Surfactant polymer formulation design targeting high salinity high-temperature giant Middle East carbonate reservoir

Chemical flooding has proven its potential in sandstone reservoirs, however, only a few case studies have been deployed in carbonate reservoirs for several reasons. These include, but not limited to, formation heterogeneities, natural fractures, oil wet conditions and most importantly, high temperature and high salinity (HTHS) conditions.These challenging conditions require either specially designed products or existing products require further formulation improvements. This study is focused on developing an effective formulation of surfactants at 167000 ppm TDS and 120 oC for a giant Middle East carbonate reservoir conditions. Earlier experimental results on biopolymer at HTHS conditions have shown its effectiveness as mobility control agent. Furthermore, an extensive screening study is performed on different surfactants to achieve low interfacial tension (IFT) and compatibility with brine at target conditions. Numerous formulations of surfactants are instigated. Experimental results show that cationic surfactants are incompatible with brine at targeted reservoir conditions. Nevertheless, lowering their pH values result in good compatibility of these surfactants but their applicability in carbonate formation is compromised and resulting high HLB values reduce their lipophilic properties. Ethoxylated alcohols type nonionic surfactants are found to be unsuitable due to a drastic lowering of their cloud points in high salinity conditions. Based on experiments and their structure-performance relationships, anionic surfactants of type alkyl ether carboxylate (AEC) were preselected for their superior salt and temperature tolerant abilities. However, their low oil solubilization capability at HTHS conditions is found unfavorable. Therefore, combinations of AEC surfactants with moderate chain length (C14-C16) internal olefin sulfonate (IOS) surfactants are designed to enhance the oil solubilization values. Further experiments are conducted to improve formulation compatibility at reservoir conditions. The addition of a short chain alkyl poly glycoside (APG) surfactant in small proportion provides the desired clear formulation with good oil solubilization values. Phase behavior tests of formulations with reservoir dead crude oil are performed at 120 oC in high-pressure glass tubes and a formulation of AEC, IOS and APG is selected. IFT tests between equilibrated crude oil and surfactant solution demonstrate the aptness of formulation showing an IFT value of 0.001 mN/m. These extensive tests and their favorable results demonstrate a workable formulation designed for carbonate reservoirs for the surfactant-polymer injection process. The studied combination of surfactants will open up the new perspectives to highly challenging tasks of surfactant selection for HTHS carbonate reservoirs.