Fluid Movement in the SAGD Process A Review of the Dover Project

Abstract The fundamentals of Steam Assisted Gravity Drainage (SAGD) steam chamber development are now well understood through Butler's analytical models, and extensive field and laboratory testing. However as industry continues to extend SAGD to new reservoirs and looks towards SAGD wind down at the end life of the projects, it is important that we recognize the value of not only understanding the steam chamber but also of the movement of fluid in the reservoir. The Dover SAGD Pilot is the most mature pilot of its kind in the world. A study of this Pilot has been undertaken in an attempt to understand the behavior of the fluid within and in front of the steam chamber. The economics of SAGD are significantly impacted by the cost of generating steam. At roughly 1mcf/bbl of bitumen produced for an SOR in the range of 2.3–2.5 m3/m3, natural gas is the single largest operating cost in a SAGD project. Water movement within the reservoir can impact the natural gas consumption wherein warm steam condensate not recovered must be replaced in the process by colder make-up water, decreasing the heat efficiency of the steam generation. Further, where water loss to the reservoir is high, the steam-oil ratio (SOR) may be negatively impacted. As we approach the 20th anniversary of the initiation of the Dover Pilot, the cold water injection test performed prior to any thermal operations taking place is revisited here. Understanding the transmissibility of water in the reservoir is key to choosing the optimal operating pressures and maximizing the value of a project. It has been widely published1,2 that the injection of non-condensable gas (NCG) into SAGD chambers will result in the accumulation of the NCG at the top of the chamber, cooling the chamber. The lower temperatures within the chamber cause the viscosity of the bitumen to increase thereby reducing the bitumen production rate. This has been suggested as a method of winding down steam chambers as they reach their economic producing limits3,4,5. From April 1998 to May 2002 NGC was injected with steam at the Dover Pilot. The gas volume injected at reservoir conditions was triple the volume of the produced bitumen over that time. The SAGD chambers did not behave as predicted. The bitumen production rate did not fall off any more than would be expected from a mature steam chamber and live steam was still detectable through the thermocouples within the steam chamber. Further, an increased overall recovery was observed, most likely from the gas assistance in the production of previously inaccessible reserves. The simulation model developed to describe, as well as further observations regarding the behavior of NCG in the reservoir, are discussed. Introduction Geographically located in northeast Alberta, the Athabasca Oil Sands deposit forms part of the western Canadian oil sands. With an estimated 1.7 trillion barrels of oil in place, it is arguably the single largest oil deposit in the world. SAGD, developed by Butler6 in the early 1980's, is to date, the most successful in-situ method of exploiting this resource.