Foamy oil flow occurs in primary depletion of heavy oil reservoirs as has been demonstrated in many laboratory experiments. It is thought to be an important recovery mechanism in several heavy oil reservoirs in Canada and Venezuela, which have shown higher recovery factors compared to what is expected from the normal solution gas drive theory. This work investigates the effects of several process parameters on oil production rate and recovery factor in foamy solution gas drive.
The parameters examined included gas-oil-ratio (GOR), saturation pressure, the rate of pressure depletion and the drawdown pressure. GOR was varied independent of saturation pressure by using different gases, namely, methane, ethane and carbon-dioxide. Each foamy oil system was fully characterized by measuring gas oil ratio, oil compressibility, live oil viscosity, surface tension and foam stability. A total of 10 solution gas drive experiments were carried out in 2-meter long sand-pack equipped with several intermediate pressure taps using different solution gases and varying pressure decline rates.
The results show that the foamy solution gas drive performance is affected by solution gas-oil-ratio in a counterintuitive manner. The oil recovery decreased with increasing gas oil ratio at fixed saturation pressure. At the same rate of pressure depletion, higher oil recovery was obtained with methane saturated oil than with carbon dioxide saturated oil. Ethane, which had the highest solubility, provided the lowest recovery factors. It was also found that the oil recovery factor did not decrease significantly when the saturation pressure was decreased from 3,500 kPa to 2,100 kPa.
An analysis of all 10 depletion tests revealed that the most important factor that affects the oil recovery performance was the drawdown pressure, which was defined as the difference between the average sand-pack pressure and the production port pressure. Results obtained with different solution gases and widely varying depletion rates fell on the same trend line when the final recovery factor was plotted against the average drawdown pressure applied during the depletion.