Surfactant treatment of oil-wet fractured carbonate reservoirs

Most carbonate reservoirs are mixed-wet/oil-wet and fractured. Conventional water flooding recovers little oil in fractured oil-wet reservoirs because water flows through the fractures and does not imbibe into the matrix due to negative capillary pressure. Surfactant solutions can be used to alter the wettability of carbonate rocks such that water can imbibe in, resulting in oil production.;Several anionic and non-ionic surfactants are identified which are stable in the brine solution up to a temperature of 95° C and 82000 ppm salinity, and which can alter the water-oil contact angle to a value below 90°. 30-70% OOIP is obtained for many surfactants at very low surfactant concentrations (<0.1 wt %) in tight (∼15 md) carbonate cores. ATR-IR spectroscopy is carried out on surfactant treated calcite plates to understand the mechanism of wettability alteration by anionic surfactants. Sensitivity of the process on reservoir temperature, salinity, surfactant concentration and type of surfactant is discussed. Wettability of carbonates can also be altered by divalent ions at high temperature (90° C and above). The effect of Ca2+, Mg2+ and SO42- ions on the wettability is studied. Various parameters are identified which can improve the oil recovery rates from oil-wet carbonate rocks. Mechanistic numerical simulations are conducted to understand the process by analyzing in-situ distribution of flow streamlines and other fluid/rock properties. The process is found to be gravity driven.;The surfactant molecules diffuse from the fractures into the matrix and change the IFT between the aqueous and oil phase This leads to the reduction in capillary entry pressure. The gravitational force overcomes the capillary pressure and aqueous phase invades from the bottom of the core pushing oil out from the top. Surfactant also changes the wettability from oil-wet to water-wet state resulting in an increase in the relative permeability of oil, thus increasing the rate of oil recovered. This process can help recover large amounts of lost oil in fractured oil-wet carbonate reservoirs.