Laboratory Evaluation of Chemical EOR Process for Viscous and High EACN Oil in East African Oilfields

Abstract

Poor conformance and unfavourable mobility ratio in the reservoir are common issues that hinder effective oil production and recovery. These issues are often exacerbated when the displacement is associated with viscous oil driven by water in highly permeable porous media. One of the reservoirs located onshore in East Africa suffers from this issue of high bypassed oil due to the presence of viscous oil, i.e. viscosity of 20 cP and API gravity of 28°. Most of the produced wells were observed to experience early water breakthrough and suffered from high water cut at present. In view of this phenomenon, Alkaline-Surfactant-Polymer (ASP) flooding was proposed to recover the remaining oil that is bypassed and trapped underneath. Through laboratory evaluation, the main objective of this study was to generate an effective chemical concoction comprising of alkaline, polymer and surfactant which are compatible to the formation fluids and rocks for the targeted reservoirs. The laboratory works were conducted based on a comprehensive and systematic workflow. Sample characterization and fluid-fluid analysis at the early phase encompass the studies of phase behaviours, physico-chemical properties, thermal stability etc. These studies aimed to screen off the chemical candidates which did not fulfill the corresponding Key Performance Indicators (KPI). The successful ASP formulations were subsequently optimized in terms of their respective concentrations without sacrificing the technical performance. Fluid-rock analysis aimed to quantify the oil recovery potential and reduction of residual oil saturation through coreflood experiments. Due to the high Equivalent Alkane Carbon Number (EACN) of the crude oil, i.e. 19 (nonadecane), and ultra-low salinity of formation water, i.e. 950 ppm, the greatest technical challenge in this study was to identify an effective surfactant which could reduce the water-oil interfacial tension (IFT) significantly under the operating range. Despite the challenges, an optimized ASP formulation, with a concentration of 0.5 wt.% of alkali, 0.2 wt.% of surfactant and 0.2 wt.% of polymer, successfully fulfilled all of the required KPI. The coreflood results indicate a reduction of residual oil saturation more than 30 % of Oil-In-Place. With these encouraging laboratory results, it could lead to the prospect of pilot and detailed Field Development Plan (FDP) studies, which could further de-risk the chemical EOR projects in this region.