Carbonated waterflooding in carbonate reservoirs: Experimental evaluation and geochemical interpretation

Abstract

Abstract Carbonated water flooding (CWF) appears to be an important means in enhanced oil recovery (EOR) in carbonate reservoirs. While a few CWF coreflooding experiments have been done to reveal the controlling factor(s) behind incremental oil recovery, few has examined the impact of calcite dissolution on the contribution of the proposed mechanisms, and fewer have looked beyond the impact of calcite dissolution on different length scale (from core to reservoir). We thus conducted a series of core flooding experiments to investigate the residual oil saturation and recovery factor during waterflooding with and without carbonation. We also imaged the core plugs using a computed tomography scanner to examine the evolution of calcite dissolution along the core plug. Furthermore, we performed 1D reactive transport modelling at core- and reservoir-length-scale to delineate the impact of calcite dissolution process during carbonated waterflooding. Coreflooding experiments confirm that lowering salinity increases oil recovery from 53% to 64.5% without carbonation. However, low salinity carbonated water at secondary mode yielded 47.6% and 52% oil recovery, between 1 and 5.4% less recovery compared to formation brine flooding without carbonation, lower than the formation brine flooding without carbonation. Carbonated waterflooding also resulted in a significantly increases of permeability. CT images clearly show the generation of wormholes along the core, accounting for the low recovery and increased rock permeability. 1D reaction transport modelling at core-scale reveals the calcite dissolution taking place throughout the core plugs, supporting the wormholes evolution from CT images. One-dimensional reactive transport modelling at reservoir-scale shows the calcite dissolution distance from wellbore increases from 13 to 45 m with increasing flow rate from 0.05 to 1 m/day. Taken together, our results imply that calcite dissolution may deteriorate heterogeneity of reservoirs particularly near the wellbore. This may significantly undermine the contribution of oil-swelling, viscosity reduction, IFT reduction and wettability alteration on incremental oil recovery, as well as wellbore stability. However, the negative impact of calcite dissolution may not prevail at in-depth of reservoirs because the calcite dissolution would reach equilibrium at a certain distance, which is also associated with injection rates.