Abstract
Many heavy oil reservoirs under solution gas drive show a significant increase in productivity. Different mechanisms have been postulated to explain the high recovery factors observed in the field. Lately, a widely accepted cause of this increase in productivity is the foamy oil flow, which dramatically decreases the gas relative permeability. However, it still remains inadequately understood.
It has been reported that the foamy flow effects observed in the laboratory depletion tests are strongly affected by depletion rate. This observed rate effect has been attributed to the influence of the pressure depletion rate on bubble nucleation. However, the researchers have mostly overlooked the capillary number, which increases significantly with increasing depletion rate. The goal of this study was to examine this capillary number effect in heavy oil reservoirs and its relationship with gas-oil relative permeability curves.
A number of experiments were carried out on a two-meter long sand-pack to determine the conditions required to produced more oil under solution gas drive. The experimental data were matched on a commercial black oil simulator to determine the relative permeability curve under varying flow conditions and at different capillary number. The experimental results show that the oil produced was a unique function of capillary number and beyond a critical capillary number no additional improvement in the recovery factor was observed. An examination of the changes of capillary number during the test provides insight into the gas-phase build up during solution gas drive and the resulting production behavior. It was also found that oil relative permeability increases and gas relative permeability decreases with increasing capillary number. At high capillary number, relative permeability ratio remains very low up to higher value of gas saturation. The results from this research suggest that the conventional correlations of gas-oil relative permeability should incorporate the effect of capillary number in order to improve the prediction of foamy oil flow mechanism.
