Carbonated water injection (CWI)–A productive way of using CO2 for oil recovery and CO2 storage

Abstract

The main advantage of CO2 is that at most reservoir conditions it is a supercritical fluid which is likely to develop miscibility with the oil. In reservoirs that miscibility cannot be achieved, CO2 injection can lead to additional oil recovery by mixing with the oil and favourably modifying the flow properties of the oil. Displacement and recovery of oil by CO2 injection has been studied and applied in the field extensively in the past three decades. Concerns over the environmental impact of CO2 have led to a resurgence of interest in CO2 injection in oil reservoirs. The injection of CO2 can enhance oil recovery from these reservoirs and at the same time help mitigating the problem of increased CO2 concentrations in the atmosphere by storing large quantities of CO2 for a long period of time. CO2 injection projects so far have been mainly limited geographically to oil fields located in areas where large quantities of CO2 have been available mainly from natural resources. Various CO2 injection strategies e.g. cyclic injection, continuous CO2 flood, alternating (WAG) or simultaneous injection of CO2 and water have been applied in these fields. With the new global interest in CO2 injection, many other reservoir settings and scenarios are being considered for CO2 injection in oil reservoirs. This may require injection strategies other than those conventionally used for CO2 injection especially for offshore reservoirs or in cases where the supply of CO2 can be variable or limited. An alternative CO2 injection strategy is carbonated (CO2-enriched) water injection. In carbonated water, CO2 exists as a dissolved phase as opposed to a free phase eliminating the problems of gravity segregation and poor sweep efficiency, which are characteristics of a typical CO2 injection project. In fact, both viscosity and density of water increase as a result of the dissolution of CO2 in water. In terms of CO2 storage, through carbonated water injection, large volumes of CO2 can be injected into the reservoir without the risk of leakage of CO2 through caprock. Using the results of a series of high-pressure flow visualisation experiments, we reveal the underlying physical processes taking place during CWI. The results show that CWI, compared to conventional water injection, improves oil recovery in both secondary (pre-waterflood) and tertiary (post-waterflood) injection modes. Several key mechanisms taking place at the pore level during CWI leading to additional recovery are presented and discussed. Both conventional (light) oil and viscous oil was used in the experiments.