Petrophysical interpretation of laboratory pressure-step-decay measurements on ultra-tight rock samples. Part 1 - In the presence of only gas slippage

Parameter-estimation results based on the proposed inversion scheme are shown to be independent of the values of the initial guess for ki in the range of 1 nd to 10 μd, ϕ0 in the range of 0.01-0.20, Cp in the range of 10−2 to 10−6 psi−1, and b in the range of 0-200 psi. Estimated apparent permeability and effective porosity of the pyrophyllite samples exhibit strong pore-pressure dependence; consequently, both the properties vary substantially along the sample length during the pressure-step-decay measurement. The pyrophyllite samples studied in this work are assumed to be homogeneous. The numerical model of the pressure step decay measurement accounts for the variation in porosity and permeability along the length of core due to the time-dependent pore pressure variation during the experiment. On an average, the effective porosity of the pyrophyllite samples increased by 100% and the apparent permeability decreased by 75% when the pore pressure increased from 50 psi to 500 psi during the measurement. The average values ki, ϕ0, Cp, and b of the nine pyrophyllite samples were estimated to be 40 nd, 0.03, 2 × 10−3 psi−1, and 100 psi, respectively.