Main Mechanism for Stability of THAI-Toe-to-Heel Air Injection
Authors: Xia T.X. , Greaves M. , Turta A. 
Published: March 2010

Abstract
THAI, or "Toe-to-Heel Air Injection," is an integrated horizontal well process for in situ recovery and upgrading of heavy oil and tar sands bitumen. During extensive studies of the process at the University of Bath, involving more than 50 three-dimensional combustion cell tests, the process has repeatedly demonstrated robust and stable operation. However, the answer to the question: "Why does oxygen breakthrough into the toe of the horizontal well not occur," has not been fully developed. This is now all the more important since the THAI process is about to be tested in the field. In order to maintain stable propagation of the combustion front, sufficient fuel needs to be available ahead of the front. This is fundamental to the in situ combustion (ISC) process, whether in its conventional form, or THAI. When the combustion front approaches close enough to the horizontal producer well, heavy residue can drain into the well. This residue, or coke material, provides a gas seal, preventing the injected air from channeling through to the well.
This paper presents post-mortem results of two THAI experiments, in which the horizontal well was cut-open to reveal the extent of heavy oil residue/coke deposition. The visual evidence is supported also by a numerical simulation of the experiment, showing the distribution of coke and oxygen through the oil layer.

Introduction
In situ combustion (ISC) has, theoretically, always held the promise of high potential rewards for heavy oil recovery. This is basically because, if a stable combustion front at high temperature (500 - 600 ° C) can be propagated through the oil-bearing formation, virtually all of the heavy oil it contacts is displaced. The fundamental principle of ISC, as applied to heavy oil recovery, is well known, as described by Burger et al.(1). ISC is achieved by burning a small fraction of the oil in the reservoir, which releases sufficient reaction energy to create a large increase in reservoir temperature in the combustion front zone, thereby mobilizing and displacing the oil ahead of it. The chemical reactions between oxygen and crude oil are extremely complex, principally because there are a large number of components which can undergo thermal cracking reactions. The overall complexity of the ISC process is increased because of the interaction between chemical reaction and transport processes in reservoir porous media.
Although considerable understanding of the ISC process has been gained in recent years, and new guidelines and strategies for field operations have been presented(2, 3), there are still a number of concerns about the process which need to be resolved if its full operational potential is to be realized. These include:Gravity segregation, or gas overriding;Oil banking in the cold region ahead of the combustion front; and,Permeability heterogeneity.
Gas overriding, due to the buoyancy effect between the combustion gases and reservoir liquids, exacerbated by permeability contrasts in the reservoir porous media, can cause severe channeling of gas through to the producer well.