Impact of oil and wettability on foam-assisted conformance control in a Middle Eastern carbonate reservoir

Abstract

Gas injection has been evaluated and implemented in the large carbonate oil field located offshore Qatar as water alternating gas or WAG. It has been seen that WAG is economically viable and a robust enhanced oil recovery (EOR) method for homogenous formations. However, using WAG in heterogeneous parts of the field has not been attempted due to issues like early gas breakthrough and unfavourable sweep efficiency. Hence, it was decided to design a foam-assisted WAG system that can provide the required conformance control to mitigate those issues and expand the use of WAG. A complete laboratory programme was carried out to screen the best foaming surfactant formulation that resulted in the selection of environmentally friendly, Alkyl polyglucoside (APG) surfactant. Surfactant testing involved foamability in porous media under oil-wet and water-wet conditions in presence of residual oil to gas flood. The chosen surfactant showed good foam strength in both cases with effective mobility reduction factors (MRF) above 20. The impact of mobile oil was also observed by co-injecting oil during linear core flood experiments, which clearly showed that good foam strength could be achieved in a lower flow fraction of mobile oil with foam. Additionally, boosting of foam strength using amphoteric Lauryl Betaine (LB) surfactant was also performed. The formulation with APG/LB (70%/30%) gives foam boosting of 2.5 times compared to foam strength using APG alone. Additionally, it was shown that altering wettability using a nonionic Tergitol surfactant (TGT-1) as pre-flush and then injecting APG-5 surfactant and methane resulted in foam strength, which is 1.5 times higher than observed in oil-wet conditions.