Thermally Induced Wettability Alteration to Improve Oil Recovery in Fractured Reservoirs

摘要

Many naturally fractured reservoirs are oil-wet and duringwater injection water will not imbibe into the matrix but willpreferentially flow through the fractures resulting in very lowoil recoveries. For instance, the Ghaba North field in Oman isan extensively fractured oil-wet carbonate which that has onlyachieved 2% recovery after over 20 years of production.Experiments on core from fields in Oman and elsewherehave indicated that the rock will undergo a transition from oilwetto water-wet as the temperature is increased. Thetemperature could be increased in a reservoir setting throughsteam or hot water injection. The wettability change is due todesorption of asphaltenes from the rock surface.It is proposed to inject steam or hot water to heat thematrix sufficiently to induce a wettability change, rendering itwater-wet. Hot water in the fractures can spontaneouslyimbibe into the matrix displacing oil and resulting in favorableoil recoveries.A one-dimensional model of the saturation andtemperature profiles during imbibition into a matrix block isdeveloped and solved analytically. Example solutions andtypical time scales for recovery are proposed. Using GhabaNorth properties the imbibition rate is limited by thermaldiffusion through the oil. The advancing water front is locatedwhere the rock temperature equals the transition temperaturefor wettability change. It is estimated that around 30% oilrecovery could be achieved in single matrix block afterapproximately 700 days. In less permeable media, theimbibition rate is limited by capillary forces, and thetemperature front would move ahead of the water, resulting inslower recovery.