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Summary Steamโassisted gravity drainage (SAGD) is a thermalโrecovery process to produce bitumen from deep oilโsands deposits. The efficiency of the SAGD operation depends on developing a uniform steam chamber and maintaining an optimal subcool (difference in saturation and actual temperature) along the length of the horizontal well pair. Heterogeneity in reservoir properties might lead to suboptimal subcool levels without the application of closedโloop control. Recently, modelโpredictive control (MPC) has been proposed for realโtime feedback control of SAGD well pairs based on realโtime production, temperature, and pressure data along with other well and surface constraint information; however, reservoir dynamics has been represented using extremely simplified and unrealistic models. Because SAGD is a complex, spatially distributed, nonlinear process, an MPC framework with models that account for nonlinearity over an extended control period is required to achieve optimized subcool and steam conformance.
In this research, two novel work flows are proposed to handle nonlinear reservoir dynamics in MPC. The first approach is adaptive MPC, and includes continuous reโestimation of the model at each control interval. This allows the evolution of the coefficients of a fixedโmodel structure such that the updated systemโidentification model in the MPC controller reflects current reservoir dynamics adequately. Another approach, gainโscheduled MPC, decomposes the subcoolโcontrol problem in a parallel manner, and uses a bank of multiple controllers rather than only one controller. This ensures effective control of the nonlinear reservoir system even in adverse control situations by using appropriate variations in input parameters based on the operating region.
The work flows are implemented using a historyโmatched numerical model of a reservoir in northern Alberta. Steamโinjection rates and liquidโproduction rate are considered input variables in MPC, constrained to available surface facilities. The well pair is divided into multiple sections, and the subcool of each section is considered an output variable. Results are compared with actual field data (in which no control algorithm is used), and are analyzed on the basis of two criteria: (1) Do all subcools track the set point while maintaining stability in input variables? and (2) Does the net present value (NPV) of oil improve with adaptive and gainโscheduled MPC? In general, we conclude that both adaptive and gainโscheduled MPC provide superior tracking of subcool set points and, hence, better steam conformance caused by adequate representation of reservoir dynamics by reโestimation of coefficients and multiple controllers, respectively. In addition, the results indicate stability in input parameters and improvement in economic performance. NPV is improved by 23.69 and 10.36% in case of adaptive and gainโscheduled MPC, respectively.
The proposed work flows can improve the NPV of an SAGD reservoir by optimizing the wellโoperational parameters while considering constraints of surface facilities and minimizing environmental footprint.
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