Carbon neutral oil assessment using a dual-porosity compositional model in unconventional reservoirs

Abstract

The combined use of CO2 for enhanced oil recovery (EOR) and decarbonization has not been viewed favorably by environmentalists because the combustion of oil emits CO2. However, the use of CO2 for EOR and the consequential decarbonization is a viable and practical option that can be viewed as a win-win situation given other alternatives. Thus, we embarked on this study to assess the benefits of the process by modeling its application in the Niobrara and Codell unconventional formations in the Wattenberg Field of the Denver-Julesburg (DJ) Basin, Colorado. The modeling results provide an inexpensive substitute for the current non-existing field pilot tests. Modeling the reservoir geology and production performance in the unconventional resources of the DJ Basin is challenging because of the presence of numerous faults, micro-fractures, and macro-fractures. In this numerical model study, we used much large fracture permeabilities and compaction coefficients in the vicinity of major faults and we obtained hydraulic fracture dimensions and properties from a credible geomechanics simulator GOHFER. Seismic data, well logs, cores, PVT analyses, and field production data were integrated to construct the numerical flow model. The input reservoir permeabilities for the model were obtained from the rate transient analyses (RTA) of production data. After achieving a history match of production data for eleven wells, theoretical RTAs were conducted on each well using the modeled pressure and flowrate data to calculate the formation permeabilities to confirm the simulation accuracy. Despite the complexity of the well completion and reservoir heterogeneity in the model, the RTA analyses of the model production results for all eleven wells yielded the input reservoir permeability within an acceptable error tolerance. Moreover, a term named “Carbon Neutrality Index (CNI)” is defined in this research where CNI = 0 indicates carbon neutrality while negative CNI values indicates that the produced incremental EOR oil is carbon negative and there is a net decrease in carbon emission. When our validated numerical model was used to simulate a 4-year huff-n-puff CO2-EOR process, the results yielded a positive CNI value—indicating that the EOR oil was not perfectly carbon neutral. However, the stored CO2 divided by the CO2 generated from burning the incremental EOR oil is 0.82 — indicating that there is 82% reduction in CO2 emissions per bbl of EOR oil when compared with the emissions of one barrel of conventionally produced oil. Thus, CO2-EOR in the DJ Basin unconventional reservoirs is an effective decarbonization method.