Investigation of Various Pressure Transient Techniques on Permeability Measurement of Unconventional Gas Reservoirs

An evaluation of fluid flow behavior in porous media is necessary considering its widespread applications in geo-engineering projects, such as reservoir productivity and hydraulic fracturing. In order to improve the understanding of gas flow characteristics in porous media, particularly permeability estimation of unconventional gas reservoirs, various pressure transient measurement techniques were numerically evaluated in this study based on the established mathematical models, closely representing each experimental design of these transient techniques. Given that both compressive storage and sorption capacity of unconventional reservoirs vary at different pressures, the practicability of the conventional pulse decay method (PDM), initially adopted by Brace et al. (J Geophys Res 73(6):2225-2236, 1968), was examined. The results showed that compressive storage and sorption effect are the limiting factors in the application of the conventional PDM for permeability measurement of sorptive rocks. Meanwhile, the degree to which these factors affect the accuracy of pressure response and permeability were investigated quantitatively. It was found that pulse size, rock porosity, gas compressibility and reservoir volumes are the main parameters influencing compressive storage and sorption effect, and thus the application of the conventional PDM. A comprehensive numerical investigation was also conducted on Metwally and Sondergeld's technique, a modified PDM, initially proposed for measuring low permeabilities of gas sands and shales. Considering limitations in the derived permeabilities of sorptive rocks when using the conventional PDM techniques and Metwally and Sondergeld's technique, an optimized PDM was then proposed by creating dual-pressure pulses to avoid the pressure disturbance due to compressive storage, and ad-/de- sorption during the course of measurement. To verify the feasibility of the optimized PDM, a suite of numerical investigation was carried out for various PDM techniques, and the pressure responses were also compared with calculated data obtained from the mathematical model and experimental results. Both numerically calculated and experimental results exhibit the practicality of the optimized PDM, providing a fast, accurate and reliable approach for permeability measurement of sorptive rocks.