An Experimental Investigation of the Effect of Rock Wettability on the Performance of Carbonated Water Injection (CWI)
Authors: Castaneda Jaime , Alghamdi Almohannad , Farzaneh Amir , Sohrabi Mehran 
Published: November 2020

Abstract
Wettability is often considered one of the most relevant variables in any conventional water injection process as it dominates the microscopic distribution of fluids in the porous medium, determines the amount of residual oil, and defines the ability with which a phase can flow. On the other hand, carbonated water injection is an enhanced oil recovery technique, where basically water saturated with CO2 is injected along the reservoir with the benefits of water displacement together with the benefits of CO2 injection, without the great disadvantages of poor sweeping causing low areal efficiency.
In addition, it has been proven that the transfer of CO2 from the aqueous phase to the oil phase, in one way, promotes the generation of what has been called a new gas phase, which is the main responsible for the incremental oil production, and which mainly attacks the residual oil saturation. Numerous experiments performed in the past on micro models, and plugs have shown that the injection of carbonated water plays an important role in the wettability of the rock. The injection has been demonstrated a change in the wettability to a water-wet because there is a reduction in the pH of the aqueous phase, and this is expected to modify the charges on the oil/water, and water/rock interfaces, and hence the wettability of the system. The dissolution of CO2, into the oil phase, and the destabilization of the polar components of the oil also may shift the wettability more towards water-wet, which favours a later water breakthrough, and a higher oil recovery factor. However, none of these experiments, as far as the author is informed, have been performed on whole cores, nor have these experiments used live crude oil with multi-component gases in solution, which would be closer to reality. This research seeks to close this gap by performing a new series of core floods to understand, from an engineering point of view, what effect the injection of carbonated water has on wettability in circumstances more realistic.
From these analyses it was concluded that rock wettability plays an important role on the differential pressure behaviour for both waterflooding, and carbonated water injection. A mix/oil-wet rock causes a greater differential pressure response. A much higher differential pressure is obtained when carbonated water injection is started. This is assumed to be due to the formation of the new gas phase. A greater oil recovery factor is obtained in a water-wet system when both secondary waterflooding, and tertiary CWI oil recovery are summed. However, when only tertiary injection of carbonated water is analysed, a higher oil recovery is obtained in mix/oil-wet systems. The new gas phase formation is facilitated in mix/oil-wet systems. The methane content dissolved in live oil plays the main role for oil recovery, and differential pressure behaviour in a carbonated water injection process. It is inversely related to the pressure behaviour, and oil recovery. This occurs because a low methane content allows a higher formation of the new gas phase, and therefore a higher production of oil; however, the differential pressure increases at the same time. Viscosity reduction due to CO2 mass transfer has a smaller effect in oil recovery, and differential pressure than the effect caused by the formation of a new gas phase. In the experiments that were conducted, the author calculated a novel linear relationship between new gas phase saturation, and tertiary oil recovery. This relationship is almost constant irrespective of the oil, and gas compositions and the wettability of the rock. This approach would allow the calculation of the additional tertiary oil recovery potential by the injection of carbonated water, based only on the saturation behaviour of the new gas phase; therefore the new objective of this recovery method would be to maximise the formation of this new phase. Although at laboratory scale there are different methods to determine the wettability of a rock, sometimes it is not possible to perform such measurements. Therefore, the author proposes a novel method that identifies trends in wettability, or better, compares trends based on Darcy's equation. This method was applied to the experiments conducted in this research, and its results were corroborated by other approaches available in the industry. Based on the results, it is possible to infer that by using a whole core the wettability change effect associated with the injection of carbonated water is not so preponderant, on the contrary, it could be more affected by the methane content in the system. The experiments conducted prior to this research had been focused on micro-models, and 1 to 2 inch diameter core evaluations, where the analysis was restricted to pore level or small scale behaviour, systems in which the impact of pore level wettability change is much greater.