Time-Dependent Physicochemical Changes of Carbonate Surfaces from SmartWater (Diluted Seawater) Flooding Processes for Improved Oil Recovery

Abstract

Over the past few decades, fieldand laboratory-scale studies have shown enhancements in oil recovery when reservoirs, which contain highsalinity 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 physicochemical 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-fold-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 timescales: (1) a rapid (<15 min) increase in the colloidal electrostatic 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 (>12 h to complete) physicochemical changes of the calcite and carbonate reservoir rock surfaces, including surface roughening via the dissolution of rock and the reprecipitation of dissolved carbonate species after exchanging key ions (Ca 2+ , Mg 2+ , CO 3 2− , and SO 4 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 preadsorbed 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 to increasing the 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 physicochemical 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 strongly 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.

Adhesive forces between two cleaved calcite surfaces in NaCl solutions: The importance of ionic strength and normal loading

“…In general, calcite and other natural mineral surfaces display some degree of roughness at a molecular scale. Several studies have shown that surface roughness affects the water wettability of calcite [45,46] and oil desorption from calcite surfaces [47]. It, also, influences the interfacial forces between mineral surfaces in molecular scale.…”

Adsorption- and Diffusion-Controlled Wettability Change in Modified Salinity Water Flooding

“…As a result, the additional volume of oil may be produced depending on the salinity level and ionic composition of intrinsic and injecting brine in the porous medium, ,− initial rock wetting preference, − chemical activity of the rock surface, ,,− thermodynamic conditions of the core flooding experiment, ,,, and physicochemical properties of the oil–rock–brine (ORB) system, − among others. The effectiveness of MSW flooding in reducing the adhesion force between oil and rock surface and possible improvement of oil recovery depends on the alteration of physicochemical equilibrium at the subpore scale. ,,, Figure illustrates different scales relevant to MSW flooding. At the pore scale, a trapped oil droplet adhered to the rock through a thin formation water film can be mobilized by the alteration of the equilibrium condition at the rock–water–oil interface.…”

“…A 25 times dilution of seawater did not show any impact on the wetting tendency of dolomite patches, while the contact angle for limestone changed in the order of 5°–17° over 100 h. In another study with a similar procedure, Mahani et al observed that kinetics of oil detachment from the fabricated clay patches during the exposure to low salinity brine was slower than the expected time for a process that is controlled by molecular diffusion. Chen et al monitored the dynamic and static contact angle of an oil droplet surrounded by MSW on a fully aged calcite surface. It was reported that in <15 min there was a rapid but slight increase in water wetness of the calcite surface that they attributed to the rapid increase in the electrostatic double-layer repulsion.…”

Wettability Reversal on Dolomite Surfaces by Divalent Ions and Surfactants: An Experimental and Molecular Dynamics Simulation Study

“…This is justified in that dolomite can be rendered oil-wet by carboxylates and that the oil-wetness can be reversed more effectively by cationic surfactants than anionic ones, similar to the other types of carbonate rocks. 11−16 However, compared to the extensive literature on the wettability alteration by potential determining ions (PDIs) and their application for EOR in calcite-based carbonate rocks (including those containing both calcite and dolomite minerals 17 ), very few systematic studies on the effect of PDIs on dolomite wettability have been reported. 18 A number of reports have identified that dolomite and calcite share the same PDIs, 19−23 namely, Ca 2+ , Mg 2+ , and SO 4 2− .…”

“…Most academic interest has been devoted to the precipitation and dissolution of the dolomite mineral, known as the “dolomite problem”. − By contrast, in terms of EOR and, particularly, the mechanism of the wettability alteration, dolomite is often grouped together with calcite and aragonite into a collective concept of carbonate rocks and is believed to exhibit the same general wetting behavior as the other two calcium-rich mineral types. This is justified in that dolomite can be rendered oil-wet by carboxylates and that the oil-wetness can be reversed more effectively by cationic surfactants than anionic ones, similar to the other types of carbonate rocks. − However, compared to the extensive literature on the wettability alteration by potential determining ions (PDIs) and their application for EOR in calcite-based carbonate rocks (including those containing both calcite and dolomite minerals), very few systematic studies on the effect of PDIs on dolomite wettability have been reported . A number of reports have identified that dolomite and calcite share the same PDIs, − namely, Ca 2+ , Mg 2+ , and SO 4 2– .…”