Optimized pressure pulse-decay method for laboratory estimation of gas permeability of sorptive reservoirs: Part 1 - Background and numerical analysis

Analysis of transient fluid flow in porous media has several applications in geophysics/geotechnics and engineering. An in-depth evaluation of fluid flow behavior is critical in order to improve the understanding, and development of, laboratory measurement techniques in geo-mechanics, particularly permeability estimation. Given that considerable errors may be yielded in the derived permeabilities of sorptive rocks when using the conventional pressure-pulse decay method (PDM), this paper proposes a new experimental design, an optimized PDM, by creating dual pressure pulses to avoid the pressure disturbance due to compressive storage, ad-/de-sorption and thermal effect during the course of measurement. In this paper, the first of a two-part series, a mathematical model established for the conventional PDM was modified to closely represent the new experimental design, thereby extending the transient technique for permeability measurement of unconventional reservoir rocks, particularly ones with large sorption potential. Considering the complexity of analytical solutions of mathematical models, a comprehensive numerical investigation was established and a detailed comparison was carried out between the conventional and optimized PDMs. The results showed how the profiles of the decay curves change in space and time domain, and how pressure responses are influenced by various parameters in the experimental design for PDM. Combining the new design with numerical simulation results in an optimized technique for fast and accurate permeability measurement of sorptive rocks. In particular, Brace et al.'s solution, the most acknowledged and commonly-used permeability calculation method, was extended for permeability estimation of sorptive rocks. In Part 2, the experimental work under best-replicated in situ stress/strain conditions using the optimized method along with the numerical pressure response results obtained from computer simulation is presented. The results verified the practicality of the optimized PDM and the validity of Brace et al.'s solution for permeability measurement of sorptive rocks.