Effect of Live-Crude-Oil Composition on Optimal Salinity of a Surfactant Formulation: Experiments and Modeling

Abstract

SummaryThe objective of aqueous-surfactant formulation design is to achieve ultralow interfacial tension (IFT) with the oil in place at reservoir conditions. Several parameters have to be investigated, and among them, it is well-known that the presence of gas dissolved in crude oil can greatly affect the surfactant/brine/crude-oil-microemulsion phase behavior. Omitting it might degrade the formulation performance. In this work, we present a combined experimental and theoretical investigation of optimal-salinity evolution as a function of live-oil compositions and conditions, varying the pressure independently of the gas/oil ratio (GOR) (i.e., the amount of gas dissolved in crude oil).A specific high-pressure/high-temperature (HP/HT) sapphire cell with a mobile piston is used to separately study the effect on the optimal salinity of the GOR by adding different amounts of n-alkanes methane (C1), ethane (C2), and propane (C3) at the corresponding saturation pressure, and the effect of pressure (up to 500 bar) is studied in a second cell by varying the cell volume (without changing the live-crude-oil composition).Using the HP/HT sapphire cell, we show that GOR variations (tested values up to 135 std m3/m3) induce important modifications of the brine/surfactant/oil-microemulsion phase behavior. In the case of the studied fluids, experimental data indicate that the optimal salinity of the brine/surfactant/oil system decreases linearly when increasing the amount of gas dissolved in the live crude oil. As a consequence of the Salager relation (Salager et al. 1979), the equivalent alkane carbon number (EACN) of the live crude oil varies linearly with the GOR. We demonstrate hereafter that the cell pressure alone (up to 500 bar), for a fixed composition (i.e., fixed GOR), affects neither the formation nor the stability of the Winsor III (WIII) microemulsion. Furthermore, results suggest that the composition of the dissolved representative gas can have an effect on the microemulsion phase behavior.Three models have been evaluated to estimate EACN values for 13 live crude oils and for the two live oils of interest in this study [Crude Oil 1 and n-tetradecane (n-C14)]. We compared predictions with our new experimental data. The Creton and Mougin (2016) model quantitatively predicts the behavior of live oils and agrees with the negligible effect of pressure on microemulsion formation and stability.