Abstract
Laboratory experiments and field trials have shown that oil recovery from carbonate reservoirs can be increased by modifying the brine composition injected during recovery in a process termed controlled salinity water-flooding (CSW). However, CSW remains poorly understood and there is no method to predict the optimum CSW composition. This work demonstrates for the first time that improved oil recovery (IOR) during CSW is strongly correlated to changes in zeta potential at both the mineral-water and oil-water interfaces. We report experiments in which IOR during CSW occurs only when the change in brine composition induces a repulsive electrostatic force between the oil-brine and mineral-brine interfaces. The polarity of the zeta potential at both interfaces must be determined when designing the optimum CSW composition. A new experimental method is presented that allows this. Results also show for the first time that the zeta potential at the oil-water interface may be positive at conditions relevant to carbonate reservoirs. A key challenge for any model of CSW is to explain why IOR is not always observed. Here we suggest that failures using the conventional (dilution) approach to CSW may have been caused by a positively charged oil-water interface that had not been identified.
Highlights
The typical CSW approach in carbonates is to switch from formation brine to seawater[15,18,19] (e.g., Fig. 1a) and/or to dilute formation brine or seawater[10,20] (e.g., Fig. 1b)
The zeta potential is a measure of the electrical potential in the diffuse part of the electrical double layer (EDL)[28] and modifies the electrostatic forces acting between the surface and the polar functional groups in the oil, consistent with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory[29]
Our results demonstrate for the first time that the oil-water interface can be positively charged at the high pH and ionic strength relevant to the formation brines found in many carbonate reservoirs
Summary
The typical CSW approach in carbonates is to switch from formation brine to seawater[15,18,19] (e.g., Fig. 1a) and/or to dilute formation brine or seawater[10,20] (e.g., Fig. 1b). Previous studies have typically utilised measurements of electrophoretic mobility to determine the zeta potential of artificial and natural calcite and carbonate[30,31,32,33,34,35,42,43,44,45,46]. In this approach, the sample is crushed to a fine powder and suspended in a solution of the electrolyte of interest. It can be challenging to maintain a stable suspension in such experiments, especially in natural brines with high ionic strength[44,50]
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