Investigation of Recovery Mechanism of Steam Injection in Heavy Oil Carbonate Reservoir and Mineral Dissolution

Abstract

Abstract Steam injection in carbonate heavy oil reservoirs is a complex process. The main challenge is that the injected steam breaks through from a fracture network resulting in poor sweep efficiency. A large amount of oil remains behind the steam front. In addition, severe mineral dissolution (or precipitation) due to steam/brine/rock interaction occurred at high temperature has a significant impact on steam injectivity and recovery mechanism. This paper presents an extensive lab study focused on understanding the recovery mechanism and relevant mineral dissolution. Around 10 reservoir cores and crude oil sample with an API gravity of 14 were used for this study. Imbibition, steam flooding, and pressure blow-down tests were conducted. The results show that the recovery mechanism of steam injection in the target carbonate reservoir is composed of imbibition, viscosity reduction, in-situ steam generation induced by pressure blow-down, and oil expansion mechanisms with different contributions. As the rock is heated to near 400°F, imbibition becomes the dominant recovery mechanism. The increased imbibition recovery is strongly dependent on mineral dissolution at a high temperature, which results in wettability alteration towards strong water-wetness. Due to the presence of micro-fractures, steam drive is less efficient compared to imbibition. In-situ steam generation can still increase oil recovery by an additional 17-32% (OOIP) following steamflood displacement at residual oil saturation. Significant mineral dissolution is observed with high resolution CT scanning and effluent geochemical analysis. CT-image analysis shows the size of vugs is increased 2-5 times due to mineral dissolution. ICP (Inductivity Coupled Plasma) and IC (Ion Chromatogragh) analysis shows that carbonate is the main dissolved mineral. Mineral dissolution does not increase the effective permeability because these vugs are mainly connected by micro fractures where mineral dissolution effects are negligible. In fact, permeability is reduced due to fines migration resulted by mineral dissolution.