Main Mechanism for Stability of THAI- "Toe-to-Heel Air Injection"

Abstract THAI -- 'Toe-to-Heel Air Injection' is an integrated horizontal wells 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 planned for field pilot testing. 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 the well. The 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. This visual evidence is supported also by a numerical simulation of the experiment, which shows the distribution of coke and oxygen through the oil layer. Introduction Despite the rapid technical developments in the reservoir engineering and enhanced oil recovery (EOR) methods, the production of heavy oil is still a challenging issue for the oil industry. Heavy oil production only accounts for about 5% of total oil production. The main obstacle to heavy oil recovery is the high oil viscosity at reservoir conditions. High viscosity, typically 10,000 to 1,000,000mPas, also prevents access to wider markets and low cost methods for upgrading are required to overcome this. 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 of high temperature (° 500–600oC) can be propagated through the oil-bearing formation, virtually all of the heavy oil which it contacts is displaced. The initial development of ISC for heavy oil recovery can be traced back to the beginning of the 20th century. For more than 30 years, from 1950 to 1980, many ISC field operations were carried out. This accompanied at the same time by a tremendous amount of research in the laboratory, in order to understand what are the main mechanisms governing the process. The fundamental principles of ISC, as applied to heavy oil recovery, is very well known, and is described in details by Burger et al (1). ISC is achieved by burning a small fraction of the oil in the reservoir, releasing reaction energy between the injected oxygen and the hydrocarbons. This creates a large increase in reservoir temperature in the combustion zone, thereby mobilising and displacing the oil. The chemical reactions between oxygen and oil are extremely complex, due to the large number of components in the heavy oil.