Carbonated water injection (CWI) is a CO 2 -augmented water injection strategy that leads to increased oil recovery with added advantage of safe storage of CO 2 in oil reservoirs. In CWI, CO 2 is used efficiently (compared to conventional CO 2 injection) and hence it is particularly attractive for reservoirs with limited access to large quantities of CO 2 , e.g. offshore reservoirs or reservoirs far from large sources of CO 2 . We present the results of a series of CWI coreflood experiments using water-wet and mixed-wet Clashach sandstone cores and a reservoir core with light oil (n-decane), refined viscous oil and a stock-tank crude oil. The experiments were carried out to assess the performance of CWI and to quantify the level of additional oil recovery and CO 2 storage under various experimental conditions. We show that the ultimate oil recovery by CWI is higher than the conventional water flooding in both secondary and tertiary recovery methods. Oil swelling as a result of CO 2 diffusion into the oil and the subsequent oil viscosity reduction and coalescence of the isolated oil ganglia are amongst the main mechanisms of oil recovery by CWI that were observed through the visualisation experiments in high-pressure glass micromodels. There was also evidence of a change in the rock wettability that could also influence the oil recovery. The coreflood test results also reveal that the CWI performance is influenced by oil viscosity, core wettability and the brine salinity. Higher oil recovery was obtained with the mixed-wet core than the water-wet core, with light oil than with the viscous oil and low salinity carbonated brine than high-salinity carbonated brine. At the end of the flooding period, an encouraging amount of the injected CO 2 was stored in the brine and the remaining oil in the form of stable dissolved CO 2 . The experimental results clearly demonstrate the potential of CWI for improving oil recovery as compared with the conventional water flooding (secondary recovery) or as a water-based EOR (enhanced oil recovery) method for watered out reservoirs.

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One of the well-known problems in CO2 enhanced oil recovery (EOR) processes is the poor sweep efficiency due to the high viscosity contrast between CO2 and the reservoir resident fluids (oil and brine). CO2-augmented waterflooding or carbonated water injection (CWI) could lessen this problem. As CO2 is dissolved in and transported by the flood water, CO2 is more evenly distributed within the reservoir thus improves the sweep efficiency. This is beneficial to watered-out oil reservoirs where high water saturations could adversely affect the performance of the conventional CO2 injections. CWI also provides a very safe method for storing large quantities of CO2 as a dissolved phase in oil reservoirs. This paper presents the results of our experimental and numerical investigations on the oil recovery and CO2 storage benefits of CWI in secondary and tertiary recovery modes through a series of coreflood experiments and detailed compositional simulation. The experiments were performed in a water-wet Clashach core with decane as well as restored North Sea reservoir core with stock tank crude oil and seawater at reservoir conditions. The experimental results demonstrate that CWI in both secondary and tertiary recovery modes can improve the oil recovery above the plain waterflooding. 45-51% of the injected CO2 was stored in the core at the end of the coreflood tests indicating the high potential of CWI not only for EOR but also as a CO2 storage injection strategy. Results of the corefloods were used to assess the capabilities and limitations of a commercial compositional flow simulator in modelling the CWI process. The simulation results show that diffusion should be taken into account to properly model the CWI process at the core scale. Using the commercially available reservoir simulators with the instantaneous equilibrium and complete mixing assumptions would lead to inaccurate evaluation of CWI process at this scale of interest.

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Surfactants have been successfully used for enhanced or improved oil recovery in reservoirs having mild conditions (low temperature, low salinity). Reservoirs having harsh conditions, however, offer unique challenges in that most surfactants precipitate and chemically degrade due to a combined effect of high temperature and hardness salinity. Industry's efforts are continuing to develop or formulate surfactants for oil recovery applications to high temperature and salinity. The aim of this study was to evaluate several modified anionic surfactants/formulations that were claimed to be able to overcome the unfavorably high-salinity brine (sea water) and high temperature and to understand the impact of high temperature to surfactant adsorption. A series of experiments were conducted to characterize and quantify the effects of aging time in high temperature (106 °C) and seawater salinity (32,000 ppm with 1600 hardness) on surfactant performance. Results for both sulfate-and sulfonate-based surfactants were deemed not to be satisfactory. Sulfate-based surfactants encountered hydrolysis problem at high temperature, whereas sulfonate-based surfactants precipitated in the presence of divalent ions. This study then focused on alkyl ether carboxylate (AEC) as the main surfactant, and blends of AEC with alkyl polyglucoside (APG). To find the optimum conditions, phase behavior tests were performed with a fixed seawater salinity but with different blending ratios of surfactant and co-surfactant, as well as overall surfactant concentrations, similar to the salinity scan. Type III microemulsion was observed for both surfactant solutions of AEC and AEC-APG blend with IFT of 10 −3 mN/m (millinewton/meter). Surfactant adsorption resulted in lower adsorption in the high-temperature region. The results of this project are urgently needed by the industry for future screening in order to find suitable surfactants for applying to reservoirs with harsh conditions. The study also intends to provide an understanding of adsorption relationship to high temperature, as a guideline in addressing surfactant losses due to adsorption at high-temperature field application.