Steam-Assisted Gravity Drainage: Concept, Development, Performance And Future
Authors: Butler R.M. 
Published: March 2010

In the Steam-assisted Gravity Drainage (SAGD) process, heated oil drains from around growing steam chambers, driven by gravity, to lower horizontal wells. As the oil drains, the steam chamber advances into the reservoir. The process has several features:The displacement or the oil is systematic and high recoveries can be obtained.In suitable applications, oil to steam ratios higher than those found for conventional steam-flooding can be achieved.The process can be used in even the heaviest of bitumen reservoirs without extensive preheating. The feature which makes this possible is that once the oil is heated, it remains hot as it drains to the production well; this is unlike conventional steam-flooding where oil which is displaced from the steam chamber tends to cool on its way to production.

Steam-assisted Gravity Drainage allows steam-flooding at economic rates without the bypass of steam. It gives high recoveries in both bitumen and heavy oil reservoirs. It has been demonstrated in Canadian field trials with results which are in reasonable agreement with prior theoretical and scaled model studies.

Introduction

Although the steam-assisted gravity drainage (SAGD) process has application in the recovery of conventional heavy oil, it was originally conceived for the recovery of bitumen where the in-situ viscosity is so high that conventional production methods are impractical(1,2).

The quantity of bitumen in place in Canadian reservoirs is as large as that for conventional crude oil in the Middle East(3). Thus, the challenge of developing suitable methods for recovery is of very great practical importance. In the main Canadian reservoirs Athabasca, Cold Lake and Peace River, the bitumen is essentially immobile and injection of fluids is usually very difficult. In-situ recovery methods generally depend upon heating the bitumen in order to reduce its viscosity; both heating with steam and in-situ combustion have been used.

One of the main problems encountered is that, even if the bitumen is heated, it cannot be pushed through cold reservoir without regaining its lost viscosity; this prevents adequate flow. It is necessary to keep the bitumen hot as it flows to the production well. One scheme for doing this which has been studied is reverse combustion(4). In this approach, the tar sand is ignited near the production well and the flame front moves away against the flow of the combustion air which is injected into another well. A major problem is obtaining adequate air injectivity. Another problem, and an even larger one, is that secondary combustion fronts tend to occur near the injection well and this has caused the process to be unsuccessful(5).

A better approach is that of cyclic steam stimulation. In this method, steam is first injected into the reservoir through the production well; in bitumen reservoir. It is frequently necessary to use injection pressure high enough to cause reservoir fracturing so that injectivity can be obtained. The reservoir adjacent to the well becomes heated and then, subsequently, the flow is reversed to allow production.