A method, apparatus, and program product model an Enhanced Oil Recovery (EOR) process in a subsurface volume by computer modeling a variable-concentration injection of an EOR agent in multiple grid blocks of a reservoir grid, where the variable-concentration injection includes an injection of the EOR agent at multiple injection concentrations. For a first grid block among the multiple grid blocks within which EOR agent is present, a viscous fingering effect is modeled by modifying a fluid mobility in the first grid block based at least in part upon the injection concentration of the EOR agent present in the first grid block.