Wettability Alteration During Surfactant Flooding of Carbonate Reservoirs

Abstract

Abstract Recovery from fractured, oil-wet/mixed-wet, carbonate reservoirs by waterflooding is poor. Dilute surfactant methods are being developed to improve oil recovery from fractured carbonates. This paper investigates the interactions of dilute alkaline anionic surfactant solutions with crude oil on carbonate mineral surfaces. Wettability, phase behavior, interfacial tension and adsorption experiments have been conducted. Anionic surfactants have been identified which can change the wettability of the calcite surface to intermediate/water-wet condition as well or better than the cationic surfactant DTAB with a West Texas crude oil. All the carbonate surfaces (Lithographic Limestone, Marble, Dolomite and Calcite) show similar behavior with respect to wettability alteration with an anionic surfactant. Anionic surfactants, which lower the interfacial tension with the crude oil to very low values (<10−2 mN/m), have also been identified. The adsorption of the sulphonate surfactants can be suppressed significantly by the addition of the alkali. Introduction About half of world's known oil reserves are in carbonate reservoirs.1 Many of these carbonate reservoirs are naturally fractured. Fractured reservoirs are poorly understood and recovery from fractured reservoirs is typically lower than those from unfractured reservoirs. Fractured reservoirs with high matrix porosity and low matrix permeability need a secondary or EOR technique to achieve maximum production, according to a recent review of 100 fractured reservoirs.2 Recovery factor in these reservoirs depends on matrix permeability, wettability, fracture intensity and fluid properties. Water-flooding is an effective technique for fractured reservoirs, if the formation is water-wet. The positive capillary force helps in imbibing water into the matrix and expelling the oil out. However, many carbonate reservoirs (about 80%) are mixed-wet or oil-wet.3 Positive zeta potential on carbonate surfaces at typical pH (~7),4 presence of sufficient number of negatively charged / asphaltenic molecules in oil,5 and high capillary pressure during oil migration into the reservoir often render mixed-wettability to the reservoir rocks.6 Waterflooding does not lead to significant recovery from the matrix porosity because the capillary pressure curve is predominantly negative. Surfactant flooding (or huff-n-puff) is being tested7 to improve oil recovery from mixed-wet, fractured carbonate formations and is the subject of this study. Surfactant-polymer flooding,8 low tension polymer flooding,9 and alkaline-surfactant-polymer flooding10–12 were developed in 1960s through 1980s. They were technical successes, but have not been implemented in many fields due to low oil prices in the last fifteen years. These surfactant flooding methods were developed for sandstone reservoirs. Fractured carbonates are different from sandstones in many respects. The zeta potential on carbonate minerals is positive (negative on silica at normal pH), pore structure is more heterogeneous (presence of fractures, vugs and dead-end pores), and permeability (of the matrix) can be significantly lower. Wettability alteration to water-wet or intermediate-wet conditions is an important mechanism for improving oil recovery in fractured media.