A New Approach To The Modelling Of Steam-Assisted Gravity Drainage
Authors: Butler R.M. 
Published: March 2010

Abstract
A method for predicting the bitumen drainage from around a spreading steam chamber above a horizontal well is described. The interface is treated as a number of segments. Oil drainage for each segment is calculated for successive time steps and these rates are used to calculate the displacement of the interface. The change in accumulated heat penetration is estimated using an approximate differential equation. The calculation is repeated in a cyclic manner.
A continuous heat balance is calculated. It is shown that the fraction of the injected heat remaining in the chamber increases with the steam pressure. However, this advantage is offset by the larger amount of heat required to raise a unit volume of the reservoir to steam temperature. Higher steam pressure gives faster production and the oil steam ratio does not change much. A major factor in determining the efficiency of the process is the horizontal spacing between wells in a confined pattern; closer spacing gives better oil to steam ratios.
The spreading of an unconfined steam chamber is controlled by factors affecting the steam flux al the upper edges of the steam chamber. Non-condensable gas may be important; experimental work on factors affecting the spreading is planned.

Introduction
The main problem in the recovery of bitumen by in-situ methods is that bitumen is essentially immobile at reservoir conditions. This can be overcome by heating the bitumen or by diluting it with solvent. A major problem for the petroleum engineer in the development of either of these approaches is the definition of a suitable means for contacting the heat or the solvent with the bitumen in a systematic and complete manner.
For example, attempts to inject steam into a reservoir to heat the bitumen are generally unsuccessful unless the pressure is raised to the level at which fracturing occurs. Steam injection above fracturing pressure, followed by production from the same injection well, has been successful in Alberta and elsewhere and is the basis for recently announced commercial projects(1, 2)
Such projects are much more economic with thick reservoirs than with thin ones because a smaller fraction of the injected eat is lost to the overburden. It seems likely that in thick reservoirs most of the oil is produced by gravity drainage even in the conventional steam-stimulated approach. This was first pointed out by Doscher(3).
Although the heat economy for steam stimulation can be reasonably satisfactory, the recovery of the oil is limited to about 15 – 20% because of the tendency for steam to bypass. The present paper is concerned with the production of bitumen or heavy oils by gravity drainage from around a growing steam chamber; it follows others which were written by the author and his former colleagues at Es50(4–7). The process and the theory differ from those described in the literature for the production of conventional oils by gravity drainage. The difference is that in steam-assisted gravity drainage it is only the oil close to the perimeter of the steam chamber which is mobile whereas in the drainage of conventional petroleum under conditions of essentially constant temperature, all of the oil within the reservoir is mobile.