Fast permeability measurements of tight and sorptive gas reservoirs using a radial-flow transient technique

The pressure transient technique has been widely used for laboratory tests on unconventional gas reservoirs; however, it is still a time-consuming process when applying this technique to rocks of ultra-low permeability in the nano-Darcy scale. The applicability of a novel radial-flow transient technique to achieve fast and accurate permeability measurements is verified by an experimental investigation in this study. Pressure transient tests were performed on two hollow cylindrical shale specimens with different inner radii under radially convergent flow conditions; and for comparison, a solid shale core was also tested via an axial-flow transient technique. The results showed that the time required for a complete pressure pulse decay by the radial-flow test is within several hours compared to a few days by the axial-flow test. It was also found that non-uniform stress distribution along the radius of the sample is the principle factor resulting in errors in the measured permeability; and thus, samples with a small inner radius should be used to reduce the error in the measured permeability. In addition, analytical solutions are derived to interpret the lab data for permeability determination. The accuracy of the proposed technique is elucidated by the small permeability difference of less than 15% compared to the axial flow tests. The permeability error analysis indicated that the effect of compressive storage and gas sorption can be eliminated, and the error is less than 6.7% even though two large pressure pulses of 0.5 MPa were generated. The study experimentally validates that fast and accurate permeability measurements can be achieved by adopting the proposed radial-flow transient technique.