Tuesday 8 October 2013

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).

Tuesday 1 October 2013

The Woodburn sands of time

I’ve been spending some time working on a project in the lower reaches of the Richmond River Valley. This project got thinking about the stratigraphy and depositional history of that area. Particularly about a unit of unconsolidated sand called the Woodburn Sands (Drury 1982). In some ways this post follows on from a couple of posts that touched on the subject of sea level changes during the Quaternary.

To begin to understand this unconsolidated sediments of the Richmond River Valley we turn to the most recent mapping of the area. Troedson et al. (2004) comprehensively mapped the coastal Quaternary sediments of the whole east coast of NSW. Troedson et al. (2004) demonstrated that over large areas of the lower Richmond Valley there are two units of coastal sand which formed in barrier environments. The most obvious coastal sands are active dune and beach systems formed from a barrier by the action of present day long-shore drift. These active barrier systems occur in many places along the coast. Troedson et al. (2004) also mapped extensive areas of what an earlier researcher Thom (1965) first identified as an inner coastal barrier. This inner barrier is comprised of an old beach system that is no longer active.

Drury (1982) undertook a comprehensive study of the Quaternary sediments of the Richmond River Valley. He confirmed the view by Thom (1965) that there was an old inner barrier system. This system was formed during a higher period of sea level than today and caused regional changes to coastal sedimentation (e.g. I previously posted on the estuarine sediments of the Lismore area). The high sea level eroded away the pre-existing beaches and formed new beach systems a significant distance inland (sometimes 15km or more). Then as the sea level retreated, the new beaches were no longer subject to erosion from the sea and were left intact. The beach systems continued to form on the sea-ward side of the old beaches and eventually built up a very large area of sand. These old beach systems are what made the Woodburn Sands.

The Woodburn Sands occur in a discontinuous zone from Broken Head National Park to the Evans River and the lowest reaches of the Richmond River (Swan Bay). The maximum thickness intersected is about 35 metres, so the sand layers can be very thick.

Like many places in eastern Australia, the action of coastal wave and wind processes can lead to concentrations of heavy mineral sand.  These deposits are called mineral placers. The Woodburn Sands is another of these areas where placers are common. Indeed, a lot of sand mining took place on the north coast to exploit the high concentrations of zircon, ilmenite and even gold. Presently, the Woodburn Sands is not mined for minerals but is used as an important source of good quality groundwater, this includes the regional town water supply authority.

Drury (1982) also included an unusual feature within the Woodburn Sands. This feature was named the Broadwater Sandrock by Mcgarity (1956). McGarity (1956) demonstrated that the Broadwater Sandrock was formed by the cementation of sand by organic rich material probably formed by changes occurring in a peat swamp environment. This sandrock is a common feature up and down the east coast of Australia. Another common feature is the diversity of names given to this material which include ‘indurated sand’, ‘coffee rock’, ‘coastal sandrock’, ‘painted rock’, ‘beach rock’, ‘humate’ and ‘B-horizon of the humus podzol’ (Drury (1982), Mcgarity (1956), Thom (1965) and Den Exter (1974)). Take your pick! I follow the terminology proposed by Drury (1982) who included the Broadwater Sandrock as a member of the Woodburn Sands, i.e. the Broadwater Sandrock member.

Since doing the above post an anonymous commenter has rightly corrected and provided further information. You can see the full comment below, the comment much more accurately describes 'coffee rock' formation but I reproduce this section specifically:

...humicrete (coffee rock) forms as the B-horizon of a fossil soil on sand (a podsol). It is NOT a sedimentary layer itself ie NOT a stratigraphic unit, so should not have been referred to as a "member" ...

As such, I have now changed my mind! The Broadwater Sandrock member is not the best name after all. It seems that 'B-horizon of the humus podzol' is indeed one of the best ones. Humicrete is another good one. Well, it seems that the diversity of names will probably continue, but we can remove the one I thought the simplest (Broadwater Sandrock member) from the list!


*Den Exter, P.M. 1974. The Coastal Morphology and & Late Quaternary Evolution of the Camden Haven District, NSW. Australia. PhD thesis. University of New England, Armidale.
*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, NSW. PhD thesis. University of New South Wales, Kensington.
*McGarity, J.W. 1956. Coastal sandrock formation at Evans head, NSW. Proceedings of the Linnean Society of New South Wales. V81 p52-58.
*Thom, B.G. (1965). Late Quaternary morphology of the Port Stephens-Myall Lakes area, NSW. Journal of the Royal Society of New South Wales V98 p23-36.
*Troedson, A., Hashimoto, T.R., Jaworska, J., Malloch, K., Cain, L., 2004. New South Wales Coastal
Quaternary Geology. In NSW Coastal Quaternary Geology Data Package, Troedson, A., Hashimoto, T.R. (eds), New South Wales Department of Primary Industries, Mineral Resources, Geological Survey of New South Wales, Maitland.