Estimation of Pore-Network Characteristics and Irreducible Saturations in Wolfcamp and Eagle Ford Shales Using Low-Pressure-Nitrogen-Adsorption/Desorption-Isotherm Measurements

Abstract

Summary Pore-network characteristics, such as pore-size distribution (PSD), pore connectivity, and pore complexity, along with irreducible saturations in shales, are important petrophysical parameters for accurate estimation of absolute and relative permeability curves of various phases. We apply a method for estimation of these petrophysical parameters in shales by processing the low-pressure-nitrogen-adsorption/desorption (AD) measurements. The method uses effective-medium theory, percolation theory, and critical-path analysis (CPA) to quantify the transport properties of shales. The method has been applied to 35 samples of the Eagle Ford and Wolfcamp Shales with different compositions and from different maturity windows. Further, samples from the gas and oil windows of the Eagle Ford Shale Formation were low-temperature plasma ashed to study the effect of the removal of organic matter on pore-network characteristics and irreducible saturations. The estimated PSDs of condensate-window samples from the Wolfcamp samples are significantly different from those of Eagle Ford samples. Our interpretation methodology indicates that the Eagle Ford samples exhibit better long-range pore connectivity and lower pore complexity compared with Wolfcamp samples. Consequently, Eagle Ford samples from oil and gas windows suggest better flow capacity compared with Wolfcamp samples from the condensate window. Moreover, the pore-network characteristics of kerogen from the gas-window samples are significantly different from those of oil-window samples. The estimated irreducible saturations for the samples collected from the 100-ft interval in the Eagle Ford gas window, the 30-ft interval in the Eagle Ford oil window, and the 60-ft interval in the Wolfcamp condensate window of shale formations exhibit minimal variation with depth. The samples exhibit large variations in organic content, pore connectivity, range of connected-pore network, and pore complexity that do not affect the irreducible-saturation estimates.