viscous heavy oil reservoirs / free gas / dissolved gas / heavy oil / high porosity sandstone oil field / gas depletion / higher oil production rates / gas slugging / gas exsolves / typical heavy oil reservoirs / shear energy / solution gas drive / foamy oil / oil / continuous gas phase / gas pressures / heavy oil wells / high-rate oil wells / gas-free / gravitational energy component acts / fluid and discrete gas channels / gas bubbles / gas cap / oil reservoir change / oil flow phenomena / gas behavior / conventional solution gas behavior / gas-oil-ratios / energy / /