Steam assisted gravity drainage (SAGD), as a commercially proven high ultimate recovery process for heavy oil and bitumen, is energy intensive and the process is estimated to be economic when oil prices are above certain level. Under conditions were oil price is below the economic threshold of the project, steam injection can be reduced and or completely stopped for a period of time; and can resume thereafter when conditions change. Although the process performance and efficiency may get impaired compared to the continuous steam injection case, it is still essential to evaluate and optimize the operating strategy after steam injection is re-initiated.
This study, thus, aims at providing insights about impact of steam injection interruptions on SAGD project performance. Optimization strategy of SAGD process in cases, where steam injection is interrupted, is also discussed. Field scale simulation of the process has been conducted with actual reservoir models of different geological formations to investigate the optimum operating strategy. An economic model is employed to evaluate the impact of operating strategy on the net present value (NPV) of the project. The parameters such as initial steam injection period, shut-in period, steam injection rate during the re-initiation period etc. are optimized for Athabasca type oil sand reservoirs.
The results indicate that there are several key mechanisms in the life cycle of a SAGD process that must be considered in the simulation model to reflect the field scale behavior and obtain the optimum operating strategy; otherwise, due to the mechanistic simplicity of the models, the results might be at best semi-quantitative and directional. Among all the mechanisms, the impact of temperature on the basic petrophysical properties of reservoir rocks, such as relative permeability, has been found to play an important role in the oil recovery and significantly affects the optimization results. Despite the complex behavior of the SAGD process, when the process is interrupted, there would be an optimum shut-in period in which the oil recovery can be maximized. The optimum length of interruption depends on the initial period of steam injection.