A fast bidirectional pressure decay method for measuring permeability and porosity in tight rocks

Abstract

Permeability and porosity parameters of tight rocks are of crucial importance to the applications ranging from extraction of geothermal and hydrocarbon resources to subsurface storage of carbon, nuclear waste, and hydrogen. Separate measurements of permeability and porosity, however, cost time and can produce erroneous results. We designed a bidirectional pressure decay (BPD) experiment to assess the porosity and permeability at the same time. A pressure differential was applied between a gas chamber and a core sample’s two end surfaces. The pressure in chamber (p_c) was recorded with time until the pressure reached an equilibrium pressure (p_e). The porosity of the core sample was computed based on the gas mass amount in chamber and core sample. The mathematical model was derived for this BPD method. The exact solution of the model was validated with numerical results. The late-time solution shows that the difference between p_c and p_e has a linear correlation with time on a semi-logarithmic plot. The permeability was produced based on the slope of the linear regression. Applying confining pressures ranging from 12 MPa to 22 MPa, the BPD method was implemented for estimating the porosity and permeability of a shale sample. The results suggest that it can efficiently measure the stress-dependent porosity (2.23%–2.61%) and permeability (2.21×10⁻⁵–3.72×10⁻⁵ mD) of the sample. This technique has two main advantages: On the one hand, it is highly efficient due to the bidirectional flow pattern, allowing gas to enter the core through both end surfaces and drastically reducing experimental duration; On the other hand, measuring porosity and permeability from a single experiment is more straightforward and precise than testing them separately.