Combustion Front Expanding Characteristic and Risk Analysis of THAI Process

Toe-to-Heel-Air-Injection (THAI™) is an in-situ combustion process that is used for the recovery of bitumen and heavy oil. It combines a horizontal production well with a vertical air injection well placed at the toe. The drainage mechanism of THAI is similar to SAGD of the vertical - horizontal well pattern. However, it is much more difficult to control the THAI process compared with SAGD as gas flow and liquid flow coexist in the horizontal well for THAI. The start-up and the combustion front expanding characteristic of THAI require critical attention, in order to ensure optimal process operation. The temperature profiles and post-mortem pictures of the sandpack during 3-D combustion cell tests performed on Z Block heavy oil are presented in this paper. The processes and the results of 3-D experiments indicate that high temperature ignition, at least 500 is essential for startup of THAI. Then the combustion front is growing in size and expanding downwards, and the high temperature ensures that the utilization factor oxidation is high. After the combustion front propagates beyond the ‘toe’ position, the temperature burning zone, deposition of coke around horizontal well, air injection rate and production rate are key factors for stable propagation of the combustion front. Experimental results also indicate that the combustion front will break through along horizontal wellbore in case of high temperature and high oxygen concentration in the horizontal well because of improper regulation of injection and production. After combustion front breakthrough, a considerable bypassed oil area will be left in the reservoir which results in low recovery efficiency. In the same time, the combustion front breakthrough along horizontal well may also lead to damage of the wellbore by means of high temperature oxidation(combustion) which will result in considerable engineering risk. In order to solve this problem supervision and control measures are researched. It can be helpful for successful planning and implementation of the field pilot testing.