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Future of Underground Coal Gasification


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UCG has been through many development phases in its 50-year history.

The basic assumptions about UCG today, are that:
  1. It must be environmentally sustainable,
  2. The process has to be controllable and reliable
  3. Production costs compare favourably with the traditional methods of fossil fuels conversion.


Process Well Construction

Various routes are being considered for the development of UCG.
  1. The recent European trials in hard coals has shown that these assumptions can be realised by a combination of greater depth and the use of directional and exploratory drilling, which has advanced rapidly in the past few years. This is the direction that UK programme is taking.
  2. An alternative route, which appears to be equally valid for shallow sub-bitumunious, lignites and coals with greater permability is to access the coal with with a succession of closely-spaced vertical boreholes.The Australian trial has shown that a controllable process producing a syngas suitable for gas turbines can be developed and gas production costs are claimed to be very low.
  3. The current Chinese UCG Programme is different again, in that it uses largely abandoned mines. The Process well circuit is constructed by vertical wells into mining galleries, which act as the in-seam connections. Construction costs would be relatively low, and the researchers appear to have mastered the long term control of the process.

    Depth of the Process

    There is no doubt from the latest European trial (1992-1999) that in-situ gasification of coal seams at depths of 500 metres is technically feasible. At this depth, the operating pressure is over 50 bar and the results show that ignition under these conditions is very rapid, and that cavity growth is probably enhanced by roof collapse. The high pressure also favours the formation of methane which significantly enhances the calorific value of the product gas. A further added bonus of depth is the increase in work energy available from the higher pressure.

    Increased drilling cost is the price for greater depth, but the benefits in terms of the UCG process are significant. Neither the drilling or performance data is sufficient to undertake a true cost benefit analysis of UCG at 500m or greater, but the indications are that depth is an advantage and the trend in Europe is towards the gasification of the deeper seams.

    In-seam Drilling

    The use of directional drilling to construct in-seam wells along the coal seam has been recognised for many years as an effective method of coal access for UCG. The problems have been accuracy, control and cost. Latest developments in down hole motors and guidance systems, from the oil and gas exploration industry are now able to demonstrate improved accuracy. The application of these techniques to coal seams (mainly for exploration and CBM) has really only taken place in the 1990s, and the realisation of its potential for UCG is only just beginning.

    Technically, the equipment and operational problems of directional drilling in coal are largely solved. Their application, however, will not succeed until the high costs of specialist drilling are substantially reduced.

    Conclusions
    The following are the Author's own views on the future of UCG

    1. As the UCG process becomes better developed, UCG has a very good chance of succeeding as the modern method of exploiting and winning the energy stored in the massive coal resources of the world.
    2. In- seam will only become more attractive for UCG if the currently high costs of specialist drilling can be substantially reduced. This has to be addressed as a priority.
    3. Environmental considerations will pre-dominate in any future development of UCG, eventually, even those located away from populated areas.
    4. Onshore UCG in populated areas, like the UK and other parts of Europe, will be permitted only at considerable depth, where the possibility of endangering aquifers and the leakage of gas can be virtually eliminated.
    5. The combination of UCG and CO2 sequestration offers an interesting possibility for the sustainable consumption of coal resources, particularly those beyond mineable depth. Further research is required.
    6. It is unlikely that a single coal producing country has the resources or incentive to fully develop UCG. International collaboration is the only realistic means of developing a major commercial UCG industry.