List of Natural Gas Posts

To try and bring some order to some subjects that have been dealt with in previous posts I think it would be useful to create some list posts. This first list is about a subject that is very topical at the moment, natural gas. Natural gas includes so called "unconventional" gas such as coal seam gas (CSG), shale gas, tight gas.

Posts on "conventional" gas

• Being tight with loose terminology?

Posts on coal seam gas

• Baseline CSG methane in groundwater
• Clarence-Moreton Basin CSG Bioregional Assessment
• What is CSG?
• Some musings on coal seam methane
• Coal seam gas gets a seismic thump!
• Why you wont find CSG here

Posts on shale gas

• Gas from Shale

Posts on tight gas

• Being tight with loose terminology?

Posts on other unconventional gas

• Fracking qualifies for aged pension
• Deeply a fire smoulders

Gas from shale

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)

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.

Being tight with loose terminology?

There has been a lot of discussion recently about a local company resuming exploration for gas in our region. In particular the announcement by the company that they intend to drill a deep borehole next to the Lismore-Kyogle Road at Bentley has raised a great deal of heated debate. For example, this story in the Northern Star shows just how intense the feelings (one way or another) can be. One thing has been clear though is people are sometimes having trouble figuring out what gas companies are doing. The news release from Metgasco and their Review of Environmental Factors report state the proposed drill hole will be for "conventional gas". Critics of gas companies say since hydraulic fracturing (fraccing, fracking etc) may be carried out in the proposed drill hole the gas must be "unconventional" tight gas. Some people (including the local members of parliament) seem to think any drill hole in the area must involve coal seam gas. It is all a little confusing.

The first thing to note is gas should not be described as either "conventional" or "unconventional". There is essentially no difference in the gas (mainly comprised of methane). The difference is in how it is extracted.

The second thing to note is "tight gas" is only termed such by an arbitrary permeability value assigned by oil and gas engineers. In the real world there is a spectrum between traditionally sourced gas and tight gas. The tighter gas is gas occurring in a reservoir but does not flow as rapidly as in other locations. Tight gas is restricted from flowing by the fill in the cross connecting voids by a material formed after the gas migrated there (usually a natural cement such as calcite or quartz). The lack of cross connection between gas filled pore spaces is what reduces the permeability of the rock.

To clarify I use filter analogies. A new filter will let a substance flow through it easily but an old one is more clogged up and doesn't let the substance flow as rapidly. In the oil and gas industry an arbitrary permeability value is used as an indication of when it is called tight gas. It usually has a permeability of less than 0.1mD (millidarcy). It is also important to be clear that permeability is not the same as porosity because even tight gas reservoirs still have high porosity.

What is a millidarcy? I should do a blog post specifically on Darcy's Law but in the mean time it is good to visualise 1 millidarcy as the permeability of water in a fine sand filter. The way a millidarcy is calculated is through measurement of the velocity of fluid flow through the filter, viscosity of the fluid, cross sectional area of the filter and the pressure. In the case we are talking about the fluid is gas. The main difference being gas has a much lower viscosity and therefore can pass through a finer filter even easier. For 1 millidarcy our analogy for gas might be a fine paper filter instead of fine sand filter for water. It is interesting to note that the measurements needed to calculate the permeability of a gas reservoir are identical for hydrogeologists trying to understand groundwater flow or brewers filtering a favorite beer.

When the permeability of a gas reservoir decreases to 0.1mD the gas flows at a lower and lower rate. This means that it becomes less economical to let the gas migrate out of the geological formation on its own. Instead companies often look to reservoir stimulation which in its simplest form is introducing acid to dissolve minerals. Such a common mineral is calcite that may have clogged the formation, opening up the blockages between the pores. This is a very useful technique in natural calcium cement rich formations. Acidification has been used for hundreds (perhaps thousands) of years to increase the flow rate of groundwater sources for drinking, irrigation and other purposes. It is still commonly used in Australia today for groundwater purposes too. However, the most well known form of reservoir stimulation is the increasingly used hydraulic fracturing (fraccing/fracking). Fraccing involves the introduction of a fluid such as water (plus other ingredients) under pressure to propagate fractures through the formation. These fractures allow gases to escape much more easily. I don't want to go into details about fraccing here but I will suffice to say that the method is controversial.

As far as the terminology goes, tight gas is a very loose term. Tight gas is not an "unconventional" gas, it is a bog standard gas that sometimes require "unconventional" techniques to extract it. It is also important to note that reservoir stimulation is an "unconventional" method of extracting gas, but this does not in itself say much because the "conventional" method of extracting gas is just sticking a big hole in the ground.

I hope this blog post makes sense. While I was writing, it became obvious that several different posts are needed to explain the different areas of gas reservoirs. In the mean time I hope that this short post makes sense. I'll see what I can do over the coming months to further delve into the hidden world of petroleum geology (while steering as far away from controversy as possible).

Coal seam gas gets a seismic thump!

Ok, time for me for foray into an area that is politically sensitive. But I hopefully do so factually.  I'm starting now because of recent developments by the Lismore City Council to first approve a Review of Environmental Factors (Which is strange since the Mineral Resources Division of the NSW Government approves these (or was it approval to use road reserves? I can't seem to figure out what authority the council has from the news reports)) and then to rescind this approval once they found that the work being undertaken could be used to target the area for further coal seam gas exploration (well... that is how I read it). Here are two newspaper reports that discuss the matter: Northern Star and Northern Rivers Echo.

So, I guess the centre of the matter is 'what is seismic exploration anyway?' Seismic exploration comes under the category of geophysics and in this case refers to the use of sound waves to try and understand what is below the ground. It is a non invasive method with the major environmental impacts believed to be limited to noise pollution and a small area of squashed grass. Practically one of the common processes used (and I understand this may have been the method outlined in the REF) is a truck with a pad under which will drop and hit 'thump' the ground. Sensors in the truck (or an accompanying vehicle) then receive data back from the ground as the vibrations made are reflected off layers of rock under the ground. The truck will then drive off a hundred or so metres further along and then do it again and so on.

The Department of Mineral Resources have previously conducted some of this work in the area since the 1970s. But I understand that Metgasco are currently undertaking more detailed work which may provide them with information that can lead to (or rule out) possible places to target exploration drilling. The Roads and Traffic Authority (RTA) frequently use this technique when planning routes for roads, the last time I heard this was between Byron Bay and Grafton about 6 months ago.

The decision by Lismore City Council is therefore one I find a little hard to understand unless it is simply a matter of the Councillors not understanding the technique itself or just deferring to the current political environment. Personally, I'd certainly love to get some more geophysical information on the Clarence Moreton Basin (for the sake of scientific knowledge itself) since I've recently had some discussions various people that prove that we know very little about faults, stratigraphy, intrusions, groundwater, deformation, metamorphic events, etc that have occurred in the basin since the rocks were laid down. Seismic exploration would go along way to answering some of these questions. But I guess if the politicians and general public don't want the information to be used specifically in coal seam gas exploration then it is important that we do not learn about these features in our region. That might be a price many in our region are prepared to undertake.