THAI - "Toe-to-Heel" Air Injection, is a new EOR process, which integrates advanced technology and horizontal well concepts, to achieve potentially very high recovery of heavy oil. It can also realise very substantial in situ upgrading by thermal cracking, producing upgraded oil to the surface. The process operates in a gravity stabilised manner by restricting drainage to a narrow mobile zone. This causes the flow of mobilised fluids to enter directly into the exposed section of a horizontal production well. The process can be operated on primary production, as a new technology, as a followup to existing technologies, or as a co-process where the advantages of high thermal efficiency are required. This is achieved by concentrating the energy required for oil mobilisation, recovery and thermal upgrading in the reservoir. Combined with clean technology design, THAI offers a pathway to future economic success for the heavy oil industry.
Three-dimensional, semi-scaled experimental tests on light "Forties Mix" oil (30.7 API), Clair, West of Shetlands medium heavy oil (20.8 API) and heavy Wolf Lake oil (10.95 API) show that a well-controlled, narrow mobile oil zone is created just ahead of the combustion front. The width of this narrow zone depends on the characteristics of the heavy oil at reservoir conditions and the degree to which the very high viscosity of the cold oil seals the horizontal producer well. Well sealing can be augmented by a novel sleeve-back technique, which allows perforated downstream sections of the well to be shut-in. The application of this technique enabled the light oil test to mimic the operation of a heavy oil reservoir using THAI. Very high oil recoveries were achieved in the tests, up to 85% OOIP. During wet in situ combustion (ISC), Wolf Lake oil was upgraded to 20 API, achieving a reduction in the cold oil viscosity from 100,000 mPas to around 50 mPas.
The concept of the Toe-to-Heel Air Injection Process (THAI) is represented by the schematic in Fig. 1. Thus, a horizontal producer well is positioned in a line drive in the reservoir and air is injected via a horizontal injection well.
This arrangement is identified as HIHP. Alternatively, the injection well can be vertical (VIHP). This may be adequate, if the horizontal to vertical permeability allows good distribution of gas into the reservoir. Generally, a horizontal injector will provide a more uniform distribution of air across the inlet reservoir face of the line drive section. HIHP is considered to be the base-line well combination, which can be extended through the reservoir in a staggered line drive, HI2HP etc., by employing additional horizontal producer wells.
In Fig. 1, the combustion front is shown (ideally) as a "moving window", which traverses the horizontal production well, from the "toe" position, to the "heel". If such an ideal process could be implemented, then an essentially 100 per cent sweep would be obtained. This is obviously a desirable target efficiency to be aimed for in any advanced EOR process.
