Heavy Oil Production By In Situ Combustion-Distinguishing the Effects of the Steam And Fire Fronts
Authors: Freitag N.P. , Exelby D.R. 
Published: March 2010

Abstract
In heavy oil production by in situ combustion, information on the relative importance of the combustion and steam fronts is very useful in the development of good production strategies. To obtain this information for two reservoirs, one containing heavy oil and the other a bitumen, a novel series of combustion tube tests was conducted. Contrary to conventional belief, the results showed that, before steam breakthrough, the produced oil properties were influenced much more by steam distillation than by cracking reactions. While the combustion front mobilized substantial amounts of oil, almost all of it remained behind the steam front. This oil displayed significance differences in properties and composition compared with the original oil. Overall, the results helped to provide some guidelines for developing improved strategies for fireflood operation, and emphasize some requirements for the prediction of fireflood behaviour.

Introduction
As a thermal method of improved oil recovery, in situ combustion has the advantage of delivering heat to a formation very efficiently. Consequently, the prospect of using in situ combustion to produce heavy oils has continually attracted interest, particularly for thin formations where heat losses to non-productive horizons are large. The results of field trials(1–3) in such reservoirs have varied widely, suggesting that the method may be strongly affected by the approaches used to apply it. To help develop efficient strategies for in situ combustion it is important to understand its many mechanisms and their impact on oil recovery.
Islam et al.(4) have demonstrated the importance of the gas flooding effect on oil recoveries during in situ combustion. Other phenomena, such as the combustion steam fronts, must also affect oil production. During dry combustion, the steam zone may be small, particularly in thin reservoirs. In this case, the combustion and steam fronts act almost as a unit. During wet combustion, however, when water is injected behind the combustion front to scavenge heat and deliver it, in the form of steam, to the oil-bearing zone, the steam front can advance well ahead of the combustion front. In this case, several important questions arise. Does the steam front dominate the oil displacement process leaving the combustion front to function only as the heat source that sustains the steam front, or does the combustion front also serve to displace oil? To what extent does each front affect the properties of the displaced oil?
In an effort to answer these questions, a series of four combustion tube tests was carried out. These tests were designed to show the effects of each front in two reservoirs, one representative of a heavy oil, and one of a bitumen.

Equipment
The experiments were conducted in a heavy-walled Incolloy 800 combustion tube with an inside diameter of 5.37 cm, a total inside length of 162.3 cm, and a wall thickness of 0.49 cm. No pressure jacket was employed. The maximum working pressure was 4.2 MPa (610 psig). Pack temperatures at the centreline were measured with 25 thermocouples inserted through fittings welded in a helical pattern on the tube wall.