A microvisual study of solution-gas-drive mechanisms in viscous oils

The literature suggests that oil-phase viscosity has a profound effect on oil production during solution gas drive. Some heavy-oil reservoirs show higher than expected production rates, produced gas-oil ratio (GOR) close to solution GOR, and relatively high recovery. The reasons for this behavior are not clear, but it has been suggested that gas-phase nucleation, growth of gas bubbles, and eventual coalescence into a continuous gas phase must be examined in detail to interpret better recovery. To this end, solution-gas-drive (depletion) experiments were conducted in micromodels, oil-lens drainage was observed and modeled in cornered capillary tubes, and bubble growth modeled. Experiments and calculations were conducted with viscous and low-viscosity liquids to probe explicitly the effect of solution viscosity. A micromodel is a two-dimensional representation of pore space. The micromodels employed are etched from silicon and exhibit pore body and throat sizes equivalent to a representative sandstone. Likewise, they capture many aspects of pore-wall roughness. Micromodels are housed in a pressure vessel to allow experimentation at elevated pressure. Pore-level events are viewed through an optical microscope. Spatial resolution of events within the micromodel is on the order of 1 mum. Nucleation occurs repeatedly and regularly at surface roughness. Nucleated bubbles grow to fill pores before they are mobilized. Interestingly. solution viscosity appears to slow considerably the coalescence of gas. For coalescence of two gas bubbles into a single bubble, the liquid lens separating the bubbles must drain. Experimental results in micromodels and triangular capillary tubes indicate that when oil-phase viscosity is high, the rate of coalescence of gas bubbles is slow. Capillary tube experiments are modeled exactly with no adjustable parameters. It is shown that the period of time for drainage is at least linearly proportional to the liquid viscosity. The implication for recovery of viscous oil is that the produced gas-oil ratio and gas-phase relative permeability remain relatively low because gas bubbles remain dispersed. Correspondingly, oil-phase relative permeability remains high contributing to relatively efficient oil recovery despite high oil-phase viscosity. (C) 2004 Elsevier B.V. All rights reserved.