Numerical Study and Economic Evaluation of SAGD Wind-Down Methods
Authors: Zhao L. , Law D.H.S. , Coates R. 
Published: March 2010

Abstract
At a certain point in a SAGD operation, it is no longer economic to continue steam injection when the steam-oil ratio (SOR) becomes too high. A less energy intensive gas injection process following the SAGD process can prolong oil production by using the energy in place. Numerical studies of the injection of non-condensable gas, and a mixture of steam and non-condensable gas following a SAGD operation, were conducted using the CMG's STARS reservoir simulator. The simulation results suggest that, after three to five years of SAGD operation, when three quarters of the targeted reservoir is hot, it is appropriate to start a wind-down process. Injecting non-condensable gas results in a much lower production cost compared to continued steam injection; however, the oil production is reduced. Coinjection of steam with gas produces more oil than the gas-only injection process without substantially increasing the production cost. This is probably the desired wind-down process. Optimization is needed for the co-injection process. The choice of non-condensable gas depends on the cost and availability.

Introduction
The huge heavy oil and bitumen resources in Western Canada have motivated efforts to develop suitable recovery processes. One of the results of these efforts is the Steam-Assisted Gravity- Drainage (SAGD) process(1), which was proposed 20 years ago. The process utilizes horizontal wells and steam injection. The relatively slower gravity drainage is compensated for by using long horizontal wells, resulting in an economic production rate. In addition, the overall recovery may be as high as 70% of the original oil-in-place (OOIP). Much research has been conducted on the process in order to obtain a better understanding of the process under various reservoir conditions, to improve the accuracy on performance prediction, and to solve operational problems. After the process was pilot tested in AOSTRA's Underground Test Facility (UTF) and showed great promise(2, 3), SAGD has been regarded as one of the leading in situ recovery processes for heavy oil and bitumen resources. Many SAGD projects are in operation, under construction, or in the planning stages in Western Canada.
In the SAGD process, as the steam chamber grows, oil is gradually recovered, accompanied by an 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 utilized. Non-condensable gas (NCG) or a mixture of NCG and steam injection has been proposed as a wind-down process. A less energy-intensive gas injection process may maintain reservoir pressure, utilize the energy in place, and prolong oil production. Such a process has been field tested at the former UTF B patterns, and good results have been reported(8).
The purpose of this study is to evaluate various possible SAGD wind-down processes, and to find the appropriate time for starting a wind-down process. Using the reservoir simulator, STARS (Computer Modelling Group Ltd.), wind-down processes of the injection of NCG, and mixture of steam, and NCG were numerically investigated. It was found that the wind-down process could produce a considerable amount of oil.