Improving the Performance of Classic SAGD with Offsetting Vertical Producers

Abstract

Abstract Both theoretical knowledge and field use of 'classic' SAGD (defined in this paper as two horizontal wells of the same length separated by a relatively short vertical distance, typically 5 to 7 m, with the injector on the top and the producer on the bottom) have greatly expanded over the last 15 years. During that period only a few modifications of classic SAGD have been proposed. The most successful was the use of vertical injectors at Sceptre (now CNRL) North Tangleflags and Strike (now Husky) Bolney. Laterally staggered injectors and producers and perpendicular injectors and producers have been proposed, but have not been successfully field tested. This paper discusses field observations and numerical simulation studies indicating that properly placed vertical production wells can capture a significant amount of oil that is not produced by classic SAGD development. These vertical wells could also first be used for steam injection high in the pay, CSS low in the pay or temperature observation purposes, adding further enhancement to the performance of nearby classic SAGD pairs. Horizontal wells could also be placed to access oil that is missed because it is below and beneath classic SAGD pairs, but practical aspects involving drilling, completing and producing horizontal wells in a non-uniformly heated and depleted environment suggest vertical wells are a better choice. Recent History of SAGD Usage Classic SAGD Classic SAGD is currently the most discussed and most proposed steam-based exploitation process in Canada. An excellent summary was published in 2001 by the inventor of the process, Dr. Roger Butler(1). Other recent publications have discussed a variety of topics about classic SAGD, including: reservoir screening criteria(2, 3), steam chest containment(4), co-injection of solvents(5, 6), optimum pressure regime(7), producer inflow problems(8), effect of solution gas(9) and late life strategies(10). Prior Proposed Spatial Alterations From Classic SAGD The literature also contains descriptions of processes for several well orientations altered from the classic SAGD configuration. The most advanced alteration is the use of vertical injectors and horizontal producers(11–15). The most successful field project to date using this configuration has been Sceptre's (now CNRL) North Tangle flags(16). Another spatial variation from classic SAGD is the addition of horizontal CSS wells(17). This configuration contains a horizontal CSS well at the same depth as the SAGD producers, and located in the middle of a typical SAGD pair-to-pair spacing, about 50 m away from each of the off setting pairs. Placement of the infill horizontal wells at the same depth as the SAGD producers likely leaves unswept oil below that depth, and possibly creates another opportunity for the addition of vertical producers. To date, only numerical simulation and physical modelling have been conducted, but those results suggest improved energy efficiency and productivity compared to classic SAGD. A third variation of well orientation is cross-SAGD or XSAGD(18). For XSAGD, the injectors are placed above the producers, but perpendicular to them.