Conventional laboratory characterization of ultra-tight reservoir rock samples involves separate laboratory measurements on different core plugs or on crushed rock samples. Heterogeneity and anisotropy of ultra-tight reservoir samples adversely influence the laboratory correlation among various measured and estimated petrophysical properties.

We apply an inversion algorithm to simultaneously estimate the intrinsic permeability, effective porosity at ambient condition, pore-volume compressibility, and Klinkenberg-slip factor of an ultra-tight pyrophyllite sample from a single laboratory-based pressure-step-decay measurement. In doing so, we circumvent the effects of heterogeneity and account for the pore-pressure dependency of apparent permeability and effective porosity. Moreover, the time and resource requirements are drastically reduced in comparison to those required when performing multiple experiments that separately estimate the aforementioned petrophysical properties.

The inversion algorithm is valid for nitrogen injection pressure in the range of 50 to 500 psi. The algorithm assumes ideal-gas behavior of the injected nitrogen, 1D isothermal laminar gas flow, homogeneity of the core, pressure-independent gas viscosity, inverse-pressure dependence of the apparent permeability, pressure-independent Klinkenberg-slip factor, non-negligible pore-volume compressibility, pressure-dependent effective porosity, negligible inertial effects, negligibly small pressure gradients, and time-invariant confining pressure.

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. 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.

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