The Effect of CO2-Rich Brine on Enhanced Oil Recovery in Sandstones

Summary Carbon capture, storage, and utilization (CCS/CCUS) is a key technology in achieving net-zero emissions. Among various CCUS methods, combining CO2 with injection water increases oil production and reduces overall emissions. However, when CO2 dissolves in injection water, the resulting acidic environment can induce chemical reactions that affect rock integrity, injectivity, and the reservoir’s wettability. This study aims to demonstrate the efficiency of CO2-rich formation water (CFW) for enhanced oil recovery (EOR) compared to conventional formation water (FW) injection. Furthermore, this study assessed the possible geochemical reactions that may occur by injecting CFW into sandstone outcrop and how those reactions might impact the injectivity and wettability of the reservoir rock. Chemical interactions within the rock-brine-CO2 and rock-brine-oil-CO2 systems in Bentheimer sandstone, consisting mainly of quartz, were assessed. First, the solubility of CO2 in brine and the resulting brine pH were estimated experimentally and through modeling. To evaluate the stability of the rock minerals, ground Bentheimer sandstone samples were exposed to CFW at 80 bar and 60 °C for one month. To detect geochemical interactions, mineral and brine samples were analyzed before and after CFW exposure using scanning electron microscopy (SEM) and ion chromatography (IC). Finally, oil recovery experiments were performed on water-wet Bentheimer cores restored with Swi = 20% and crude oil. The restored cores were initially flooded with FW to establish the baseline recovery. The same cores were cleaned and restored to the initial condition, and this time, CFW was directly injected into the core in secondary mode. The results confirmed that the solubility of CO2 in brines is limited, <5%, while the brine’s pH was significantly decreased as CO2 dissolved in it. The Bentheimer rock minerals showed minimal reactivity toward CFW in the form of negligible mineral dissolution influencing rock stability. A small difference in oil recovery was observed, in which CFW injection resulted in 2 to 3% OOIP more oil produced compared to pure FW injection. Before conclusions can be drawn about the reason behind the extra oil produced by CFW flooding, further studies are needed as the results demonstrate limited contribution from rock dissolution and oil swelling. The possible contribution from wettability alteration on oil recovery needs to be further investigated in less water-wet sandstone rocks.