Enhanced Oil Recovery and CO2 Storage by Carbonated Water Injection
Authors: Sohrabi Mehran , Riazi Masoud , Jamiolahmady Mahmoud , Ireland Shaun , Brown Christopher 
Published: May 2016

This reference is for an abstract only. A full paper was not submitted for this conference.

Abstract
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. Displacement and recovery of oil by CO2 injection has been studied and applied in the field extensively. The main advantage of CO2 is that at most reservoir conditions it is a supercritical fluid with high solvency power to extract hydrocarbon components and displace oil miscibly. Furthermore its high density makes it quite compatible with oil alleviating gravitational segregation. However, CO2 injection lacks acceptable sweep efficiency due to large viscosity contrast between CO2 and reservoirC_'s resident fluids. Various CO2 injection strategies e.g. alternating (WAG) or simultaneous injection of CO2 and water have been suggested to alleviate the problem of poor sweep efficiency. An effective 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 and hence eliminating the problem of gravity segregation and poor sweep efficiency which are characteristics of a typical CO2 injection project. In fact, both the viscosity and density of water increase as a result of the dissolution of CO2 in water which will favourably affect oil displacement and recovery. 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 which is considered one of the most serious concerns associated with the safety of long term storage of CO2 in underground reservoirs. This paper describes the results of an integrated experimental and theoretical investigation of the process of carbonated water injection (CWI) as an injection strategy for EOR with the added value of CO2 storage. High-pressure micromodel technology was used to physically simulate the process of CWI and visually investigate its EOR potential at typical reservoir conditions. Using the results of a series of high-pressure flow visualisation (micromodel) experiments, we reveal the underlying physical processes and the pore-scale mechanisms of fluid-fluid and fluid-solid interactions during CWI. The results show that CWI, compared to unadulterated water injection, improves oil recovery in both secondary (before water flooding) and tertiary (after water flooding) injection modes. The improvement is, however, higher when carbonated water is injected before water flooding. Several mechanisms that take place at the pore level during CWI and cause oil recovery improvement are presented and discussed. Both conventional (light) oil and viscous oil was used in the experiments. The mechanisms of oil recovery observed for light oil was vastly different from viscous oil. In light oil, significant swelling of the oil was observed during CWI as a result of the partitioning of CO2 from CWI and its dissolution into the oil. This oil swelling brought about reconnected of the isolated oil ganglia left behind after conventional water flooding. The reconnection of the oil in turn caused remobilisation and recovery of the trapped oil and hence increased the amount of oil recovery. In viscous oil, while oil swelling was again observed, the amount of the swelling was much less than the light oil. However, the degree of oil viscosity reduction in viscous oil was much more than the light oil. This significant reduction of viscosity of the viscous oil led to the mobilisation and recovery of the trapped oil. Based on the results of our flow visualisation, a mathematical model was developed that accounts for the pore-scale mechanisms observed during the experiments. The results of the mathematical model and a sensitivity analysis which was carried out to investigate the impact of the key pertinent parameters are also discussed. The results of this research show that carbonated water injection is an effective and efficient injection strategy that offers a unique solution for the problem of increasing oil recovery while at the same time reducing CO2 emission.