Mechanistic simulation of continuous gas injection period during surfactant-alternating-gas (SAG) processes using foam catastrophe theory

The use of foam for mobility control is a promising means to improve sweep efficiency in subsurface applications such as improved/enhanced oil recovery and aquifer remediation. Foam can be introduced into geological formations by injecting gas and surfactant solutions simultaneously or alternatively. Alternating gas and surfactant solutions, which is often referred to as surfactant-alternating-gas (SAG) processes, is known to effectively create fine-textured strong foams in situ by repeating drainage and imbibition processes. Recent studies show that foam rheology in porous media can be characterized by foam catastrophe theory which exhibits three foam states (weak-foam, strong-foam, and intermediate states) and two strong-foam regimes (high-quality and low-quality regimes).Using both mechanistic foam simulation technique and fractional flow analysis which are consistent with foam catastrophe theory, this study aims to understand the fundamentals of dynamic foam displacement during gas injection in SAG processes. The results revealed some important findings: (1) the complicated mechanistic foam fractional flow curves (f_w vs. S_w) with both positive and negative slopes require a new way to solve the problem analytically rather than the typical method of constructing a tangent line from the initial condition; (2) none of the conventional mechanistic foam simulation and fractional flow analysis can fully capture sharply changing dynamic foam behavior at the leading edge of gas bank, which can be overcome by the pressure-modification procedure suggested in this study; (3) four foam model parameters (?p_o, n, C_g/C_c, and C_f) can be determined systematically by using an S-shaped foam catastrophe curve, a two flow-regime map, and a coreflood experiment showing the onset of foam generation; and (4) at given input data set of foam simulation parameters, the inlet effect which explains a delay in strong-foam propagation near the core face is scaled by the system length, and therefore the change in system length requires a new set of individual lamella creation and coalescence parameters (C_g and C_c).