The development of THAI
Professor Greaves became involved with academic work relating to enhanced oil recovery (EOR) in 1981 when one of his PhD students at the University of Bath, UK, was researching polymer flooding. He became aware of the benefits of in situ combustion (ISC) at the first European EOR Conference in Bournemouth, UK. Subsequent discussions in the US and Canada confirmed that in situ combustion had the greatest potential as a research area, and was at that time still regarded by the oil industry as “the great hope” for heavy oil recovery.
Following the US/Canada trip, Greaves’ team in Bath built the first combustion tube system in the UK. Two more low pressure systems were built before he received funding from the EU and BP to construct a fully-automated, combustion tube system operating at up to 240 bar pressure.
Although the initial experiments were on heavy oil, the subsequent dramatic collapse in the crude oil price diverted attention to light oil reservoirs, in which ISC has since achieved field success as an improved oil recovery (IOR) technique. The Bath HP Combustion Tube Facility, although having no direct link to THAI developments, was an important research precursor, providing significant insight and learning experience for subsequent ISC experiments.
In 1989, another of Greaves’ PhD students was researching horizontal wells, this prompted the Professor to consider, for the first time, the multi-phase flow implications of an ISC setup comprising a single vertical air injection well, offset and in line from the toe of a single horizontal thermal production well.
The oil price collapse of the early 1990s was one of several reasons why the commercialization of THAI has been a long process. Reduced income led to a decline in interest (and investment) in heavy oil, with many large companies switching their focus to light oil. In addition, experience to date had made reservoir engineers very skeptical about ISC. Of around 160 ISC field pilots during the 1970s and 1980s only about one-third were considered a technical and economic success. Another one-third of cases were only partially successful, and the remaining third were deemed to be failures, encountering unstable or uncontrollable combustion.
"THAI is very simple," says Greaves, and although many companies expressed keen interest, no one wanted to fund the research. Until very recently only a few people thought it would actually work. “No-one gave Frank Whittle (inventor of the jet engine) any money either,” noted Greaves. Greaves considers it ironic that, during this period, he was able to obtain substantial funds from the UK Engineering and Physical Sciences Research Council (EPSRC) for conventional heavy oil ISC research. This was very useful because it helped considerably towards developing operating expertise on 3D combustion cell geometries used to investigate ISC processes. The research also provided fundamental understanding which was important for the subsequent development of THAI. However, throughout the 1990s, Greaves and his colleagues battled to convince companies and funding bodies that this odd-looking well arrangement would work. PhD students supported by oil companies made significant progress, but it was not until 1997 that the next intensive phase of testing could be performed, enabled by a special award from the EPSRC. This was specifically earmarked for work on downhole upgrading of heavy crude (the CAPRI process). Greaves saw this as an indication that industry experts had, by now, reached the conclusion that THAI might work after all!
A 3D combustion cell can provide a much more realistic physical simulation of the combustion front propagation and fluid flow occurring in a real reservoir than the artificially constrained 1D flow in a combustion tube arrangement. In total, over 130 3D laboratory ISC experiments were performed between 1990 and 2002, each lasting up to 15 hours. The tests produced temperatures of 500—800 degC and achieved recovery rates up to 84%. Approximately 10% of ooip was burned as fuel and 6% left as heavy residue and coke. For heavy crudes, there was never any occurrence of instability in any of the tests. At the experimental level therefore, the THAI process was very stable and robust. Greaves presented results in a paper at an SPE/DOE IOR Conference in 2000 and says that the first question at any conference presentation has been why injected air does not channel through, directly into the toe of the horizontal well. The answer was not discovered until 2002 (Paper 2003-030 - Proc. Canadian International Petroleum Conference). It was observed that, towards the end of an experiment, the air injectivity fell dramatically. The horizontal producer well was subsequently cut open, revealing a heavy coke-like residue plugging the heel of the horizontal well. Numerical simulations and more tests indicated that, just ahead of the combustion front, as the draining heavy oil gets hotter, it starts to coke forming a plug that provides a flow resistance barrier inside the horizontal well preventing air breakthrough.
ISC methods share many of the challenges inherent to other EOR methods and also present some particular complications. Greaves considers poor, or irregular, inter-well communication to be at the root of many of the problems that plagued conventional ISC projects using two vertical wells placed 100s of meters apart. Banking-up of the oil and water reduces gas permeability and so restricts air injectivity. Inadequate combustion can lead to low-temperature oxidation and emulsions. A key to the success of THAI is its vigorous high-temperature combustion.
Greaves shares the basic patent for THAI with Dr. Alex Turta, a Romanian engineer. Romania has the biggest ISC operation in the world—at Surplacu du Bacu—which has been operated continuously for more than 30 years. In this case, the conventional well geometry works well thanks to a uniform reservoir character and a near optimum 17º dipping reservoir. Other successful conventional ISC fields are producing in India and Louisiana. Many other projects have been tried but, according to Greaves, some failures have occurred due to poor reservoir selection. Additionally, in many cases, it was not very well understood during the early phase that heavy oil ISC must be operated in a high temperature oxidation mode, i.e., vigorous combustion at high temperature (greater than 500 degC).
Dr. Tian Xia, who worked as a research officer in the IOR Group from 1997 until 2004, was instrumental in advancing many refinements to the 3D experimental methodology, enabling significant new understanding to be gained about THAI.
Another associate in the development and promotion of THAI was Dr. Conrad Ayasse, President of the Petroleum Recovery Institute (PRI) in Calgary until it was acquired in 1999 by the Alberta Research Council (ARC). Before PRI, he was a senior research manager in the chemical industry. Greaves, Turta and Ayasse all shared a personal interest in the development of ISC technologies and recognized the potential of THAI. Petrobank Energy and Resources Ltd. owns the patents to THAI and CAPRI and have developed addition patents around both technologies.
THAI and CAPRI are trademarks of Petrobank Energy and Resources Ltd.
