Design of a Novel Flooding System for an Oil-Wet Central Texas Carbonate Reservoir

Abstract

ABSTRACT Alkaline-surfactant-polymer flooding (ASP) as a method for oil recovery has traditionally been applied to sandstone reservoirs. This paper describes the first application of ASP technology with wettability alteration in a carbonate reservoir and compares laboratory performance of alternate chemical flooding processes. The oil-wet Cretaceous Upper Edwards reservoir, located in Central Texas, has characteristics generally favorable for chemical flooding. The reservoir has low brine salinity, low temperature, moderate oil viscosity, high residual oil saturation, and a diverse carbonate lithology without anhydrite or gypsum. This reservoir targeted for EOR, is currently produced by a strong, natural water drive from wells on a 2-acre spacing. Extensive reservoir characterization has been performed to establish the extent of the resource. A laboratory study was conducted to compare alternative EOR processes for individual geologic zones identified by describing and subdividing the reservoir. Coreflood experiments using fresh Upper Edwards reservoir core, have given post-waterflood oil recoveries of 45.3%, 10.0%, and 11.6% from small, low-concentration slugs of alkaline-surfactant-polymer, alkaline-polymer, and polymer formulations, respectively. These laboratory evaluations have shown that ASP formulations of sodium tripolyphosphate and sodium carbonate along with a low concentration of petroleum sulfonate maintain interfacial activity upon extensive dilution with reservoir brine and give significantly better oil recovery than alkaline-polymer or polymer formulations. The results indicate that a cost effective alkaline-surfactant-polymer process can be designed for application in a carbonate reservoir.