Thermally Induced Wettability Alteration to Improve Oil Recovery in Fractured Reservoirs

Abstract

Abstract Many naturally fractured reservoirs are oil-wet and during water injection water will not imbibe into the matrix but will preferentially flow through the fractures resulting in very low oil recoveries. For instance, the Ghaba North field in Oman is an extensively fractured oil-wet carbonate which that has only achieved 2% recovery after over 20 years of production. Experiments on core from fields in Oman and elsewhere have indicated that the rock will undergo a transition from oil-wet to water-wet as the temperature is increased. The temperature could be increased in a reservoir setting through steam or hot water injection. The wettability change is due to desorption of asphaltenes from the rock surface. It is proposed to inject steam or hot water to heat the matrix sufficiently to induce a wettability change, rendering it water-wet. Hot water in the fractures can spontaneously imbibe into the matrix displacing oil and resulting in favorable oil recoveries. A one-dimensional model of the saturation and temperature profiles during imbibition into a matrix block is developed and solved analytically. Example solutions and typical time scales for recovery are proposed. Using Ghaba North properties the imbibition rate is limited by thermal diffusion through the oil. The advancing water front is located where the rock temperature equals the transition temperature for wettability change. It is estimated that around 30% oil recovery could be achieved in single matrix block after approximately 700 days. In less permeable media, the imbibition rate is limited by capillary forces, and the temperature front would move ahead of the water, resulting in slower recovery.