Underground Coal Gasification

Good evening readers and welcome to 2012. I write having just seen this report on Channel 4 news about Underground Coal Gasification (UCG). I found it fascinating, and I must admit that, for someone who claims to be in-the-know about this sort of thing, it caught me somewhat by surprise. It seems developers are keen to give the technique a go in the Swansea area, which was news to me.

So, what is UCG? Let's ask wikipedia. In short, the basic idea is that you take a coal seam that is too deep or too thin to mine, and inject oxygen. The oxygen combusts with the coal (at temperatures of 700C or more), producing CO2, CO, H2 and CH4. Once produced, the CH4 and H2 are burned to produce energy. Yes, you heard correctly, we're going to drill down into some coal seams, pump in oxygen and then set the coal on fire! It's so crazy, it just might work! Actually, it does work, but obviously there are issues of groundwater contamination, air pollution (CO2 'only' causes global warming, CO is far nastier stuff) and subsidence. 

But wait - there's more! UCG is not the only process where we set fire to hydrocarbons while they're still in the ground. One method of extracting heavy (very viscous) oil is a process called Toe-Heel Air Injection (THAI). By injecting oxygen and burning a portion of the hydrocarbon, we increase the temperature of the rest of the oil, 'melting' it, making it less viscous, and therefore easier to suck out through the production well. Here's a picture:

An aside: sometimes the things we do in pursuit of hydrocarbons blows my (admittedly tiny) brain! As an engineer/scientist, I'm proud and amazed that humanity is capable of doing such incredible things. From a more social/environmental angle, I'm disgusted that our addiction to oil has forced us to do such incredible things.


However, my main interest in discussing UCG is to make a comparison with hydraulic fracturing and shale gas. Firstly, note that in order to generate sufficient permeability in the coal beds, fracking will almost certainly be required for UCG as well. Now consider that in shale gas the workflow goes something like: drill - frack - produce the gas. For UCG it goes: drill - frack - inject oxygen - combust coal at 700C - produce gas (as well as CO2 and CO, and smaller amounts of sulphur oxides etc). From a global warming perspective, I'd be surprised if UCG (which is, ultimately, coal-based) doesn't produce a lot more CO2 than shale gas. I'd be more worried about the possibility of CO leakage (although in my brief trawl of the internet I've not seen this raised as a major issue). Also, the volume of the remnants (coal ash) is less than the initial volume of untreated coal, creating the possibility of subsidence during production.

The combination of heating and possible subsidence means that controlling fracture propagation during UCG could be a serious challenge. If you want to stop a hydraulic frack (as for shale gas), you turn the pump off, draw down the pressure and it stops. Not so sure how you'd stop fracturing caused by an overheated zone, or by subsidence. Fractures can provide pathways for fluids to migrate beyond the target zone.  So if you can't control your fractures, how can you ensure that any potential pollutants stay 'in-zone' and do not escape, to eventually end up in someone's water supply?

With these disdvantages, I find it difficult to believe that UCG can be as safe, as controlled, with a lower environmental impact (both local air and water quality, and global GHG emissions) as shale gas. However, what it does have going for it (in the UK at least) is that it will likely happen in areas accustomed to large scale coal mining, meaning the people there are less likely to complain about new industrial developments, and these are regions that, with the decline of the UK coal industry, are more likely to take the opportunities for jobs and economic development over environmental concerns. In contrast, the major shale gas areas considered so far - Fylde, the Mendips, the Vale of Glamorgan, Kent - are some lovely unspoilt areas of countryside, and are also much more prosperous, and so the local people there are far more likely to object to shale gas (or any other industrial) developments.

I think it'll be very interesting to see how these things play out. Will we go for both shale gas and UCG? One or the other, or neither?