Laboratory pulse-decay techniques are effective for determining rock permeabilities ranging from 0.01 μd to 0.1 md. However, the measurement time is fairly long, reaching up to several hours to days when testing gas shales because their permeability can be as low as dozens of nano-Darcys. This paper proposes an alternative pulse-decay technique that offers accurate permeability measurements within a considerably short time frame.
A radial-flow pulse decay method with a new experimental design is developed, which employs two gas reservoirs of the same volume and two pressure pulses with the same magnitude, one "positive" and one "negative". A mathematical model is then developed to numerically verify the feasibility of the newly developed technique and analytically solved for permeability estimation. Pressure responses with time in the radial direction of the test sample are analyzed and compared with those in the axial direction by solving the mathematical model. A concentric circular cavity with a diameter of 0.51 cm is created along the core samples, which are tested to experimentally demonstrate the applicability of the technique.
The numerical results showed that the pressure pulse of the radial flow decays much faster than the axial flow and at least a 10-fold reduction in measurement time can be achieved. Numerical results indicated that the proposed technique is capable of eliminating the effects of the compressive storage and gas sorption in the permeability measurement; while great efforts need to be put to correct such effects when employing conventional transient techniques. An in-depth comparison is experimentally made between the proposed method and the conventional axial-flow method. The experimental results presented in the study confirm that fast and accurate measurements can be achieved using the proposed technique.