THAI-"Toe-to-Heel" Air Injection, is a new EOR process, which integrates advanced technology and horizontal well concepts, to achieve a potentially very high recovery of heavy oil. It can also realize very substantial in situ upgrading by thermal cracking, producing upgraded oil to the surface. The process operates in a gravity stabilized manner by restricting drainage to a narrow mobile zone. This causes the flow of mobilized fluids to enter directly into the exposed section of a horizontal production well. The process can be operated on primary production, asnew technology, as a follow-up to existing technologies, or asa co-process where the advantages of high thermal efficiency are equired. This is achieved by concentrating the energy required for oil mobilization, recovery and thermal upgrading in the reservoir. Combined with clean technology design, THAI offerspathway, to future economic success for the heavy oil industry. Three-dimensional, semi-scaled experimental tests on light "Forties Mix" oil (30.7 °CDATA[API), Clair, West of Shetlands medium heavy oil (20.8 °CDATA[API), and heavy Wolf Lake oil (10.95 °CDATA[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 °CDATA[API, achieving a reduction in the cold oil viscosity from 100,000 mPas to around 50 mPas. Introduction Horizontal and multi-lateral wells are being introduced in ever increasing numbers to achieve further gains in reservoir productivity. Two hundred horizontal wells were drilled a decade ago. Now, one company has reported drilling more than 2,100 horizontalwells in North America(1). Advances have also been achieved in the depths drilled, reaching 8,761m(2) and lengths now reaching up to 11 km. Multi-lateral wells, with four or more side-tracks, are not uncommon and offer further optimization gains. Moreover, this technology will accommodate reservoir faults and problems of structural geology as regular design factors so that strategic placement of the well path is achieved. More recently, a new technique to control gas and water production rates by varying the fluid entry points has been successfully tested(3). The most important advantage of horizontal wells is the increase in the reservoir contact area compared to a vertical well. Horizontal wells have created new possibilities for the implementation of advanced IOR methods, especially in thermal recovery.

This paper was prepared for presentation at the 1998 SPE International Conference on Horizontal Well Technology held in Calgary, Alberta, Canada, 1-4 November 1998.

There is growing interest in the application of air injection in Jeep l ight oil reservoirs, especiaily in r emote locations and offshore. Air represents a cheap source of injection gas, and witti low temperature oxidation (LTO) the oxygen is generating an in situ source of N2 or `flue gas' to carry out immiscible or miscible displacement processen. If the oil and reservoir matrix are sufficientiy reactive, high temperature oxidation (HTO) can be implemented to sustain fully-propagating in situ combustion, witti excellent improved oil recovery (IO R). One problem observed witti light lil reservoirs is the tendency to desaturate the oil layer dotivnstream of the moving front, particularly for a thermal IOR process because of the additional vaporisation which occurs. A low pressure test was carried out in a 3-D cell equipped witti an extensive thennocouple array. In order to restrict desaturation of the oil Layer, a modified hórizontal producer well, incorporating a `sleeve-back' principle, was used. The idea was to mimic the proces occurring during heavy oil recovery wherein the downstream oil is essentially immobile, due to its high viscosity. The `sleeve-back' of the well was achieved using a co-aligned, two-well assembly, so that the upstream section of the horizontal producer well is active, and adjusted during propagation of the combustion front. Sufficient improvement in overall stability of the proces was observed using the `sleeve-back' modification, as witnessed by sustained propagation of the combustion front and high levels of C02 in the produced gas , IN TRODUCT ION The introduction of horizontal wells hos been responsible for tremendous advances in oil field development. Horizontal and multilateral wens are being introduced in ever-increasing numbers to achieve further gains in reservoir productivity. Two hundred horizontal wens veere drilled a decade ago. No v, one company hos reported drilling more than 2,100 horizontal Wells in North Americat'1. Advances have also been achieved in the clepths drilled, reaching 8,761m12], and allo the length, reaching up to 8km. Multi-lotsral Wells, witti four or more sidetracks , are not uncommon now and offer further optimisation gains. Morcover, this technology will accommodate reservoir faults and problems of structural geology as regelar design factors so that strategie placement of the well potti is achieved. More recently, a new technique to control gas and water production rotes by varying the Huid entry points along the well length hos been successfully tested[31. The most important advantage o£harizontal veelis is the increase in reservoir contact area, which is limited to the reservoir thickness for a vertical veel]. This property hos also created new possibilities for the implementation of advanced EOR methods, especially in thermal recovery. A horizontal well con increase both injectivity and productivity. 9th E uro p ean S ymposium on Improved Oi l R ecovery, The Hague-The Netherlands, 20-22 O ctober 199 7 M A LCO LM GRE AV E S and A BDULBA SET M. SA G...