Oil Recovery Improvement Using CO2-Enriched Water Injection

Abstract

Abstract CO2 injection is increasingly considered as having potential applications as a possible enhanced oil recovery (EOR) process for oil reservoirs. However, poor sweep efficiency has been a problem in many CO2 floods and hence, the injection strategies like WAG (water-alternating-gas) injection have been proposed and applied in the field as a way to mitigate the problem. An alternative injection strategy is CO2-enriched (carbonated) water injection (CWI). This paper presents the results of an integrated experimental and theoretical study on the application of CO2-enriched water flooding for enhanced oil recovery. Direct flow visualisation experiments were carried out using high-pressure transparent porous media. The results of our visualisation experiments demonstrate that CWI, compared to unadulterated water injection, improves oil recovery. The additional oil is recovered as a result of an improved sweep efficiency, due to the oil swelling, viscosity reduction and coalescence of the isolated oil ganglia as a result of CO2 diffusion. This injection strategy is particularly attractive in waterflooded oil reservoirs in which high water saturation adversely affects the performance of conventional CO2 injection methods. CWI can also be carried out in combination with reservoir depressurisation carried out subsequent to CWI or in a cyclic manner in which carbonated and plain water cycles are injected in succession. The results of a mathematical model are also presented which honours our experimental observations and simulates the dynamic process of oil swelling and shrinkage due to CO2 transfer during Carbonated water and plain water injection. Introduction In many reservoirs, after waterflooding, a large volume of oil is still left behind. There is thus scope for processes that can unlock some of the remaining oil to maximise oil recovery from these reservoirs. The use of CO2 injection to enhance oil recovery is often associated with poor sweep efficiency (due to high CO2 mobility) [1]. Therefore, direct injection of CO2 (both continuous flooding and WAG) might not result in economically significant amount of additional oil recovery. In terms of CO2 storage potential, poor sweep efficiency also implies lower storage capacity. An alternative injection strategy is carbonated water (CO2-enriched water) injection. Carbonated water has advantages over direct CO2 injection as it has a better sweep efficiency. In direct CO2 injection, it has been shown that due to low sweep efficiency and gravity segregation, the time scale for CO2 diffusion in oil can be several years [2]. In terms of CO2 storage, since in CWI (carbonated water injection) CO2 is dissolved in water (and later oil) rather than existing as a free phase, CWI would provide a very safe method for CO2 storage. CWI causes the oil to swell and the viscosity of the oil to drop. It can reduce water-oil interfacial tension and can also favourably affect wettability of the reservoir. Swelling of the oil can reconnect the discontinuous residual oil and result in additional oil recovery. Additional oil recovery might also be achieved through a blow down phase subsequent to a period of carbonated water injection. Another water injection period after CWI stage could also cause more fluid redistribution and oil recovery as the rate of CO2 dissolution in the oil during CWI is not necessarily the same as the rate of CO2 stripping from the oil during WI. The objective of this study was to investigate the process of carbonated water injection through conducting flow visualisation experiments and mathematical modelling. To achive this objective, we have performed a series of high-pressure flow visualisation experiments. In this paper we show and discuss the results of one of our experiments carried out to investigate the performance of carbonated water injection (CWI), as a tertiary oil recovery method, after water injection (WI). The experiment was conducted using a high-pressure transparent porous medium (micromodel) at 2000 Psia and 38 °C.