Thermally Induced Wettability Alteration To Improve Oil Recovery in Fractured Reservoirs

Abstract

Summary Many naturally fractured reservoirs are oil-wet; during water injection, water will not imbibe into the matrix but will flow preferentially through the fractures, resulting in very low oil recoveries. For instance, the Ghaba North field in Oman is an extensively fractured, oil-wet carbonate that has achieved only 2% recovery after more than 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 increases. The temperature could be increased in a reservoir setting through steam or hot-water injection. It is proposed to inject steam or hot water to heat the matrix sufficiently, inducing a wettability change and rendering the matrix water-wet. Hot water in the fractures can spontaneously imbibe into the matrix, displacing oil and resulting in favorable oil recoveries. A 1D model of the saturation and temperature profiles during imbibition into a matrix block is developed and solved analytically. Using Ghaba North properties, it is shown that the imbibition rate is limited by the diffusion of heat through the oil. The advancing water front is located where the rock temperature equals the transition temperature for wettability change. It is estimated that approximately 30% oil recovery could be achieved in a single matrix block after approximately 700 days. In less permeable media, the imbibition rate is limited by capillary forces, and the temperature front moves ahead of the water, resulting in slower recovery.