Seawater flooding has recently been investigated as a new emerging method on improving oil recovery in Middle East carbonates. This paper presents a mechanism study about seawater flooding in carbonates. The objective of this paper is to identify the key controlling factors for improving carbonates oil recovery by seawater injection.

We briefly describe the analytical model for Halfaya (HF) carbonate reservoir. A new parameter is introduced to analyze the salinity affecting on oil recovery. We carry out core flood experiments on selected carbonate core samples and formation water (166840-192450 ppm) from HF carbonates, Iraq. The formation water, seawater and treated seawater with different salinities and ionic compositions are injected to evaluate the effect on oil recovery. The relationship between brine salinity and recovery factor shows that the lower the salinity, the higher the recovery factor, and the optimum salinity (30150 ppm) is much close to that of seawater. Ion composition of seawater is modified to analyze the effects on oil recovery. Ion compositions of both injected and produced water are analyzed. Reservoir simulation predicts the production performance of well group to investigate the improvement of water cut and recovery factor for different kind of injected water.

Results indicate that seawater flooding is feasible for HF carbonate reservoirs. Low salinity of seawater samples with higher SO42- and lower Ca2+ concentration can dramatically improve the oil recovery 5-10%. The mechanism of enhanced oil recovery by seawater is mainly attributed to ion exchange happened during treated seawater flooding in carbonate rocks and the wettability alternation to more water-wet condition.

This study presents a method and parameters to handle core flood data and reservoir simulation results. Furthermore, this paper can be extended to assess other carbonate reservoirs developed by seawater flooding in the Middle East.

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