Alkali Surfactant Gas Injection: Attractive Laboratory Results Under the Harsh Salinity and Temperature Conditions of Middle East Carbonates

摘要

Alkali-surfactant-polymer (ASP) injection is an attractive enhanced oil recovery (EOR) technique that allows achieving almost zero residual oil saturation at the microscopic scale when well designed. In this combination of chemicals, the role of polymer is to achieve the necessary mobility control of the microemulsion / oil fronts which are formed and propagated through the reservoir. Foam has been recently identified as an alternative to polymer to achieve such mobility control. This paper describes the alkali-surfactant-gas (ASG) and surfactant-gas (SG) laboratory results which have been obtained on carbonate core samples under harsh salinity (~230 g/L) and temperature (83°C) conditions representative of some Middle East reservoirs. The starting point was the development of a surfactant formulation to achieve ultra-low interfacial tension between the oil and injected solution in these particular salinity and temperature conditions, using the classical microemulsion phase behavior approach. This formulation used a class of surfactants newly developed at The University of Texas at Austin (UTA), compatible with and without divalent ions. The efficiency (in terms of oil recovery) of this chemical formulation was demonstrated with SP core floods. The same chemical formulation was used for SG as the starting point, and was further enhanced; the polymer was replaced by nitrogen or methane co-injected with surfactant to create foam. Extensive studies of the ASG process have been performed. This includes phase behavior with and without alkali, screening laboratory studies to pre-select the surfactant with adequate foam properties, and carbonate coreflood experiments to measure the residual oil saturation to SG injection, including the consumption of chemicals. There is still room for optimization, but very promising results have already been obtained on that particular case leading to high recovery of the remaining oil after waterflood.