| Summary: | Low salinity and composition effects in improving oil recovery in sandstone reservoirs are known. However, these effects have not been thoroughly studied for the carbonate reservoirs. Because of the lack of the clay minerals in the carbonate rocks, the mechanisms for the improved oil recovery with low salinity, brine composition, and temperature may not be the same as those for sandstones. This experimental study attempts to investigate the effects of low salinity, brine composition, and temperature on wettability and oil recovery in a dolomite reservoir. Also, it is attempted to confirm that wettability alteration is the main mechanism for improvement of oil recovery.
The experiments for this study were performed at both ambient and reservoir conditions as well as at a temperature of 250°F using two different techniques, Dual-Drop Dual-Crystal (DDDC) and coreflooding. Water-advancing contact angle was measured using the DDDC technique to characterize reservoir wettability with different salinities including twice, 10, 50 and 100 times diluted brines. Also, the effect of brine composition on wettability was investigated with Yates synthetic brine, Yates synthetic brine without sulfate, and brines containing sulfate in different concentrations. In addition, the effect of temperature on wettability was investigated using DDDC technique. Coreflood experiments were carried out using a dolomite core to determine aging time, to measure the oil recovery, and to confirm whether an optimal salinity brine and an optimal composition of brine obtained contact angle measurments improve the oil recovery compared with Yates synthetic brine. Oil-water relative permeabilities were generated by history matching the oil recovery and pressure drop data obtained from the coreflood experiments.
The experimental results showed that the wettability was altered from strongly oil-wet to intermediate-wet by diluting the Yates synthetic brine by about 50 times and increasing the amount of sulfate in Yates synthetic brine from 2.2 g/l to 4.4 g/l. Also, increasing the temperature to 250°F had a significant effect on wettability and changed the wettability from oil-wet to intermediate-wet. Coreflood results confirmed the wettability alteration to intermediate-wet and also demonstrated improvements in oil recovery induced by the optimal salinity and optimal brine composition.
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