Gas permeability measurements from pressure pulse decay laboratory data using pseudo-pressure and pseudo-time transformations

Abstract

This paper presents a pseudo-pressure and pseudo-time straight-line approach to interpret laboratory pulse decay data in order to estimate rock core permeability using gas as the pore fluid. The implementation of the straight-line approach provides a practical method to estimate gas permeability from experimental data, as long as changes in gas viscosity and compressibility are negligible. On the other hand, pseudo-pressure and pseudo-time allow the transformation of the compressible flow equation from its highly nonlinear form to a quasi-linear partial differential equation, where changes in gas viscosity, gas compressibility and compressibility factor are accounted for. The purpose of this work is to combine both, pseudofunctions and the straight-line approach to estimate gas permeability from pressure pulse laboratory data with a more rigorous treatment of gas properties. Five pulse decay experiments were performed in Marcellus shale cores at pore pressures ranging from 130 to 700 psi in a triaxial cell to estimate permeability and porosity of ultra-low permeability cores. The experiments were made in an increasing order of equilibration pressure starting from 130 until 700 psi, and vertical and radial stresses kept constant at 1500 psi. Permeability estimates were compared against the P 2 -approach to show the validity of the proposed method at low gas pressures.

Image-based micro-continuum model for gas flow in organic-rich shale rock

“…The apparent permeability for shale has been a focus of many recent experimental and modeling work. Pressure pulse-decay method, a technique designed to measure permeability of tight rocks [51,52,54], has been used to measure the apparent permeability of shale core plugs [e.g., 53,55,56]. This is done by fitting a 1D model (similar to the 1D model in section 4.3.1 when non-Darcy effects, adsorption/desorption, and surface diffusion are not included) to the pressure curve from the dynamically changing flow due to the pressure pulse.…”