Relative Permeability and Residual Saturation Estimates for Organic-Rich Shale Samples from Bakken, Wolfcamp, Eagle Ford and Woodford Formations
Shiv Prakash Ojha, Siddharth Misra, Ankita Sinha, Son Dang, Ali Tinni, Carl Sondergeld and Chandra Rai, The University of Oklahoma
Relative permeability and residual saturations of organic-rich samples were determined from low-pressure nitrogen adsorption-desorption measurements on the samples. Robustness of this relatively new estimation technique for shale-reservoir samples is evaluated in this study. Petrophysical correlations of relative permeability and residual saturations with kerogen maturity, kerogen content, kerogen removal, and sample cleaning are developed for purposes of improved understanding of the saturation-dependent transport behavior of shale reservoirs. These correlations enable comparative predictions of intra-well and inter-well production performances.
Adsorption-desorption isotherm measurements were performed on 100 organic-rich shale samples from Bakken, Wolfcamp, Eagle Ford and Woodford formations. Total organic content varied in the range of 1 to 9 wt.% and the porosity varied in the range of 1 to 10 %. The adsorption-desorption measurements were interpreted to obtain pore size distribution (PSD) using a modified Barrett-Joyner-Halenda method. Bimodal fractal model was then applied to the PSD estimates to compute certain percolation and fractal parameters, which were implemented into percolation theory, effective medium theory and critical path analysis models to generate the relative permeability curves and residual saturations of the 100 samples.
Kerogen maturity was found to be the dominant factor affecting the relative permeability and residual saturation estimates. Irreducible water saturation and residual hydrocarbon saturation estimates increased from 30% to 60% and 20% to 40%, respectively, with an increase in kerogen maturity from oil window to late-condensate window. This implies a sharper decline in hydrocarbon rates (lower cumulative hydrocarbon production) and a prolonged period with lower water rates (lower cumulative water production) for shale reservoirs with higher kerogen maturity. Estimates of relative permeability of hydrocarbon and aqueous phases at 80% saturation of the corresponding phases increased from 0.7 to 0.75 and 0.05 to 0.2, respectively, with the decrease in kerogen maturity. This can be explained by decrease in relative contribution of micropores to total porosity with decrease in kerogen maturity. Higher mean pore size for samples with lower thermal maturity leads to better saturation-dependent transport properties in these samples. When the samples were cleaned with toluene-methanol mixture to remove soluble, dead hydrocarbons and bitumen from the samples, the hydrocarbon phase residual saturation estimates decreased by 20%, whereas the aqueous phase irreducible saturation estimates increased by 9%. Simultaneously, cleaning resulted in decrease in wetting phase relative permeability with increase in non-wetting phase relative permeability.