Evaluation of various CO2 injection strategies including carbonated water injection for coupled enhanced oil recovery and storage

In view of current interest in geological CO2 sequestration and EOR, this study investigated water-based and gas-based CO2 injection strategies for coupled EOR and storage purposes. For water-based CO2 injection strategy, carbonated water injection (CWI) was investigated as an alternative injection mode that could improve sweep efficiency and provide safe storage of CO2. Despite its potential, CWI has not been very much studied. This thesis presents the details on the performance of CWI of moderately viscous oil (>100 cP), which has not been reported before. The effects of oil viscosity, rock wettability and brine salinity on oil recovery from CWI were also studied and significant findings were observed. To the author’s knowledge, no attempt has been made to experimentally quantify the CO2 storage by CWI process and to model the nonequilibrium effects in the CWI at the core scale using the commercial reservoir simulators. These are amongst the main innovative aspects of this thesis. The experimental results reveal that CWI under both secondary and tertiary recovery modes increase oil recovery and CO2 storage with higher potential when using light oil, low salinity carbonated brine and mixed-wet core. In this study, the compositional simulator overpredicts the oil recovery. The instantaneous equilibrium and complete mixing assumptions appear to be inappropriate, where local equilibrium was not in fact achieved during the CW process at this scale. The author evaluated the use of the transport coefficient (the a-factor) to account for the dispersive mixing effects, and found that the approach gives a more accurate prediction of the CWI process. For the gas-based CO2 injection strategies, a practical yet comprehensive approach using reservoir simulation, Design of Experiment (DOE) and the Response Surface Model (RSM) to screen for and co-optimize the most technically and economically promising injection strategy for coupled EOR and CO2 storage is presented. For the reservoir model used in this study, miscible WAG was found to be most economically promising, while miscible continuous CO2 injection was ranked as the most technically viable. The duration of the preceding waterflood, relative permeability (wettability) and injected gas composition are the three most significant factors to the profitability of oil recovery and CO2 storage through tertiary WAG injection

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

AbstractThe 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