The reservoir matrix is important – some clay content is required to help thermal cracking of the oil ahead of the combustion front. The process can break down intermittent shale breaks that could otherwise obstruct permeability. THAI can also cope with a bottom water zone, as exists in part of the Christina Lake pilot, by effectively steaming it out. Tests also indicate that THAI can handle a gas cap; gravity and pressure draws down all the gasses ahead of the combustion front.

Ahead of the combustion front (typically around 600 degC), is the coking zone, in front of which is a 3–5m wide mobile oil zone through which drainage takes place into the horizontal well. These zones move through the reservoir at about 0.5 to 1 meter per day, depending on the air injection rate. The temperature drops to between 200 and 350 degC at the front of the mobile oil zone, with a corresponding reduction in the rate of drainage. Ahead of the mobile oil zone is the cold immobile virgin oil layer, through which there is no communication for gas. This characteristic of THAI geometry means that the only way out is down into the open section of the horizontal well. The horizontal well trajectory is thus an inbuilt self-controlling guidance system for fluid flow. This makes THAI much more controllable than conventional ISC systems, in which fluids and gas can roam anywhere in the reservoir.

Greaves considers that THAI is ideal for producing oil less than 15 API. Laboratory tests performed on a 20 API sample supplied by BP from its Clair field in the UK North Sea indicate that it could also bring benefit to medium heavy oils. In common with any EOR process, offshore application of THAI would require very careful reservoir simulation studies and detailed design evaluation. Compared to SAGD, it would avoid the need for offshore steam generation.
Well costs would, however, be high. The Petrobank pilot in Athabasca has pairs of wells 100m apart, with each horizontal production and vertical injection well costing about two million dollars to drill and complete. For medium heavy oil, spacing could be increased to about 200m, but a North Sea well is likely to cost about $15 million, which will radically impact the economic feasibility unless innovative, new THAI well arrangements can be found.

Greaves observes that, while 10 years ago the main focus was for higher production, today it has moved towards maximizing recovery. THAI has the potential for delivering better recovery rates than SAGD (<50%), CSS (<20%) and Waterflood (10%) and should be profitable, even at prices less than $30 per barrel. In the lab, THAI consistently achieved oil recoveries approaching 80% OOIP, or more, 

Greaves estimates that THAI can be used for residual EOR in situations where initial production has produced up to 40% ooip—particularly where suitable wells are already in place. Laboratory tests using Athabasca oil showed THAI to effectively produce remaining oil after SAGD efforts were exhausted. A Toe to Heel Steam Flood (THSF) produced 24% ooip (Greaves notes that this was under low pressure conditions and predicts that a high pressure reaction cell, with higher steam temperatures, could have doubled recovery). THAI was applied after the THSF test, increasing the total recovery rate to over 80%.

THAI is a trademark of Petrobank Energy and Resources Ltd.

CAPRI in situ upgrading