THERE is potential for 3446 shale gas wells to be drilled onshore in the Otway Basin, covering 44,105 square kilometres in SA and Vic.
The Australia-based natural resources consultancy firm Frogtech outlined the region's potential in its January 2013 Potential Geological Risks Associated with Shale Gas Production in Australia report, which was commissioned by the Australian Council of Learned Academies.
It reported that these wells would require 51.7 gigalitres to frack over a lifespan of 25 years, at a rate of 15 megalitres a well.
It would result in an overall annual groundwater take of 2.1gL from a sustainable groundwater yield of 1998gL/yr.
Frogtech warned that while there was little risk of chemicals contaminating aquifers as a result of fracking – largely because the gas tended to sit well below potable aquifers and shales acted as aquitards – a zone that restricted the flow of water from one aquifer to another – there was still a risk of contamination from the process itself.
This was because of well failure, stimulating fractures and faults, and the poor handling of produced water.
Well failure could include the sudden escape of gas and fluids from a well, poor cementation allowing contamination to move vertically between the well casings and the surrounding rock, and radial leak, when a casing failure allowed fluid to move horizontally out of the well and into the surrounding rock and earth.
"Because of the high potential for groundwater contamination from the wells, decommissioned wells need to be effectively sealed for hundreds if not thousands of years," the report said.
"Typically sealing a well is accomplished by filling the well with cement to a pre-determined level.
"To help give certainty in the future, well operators could submit an abandonment plan to the relevant authorities, with open-ended liability for failures into the future."
The risks of failure could be reduced by following industry "best practice", but Australia's guidelines had to keep pace with ongoing developments.
It recommended that authorities in charge of inspecting wells had the necessary qualifications, appropriate levels of impartiality, and independence from operators and those with financial interests in the project.
"Ideally, inspections should continue after decommissioning, with trigger values developed linked to remedial actions," Frogtech said.
Shale gas relied entirely on fracking, as opposed to coal seam gas extraction, which relied only on the controversial technique between 10 per cent and 40pc of the time.
A smaller proportion of the water used for shale gas could be recovered because shales acted like aquitards and were much deeper – 1000 metres to 3000m – than an average CSG well.
"It may be very saline – greater than three times seawater – and the water may contain a range of harmful chemicals, which will limit treatment and reuse possibilities," Frogtech said.
* Full report in Stock Journal, July 3, 2014 issue.