Effect of Temperature on Enhanced Oil Recovery from Mixed-Wet Chalk Cores by Spontaneous Imbibition and Forced Displacement Using Seawater

Seawater, as wettability modifier in chalk, has been studied at different temperatures and flooding conditions. Without sulfate initially present in the formation water, previous studies by spontaneous imbibition have shown that a temperature of about 100 °C is needed for the wettability modification to take place. The temperature plays a very important role for the chemical reactions taking place at the chalk surface and for the diffusion of active ions (Ca2+, Mg2+, and SO42−) into the chalk matrix. In the present study, the enhanced oil recovery using seawater was studied by both spontaneous imbibition and viscous flooding at 90, 110, and 120 °C. A crude oil of high acid number (AN = 1.90 mg of KOH/g) was used to ensure a low, initial water wetness. At 90 °C, no difference in oil recovery using seawater and formation water in a spontaneous imbibition process was observed, but in the following viscous flooding process, seawater enhanced the oil recovery by 14% compared to formation water. At 110 and 120 °C, the impact of wettability modification using seawater became even more significant regarding oil recovery, but still, the viscous flood by seawater improved the oil recovery significantly compared to formation water. As a conclusion, improvements in oil recovery using seawater as a wettability modifier are obtained even at 90 °C, when displacing the oil by a viscous flood at low flooding rate.