The unusually high primary recovery factors observed in many heavy oil reservoirs are often attributed to foamy oil flow, i.e. the non-Darcy flow involving formation and flow of gas-in-oil dispersion. It occurs when the wells are produced aggressively at high drawdown pressures that lead to conditions in which the viscous forces become strong enough to overcome the capillary forces in pushing dispersed bubbles through pore throats. The role of gravitational forces in generating such dispersed flow has not been adequately studied. This work was aimed at evaluating the contribution of gravitational forces in primary depletion of heavy oil formations under foamy flow conditions.

Primary depletion tests were conducted in a 200 cm long sand-pack that was held in either horizontal or vertical orientation. The results of horizontal depletion tests were compared with the depletion tests conducted with the sand-pack in vertical direction. Vertical depletions showed better recoveries at slower depletion rates compared to horizontal depletions.

The recovery factors of both horizontal and vertical depletions were correlated against the average drawdown pressure available to move the oil. It was found that the recovery factor show a strong dependence on the average drawdown pressure. It was also found that the curve of recovery factor versus average drawdown pressure moves slightly towards higher recoveries in the presence of an added foaming agent which increases the oil foaminess.

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