A Novel Pore Scale Visual Investigation of the Oil Recovery Mechanisms of Low Salinity Carbonated Water Injection (LSCWI)

Two multiple injection stage experiments were conducted with the objective of investigating the possible presence of both mechanisms that are associated with Low Salinity Water Injection (LSWI - wettability alteration) and Carbonated Water Injection (CWI - in situ gas formation-phase behavior); as well as visually confirm the investigation carried out by (Castañeda et al., 2022a) that the formation of the new gas phase is in fact responsible for the increase in the Dp in CWI. Temperature controlled unit ovens are used in this investigation; one where the micro-model is installed; and another where the oil, injection, overburden, and recovery cells are housed. The experiments were conducted at 50°C and 3100 psig, and dual pumps were used to inject/retract fluids with pressure transducers installed to monitor pressure profiles. Each injected fluid is setup to be injected through the micro-model in the desired direction (bottom to top or vice versa). A high-resolution camera is set up to take images through the glass sight that is installed on the micro-model housing chamber. The results show that the two mechanisms of wettability alteration (micro-dispersion) and phase behavior (in-situ gas formation) are present at the same time, when injected into a porous media with a multi-component gas live crude oil. CO2 mass transfer from the injected brine to the oil can occur through the micro-dispersion film that has already been formed around the oil ganglia. As the new gas phase increases in size, and the oil blob swells up, the intensity of the micro-dispersion reduces. However, when the oil blob reduced in size due to the movement of the oil and new gas phase with it, the micro-dispersion could be formed as well as increase in size and intensity. This implies that the phase behavior mechanism has a higher domination over the micro-dispersion. When the oil is pre- saturated with CO2, mass transfer is no longer possible, and the injection behaves similarly to a conventional waterflood in the porous media. When LSCSW* flood is performed with oil fully saturated with CO2, the reduction of the salinity of the injected brine successfully triggered the wettability alteration mechanism, which strongly supports the independence claim of both the LSI and CWI mechanisms. Few authors have evaluated the behavior of the Dp in small plugs CWI flooding experiments; and in those experiments, a slight increase in pressure was identified. Only a small portion explained this behavior, having assumed it was due to the formation of the new gas. This study visually confirms the hypothesis (for the first time) that both the mass transfer and CO2 dissolution from the injected brine into the oil, which caused the new gas phase to form, was indeed responsible in the pressure increase during CWI.