The results of the numerous global tests and projects to date can be summarised as follows: There are three primary methods for undertaking UCG projects:
• The vertical well (Soviet) method
• The enhanced vertical well method with wells linked by horizontal boreholes
• The CRIP methodology
Of these, the CRIP method offers the opportunity for better process control by injecting the oxygen or air where needed and has the ability to exploit deeper seams which would not be economic using vertical wells.
There are clear conclusions that can be drawn from the work to date:
• Shallow seams are not suitable for gasification because of high gas losses, potential
breakthrough to surface and possible contamination of groundwaters;
• Thin seams, less than 3m thick, will be difficult to exploit economically unless
in a multi-seam environment;
• Thick seams work well using the CRIP method;
• Lignite and sub bituminous coal are ideal for gasification, as is bituminous coal provided
no significant swelling characteristics exist;
• Deeper coals offer the opportunity to have much higher pressures in the reactor resulting
in higher methane content and resultant higher heat value gas.
• CV’s in the range of 12-14 MJ/m3 are recorded when using oxygen feed and this may be
slightly increased as the process develops at a greater depth;
• The CRIP concept has lead to the highest gasification efficiency in terms of oxygen usage
and will allow subsidence to be minimised or possible eliminated, by using wider barrier
pillars between panels
The concept of UCG as a process is relatively simple and is similar in principle to the surface gasification system that has been used extensively in the world, such as by SASOL in South Africa. The early work in Russia focussed on exploiting shallow and often thin coal seams that were relatively easy to drill into but would not be considered suitable today.
The shallowness of the coal seams means the pressure in the reactor is low and the quality of the gas produced is reduced compared to deeper systems. In addition, the primary difficulty is that a physical connection needs to be created between the vertical wells and much of the early work focussed on this aspect.
Although several ways have been attempted to achieve this link, including electronic charring, the most appropriate systems are hydro-fracturing the coal, to create a series of physical cracks in the coal linking the boreholes or drilling a connecting borehole.
The hydro-fracturing system is being used by Eskom at the Majuba coalfield in South Africa. In this case, it is believed that vertical boreholes are drilled about 30m apart. Connection by borehole is not possible as the high incidence of vertical or sub vertical igneous intrusions prevents this in the same way that the original concept of long wall mining was not feasible. Therefore, in this case, there was little other possibility to exploit the coal than using a linked vertical well method. Little information is available on this project.
An Australian Company has also adopted a linked well method, but using an in- seam borehole to link the vertical wells. It is a system that is probably the best given the relative shallowness of the coal (just over 100m deep) but has certain disadvantages when compared to the CRIP method, discussed below.
In summary, the experience of vertical well-based UCG projects is that the system can work but the key issue is the nature of the physical connection between the vertical wells. The historical evidence is that a borehole is the best system and that whilst hydro-fracturing can work, it depends heavily on the geology and characteristics of the coal seam. Moreover, as the spacing of the vertical wells is probably 20 – 30m at best (again, dependent on the geology), it becomes an expensive technique when the coal is deeper than 250m. Although drilling vertical wells is technically simple and non-cored percussion methods can be used, the amount of coal gasified relative to the meterage drilled is significantly less than using the CRIP method.
The most significant developments in the last thirty years have been the successful Rocky Mountain 1 (USA) in the late 1980’s, the El Tremedal trials in Spain in the 1990’s and the recent Carbon Energy pilot in Australia, and the improvement in directional drilling technology. El Tremedal proved that gasification of coal at depth was possible, that it was possible to extinguish and restart the process and that the drilling process could be completed satisfactorily. The Rocky Mountain 1 proved operating at low pressures and post-burn cavity venting and cooling are effective in terms of efficiency and environmental impact when using UCG technology.
Whilst LLNL had already shown that coal gasification using the CRIP technology produced better quality gas than linked vertical wells, the importance of El Tremedal was that it proved deep coals could be successfully gasified. This meant that with the use of oxygen and the higher pressures in the reactor, deep coals produced a far better product whilst opening up a significantly large resource base of coal that was unsuitable to be exploited by conventional mining.
In addition, the greater depth significantly reduced the risk of groundwater contamination and minimised the impact of surface subsidence, whilst preventing any possible breakthrough to surface that could offer an uncontrolled exposure to air intake. When this is added to the fact that trials have indicated that most coals can successfully be gasified, El Tremedal formed the basis of the best technology to take the industry forward and this has been affirmed by the recent successful pilot project completed by Carbon Energy. The future of UCG will focus on gasifying deeper coals, below 250m, and with a thickness of over 4m which means a substantial resource base is available on a global basis.