In many steam-based heavy oil/bitumen recovery processes, non-condensable or condensable gas is either present in the reservoir or is co-injected with steam. The distribution of non-condensable gas in the oil-depleted region can be very complex and may greatly influence process performance. The mechanisms are not fully understood. The current numerical study attempts to better understand the SAGD process where gas is either present or co-injected. Numerical simulations were based on comparison with published laboratory experiments. A sensitivity study was carried out to examine the effects of gas diffusion, heat loss, spatial aspect ratio, and heterogeneity. The study showed that gas diffusion caused by concentration gradient is minimal, and the gas flow appears to be dominated by forced convection. Noncondensable gas accumulates and concentrates in the region where steam condenses. The gas distribution in the oil-depleted zone is determined by steam/gas injection rate and heat loss rate.
The Steam Assisted Gravity Drainage (SAGD) process1,2 is regarded as one of the leading in-situ recovery processes for heavy oil and bitumen resources. After the success of the UTF3,4 field pilot, many SAGD projects are in operation, construction, or planning stages in western Canada.
In the SAGD process, as the steam chamber grows, oil is gradually recovered, accompanied by increasing steam-oil ratio. At a certain point, it is no longer economic to continue steam injection; however, the reservoir is still hot, and the energy in place can be used. Non-condensable or condensable gas injection has been proposed as a follow-up process. A less energy-intensive gas injection process can maintain reservoir pressure, utilize energy in place, and prolong oil production. When a new SAGD operation is implemented adjacent to a depleted zone, it is likely that steam may escape to the depleted zone, thereby greatly reducing process efficiency. One of the methods proposed to overcome this problem is to inject non-condensable gas into the depleted zone to increase and maintain reservoir pressure, and to prevent steam from escaping. In order to extend the range over which SAGD can be applied, it has been proposed to coinject steam with non-condensable gas to reduce heat loss to the overburden5. In fact, in SAGD operations, the injected steam often contains a considerable amount of noncondensable gas, such as carbon dioxide or nitrogen.
When SAGD is applied to a live oil reservoir, the solution gas in the oil may mix with the steam, and hence make the process more complicated.
All of the above processes and applications involve the presence of a steam-gas mixture in the reservoir. Gas mixing in porous media is a complex process, involving diffusion and convection. Depending on the operating conditions, the distribution and movement of gas and steam in the reservoir may greatly influence process performance. Unfortunately, our understanding of the mechanisms is incomplete. This is because it is very difficult to measure the distribution of gas concentration directly, even under well-controlled laboratory conditions. It is even harder, if not impossible, to measure gas velocity in the reservoir.
