So far I have very quickly covered a couple of natural gas ‘types’. These types are ‘tight gas’ and in-situ ‘syngas’. Before I examine coal seam gas I need to discuss another major gas source called shale gas. Shale gas is not very common in Australia at the moment. It is mainly exploited in the Moomba gas fields in South Australia and Queensland (The Cooper Basin). This gas field is where ‘conventional’ gas has all but run out and the companies operating there are exploring new ways to keep gas supply going. Shale gas is a very well-known source of gas because of the film Gasland which discusses many shale gas basins in the United States. Economical shale gas resources are not currently known in the Northern Rivers. The conditions of formation are not suitable in the Northern Rivers when compared to other gas sources. So, geologically, what is it?
Once again, comparison between ‘conventional’ gas and ‘unconventional’ shale gas is probably the best way to demonstrate how and why shale gas forms and the methods needed for extraction. The first point to make is that chemically there is very little (if any) difference between ‘conventional’ and shale gas. The gas is composed mainly of methane with very small amounts of carbon dioxide and sometimes heavier compounds such as ethane. The second point is ‘conventional’ gas has migrated to a reservoir (a porous and permeable rock that is capped by impermeable rocks). Shale gas, however, remains where it is formed. It is trapped within the natural micro-pores and fractures, or adsorbed to organic matter and clay's in the shale. The gas is formed by the heating and compression of organic rich sediments as they are buried in a sedimentary basin.
Shale gas behaves very similar in ways to shale oil both in terms of formation (except temperature differences) and the way it is trapped in the rock. Interestingly, shale oil was produced in large quantities in the eastern states of Australia during the oil shortages of the Second World War. But, I digress.
Shale gas is trapped tightly on a very small scale it does not flow under natural conditions which is where it differs most from ‘conventional’ gas. Shale gas in Australia is typically very deep gas, rarely found less than a kilometre underground and usually found about twice that depth. Because of the natural conditions, stimulation to get the gas flowing is required. Stimulation in the case of shale gas is almost entirely hydraulic fracturing. In fact, numerous fraccs are required for shale gas exploitation, generally even more so than ‘tight gas’.
Another aspect used all the time is directional drilling. Directional drilling is used for other gas sources such as tight, syngas sometimes ‘conventional’, and rarely for coal seam gas (CSG). Directional drilling allows an increase in the well surface area and therefore increase the rate of gas extraction. The drilling of a directional well follows a ‘conventional’ vertical borehole. Then at the required depth the well is deflected to follow the target formation in a horizontal manner. The interesting thing about shale gas is that directional drilling and hydraulic fracturing make this gas viable. Without both of these technologies this gas type would be an obscure novelty.
Due to the depth of shale gas sources and the surrounding geology the techniques required to extract the gas are tricky. These challenges include the high pressure fraccing required combined with the relative difference in the rock strength of adjacent formations to the shale. This means there is an increased risk to fraccing causing fractures to extend into adjacent non-target rock and therefore connecting other porous or permeable systems with the gas formation. Repeated fraccing may increase the risk further. However, the depth of shale gas in Australia is such that if damage does occur it may mitigate some of the affect to adjacent formations.
A good summary of shale gas in Australia can be found on the CSIRO website
here.
The next post I will do relating to hydrocarbon geology will be on coal seam gas (CSG). CSG is one of the most likely sources for gas in the Northern Rivers.
Previous posts on the topic of petroleum geology (includes gas resources)
