Diagenetic Capillary Heterogeneity Influences Multiphase Flow, Enhanced Oil Recovery, and CO2 Storage in a Depleted Brownfield Reservoir

Pennsylvanian Morrow Sandstones, part of the Southwest Regional Partnership on Carbon Sequestration's (SWP) Farnsworth Unit CO₂ injection project in the Texas Panhandle, USA, have been targets of decades of enhanced oil recovery with both water and CO₂ flooding. We investigate CO₂-brine relative permeability in core obtained through the SWP at both in situ residual oil saturation and cleaned via Dean-Stark extractions. SEM, optical microscopy, laser scanning confocal microscopy, and mercury porosimetry show that all core contain abundant diagenetic microporosity, and that most residual oil resides within the microporosity, commonly associated with abundant kaolinite and illite-filled pores. We classify the pore heterogeneity within the Morrow-B unit at well 13-10A in terms of five hydraulic flow units based on mercury porosimetry, gas permeability, and porosity of cleaned core. Using dual focused ion/scanning electron beam and micro-CT analysis, we quantity properties and three dimensional distributions of macro- and microporosity in the five units, and show via pore network modeling how micropore distribution, and not just extent of heterogeneity, controls effective stress-dependent absolute permeability and flow paths in and around hydrocarbon-containing micropores. A parallel experimental effort involves co-injection of brine and super-critical CO₂ into core at in situ conditions. Tests at capillary numbers near the viscous limit yield brine curves that follow a Corey-type relative permeability curve, whereas CO₂ curves during drainage do not. CO₂-flooding at similar injection rates near irreducible water saturation yield capillary numbers approaching the capillary limit, apparent flow-rate dependent relative permeability, and low end point CO₂ permeability related to the degree of capillary heterogeneity in the flow units. These observations are linked to influences of microporosity. We examine effects of extreme capillary heterogeneity with reservoir simulations of CO₂ injection into a model Farnsworth reservoir using a five-spot injection-producer well pattern. Relative permeability relationships derived from low capillary number data generally show poor sweep and storage efficiency compared to history-matched relative permeability models, corresponding to much higher capillary numbers. We discuss the potential for fast paths, residual CO₂ trapping, and enhanced oil recovery in light of these results.

Funding for this project is provided by the U.S. Department of Energy's (DOE) National Energy Technology Laboratory (NETL) through the Southwest Regional Partnership on Carbon Sequestration (SWP) under Award No. DE-FC26-05NT42591. Imaging and pore network analysis was funded as part of the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001114. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.