(360h) How Molecular Structure of Surfactants Determine the Dynamics and Viability of Wettability Alteration

Only about 35% of oil in place is recovered from carbonate reservoirs through primary and secondary flooding because of oil wet surfaces and resulting unfavorable capillary pressures. Surfactants can improve oil recovery significantly by altering wettability of surfaces. However, the process of selecting an effective surfactant for wettability alteration is dependent on a number of factors, including mineral type, temperature, salinity, and nature of adsorbed oil and ultimately how the molecular structure of the surfactant interacts with all of these. Here we present an experimental study of the effectiveness of nonionic surfactants with different hydrophobic groups and different lengths of hydrophilic ethylene oxide oligomers. The surfactants selected alter the wettability of the rock primarily by acting on the water-rock and oil-rock interfaces and have minimal effect on the interfacial tension of the oil-water interface. The dynamics of wettability alteration is measured by the evolution of contact angles of oil-drops on initially oil-wet surfaces and film coverage for surfactant-induced dewetting of oil-films. The hydrophobic groups are found to determine which surfactants primarily act on rock interfaces. The rate and degree of wettability alteration are strongly controlled by the length of the ethylene oxide oligomers. Shorter oligomers lead to faster and greater wettability alteration and are least sensitive to the effects of temperature and salt concentration. Experimental observations are efficiently summarized by a few thermodynamic and kinetic parameters. A theory is presented to describe the dynamics and provide a framework to extract rate constants for the adsorption and desorption of the surfactants to the rock interfaces. These rate constants and the surfactants effect on contact angle are used in a pore-scale model for imbibition in the presence of surfactant to determine which surfactants structures are practically viable for wettability alteration in oil-wet carbonate reservoirs.