Fluid Movement in the SAGD Process: A Review of the Dover Project
Authors: Aherne A.L. , Maini B. 
Published: March 2010

The fundamentals of Steam Assisted Gravity Drainage (SAGD) steam chamber development are now well understood through Butler's analytical models, as well as extensive field and laboratory testing. However, as the industry continues to extend SAGD to new reservoirs and look towards SAGD wind-down at the end life of 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 project has been undertaken in an attempt to understand the behaviour 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 283.17 m3/bbl (1 mcf/bbl) of bitumen produced for a steam-oil ratio (SOR) in the range of 2.3 to 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, whereas warm steam condensate not reproduced 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 SOR may be negatively impacted. On 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 published(1,2) 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 limits(3–5). From April 1998 to May 2002, NGC was injected with steam at the Dover Pilot. The gas volume injected 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. Furthermore, an increased overall recovery was observed, most likely from gas assistance in the production of previously inaccessible reserves. The simulation model developed to describe the behaviour of NCG in the reservoir, as well as further observations regarding this behaviour, 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 Butler in the early 1980s(6) is, to date, the most successful in situ method of exploiting this resource.