Time-dependent physico-chemical changes of carbonate surfaces from SmartWater (diluted seawater)-flooding processes for improved oil recovery.

Over the past few decades, field and laboratory scale studies have shown enhancements in oil recovery when reservoirs, which contain high-salinity formation water (FW), are waterflooded with modified-salinity salt water (widely referred to as the "low-salinity", "dilution", or "SmartWater" effect for improved oil recovery). In this study, we investigated the time-dependence of the physico-chemical processes that occur during diluted seawater ( i.e., SmartWater) waterflooding processes of specific relevance to carbonate oil reservoirs. We measured the changes to oil/water/rock wettability, surface roughness, and surface chemical composition during SmartWater-flooding using 10-times-diluted seawater under mimicked oil reservoir conditions with calcite and carbonate reservoir rocks. Distinct effects due to SmartWater-flooding were observed and found to occur on two different time-scales: (1) A rapid (takes less than 15 minutes) increase in the colloidal "double-layer" repulsion between the rock and oil across the SmartWater, leading to a decreased oil/water/rock adhesion energy and, thus, increased water-wetness; and (2) slower (take longer than 12 hours to complete) physico-chemical changes of the calcite and carbonate reservoir rock surfaces, including surface roughening via dissolution of rock and re-precipitation of dissolved carbonate species after exchanging key ions (Ca2+ , Mg2+ , CO3 2- , SO4 2- in carbonates) with those in the flooding SmartWater. Our experiments using crude oil from a carbonate reservoir reveal that these reservoir rock surfaces are covered with organic-ionic pre-adsorbed films (ad-layers), which the SmartWater removes (detaches) as flakes. Removal of the organic-ionic ad-layers by SmartWater-flooding enhances oil release from the surfaces which was found to be critical for increasing water-wetness and significantly improving oil removal from carbonates. Additionally, the increase in water-wetness is further enhanced by roughening of the rock surfaces, which decreases the effective contact/interaction area between the oil and rock interfaces. Furthermore, we found that the rate of these slower physico-chemical changes to the carbonate rock surfaces increases with increasing temperature (at least up to an experimental temperature of 75 °C). Our results suggest that the effectiveness of improved oil recovery from SmartWater-flooding depends highly on the formation of the organic-ionic ad-layers. In oil reservoirs where the ad-layer is fully-developed and robust, injecting SmartWater would lead to significant removal of the ad-layer and improved oil recovery.