Intermittent Alkali Flooding in Vertical Carbonate Reservoirs (SPE-121832)

摘要

Description and Application Intermittent alkali flooding can significantly enhance oil recovery in oil-wet carbonate reservoirs. The method basically acts in two ways, by reducing the interfacial tension and reverse the wettability to a more favorable condition. However, the reversal of wettability requires aging time to reach the equilibrium. An intermittent or a pausing period is then added into an alkali flooding process to let the surface reach maximum favorable wettability. As the injected slug is paused during flood process, vertical or inclined reservoirs are suitable for this combination because water tongue effect will not cause an early breakthrough. Results Laboratory results show that one-week-intermittent alkali flooding in homogeneous carbonate rock yields greater oil recovery about 10 - 15 percent greater than conventional continuous alkali flooding in a proper range of injected alkali concentration. Very low alkali concentration causes alkali depletion as time increases, while very high alkali concentration causes pore plugging by insoluble in-situ soap. The alkalinity of injected slug has to be kept as high as possible. Therefore, strong base such as sodium hydroxide is recommended for this combination. High acid concentration in crude drives in-situ saponification forward hence; the design for alkali concentration range has to be more specified. High initial water saturation prevents the system from alkali accumulation and as a consequence, insoluble soap is less concerned. Fractured carbonate reservoir is possibly a good candidate since aging time would allow alkali to diffuse to, and reverse the wettability of the inaccessible zone. Significance The proposed technique seems very effective to increase the ultimate oil recovery in oil-wet carbonates. The drawbacks are proven to be acceptable and expected result is economical.