You can write a book on answering this issue. I'll try and be...

You can write a book on answering this issue. I'll try and be shorter than that.

A good resource is the LLNL library, goto www.llnl.gov, find the library and seach for UCG. You can download all the US reports if you wish.

The Soviets developed UCG from the 1930s but they didn't make large scale installations until around the 1950s and they have quite a number by the 1960s. The scale of these installations was up to 100 - 200 MW or around 1000 t/d.
Most Soviet UCG stations were shut down in the 1970s - why? well I think the manpower required for UCG was just too great compared with the low cost natural gas which started being exploited at that time. Before that coal was all they had, and the Soviets (like Lenin) also liked the idea that UCG didn't require men underground. Many UCG stations also had problems with subsidence, gas leakage etc. But I think most of these problems were resolved. The scale of the Soviet effort was pretty large.

Now outside the USSR there have been many many trials - starting in fact in 1911 I believe. In the 1950s the UK did trials; there were others in France in the 1940s. All these were largely unsuccessful but mostly cos, technology for linking the wells was poor and the European coal seams are too thin IMO.

The US started a major program in the 1970s. The LLNL website has some great reports on it. However due to cold war its my belief they didn't fully understand the developments made in the USSR. Hence most of their trials, eg Hoe Creek etc., were a period to find out the learnings already made elsewhere. In terms of site selection, design and operation they made many mistakes. They had gas leakage and aquifer contamination (Hoe Creek) etc. But I would argue these are not representative for future UCG.

The US program also developed CRIP methods, which will be adapted by MEE and anabale much better control of the burn.

The key learnings from all this are site selection is key (often not done well in past), then gasifier should be operated at or below hydrostatic pressure, you must inject oxidant at bottom of seam. Techniques exist to manage the migration of heavy organics into the aquifer and to remediate the site to acceptable quality. LNC did this at Chincilla.

However, the subsurface is not a quantify which can be known. So new surprises can develop. Who know's how likely a successful trial burn is. But given location, success of Chincilla and proven USSR methods I think there's a good chance of success.