Simultaneous determination of permeability and diffusivity subject to dynamic sorption in gas shales

Abstract

Gas storage and transport mechanisms are essential for the shale gas development, while the three critical parameters are fracture permeability, adsorption capacity, and diffusivity. However, significant errors on the measured permeability may occur due to the gas sorption in the pores during the lab tests, which also affects the in-situ pore pressure. In this paper, a new approach is proposed to simultaneously determine the permeability, adsorption capacity, and diffusivity based on a modified pressure transient technique in the shale formation. Results showed that the shale permeability would be overestimated by >27.3% for Barnett shale and 55.5% for Eagle Ford shale when ignoring the sorption effect. The factors leading to errors in the measured permeability is numerically investigated and it was found that the sorption effect on the measured permeability can be reduced by using short core samples. In addition, the volume of the reference cell should be at least 6 times larger than the pore volume of the sample in order to control the permeability error within 5%. This study provides a methodology which can be readily applied in simultaneous determination of gas permeability, gas storage, and gas diffusivity, and sheds lights on better understanding of the gas storage and transport mechanisms in gas shale reservoirs.