Optimization of Immiscible Alkaline-Surfactant-Alternated-Gas/CO2 Flooding in an Upper Assam Oilfield

Abstract

Abstract The immiscible alkaline-surfactant-alternated-gas/CO2 (ASAG) flooding comprise of alternately injecting alkali-surfactant (AS) formulations and gas/CO2 into the formation leading to the generation of in-situ foam. Optimization of the ASAG process is a necessary step prior to its application in a particular oil field for cost-effective enhanced oil recovery (EOR) approach. In this article, optimization of the ASAG flooding in an Upper Assam oilfield was studied through lab-scale core flood experiments. First, reservoir core samples, crude oil, and formation water of the oilfield were characterized. Second, AS formulations were designed through foam stability tests, phase behavior tests, and equilibrium interfacial tension (IFT) measurements. Third, a total of 15 lab-scale core flooding tests were conducted through different ASAG injection schemes. Crude oil of the oilfield was medium gravity with acid number equaling 2.1 mg of KOH/gm. The average porosity and permeability of core samples were found to be approximately 20% and 6 md respectively. XRD and FESEM analysis showed the presence of silica, quartz and smectite clays in the rock samples. The total salinity based on the ionic composition of the formation water was found to be 3458 ppm. The foam stability tests indicated that maximum foaming behavior in presence of crude oil occurred at specific combination of surfactant, alkali, and brine salinity. The optimal salinities determined by salinity scan were found to concur with the salinities at which lowest equilibrium IFT values were obtained for the AS formulations. The lowest IFT values were in the ultra-low range (less than 0.01 mN/m). The favorable operating parameters with best oil recovery during ASAG flooding were obtained with 1:1 slug ratio, 0.25 pore volume (PV) slug size, 0.2 ml/min gas injection rate, 2 PV injected fluid volume and combined gas-liquid tapering. The application of negative salinity gradient during ASAG flooding could further improve the additional oil recovery to 29.19 % original oil in place. Moreover, the use of a sacrificial agent during waterflooding proved to be valuable in economically improving the cumulative oil recovery by ASAG flooding.