Steam-Assisted Gravity Drainage Model Incorporating Energy Recovery From A Cooling Steam Chamber
Authors: Ferguson F.R. Scott , Butler R.M. 
Published: March 2010

Abstract
A new calculation procedure has been developed which allows the prediction of the effects of varying steam injection rates, pressures, and the duration of injection cycles on heavy oil recovery by gravity drainage. Previous models assumed maintenance of a constant steam chamber pressure and temperature.
The model uses an approximate mathematical method to obtain the heat transfer to the receding oil bank and to the overburden. The heat injected is balanced against the total losses from the system and surplus and deficiencies cause variations in the temperature and pressure within the steam chamber.
Modelling results indicate that considerable increases in cumulative oil-steam ratios are to be gained by stopping steam injection before the recoverable reserves in a pattern have been completely produced. It is also found that higher injection pressures and rates accelerate production and do not reduce the cumulative oil steam ratio significantly. Little benefit was found to result from multiple injection cycles within the same production pattern. These results should be of practical importance in the application of this process in the field.

Introduction
Steam-assisted gravity drainage in heavy oil recovery is a production method which exploits the tendency of oil heated by injected steam to flow from the top to the bottom of the reservoir. Steam injected into a reservoir rises to the top due to gravity differences between the steam and the natural reservoir fluids. The heat concentration at the top of the reservoir reduces the oil viscosity in the vicinity of the steam. The mobile oil flows to the base of the reservoir driven by gravity.
Gravity drainage is likely responsible for much of the oil produced in conventional heavy oil production methods(4). In horizontal steam-assisted gravity drainage, steam is injected along the length of and above a horizontal production well which is at the base of the reservoir and parallel to it. Esso Resources Canada Limited has chosen to use vertical steam injection wells spaced along the length of horizontal producers temperin some pilot experiments at Cold Lake, Alberta, where steam-assisted gravity drainage to horizontal wells is being tested in the Clearwater formation(5,6).
For application to the recovery of bitumen from tar sands deposits, such as those that exist in the Cold Lake, Athabasca and Peace River deposits in Alberta, steam can be introduced into the reservoir by the creation of a fracture(1,5). The bitumen cannot be displaced because it is not mobile at reservoir conditions. The assumption of an initial vertical fracture extending from the injection well to the top of the reservoir is convenient to the modelling process because it avoids the difficulties associated with instabilities of a steam chamber rising vertically counter-current to a flow of oil(1).
In a large-scale, horizontal-well, gravity drainage system, groups of injection and production wells would be placed in parallel rows so that steam chambers growing from adjacent patterns would eventually encounter each other and merge.