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European Trials
There have been a number of trials for UCG in Europe over the years, summarised in Table 1. The UK trials undertook several innovations, including attempting to connect boreholes by electrically charring the coal, which was not successful. In-seam linkages through drilling from a coal face using several boreholes per panel was successful, given the drilling technology at the time, although the product gas was of low quality. Tests using a single in seam borehole with an air pipe produced better results with gas heat values at just below 4MJ/m3. A small commercial project of about 5MW was planned, using in-seam holes about 6m apart and 100m deep as individual reactors, but this was not successful and the project was eventually abandoned in 1959.In France, two main trials took place between 1979 and 1985. The trials used vertical wells and focused on the options to create the physical link between the boreholes to aid the gasification process. A combination of electro- linkages, hydro-fracturing and reverse circulation produced mixed results, partially due to the design of electrodes and the physical characteristics of the anthracitic coal. The UCG industry now recognises that anthracite is not an ideal coal to use these linking techniques due to its very low volatile matter content.
In Spain, the trial at El Tremedal was initiated in 1991 and finally ended in 1998. Its aim was to demonstrate the technical feasibility of carrying out UCG at intermediate depths between 500m and 700m using deviated in-seam drilling and the CRIP technology. Three exploratory boreholes established the characteristics of the target seam, which is 2 m to 5 m thick, dipping at 30␣ at a depth of some 530-580 m. The coal was high sulphur (7.6%) sub-bituminous with 22% moisture, 27.5% volatiles and 14% ash with a high heating value of 18 MJ/kg.
At El Tremedal, the linkage between the injection and production wells was achieved by using directional drilling techniques. The production well was drilled to within 1m of the injection well and connection achieved by using high- pressure water injection.
The Spanish trial employed coiled tubing to transport the gasification agents (in this case, oxygen and water) and to position the injection head inside the pressurised in-seam well. The tubing was fully retractable onto a drum at the surface, and the coiled tubing equipment, which is commonly used in the oil and gas industry, was supplied with a wellhead injection assembly, pressure seals and a manifold for connecting the injection gas mixtures to the tubing. The well was cased and there was an in-seam liner which burned through when the CRIP was ignited.
Ignition of the gasifier was started 4 m from the end of the production well by the introduction of a pyrophoric compound, tri-ethyl borane, to ignite methane in a burner located at the end of the coiled tubing. Once the liner was burned through to expose the coal surface, the gasification agents were introduced.
The product syngas was generated as the coal surrounding the ignition point was gasified, creating a caved zone (Figure 1) which post-burn investigations indicated
extended horizontally to at least several times the seam thickness. Over a period of days, as the consumption of coal created a cavity, the gasification rate/efficiency declined, and the ignition point was retracted so that fresh coal was accessed. During the trial the coiled tubing was retracted progressively several times and three ignitions made.
The tests took place during July and October 1997, and involved one run lasting nine days, and another lasting four days. The experiments were terminated when there was an explosion during attempts to ignite the gasification channel for a third time. This was because there had been a build up of methane after the pilot flame had self-extinguished. The absence of the flame was not detected because the thermocouple at the ignitor had been damaged during the previous ignition. This is an important lesson learned, and care will be taken in future trials to maintain the integrity of instrumentation. There is no evidence to suggest that this situation has re-occurred in other projects.
The dry product gas consisted roughly of 40% CO2, 12% CO, 25% H, 13% CH and 8% H2S. The test established the potential viability of in-seam drilling, with final linking by high pressure water injection, and of the use of the CRIP in the relatively deep, thin coals seams that are typical of European coal fields.
FIGURE 1 - The Spanish trial depicting the characteristics of the cavity development