Relative Permeability and Production-Performance Estimations for Bakken, Wolfcamp, Eagle Ford, and Woodford Shale Formations

Abstract

Summary Relative permeability and residual water/gas saturations of organic-rich samples were determined from low-pressure nitrogen-adsorption/desorption measurements on shale samples. Adsorption/desorption isotherm measurements were performed on 100 organic-rich shale samples from the Bakken, Wolfcamp, Eagle Ford, and Woodford Formations, and were then interpreted to obtain pore-size distribution (PSD) using a modified Barrett-Joyner-Halenda (BJH) method (Barrett et al. 1951). A bimodal-fractal model was then applied to the PSD estimates to compute percolation and fractal parameters, which were incorporated in percolation theory (PT), effective-medium theory (EMT), and critical-path analysis (CPA) models to generate the relative permeability curves and residual saturations of the 100 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, and kerogen removal through sample cleaning are developed for improved understanding of the saturation-dependent transport behavior of shale reservoirs. These correlations enable comparative predictions of intrawell- and interwell-production performance. 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 maturity from oil window to late-condensate window. 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. When the samples were cleaned with a 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%. The estimated relative permeability curves allow us to predict the production performance using a numerical simulator. Lower Bakken and Wolfcamp oil formations have the best hydrocarbon-transport characteristics because of lower thermal maturity. These formations show slower decline in hydrocarbon-production rates and more-uniform increase in water production. Hydrocarbon-phase production of the Lower Bakken oil formation is better than that of the Wolfcamp oil formation, but the water-production trend is significantly reversed. On the contrary, Wolfcamp and Woodford late-condensate formations exhibit the poorest transport characteristics and lower cumulative hydrocarbon and water production because of higher thermal maturity. Wells in these higher-maturity formations should show steeper decline in hydrocarbon-production rates and faster and sudden increase in water production after longer periods of sustained lower water production.