Artificial Lift-A Major Unresolved Issue for SAGD

Abstract

Abstract Lower-pressure SAGD may be required because of thief zones, or to improve the SOR, emissions, and water use. Simple 3D simulation examples representing real-world conditions illustrate the need for low steam-trap subcool values. A low mixed subcool value is needed to minimize local subcool values along the well pair and maximize steam chamber development. Lower pressures magnify the need for low subcool values. For low mixed subcool values, there is zero or very low liquid head available above a bottomhole pump. However, a high average head is required, much higher than the minimum head, due to SAGD flow instabilities. Using typical field values, a simple analysis shows that a standard bottomhole pump configuration cannot provide low subcool values for lower steam chamber pressures. A lift system designed to provide low subcool values at lower pressures is a two-stage lift system called ELift. This was used at Gulf's Surmont SAGD pilot for three years. A simulation example of the two-stage lift system is provided showing excellent conditioning of fluids prior to the pump inlet. This allows the use of standard pumps, and extends pump service life, in addition to providing the benefits of lower-pressure operation and improved recovery performance with low mixed subcool values. Introduction Steam-assisted gravity drainage (SAGD) is now used in large commercial operations as well as pilot operations. However, one of the major issues that has not yet been resolved is the issue of artificial lift for lower-pressure SAGD operations. The main focus of this paper is to summarize why lower-pressure SAGD is important, describe the impact of steam-trap control and unstable SAGD flow on artificial lift requirements, and describe a lift system which, in principle, meets the requirements for SAGD lift at lower pressures. Reasons for Lower-Pressure SAGD Operations Lower-pressure SAGD may be required because of the presence of thief zones. Associated gas sands and/or water sands in the upper part of the McMurray formation are common features of the Athabasca oil sand deposit, and may constitute thief zones to SAGD operations. The upper gas and water sands are generally under-pressured at virgin conditions, and gas production operations have further lowered the pressure. When SAGD steam chambers achieve hydraulic communication with associated thief zones, the pressure in the steam chamber must be close to that in the overlying thief zone, and artificial lift will generally be required. There is extensive information on this topic from several gas/bitumen hearings and in decision reports by the Alberta Energy and Utilities Board(1). Even when thief zones are absent, lower-pressure SAGD may be required to improve the steam-oil ratio (SOR). Reduced SOR provides savings in natural gas consumption, reduced emissions per unit of production (specific to Kyoto regulations), and reduced usage of valued water resources. Because reduced SOR also reduces water handling requirements, it provides important savings in the separation and water-recycle facilities. A SAGD plant is mainly designed with regard to steam-generation/water-handling capacity. Once a plant is built, with relatively minor modifications it will provide increased oil rates as the SOR declines.