Effect of Initial Sulfate in Reservoir Chalks on the Spontaneous Imbibition of Sea Water

Sea water (SW) with sulphate as one of the active ions, has been reported to alter the wettability of outcrop chalk to more water-wet. Reservoir chalk samples contain higher sulphate concentration than found to affect the wettability of outcrop chalk. The main objectives for the study were to determine the effect of the initial sulphate in reservoir chalk on the spontaneous imbibition of SW and how representative wettability conditions can be established. Reservoir chalk with and without initial sulphate were prepared using synthetic formation water without sulphate and with sulphate concentration as in real formation water. The spontaneous imbibition of SW was faster and higher for the reservoir rock with initial sulphate than for the rock without initial sulphate. This means that the initial sulphate made the reservoir chalk more water-wet. Reservoir chalk should be prepared with the same initial sulphate concentration as in the reservoir area where the samples are taken. This is required to obtain correct potential estimates for water flooding and enhanced oil recovery methods. The amount of sulphate in the original reservoir chalk and in the chalk after restoration should be determined by analysis of effluent samples during cleaning and restoration of core plugs.