Reservoir Wettability Formation And Alteration: A Molecular Simulation Study

Reservoir wettability is an imperative factor for oil recovery. At present, using surfactants is one of the most common methods to alter the reservoir wettability. However, due to the complex interaction, including solution/solution and solution/rock, the process of wettability alteration is not clear. Therefore, it is necessary to develop the fundamental research for the wettability alteration. In this paper, molecular simulation is utilized to study the wettability formation and alteration of reservoir, and the oil recovery improved by wettability alteration is discussed as well.During the reservoir formation process, adsorption of oil compounds can alter the wettability of originally water-wet reservoir. The simulation results show that the formation of reservoir wettability can be divided into two processes. Firstly, the polar components in crude oil have a priority to migrate through water film and adsorb on the mineral surfaces, and the wettability turns from water-wet to oil-wet preliminary. And then, the pre-adsorption polar molecules would promote the adsorption of apolar components, which makes the oil-wet reservoir formed.The surfactants can alter the reservoir wettability effectively, and changing the wettability from oil-wet to water-wet. The simulation results show that there are two kinds of methods for surfactants to alter the wettability. Firstly, the tails of surfactants have strong hydrophobic interaction with oil-wet surface, and making the surfactants form self-assembly monolayer on the rock surface. And then, the hydrophilic groups of surfactants exposed on the surface, and making the oil-wet surface changed to water-wet surface. Secondly, the surfactant decreases the adhesive work between oil film and rock surface, which make the oil detach from rocks. And then, the hydrophilic rock surface exposed, making the oil-wet surface changed to water-wet surface.The wettability alteration can improve oil recovery. The simulation results show that as the enhancement of surface hydrophilic, the interaction strength between water molecules and surface would increase, and the water molecules would form a film on rock surface. This tightly bounded interfacial water would provide physical and energy barrier to impede the oil spreading on surface, and making the oil detached from surface. Therefore, the oil can be more easily exploited from the hydrophilic rocks.