Optimized pressure pulse-decay method for laboratory estimation of gas permeability of sorptive reservoirs: Part 2-Experimental study

Since pressure measurements can be made more conveniently and accurately compared to fluid flowrate, the pressure transient technique is considered an advanced method for laboratory permeability measurement of unconventional gas reservoir rocks, specifically, gas shales, tight gas sand, and highly-stressed coalbed methane (CBM) reservoirs. However, two major factors, compressive storage and sorption effect, can lead to enormous error when testing sorptive rocks using the pulse-decay method (PDM). In order to overcome these, an optimized PDM experimental design of the transient technique for permeability testing of unconventional gas reservoir rocks was introduced in Part 1 of this two-part series. A modified mathematical model, describing one-dimensional fluid flow in porous media and best representing the optimized PDM design, was then presented to numerically investigate the fluid flow behavior and theoretically verify the applicability of Brace et al.' s solution in permeability calculation of sorptive rocks. To experimentally illustrate and verify the applicability of the optimized experimental design under best replicated in situ stress/strain conditions, a core of coal from San Juan basin was tested under triaxial stress. The experimental results showed that accurate and time-saving measurement can be achieved by choosing appropriate fluid reservoir volumes. Based on characterization of the pressure pulse decay plots, it was concluded that the effect of compressive storage on pressure variation in the up-/down-stream reservoirs and permeability calculation can be avoided when applying the optimized PDM. CO2 was used as the test fluid to investigate the effect of sorption on pressure variation due to its higher affinity towards coal than methane. The experimental results showed that sorption effect was eliminated, leading to accurate permeability values. A similar permeability trend with pressure drawdown, as previously measured for San Juan basin coal, was established. Also, comparison of the measured pressure decay plots and those obtained from computer simulation exhibited perfect matches, indicating that Brace et al.' s solution can be used for permeability estimation of sorptive rocks by adopting the optimized transient technique. (C) 2016 Elsevier Ltd. All rights reserved.