Comparative Study on Stress-Dependent Permeability of Ultra-low Permeability Sandstone Rock using Different Types of Fluid Media

During the production lifecycle of a reservoir, rock permeability may change due to the increase of the effective stress which could significantly affect well productivity. The main objective of this paper was to investigate stress-dependent permeability of ultra-low permeability reservoir rocks using different types of fluid media. Meanwhile, wettability and its effect on stresssensitivity permeability were also studied in this research. A total of twenty-seven sandstone rock samples selected from Daqing oilfield reservoir rock and Changqing outcrop rock was used to conducted permeability stress-sensitivity experiments. The gas permeability of those rock samples ranges from 0.0029mD to 7.7603mD. Experimental results indicate that gas permeability, brine permeability and oil effective permeability at irreducible water saturation all decrease with the increase of the effective stress. Most of the reduction of gas permeability and oil effective permeability takes place over the range of 2 to 16MPa effective stress. Comparative results show that reduction of brine permeability is larger than that of gas permeability over the same range of effective stress when the rock gas permeability is greater than 1mD. However, decrease of gas permeability is larger than that of brine permeability when rock gas permeability is less than 1mD. The oil effective permeability is more stress-sensitivity than gas and brine permeability. Reduction of the oil effective permeability is much greater with the increase of the effective stress when the rock samples have been aged in crude oil. This research suggested that it may lead significantly error when gas was used as the fluid media in evaluating permeabilitydependent of ultra-low permeability reservoir rock and its effect on well productivity. In addition, reservoir wettability should be considered when oil was used as the fluid media in permeability stress-sensitivity evaluation experiment.