Enhancing Oil Recovery from Dolomitic Reservoirs- Investigation of Temperature Limitations

Abstract

Abstract EOR by Smart Water injection in calcite, CaCO3, is dependent on factors like initial wetting, injection brine composition and temperature. Because of the chemical difference between CaCO3 and dolomite, CaMg(CO3)2, the relationship between EOR effects and the above-mentioned factors could be different. Smart Water EOR potential was experimentally tested for two dolomitic reservoir systems, at low and high reservoir temperature, to determine if there was a temperature limitation for observing EOR effects by Smart Water injection. Oil recovery tests by spontaneous and forced imbibition experiments were performed to evaluate initial wetting and wettability alteration from two dolomitic reservoir systems. A fractured reservoir system at 65 °C was investigated by spontaneous imbibition tests, while a non-fractured system at 115 °C was investigated by forced imbibition tests. The reservoir cores were saturated with their respective formation waters and crude oil, then imbibed with diluted seawater or modified seawater brines. Oil recovery responses at the different temperatures and by using different imbibing brines were evaluated and compared. Spontaneous imbibition tests, performed in the fractured reservoir system, showed that a diluted seawater brine increased the oil recovery from the dolomitic core material at 65 °C. When a modified seawater brine, which is a Smart Water for CaCO3, was used, there was no EOR effect observed. Forced imbibition tests with diluted seawater brine showed increased oil recovery for a non-fractured dolomitic reservoir system at 115 °C, while a modified seawater resulted in no extra oil produced. The results showed that there was no temperature limitation for observing EOR effects in dolomitic reservoir core material within the temperature range 65 - 115 °C. It was confirmed that diluted seawater brines altered the initial core wettability towards more water-wet conditions and enhanced oil recovery was observed both in spontaneous imbibition and forced imbibition experiments. Due to the differences in mineralogy between CaCO3 and dolomite, CaMg(CO3)2, and in their chemical reactivity, the optimal injection water, i.e. Smart Water compositions, were different. Smart Water EOR effects by wettability alteration in dolomitic reservoir material were observed both at low and high reservoir temperatures. The final oil recovery was dependent on the injection brine composition. Increased oil recovery was experienced in both spontaneous and forced imbibition tests.