Experimental Investigation of Effect of Depletion Rates on Recovery of an Iranian Heavy Oil System

Some of the heavy-oil reservoirs in western Canada and Venezuela show high primary recovery under solution gas drive. Pressure decline rate in these reservoirs is low compared to that expected under solution gas drive in conventional oil reservoirs due to high viscosity. Several studies have been reported a dramatic effect of depletion rate on performance solution gas drive in heavy oil system.The objective of this experimental study was investigation about recovery dependency of Iranian heavy oil reservoir on depletion rate which has lower viscosity rather than Canadian heavy oil reservoir.The recovery factor is reported to be much higher when the flow mechanism is foamy. The foamy oil flow occurs when liberated gas from solution during pressure decline can flow through porous media while still remaining dispersed in the oil. Dispersed gas in liquid is occurs when high pressure gradient is used. When low rates of pressure decline is used, the depletion cause to occur by classical two-phase flow and results in low recovery factor.Four experimental depletion tests were carried out by Long Core apparatus respectively ultra fast, fast, slow, and combination of slow and fast depletion test.The results for ultra fast, fast and slow depletion tests confirm the effect of depletion rate on recovery factor. The faster depletion rate, the more recovery was achieved; the reason was when pressure gradients become high enough to mobilize the gas bubbles before forming the continuous gas phase. This keeps the gas mobility low and improves system recovery.For combination of slow and fast depletion test, slow depletion continues to establishment of single gas phase and subsequent fast depletion was ineffective to improve oil recovery.

Keywords:

Depletion rate , Bubble point pressure , Oil viscosity , Gas mobility , saturation pressure