Reservoir simulation of carbon storage associated with CO2 EOR in residual oil zones, San Andres formation of West Texas, Permian Basin, USA

Abstract

Residual oil zones (ROZs) are reservoirs in which oil is largely at levels of residual saturation. Such reservoirs cannot be produced by conventional techniques; rather some forms of enhanced oil recovery (EOR), such as CO2 injection is required. As a result, these zones have a potential for CO2 storage associated with EOR activities. In West Texas, the oil production potential of these zones, associated with the San Andres Formation alone, has been estimated as on the order of tens of billions of barrels. A series of numerical simulations of CO2 miscible flooding were conducted on 11 Sub-Volumes cut from a larger static reservoir that represents the range of heterogeneity in permeability and porosity found in San Andres ROZs. This work set out to evaluate the effects of injection strategies and reservoir heterogeneities on the performance of CO2 sequestration. The injection techniques investigated were: continuous CO2 injection and water alternating gas (WAG). Multiple factors were examined, including domain boundary conditions, well patterns, injection rates, permeability anisotropies, and natural fractures. It was found that ROZs could have higher retention fractions (i.e., volume fraction of injected CO2 retained in ROZs) for a combination of inverted five-spot well patterns and large WAG ratios. Based on the results of these numerical simulations, the long-term potential for CO2 storage associated with CO2-EOR of ROZs can be assessed. Our results provide key insights into how future CO2 storage projects associated with EOR in ROZs within carbonate sequences may be implemented.