|Experiment on in-situ gasification carried out in Belgium on the Thulin site in a coal deposit lying at 860 m depth. It aims at achieving an underground gasifier working at a pressure ranging between 20 and 30 bar.The first reverse combustion experiment was executed from April to October 1982 without the formation of a linking channel. The test had to be halted due to self-ignition of the coal, after 3. 5 days. In November 1982 the fire was extinguished by injection of water and nitrogen.Before starting a second experiment of reverse combustion, the wells have been restored and various improvements were brought to the equipment to eliminate the self-ignition of the coal in the vicinity of the injection hole and to prevent the accumulation of water at the bottom of the gas recovery hole. This second experiment started in September 1982 and was stopped in early May 1984. The experiment suffered from 4 interruptions due to tubing breakage by corrosion, but it has been possible to mater the problem of self-ignition of the coal.During the trial of April 1984, the combustion evolved to gasification, producing 150 rubic meters per hour of lean gas during 12 days. A careful analysis of the available data (flow /pressure, tracer tests) did not show any evidence that a channel had been started. The experiment has been stopped, while intensive work was devoted to solving the corrosion problems (the rubing has to withstand both cold and hot corrosion), and to preparing a new trial with strongly deviated drillings, starting from the existing wells.This trial has been made with success in September 1985 : a drainhole of 40 meters length (27 meters in coal) has been drilled from well I in the direction of well II, and a flexible liner has been set inside this drainhole. However, the azimuth control was poor.In December 1985 to January 1986, a sidetracked hole was drilled from the level 670 of well II and crossed the sean at about 1 m from the drainhole. The coal plug between the wells has been expelled by high pressure water on 4. 2. 86. In 1987, some 340 tonnes of coal were successfully gasified making a good quality gas. The results were considered sufficiently encouraging to proceed to further trials.The project originally had 5 phases.Phase 1 : Reverse combustion (completed May 84)Phases 2, 3 and 4 : respectively trials by IDGS (Belgium), G. E. G. S. (France) and N. C. B. (U. K. ).Phase 5 : concurrent with these involving study of common problems. The main innovation is the combination of deviation and horizontal in-seam drilling. Another is the insertion of a flexible "casing" into the curved part and in the horizontal drilling (completed 87).Technical risks are related to the inaccuracy of the azimuth control, use of a flexible casing, low coal permeability at great depths, corrosion, and high temperature. Economic risks include the high cost of deep boreholes and uncertainty about the heating value of the produced gas, efficiency of gasification and % unburned coal.