Modeling the Impact of Capillary Pressure Reduction by Surfactants

Abstract

Abstract Fluid recovery after fracturing treatments in tight gas reservoirs is often critical in obtaining maximum production rates. Fluid recovery aids in washing broken fracturing fluid out of the propped fracture and restores the relative permeability to gas in the invaded matrix of the fracture face. However, recovery of fracturing fluids from the invaded matrix requires overcoming both capillary pressure-driven imbibition and the capillary exit pressure that occurs at the fracture boundary between the matrix and propped fracture. Surface active materials, such as alcohols and surfactants, are often used to reduce the surface tension, and hence the capillary pressure functions in the matrix. These materials are expected to enhance recovery of the fracturing fluid from the matrix, improving load recoveries. Modeling this process with classical reservoir simulators is not possible because they do not allow for the impact of surface tension reduction on capillary pressure functions. A previously reported simulator for modeling fracturing fluid movement has been modified to accommodate the impact of surface tension reduction on capillary pressure. This simulator uses the simultaneous solution (SS) method for solving the two-phase flow equations, followed by calculation of the movement of chemicals based on the pressure solution at each time step. A new modification to the SS method has been developed to allow proper modeling of the impact of capillary pressure reduction by surfactants. Surfactants can reduce capillary pressure in the matrix by reducing surface tension and/or by changing the wettability of the rock surface when they adsorb on a surface. The classical SS method does not maintain proper "mass balance" when surfactants are allowed to change the capillary pressure; the reason for which is demonstrated in this work. The new modification allows surfactants to impact capillary pressure through surface tension reduction and wettability changes.