Impact of Low-Cost Dilute Surfactants on Wettability and Relative Permeability

Abstract

Abstract The decline of surfactant flooding can be attributed to the expense of reducing oil-water interfacial tension by several orders of magnitude. This study examines the potential for enhancing oil recovery by using low-cost surfactants to alter wettability. The wettability alterations by the surfactants have been characterized using contact angles and oil-water relative permeabilities. Core flooding experiments were conducted using three different rock-fluids systems:Berea sandstone, Yates stocktank crude oil, Yates synthetic brine;Yates dolomite rock, Yates stocktank crude oil, Yates synthetic brine; andYates dolomite rock, Yates live crude oil, Yates synthetic brine. Two types of surfactants, one nonionic and the other anionic have been used for wettability alteration. The experiments with Berea were conducted at ambient conditions, while dolomite rock tests were conducted at Yates reservoir conditions of 82oF and 700 psi. Contact angles were measured using Yates reservoir fluids and dolomite crystal surfaces using the Dual-Drop Dual-Crystal technique and oil-water relative permeabilities were computed by history matching oil recovery and pressure drop data from the coreflood experiments. The shifts in relative permeability ratio curves indicate the ability of both surfactants to develop mixed-wettability in Berea sandstone cores. In the Yates dolomite rock, Yates stocktank oil and Yates brine system, both surfactants altered wettability from an initially strongly oil-wet to a weakly oil-wet state. For Yates live oil containing rock-fluids system, the nonionic surfactant altered the wettability from an initial weakly water-wet to a mixed-wet state, while anionic surfactant resulted in wettability alterations to a strongly oil-wet state. The contact angle measurements corroborated well with oil-water relative permeabilities to confirm the wettability shifts except for Yates live oil containing system with nonionic surfactant, where contact angles indicated wettability alterations to an intermediate-wet state. This study thus demonstrates the relative benefits of surfactant-induced wettability alteration mechanism to develop either intermediate or mixed wettability in reservoir rock-fluids for significant oil recovery enhancements. 1. Introduction 1.1 Background Surfactants are polar compounds, consisting of an amphiphilic molecule with both hydrophilic and hydrophobic parts. The surfactants are classified as; anionic, cationic, amphoteric and nonionic depending upon the nature of the charge present on hydrophilic group. The two basic features of the surfactants that make them unique for use in oil industry are their ability to lower oil-water interfacial tension and alter the reservoir wettability. The suitability of surfactants for improved oil recovery applications in oil industry can be well understood through the definition of a dimensionless number called "capillary number". The capillary number is defined as the ratio of viscous to capillary forces and is given by, …………………………..………………… (1) where v and µ are the velocity and viscosity of the displacing phase, respectively and s is interfacial tension between the fluid phases, and ? is contact angle between the fluid-fluid interface and the solid surface.