Thermal recovery process is widely adopted in exploitation of heavy oil reservoirs. But for extra-heavy oil reservoirs with bottom water and depth more than 800m, effective recovery is not achieved with steam injection. SAGD (Steam assisted Gravity Drainage) process is tested in such reservoirs but showed poor performance because of high reservoir pressure and bottom water. This paper proposed a new approach which is utilizing super critical hydrocarbon gas to replace most part of steam during steam injection and SAGD process for exploitation of such reservoirs.

Calculations of heat distribution along the steam injection tube were carried out. The solubility of supercritical hydrocarbon gas in heavy oil was tested with PVT apparatus. Also the viscosity reduction effect of super critical gas is also tested. SAGD simulation with high mole percent of gas injection was carried out with theoritical model. Different operation strategy including pure steam, supercritical CO2 and steam co-injection, wet gas and steam co-injection was compared with numerical simulation.

The main theory of this approach is that injected gas can greatly decrease partial pressure of steam hence reduce the heat loss and drawdown of quality during injection. The results shows that the heat loss can be decreased by 40% in steam injection tubing. At super critical conditions, solubility of gas in heavy oil is almost 100m3/m3. Viscosity of heavy oil saturated with gas will be lower than 200mPa.s at 150℃which is 80% less than that without gas. The viscosity is already low enough for drainage process. High pressure SAGD experiment shows that temperature decreases gradually from the inner part to the out range of steam chamber when injected mole percent of gas is 20%. This means the decrease of steam saturation pressure related to partial pressure of steam because injected gas will take the corresponding pressure in steam chamber. The recovery process shows high recovery factor which is similar to SAGD process. And steam oil ratio is 0.8~1.2 which is much lower than any existing pure steam injection SAGD process. Simulation results shows that during the operation process bottom water will not flux into steam chamber after the balance between reservoir and bottom water was achieved.

This paper provides an effective approach to exploit the heavy oil reserves with great depth which is meaningful for reducing the steam consumption and operation cost for oil industry.

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