Investigation of Key Parameters in SAGD Wellbore Design and Operation
Authors: Vander Valk P.A. , Yang P. 
Published: March 2010

A simulation study was carried out to evaluate the impact of wellbore pressure drops and subcool control on SAGD reservoir performance using a three-dimensional fully coupled reservoir/wellbore model. This work builds on several concepts introduced by others and attempts to provide a method to evaluate these concepts and the relationships between them.

The results indicated a significant impact on steam chamber conformance, productivity and SOR at various operating pressures due to wellbore pressure drops. Pressure profiles within the injection and production wells were transferred into the reservoir, skewing the predominantly gravity drainage process. The chamber shape and conformance influence performance as it relates to subcool control. Sensitivities were run to evaluate the impact of key parameters.

This paper highlights some critical factors that affect SAGD performance and behaviour. Potential mitigating measures are introduced, such as liner and tubing design, variable perforations and injection ports, along with practical operating implications of each. The impacts of the findings on artificial lift selection and operation are discussed with regard to subcool control and net positive suction head available (HPSH). Potential implications to low pressure (LP-SAGD) operations are also considered in that light.

Introduction

It is generally accepted that optimal SAGD performance requires control of produced fluids to some subcool value so that a liquid level is maintained above the production well. This liquid level reduces the tendency for steam to flow directly into the production well liner, which ensures that steam is used efficiently in the SAGD process. However, too much liquid accumulation can reduce productivity, primarily due to a lower fluid temperature and corresponding higher fluid viscosity at the drainage faces and at the critical points of convergence to the production liner.

Optimum performance of the SAGD process can be defined for each project and reservoir type, however optimization is generally based on bitumen production rate (CDOR) and steam-to-oil ratio (SOR), often with conflicting imperatives.

One critical parameter in this optimization is subcool. Nasr's work indicates that there is no appreciable difference in CDOR between 5 ° and 30 °C of subcool for two-dimensional models (Tawfik Nasr, Alberta Research Council, Personal Communication, February 2005). However, Ito and Suzuki(1) demonstrated that the optimum SOR for the Hangingstone reservoir occurs at subcool ranges of between 30 ° and 40 °C with reasonable productivity. Edmunds(2) indicated that 20 ° to 30 °C subcool was a reasonable operating target for the 2D cases. However, he discussed the real world complications of the three-dimensional case in some detail as well.

Edmunds' work hinted that localized variability in subcool would occur, so that control of production rates to some optimum mixed or average subcool would result in steam production at some points along the liner, and large liquid level accumulations at other points. This is primarily due to the fact that the flow capacity of the wellbores is much greater than that of the reservoir in the same direction, making compensating steam movements in the reservoir difficult. Also, local liquid levels cannot effectively drain parallel to the well due to the very low drainage angles(2).