The impact of geological heterogeneity on coupled CO2 storage and geothermal extraction in inclined reservoirs

Abstract

CO2 plume geothermal (CPG) production is an emerging CO2 utilization approach coupled with CO2 geological sequestration. Geological heterogeneity plays an important role in CO2 fluid and heat flow in the formation. Although the impact of heterogeneity has been investigated during CO2 injection for many sequestration projects, few studies have been reported for CO2-circulation and associated heat extraction. This study aims to numerically assess the impact of spatially correlated heterogeneity on both CO2 plume development (CPD) and subsequential CPG operations in inclined and thinly layered geothermal reservoirs. The simulated field is simplified from a typical depleted oil/gas reservoir block located in a potential geothermal area in North Oman, which had undergone horizontal water flooding as the second oil recovery phase. Formation slope and heterogeneity properties, including variance, azimuth, and correlation lengths, are sampled using the Latin-Hypercube approach in a high dimensional parameter space defined by their ranges. A suite of numerical models simulating CPD-CPG operations by a horizontal well doublet is set up using the slope and heterogeneity parameter samples. Performance metrics for CPD-CPG, including CPD time, CPG time, net injected CO2, maximum pressure decline, minimum CO2 mass fraction in produced fluids, total recovered heat energy, and heat extraction rate, are calculated from each of the numerical models. A response surface model mapping the input (slope and heterogeneity parameters) and output (performance metrics) is developed using these model results. Using the response surface, global sensitivity analysis is efficiently conducted to quantitatively evaluate the impact of slope and heterogeneity on the performance of CPD-CPG operations. It is found that steep slopes can enhance CO2 injectivity, facilitate high CO2 fraction in produced fluids and heat extraction rate; However, it causes early cold CO2 breakthrough to the production well and reduce total recovered heat energy. High variance and x-correlation length of heterogeneous fields create prominent high-permeable preferential paths in the major flow direction (x-axis). Low-permeable lenses created by large y-correlation lengths are perpendicular to the x-axis, acting as flow barriers to both CO2 and brine towards the production well. As for z-correlation length, 15 m is found to be the critical value relative to 20 m of the reservoir thickness. Large z-continuity scale facilitates fluid flow owing to the thick preferential paths when it is below 15 m, but low-permeable lenses tend to significantly block flow as they approximate to the reservoir thickness.