A Bundle of Capillary Tubes (BOCT) Model for Carbonated Water Flooding (CWF); a Promising Technique for Simultaneous CO₂ Storage and Enhanced Oil Recovery Purposes

Carbonated water flooding (CWF) is a promising technique for EOR and a suitable methodology for permanent geological sequestration of CO₂. As carbonated water contacts oil, CO₂ preferably diffuses from water into oil, resulting in oil displacement mainly due to oil swelling and viscosity reduction. Moreover, as CO₂ remains dissolved in the fluids rather than staying as a free phase, the risk of buoyancy-driven leakage is minimised. To date, extensive experimental studies have been performed on the EOR and CO₂ storage potential of CWF. Nonetheless, due to the complexity of this process, numerical studies on accurate modelling of CWF are limited. Using commercial and in-house simulators, a number of studies have been conducted which exhibit promising results in investigating the underlying mechanisms of this process and predicting experimental observations [1-3]. However, scarcity of experimental data for predicting important parameters and model validation, as well as the intrinsic complexity of the CWF process, cause severe challenges for conducting numerical studies. Accordingly, the deficiencies of previous models, such as over-prediction of oil recovery, unrealistic conditions or materials, over-complex numerical codes, etc. necessitate new avenues of investigation. Here, we present a convenient, versatile numerical model capable of capturing the complex nature of CWF’s EOR mechanisms with relatively low runtimes. We developed a bundle of capillary tubes (BOCT) model to numerically simulate the results of a previously published experimental study on CWF in core samples [4]. In this numerical study, a pseudo-compositional approach is applied. Firstly, a BOCT model is employed to be a representative of the porous media. Subsequently, the fluid flow equations were derived using momentum balance. Then, through the utilization of a lumped mass transfer approach, CO₂ distribution through the media is captured by considering two phases (water and oil) and three components (water, oil, and CO₂). Finally, the contribution of CO₂ in the EOR process is modelled by using the calculated CO₂ distribution throughout the medium and employing valid, widely used correlations. It was shown that the developed model is adequately capable of matching the oil recovery levels outlined by the experimental results. In addition, sensitivity analyses were performed to investigate the influence of different parameters on oil recovery by CWF and validate the main underlying EOR mechanisms, specifically oil swelling and viscosity reduction. These analyses confirmed oil viscosity and API as the main parameters affecting the performance of the EOR process. Therefore, the BOCT model can be considered as a ‘physical experiment’ and the assessed performance (pressures, flow rates, and production rates) of such ‘samples’ of the porous media as ‘experimental results’. Finally, by using this model to predict the performance of the CWF process, implementing further costly and time-consuming experimental studies to attain direct measurements can be avoided.