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Environment & Science Writer
SOME hail shale gas as the miracle answer to the world’s energy needs for centuries to come, and as a key means to significantly reduce fossil fuel emissions that are the major driver of human-induced climate change.
Others dismiss such claims as exaggeration and hyperbole, and point to research suggesting the carbon footprint of shale gas extraction through hydraulic fracturing – or fracking – is equal to that of coal or even greater because of “rogue” methane leaks.
They warn of severe environmental threats – notably the potential for water supplies to be contaminated by methane and by toxic chemical residues from fracking fluids introduced as part of the drilling process.
What is not contested is that there has been an explosion of activity in the field of shale gas exploration and extraction since the combination of fracking and horizontal deep drilling was perfected during the 1990s.
In the US, 3 500 wells are being drilled every year, and exploration and extraction using fracking is also occurring in Alaska, Australia, China, Sweden, England and Ireland.
The exploitation of shale gas has revolutionised America’s energy mix, seen as a “game-changer” for US energy markets. The 0.39 trillion cubic feet of gas extracted in the US in 2000 soared to 4.87 trillion cubic feet in 2010, and some 23 percent of dry gas production there now comes from shale – up from 1 percent a decade ago.
But fracking has been banned in France and in Quebec, Canada. South Africa has introduced a moratorium, pending a full scientific investigation, and in the US a probe is underway. There is a moratorium on horizontal fracking in New York State until July 1.
Three major energy companies – Shell, Bundu Gas and Oil Exploration, and Falcon Oil and Gas – have applications in for shale gas exploration across vast areas of the Karoo, although drilling would be limited to relatively few test wells if these applications were to be approved.
There is shale gas in the Karoo: mostly methane but also the dry gases ethane, propane, butane and naphtha. Probably a lot, gauging by earlier geological exploration for oil by state exploration company Soekor during the 1960s and early 1970s.
One of these wells near Graaff-Reinet – a borehole drilled in 1968 – had an unstimulated flow rate of 1.84 million cubic feet of gas a day from fractures at a depth of 2 531m.
Shale gas is derived from ancient organic material in the form of algae, spores and pollen deposited in mud in marine or lacustrine (lake) systems. This became buried and lithified – turned into organic-rich shale rock – over hundreds of millions of years, says Dr Doug Cole of the Council for Geoscience.
Hydrocarbons – organic compounds which contain only carbon and hydrogen, found in coal, crude oil, natural gas and plant life – are generated as temperature increases under an increasing burial depth in a process known as thermal maturation.
Oil is produced at a depth of 2km to 4km where the temperature is between 60 and 120ºC, while wet gas – gas with a small amount of condensate in it – is produced at a depth of 4km to 5km and a temperature of between 120 and 150ºC. Dry gas, including methane, is produced only at a depth of between five and six kilometres, where the temperature is in the critical range of 150 to 250ºC.
A deeper burial ends hydrocarbon generation and causes the organic material to turn into graphite.
In the Karoo, shale with the potential to generate dry gas occurs mainly south of latitude 30ºS (Durban lies near this line). Shales with a potential to generate oil occur north of 29ºS, and shales that might generate wet or dry gas occur in a narrow band between these two and in a long arc through the former.
The billion-dollar question is whether shale gas occurs in the Karoo in commercially viable quantities and, if so, whether it’s possible to extract it at an environmentally acceptable cost.
In South Africa, says Cole, by far the best methane-producing shales occur in the Ecca Group of rocks from the Permian era, between about 299 and 251 million years ago, named after the Ecca Pass north of Grahamstown.
In the Ecca Group, one band stands out as having a high potential for methane generation: the Whitehill Formation, named after the railway siding between Matjiesfontein and Laingsburg, and
to a lesser extent the shales underlying the Whitehill Formation.
But it will be extremely difficult to quantify these Karoo shale gas deposits because of the presence of dolerite, which is concentrated in this Whitehill Formation.
Dolerite is an igneous rock which takes the form of bodies such as dikes and sills. The Karoo dolerites date from the break-up of the super-continent of the southern hemisphere, Gondwanaland, during the Jurassic era.
Where dolerite intrudes into the shale bands, it destroys the ability of the shale to produce methane due to its heating effects.
So when calculating gas reserves a good deal of shale affected by dolerite must be excluded.
“In order to determine the position and morphology of the dolerite sills, 3D seismic scans would have to be undertaken,” says Cole. “There’s a huge area that is involved, and you would have to do numerous scans in order to obtain accurate data. I don’t think anyone can really put an accurate figure on how much gas can be generated from these shales.
For exploitation in the Karoo, there would have to be the extra significant expense of 3D seismic scans to delineate the dolerites, and this may well reduce the economic viability of extracting gas from these shales,” Cole suggests.