Recent Laboratory Results of THAI and Its Comparison with Other IOR Processes

Abstract

Abstract The integration of reservoir processes with horizontal well technology has led to the development of more efficient techniques for recovering heavy oil and tar sands bitumen. Whilst current in situ applications have concentrated primarily on steam injection processes, such as SAGD, new developments in air injection technology have been made using ‘Toe-to-Heel’ line drive. THAI - ‘Toe-to-Heel’ Air Injection, creates unique operating conditions in the reservoir, that have special advantages for heavy oil recovery. The superiority of SAGD, compared with other steam processes, is manifested primarily in its gravity-assist mechanism, which allows a stable steam chamber to grow. The oil which is mobilised at the steam-oil interface drains down towards the horizontal producer well. In THAI, a similar gravity-assist mechanism exists by virtue of a ‘narrow mobile zone’ that develops immediately ahead of the combustion front. This occurs in a heavy oil reservoir when the vaporised oil and gases are prevented from communicating through the downstream oil zone and instead are forced to flow down into the exposed part of a horizontal producer well in line drive. The gravity-assist prevents gas override, ensuring an upright combustion front and hence stable process. Semi-scaled 3-D combustion cell tests using heavy Wolf Lake oil have revealed the essential characteristics of THAI. Temperature profile measurements indicate that a ‘narrow mobile oil zone’ is formed, which allows hot vaporized oil, steam and combustion gases to be easily drained into the horizontal producer well. Cold oil in the downstream layer is effectively immobile due to its very high viscosity and consequently the oil saturation is maintained constant. This provides for constant, stable process conditions. The thermal sweep efficiency of the process is very high and virtually all of the oil in the zones contacted by the combustion front is produced, except for the small fraction consumed as fuel. The potential oil recovery is very high compared to other processes. Introduction In situ combustion is achieved by burning a small fraction of the oil in the reservoir in order to enable flow of the unburned fraction. The history of in situ combustion for heavy oil recovery can be traced back to the turn of 20th century in the United States1. In situ combustion (ISC) is a thermal enhanced oil recovery (EOR) process which uses air injection for improving oil recovery2,3. Because of the strong exothermal oxidation reactions between hydrocarbons and oxygen, oxygen is consumed to produce flue gases and, at the same time, the temperature of the oil-bearing matrix is greatly increased. In situ combustion is particular favourable for heavy oil reservoirs because the increase in temperature reduces the oil viscosity by several orders of magnitude. By generating the thermal energy in situ in the reservoir, in situ combustion has many advantages over other heavy oil EOR processes, such as a high efficiency in terms of heat utilisation, highly efficient displacement drive mechanism, and less total environmental impact.4. This technology has been extensively studied in both the laboratory and the field. However, the conventional ISC process has not achieved general acceptance due to many apparent failures, mainly to do with inappropriate reservoir application and poor control of the process. The most significant operation problems affecting recovery from heavy oil reservoirs using vertical-vertical well pattern in situ combustion are:Gravity segregation, or gas overriding, due to the difference between the gas and oil densities.Channelling, due to the unfavourable rock heterogeneity.Unfavourable gas/oil mobility ratio.