Abstract
When low-salinity water containing sulfate ions is injected into carbonate reservoirs, rock dissolution and in situ precipitation occur, altering rock permeability and wettability. Particularly, when barium ions are present in formation water, they react chemically with {text{SO}}_{4}^{2 - }, and BaSO4 is precipitated. These reactions can cause a serious impact on the efficiency of enhanced oil recovery (EOR). Therefore, the main purpose of this study was to identify EOR efficiency induced by low-salinity waterflooding (LSWF) when Ba2+ is present in carbonate reservoirs. From the experimental results, it was confirmed that the permeability calculated by the measured pressure difference was improved because of rock dissolution predominating over in situ precipitation for the case of low Ba2+ concentrations. In the analysis of wettability alteration through the measurements of relative permeabilities before and after LSWF, the higher Ba2+ concentration case consumed more {text{SO}}_{4}^{2 - } in precipitating the BaSO4, resulting in weaker wettability alteration due to the reduction of sulfate activity. These phenomena ultimately influenced EOR efficiency, i.e., the oil recovery was greater for the lower Ba2+ concentration.
Highlights
A significant proportion of the world’s oil is found in carbonate oil reservoirs
We investigated the changes in permeability and wettability initiated by rock dissolution and in situ precipitation, in response to lowsalinity waterflooding (LSWF) of carbonate reservoirs with Ba2? in the formation water (FW)
When lowsalinity water is injected into a carbonate reservoir containing Ba2? ions, rock dissolution and in situ precipitation occur via the following chemical reactions: CaCO3ðsÞ þ HþðIWÞ $ Ca2þðaqÞ þ HCOÀ3 ðaqÞ
Summary
A significant proportion of the world’s oil is found in carbonate oil reservoirs. More than 80% of carbonate reservoirs have oil-wet or mixed wettability characteristics (Hawez and Ahmed 2014; Dong et al 2017). Oil-wet systems typically have water breakthrough and high water production rates, which may result in high residual oil saturation. To improve oil recovery from carbonate reservoirs, wettability alteration is essential. One promising method to improve oil recovery is low-salinity waterflooding (LSWF). The mechanisms of LSWF proposed by various studies are summarized below. The mechanisms of LSWF proposed by various studies are summarized below. (1) Fines migration
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