Surfactant addition could improve oil recovery of water flooding in mixed-wet/oil-wet naturally fractured reservoirs by changing wettability or reducing water/oil interfacial tension (IFT). But the mechanisms of surfactant enhanced oil recovery (EOR) in fractured reseroirs are not fully understood. One objective of this work is to analyze the effects of surfactant properties (viscosity, concentration, and adsorption) on surfactant EOR at laboratory scale. The other objective is to obtain the functional relationships between surfactant properties and oil recovery of surfactant flooding.

A core with length of 8 cm and diameter of 4 cm is cut into two parts from the middle to imitate the matrix with a horizontal fracture whose space is 0.1 cm. The middle slide with a width of 0.1 cm is used as the model in this study. Fluid is injected from left side of the fractured and produced from the right side. The original properties of matrix, brine, and oil are from Ekofisk Field. The properties of surfactant are assumed based on literatures. Eclipse is used as the simulator.

The viscosity of surfactant solution has no obvious effect on ultimate oil recovery. But the time to obtain ultimate oil recovery, which is called the ultimate oil recovery time in this paper, is linearly increasing with the increase of viscosity. Since most of surfactants have no significant effect on viscosity of brine, the viscosity of surfactant solution is not a key parameter of surfactant screening for surfactant EOR in fractured reservoirs. The increase of surfactant concentration results in a decrease of oil recovery rate, and an increase of ultimate oil recovery. The ultimate oil recovery at 364 days of surfactant flooding has a negative linear relationship with inverse square root of surfactant concentration. The surfactant concentration has a more significant effect on oil recovery from lower matrix than the oil recovery from upper matrix. The ratio (RC) of the maximum mass of adsorbed surfactant to the mass of surfactant at critical micelle concentration (CMC) is used to describe the effect of surfactant adsorption on oil recovery. Two cases are studied and results imply that the surfactant capability of wettability alteration should be considered first for surfactant screening. If surfactants have the same capability of wettability alteration, then the surfactant adsorption is a key parameter, and the smaller surfactant adsorption concentration leads to a larger ultimate oil recovery.

This work helps to understand the mechanism of surfactant flooding in fractured reservoirs, and could be used as a reference for surfactant screening for surfactant EOR in fractured reservoirs. The functional relationships between surfactant properties and oil recovery help to improve upscaling methods.

You can access this article if you purchase or spend a download.