Laboratory Studies for Surfactant Flood in Low-Temperature, Low-Salinity Fractured Carbonate Reservoir

The objective was to identify surfactants for Enhanced Oil Recovery by brine-oil interfacial tension reduction for a carbonate reservoir at ~ 25oC and salinity of ~11,000ppm TDS; thus, Alkyl Propoxy Sulfates and their blends with sulfonates were evaluated to determine optimal salinity and solubilization parameters with dead crude. Imbibition experiments were performed in reservoir and dolomite outcrop cores to determine the oil recovery efficiency of surfactant systems, selected from their phase behavior test results, with potential to recover oil. Tridecyl alcohol 13 propoxy sulfate (TDS-13A) with an oil solubilization parameter of ~8 at reservoir salinity was found to recover greater than 75% oil in imbibition experiments, at a concentration as low as 0.5wt%. The adsorption of surfactants on dolomite was measured at static and dynamic conditions; the adosprtion of TDS-13A was found to be ~ 0.26 – 0.34 mg/g reservoir rock. The effect of solution gas on surfactant phase behavior, up to 600-psi, was evaluated for methane, ethane, carbon-dioxide, and separator gas at 30oC. Methane had minimal effect on surfacatant optimal salinity, lowering it by ~2%/100psi solution gas, followed by carbon-dioxide which reduced it by ~11%/100psi. Ethane had a much more pronounced effect, reducing optimal salinity by ~46%/100psi solution gas. Introduction Enhanced Oil Recovery (EOR) in carbonate reservoirs with the use of surfactants has previously been demonstrated in both laboratory (Standnes et al., 2003; Hirasaki et al., 2004; Lu et al. 2012) and field studies (Chen et al., 2001; Rilian et al., 2010). Fractured oil-wet carbonate reservoirs tend to have a high percentage of original oil in place (OOIP) after forced displacement because, the injected fluids flow predominantly in the high permeability fractures, while most of the oil remains trapped in the low permeability matrix by capillary forces. Surfactants can reduce the brine-oil interfacial tension (IFT) to ultra-low values (<10 -3 mN/m) and/or alter the wettability of the rock to being preferentially water-wet, thereby overcoming the capillary forces that trap the oil. To achieve this, a surfactant or surfactant blend has to be tailored to the reservoir conditions: temperature, salinity (total dissolved solids (TDS), total hardness (TH)), crude oil. This paper describes laboratory studies that were performed to select a surfactant formulation, for a constant salinity EOR process in a fractured carbonate reservoir, at low temperature (~ 25°C) and low salinity (~ 11,000 ppm TDS; ~1,700ppm TH) conditions, and to determine the effect of solution gas on surfactant phase behavior at 30°C. Some of the desirable characteristics of a surfactant flood in a fractured carbonate reservoir are: ultra-low brine-oil IFT, wettability alteration from oil-wet to water wet condition, clear solutions in injection and reservoir brine, tolerance to divalent ions, thermal stability, no generation of viscous phases or emulsions, low surfactant rentenion by adsorption and phase trapping, mobility control for improved distribution of injected fluids in the fracture network, and easy separation of oil from produced emulsion. Wettability alteration, mobility control and emulsion separation were not addressed in this study. Anionic, cationic and nonionic surrfactants have all been used for IFT reduction. In this study we evaluated Alkyl Propoxy (PO) Sulfates (APS) and their blends with Internal Olefin Sulfonates (IOS), an Alkyl Benzene Sulfonate (ABS) and an Alkyl Xylene Sulfonate (AXS). For a given set of conditions (temperature, oil type, etc), surfactants may achieve ultra-low IFTs only in a range of salinitiy near optimal salinity. APSs and their blends with IOSs reported in other studies (Aoudia et al., 1995; Levitt et al., 2009; Barnes et al., 2010) have shown high solubilization parameter, an indicator of IFT reduction, but at optimal salinities or temperatures higher than those of this study. The PO groups of APSs lend tolerance to divalent ions, and provide a way to increase lipophilicity of the surfactant by adding more POs. IOSs by themselves have poor divalent ion tolerance (Liu et al.,