolation. Several experiments were carried out, such as wettability measurement, surface tension measurement, water-phase imbibition, drilling-fluid filtration and flow-back, gas-phase permeability recovery test, and etc. Atomic force microscopy was applied to characterize the fluoride adsorption morphology on rock surface. In combination with experiments, the solid surface energy and solid-liquid interfacial adsorption cohesive force were calculated to analyze the mechanism of fluoride interfacial modification to enhance gas percolation ability. The results show that fluoride is able to change the rock wettability from water wetting to gas wetting, effectively inhibit the water-phase capillary imbibition and improve the average water-phase flow-back rate from 46.6% to 85.2% in the displacement process, so as to upgrade the gas-phase permeability recovery rate from 60% to 80%. As a result of fluoride adsorption, the solid surface micro-roughness is increased with the reduction of solid surface energy and solid-liquid interfacial adsorption cohesive force, and thus the contact area between original solid surface and water phase is shrunken with the interaction weakened. It is beneficial for water phase to detach from surface boundness and further increase the flow-back ability of intrusive water phase, thus reducing the damage from water phase on gas permeability and improving the gas-phase percolation ability. ©, 2015, Science Press. All right reserved.