Impact of Live Crude Oil Composition On Optimal Salinity of A Surfactant Formulation

Abstract

Abstract The objective of surfactant formulation design is to achieve ultra-low interfacial tension (IFT) with the oil in place in reservoir conditions. Several parameters have to be investigated, the presence of dissolved gas in crude oil can greatly impact the surfactant/brine/crude oil microemulsion phase behavior and omitting it may degrade the formulation efficiency. We propose an experimental investigation of optimal salinity evolution as a function of live oil compositions and conditions varying the pressure independently to the gas to oil ratio (GOR), i.e. the amount of gas dissolved in crude oil. A specific High Pressure - High Temperature (HPHT) sapphire cell with a mobile piston is used to separately study the impact on the formulation optimal salinity of: (i) the GOR by adding different amounts of C1 to C5 n-alkanes at the corresponding saturation pressure; and (ii) the pressure cell – up to 500 bar – by varying the cell volume (without changing the live crude oil composition). Experiments were performed at 40°C and at the saturation pressure or above. Using the HPHT sapphire cell we show that GOR variations up to 135 Sm3/m3 induce important modifications of the {surfactant/brine/oil} microemulsion phase behavior. In the case of the studied fluid, experimental data indicate that the optimal salinity of the {brine/surfactant/oil system} decreases when increasing the amount of gas dissolved in the live crude oil. As a consequence, the equivalent alkane carbon number (EACN) of the live crude oil strongly depends on the gas to oil ratio. We demonstrate hereafter that the cell pressure– for a fixed composition (i.e. fixed GOR) –impacts neither the formation nor the stability of the microemulsion. Furthermore, preliminary results suggest that the composition of the dissolved gas has a slight effect on the microemulsion phase behavior. In this work, using a specific HPHT sapphire cell, we are able to dissociate the impact of the amount of added gas from the impact of the cell pressure and to consider pressures up to 500 bar. Whereas the pressure alone has a negligible influence on the surfactant/brine/oil microemulsion phase behavior, the dissolution of gases in crude oil leads to a decrease of the optimal salinity and a variation of live crude oil EACN.