There is a fast growing body of research and publications on the application of "smart waters" for improvement of recovery from oil reservoirs. The results of a wide range of investigations on this topic, which are sometimes contradictory, have enabled us to hypothesise certain mechanisms for the observed improvement in oil recovery factors. More specifically, smart waters have been considered as natural wettability modifiers, gaining significant attention from both research community and the oil industry. Several mechanisms have been proposed, but the real mechanisms or combination of mechanisms which lead to improving oil production have not been plainly defined or identified. For instance, rock dissolution in carbonate oil reservoirs, as a mechanism for oil production, has previously been documented; however, the reasons for its occurrence have not entirely been found or described. Most of the studies on smart water injection have been conducted on light oil and have been focused on both sandstone and carbonate reservoirs. However, there are insufficient published reports in the literature on the application of smart water injection for improvement of heavy oil and extra-heavy oil in carbonate reservoirs. The main focus of the research presented in this paper is the impact of rock dissolution on oil recovery by smart water injection in carbonate reservoirs due to the natural generation of acidic water.

Samples of carbonate rocks, brines and heavy crude oil were individually analysed and their mutual interactions were systematically investigated to gain a comprehensive understanding of their interactions. Five crude oil samples were utilised in this research together with limestone and dolomite cores. The oil samples had different physical properties and were tested under the same reservoir conditions, involving high temperature (92o C). Additionally, a part of this paper is devoted to direct wettability investigation using contact-angle values for two specific crude oil samples under several different aqueous phases and rocks. Subsequently, spontaneous imbibition tests were performed using both limestone and dolomite cores with the same group of brines and a basic crude oil.

The findings from our experiments firstly reveal the generation of acidic water, which is derived from the interaction between injected fluids and crude oil. It was also found that not all crude oil tested in this study could generate the acidic water which is the cause of rock dissolution. We show that dissolution occurs as a direct and strong result of the contact between injected fluids with the crude oils. This research suggests that the chemical interaction between crude oil and injected water may be one of the main reasons for the increased oil recovery efficiency in response to smart waters. Basic analyses that are presented here provide further insight into the impact of the chemical interaction between crude oil and injection water on the rock in carbonate reservoirs and the links with additional oil recovery by smart water injection.

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