Experimental investigation of CO2-foam stability improvement by alkaline in the presence of crude oil

Abstract

Dissolution of CO2 in heavy crude oil results in a significant reduction in the oil viscosity. However, CO2 injection in viscous oil reservoirs lacks acceptable sweep efficiency due to the high mobility of CO2. Co-injection of CO2 and an appropriate surfactant can generate CO2-foam in situ providings a means of controlling CO2 mobility and hence, improving sweep efficiency and oil recovery. Foam injection has been studied in the past but in the majority of these studies, foam has been investigated in the absence of crude oil. It is known that the stability of foam is adversely affected by crude oil and this problem has been a major obstacle for the application of foam injection for enhanced oil recovery (EOR).In this paper we present the results of a comprehensive series of visual experiments on foam stability in the presence of different heavy crude oil samples. Through these experiments, we have evaluated and compared the performance of a number of surfactants for foam generation. We have also tested the impact of three different alkalis (NaOH, Na2CO3, and Borate) on foam stability. The effect of gas type (CO2 versus N2) has also been investigated experimentally.The results show that while addition of NaOH drastically decreases the stability of CO2-foam, Na2CO3 and, in particular, Borate can significantly increase the stability of CO2-foam. For these alkalis, there is an optimum concentration which corresponds to maximum foam stability. The results also reveal that higher oil viscosity increases the stability of CO2-foam. Anionic surfactants with smaller carbon number produce better foam stability compared to non-anionic surfactants. As expected, foam stability increased with increasing surfactant concentration but there was an optimum surfactant concentration corresponding to maximum foam stability beyond which foam stability decreased. The results also revealed that N2-foam was more stable compared to CO2-foam.