Effect of Rate and Viscosity on Gas Mobility during Solution-Gas Drive in Heavy Oils

Abstract

Abstract Some heavy oil reservoirs under primary production have shown higher recovery and production rates than expected. Many studies have shown that gas mobility in these reservoirs is low contributing to improved oil displacement and also that gas mobility may depend not only on gas saturation but also on depletion rate and oil viscosity. Despite the observed effect of oil viscosity and depletion rate on gas mobility, it is not yet clear to what extent these factors affect relative permeability and critical gas saturation. In this study, we have conducted depletion experiments in a linear unconsolidated sand-pack. Three oils were used, with oil viscosities that varied by a factor of 30. The experiments were conducted at two different depletion rates for each of the oils. Experimental results show that as oil viscosity or depletion rate increases, critical gas saturation increases, gas mobility decreases and oil displacement becomes more effective. Critical gas saturation for all the runs was in the range of 0.2 – 5±1%. Furthermore, it was found that relative permeability to gas decreased as withdrawal rate or oil viscosity increased. The effects of oil viscosity and withdrawal rate were combined in the form of a depletion index, such that oil recovery increased as the depletion index increased.