Sunday 1 December 2013

Deeply a Fire Smoulders

In a previous post I discussed how tight gas is defined and how it behaves. I promised in that post I would expand on the theme of hydrocarbon geology and discuss other sources or techniques for obtaining natural gas. These include the aforementioned tight gas, but I will eventually cover all aspects including coal seam gas, basin gas and lastly “conventional” gas.

In this post I’ll quickly cover underground coal gasification (UCG) producing “syngas”. This is to make clear that it is different from coal seam gas. Presently, syngas is not produced in Australia and recent trials in Queensland and South Australia have ceased and moved off-shore to China. The companies cited a more conducive research and regulatory environment there than Australia. I only cover UCG because the geology of the southern Clarence-Moreton, Ipswich and maybe Lorne basins may be seen as sources for syngas in the distant future.

Syngas is produced through the process of underground coal gasification. This is a relatively new and novel way to turn coal into gas, though the concepts are in many ways similar to the older concepts of shale oil extraction and town gas production. these techniques having been used for more than a hundred years. Like most aspects of science, something new builds upon something old.

The first step in UCG is to find coal rich strata confined by a high pressure of natural water in the coal seam. A vertical drill hole is installed in one end of the coal seam and is terminated at the bottom of the target coal seam. A second drill hole is drilled at the other end of the gas field, possibly 2 or 3 kilometres away. This second hole is however, directionally drilled and follows the bottom of the target coal seam all the way until it intersects the first vertical drill hole. A well head is then set up at the first vertical drill hole and gasification infrastructure set up at the horizontal (directional drill hole). It is from the directionally drilled hole that all the interesting action takes place. The vertical one is just used for pumping the gas to the surface.

Gasification infrastructure is comprised of pumps for forcing air and guiding an ignition source into the ground. The the actual process of UCG occurs in-situ, that is, in the coal seam itself. The coal is first ignited underground at the point where the horizontal and vertical drill holes intersect. Air is pumped into the coal seam to displace some of the water which allows the process to continue. If air is not injected the water occurring in the rock extinguishes the gasification process. The coal is continually kept ‘burning’ underground and slowly moves along the directional drill hole as air and ignition is applied.

This process is essentially incomplete combustion. A process that was used to produce town gas in most major towns and cities in Australia up until the 1970’s. The incomplete combustion leads to production of CO and CH4. Adding too much air into the process simply produces more CO2 and so a balance of water pressure, air pressure and gas production is needed.

UCG differs from coal seam gas (CSG) in that water is only partially displaced from the coal seam. CSG requires as much water as possible to be removed to stimulate the natural flow of gas. Groundwater in CSG can be considered a waste product of the extraction process, a bit like overburden in a coal mine. UCG leaves the “overburden” water essentially intact.

UCG is an interesting, challenging and clever way to turn coal into a gas resource. It has been marketed as an alternative to digging a huge hole in the ground to extract the coal in a mine. The groundwater issues are regarded as less invasive than direct mining but there is added potential for incomplete burning residues to contaminate the groundwater. For example incomplete combustion can produce chemicals such as polycyclic aromatic hydrocarbons (PAH). Although generally poorly soluble, the presence of these chemicals is perceived as a concern by many people. Whether or not there is an avenue for these chemicals to become a risk to the environment is hotly debated. It is therefore now surrounded by a lot of controversy. Like underground coal mining there is also the possibility of ground subsidence. But regardless, it appears that in the short run this process will not be used in our region.