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, pseudo-functions 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 P2-approach to show the validity of the proposed method at low gas pressures.
Highlights
Laboratory measurements of permeability in tight and ultra-tight rocks are of great importance in view of increased interest in the characterization of unconventional oil and gas reservoirs
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
Complex pore pressure oscillations method is similar to the pore pressure oscillations methods, but instead of sending a sinusoidal pore pressure pulse from the upstream, different pore pressure pulses that are more distinct are sent (Boitnott 1997)
Summary
Laboratory measurements of permeability in tight and ultra-tight rocks are of great importance in view of increased interest in the characterization of unconventional oil and gas reservoirs. The data from the pulse decay experiment can be manipulated in two different ways: a straight-line method (Brace et al 1968; Dicker and Smits 1988; Jones 1997) and type curve matching (Hsieh et al 1981; Neuzil et al 1981; Haskett et al 1988; Kamath et al 1992). The only method that attempted to apply the pseudo-pressure approach (Haskett et al 1988) has some limitations Those limitations, as mentioned previously, had to do with converting the difference in pressure between the upstream and downstream to pseudo-pressures and the significant amount of work for the type curve matching. A pseudo-pressure and pseudo-time straight-line approach is proposed to resolve the treatment of gas viscosity and compressibility in the estimation of gas permeability from pulse decay experiments. The solutions for dimensionless pseudo-pressures for upstream and downstream are given in Eqs. (11) and (12), which were first developed by Hsieh et al (1981) and Haskett et al (1988) adapted them for the pseudo-pressure approach
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