Micro-optical analysis of carbonated water injection in irregular and heterogeneous pore geometry

Abstract

In this study, the performance of carbonated water injection (CWI) was examined through sequences of carefully designed micromodel tests in order to visualize the CWI process and investigate the associated oil recovery mechanisms. Wettability, contact angle, and residual oil trapping mechanisms were studied during water flooding and CWI. Strong viscous fingering phenomenon was observed up to breakthrough. In CWI runs, subsequent growth of the fingers observed due to unceasing dispersion-type CO2 mass transfer from fresh carbonated water to the oil phase. The main oil trapping mechanism was wettability trapping due to oil-wet nature of the micromodel. Evidence of other trapping mechanisms such as snap-off and pore-doublet were also detected. Micromodel observations revealed that the wettability of the system was modified gradually as the CWI proceeds. It was concluded that CWI can be considered as a beneficial improved oil recovery (IOR) technique that is capable of effectively changing the wettability of mixed- or oil-wet reservoirs to a more favorable condition, i.e., water-wet. Continuation of CWI for higher pore volumes also found to be advantageous in recovering an additional 3.1% of original oil in-place. Therefore, in contradiction to water flooding where oil recovery improvement is not noticeable after water breakthrough, CWI was found to be successful in improving oil recovery for a longer period even after breakthrough. The extent of this recovery improvement can be correlated to CO2 solubility and mass transfer rate. The main mechanism contributing to the additional recovery in CWI after breakthrough was found to be continuous dissolution of CO2 into the oil phase and subsequent oil swelling and viscosity reduction, which ultimately led to improved mobility of the residual oil ganglia in the micromodel.