Although over 60% of the world-wide hydrocarbon reserves are held in carbonate reservoirs, chemical flooding application and research endeavors have been focused on sandstones rather than carbonates. Challenges to develop chemical EOR formulations for carbonate reservoirs are significant and unique because of the complexity of rock mineral compositions, matrix pore structures, rock surface properties, fracture density, aperture and orientation, as well as oil types. The fact that some chemical EOR technologies have been successfully used in sandstone reservoirs cannot be simply extrapolated to carbonate reservoirs. Carbonate reservoirs are usually characterized as low permeability matrix with fractures, while a considerable portion present high permeability matrix with fractures as shown in the Middle East. This paper presents the development of chemical formulations addressing the challenges of chemical EOR in a representative Middle East carbonate reservoir, which has high reservoir temperature and high brine salinity. In the screening process, more than 50 surfactants and 30 polymers were studied at both ambient and reservoir conditions. Few surfactant-polymer (SP) formulations were optimized in terms of good compatibility with field brines, low interfacial tensions between the crude oil and chemical solution, and low adsorption of chemicals on the carbonate core samples. The results of the compatibility tests showed that an effective SP slug can be prepared in regular Arabian Gulf seawater without the requirement of water softening. Amphoteric surfactants showed a promising performance for the given reservoir conditions. Oil displacement tests using selected SP formulations demonstrated significant recovery potential in tertiary mode. This paper presents an overview of the chemical EOR research for the carbonate reservoir and provides insights of chemical enhanced oil recovery from carbonate rather than sandstone reservoirs.


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