Restoring coal seam gas water to ancient underground rocks for future use

CSIRO researchers at a groundwater monitoring bore on a coal seam gas site. Image: CSIRO

Injecting massive amounts of water purified after coal seam gas has been extracted may provide the irrigation water of the future as it seeps slowly through ancient sandstone rocks, according to CSIRO research.

With APLNG expanding its coal seam gas explorations into Queensland’s Surat and Bowen Basins, researchers from the Gas Industry Social and Environmental Research Alliance (GISERA) are assessing the likely impacts of such a development.

CSIRO scientist, Dr Sreekanth Janardhanan, is leading a groundwater modelling project as part of the GISERA team researching possible impacts of CSG on surface and groundwater.

“CSG production means that large volumes of water need to be extracted from underground to release the gas from coal seams,” he says. “We are talking about up to 100 gigalitres of water a year, which is about the same as 40,000 Olympic swimming pools.”

The good news is that modelling research by Sreekanth’s team shows that some of this water can be safely reinjected into wells after it has been purified using reverse osmosis.

The Queensland Government’s water management policy prioritises the use of CSG water for the benefit of the environment or making it available to industries that use water, for example as a substitute for irrigation water.

Lake Broadwater Conservation Area is near coal seam gas operations on the Darling Downs in Queensland. Image: Sreekanth Janardhanan

“Injecting water into aquifers [rocks that hold water] is not new technology and has been used since the 1960s,” says Sreekanth. “Stormwater has been injected on a large scale to recharge the groundwater in Western and Southern Australia; the National Water Commission has well established guidelines for doing this.”

However, with community concern about the potential impacts of CSG production the GISERA team sought to find out if there were any environmental impacts from large-scale injection of treated water resulting from CSG production.

There were two key aims of this GISERA project. Firstly, to see if large-scale water injections changed the levels of water in farmer’s wells or the flow of natural springs; and secondly to see if it affected water quality in farmer’s wells by reaction of the injected water with clays and rocks within the aquifers.

The proposed CSG scheme in the Surat Basin would inject about 22 ML of treated water a day into the Precipice Sandstone aquifer at Reedy Creek.

Injection well for the Precipice Sandstone. Image: Sreekanth Janardhanan

Researchers used a sophisticated modelling approach to look at the likely effects of such an injection on the groundwater wells (bores) in the Precipice Sandstone that are being used by farmers, the nearest of which was 15 kilometres away from the injection site.

“Our models predicted that the pressure head of water in the aquifer at this bore location could increase by a maximum of 4.3 metres,” says Sreekanth. “This is of no concern as water pressure changes of this magnitude can occur naturally without any injection and an increase in water pressure level is indeed a positive outcome indicating increased availability of water.

“The water flows away very slowly in these aquifers meaning that it is staying in the aquifer for farmers to use over many decades.”

Some of the sandstone aquifers found in the Surat Basin have high quality water, which has been used for many decades to provide water for cattle.

“However, injecting reverse osmosis water, which is even purer than that found in the aquifer, can sometimes trigger chemical reactions with the rock and release possible contaminants into the water,” explains Sreekanth.

“We simulated a worst case scenario to see how long such contaminants if they were produced may stay in the groundwater and how far they might travel. We found that should any contaminants be produced they are likely to dilute to very low levels away from the injection wells and are very unlikely to be found in the nearest water bore 15 kilometres away, even for the worst case scenario.”

Correct treatment of water prior to aquifer reinjection, though, is likely to reduce the potential for contaminant production to zero. Sreekanth stresses that if industry follows best practice and regularly monitors the water near the injection wells for contaminants, the risks will be negligible.

The modelling done by the GISERA team used a risk-based approach and took into account any uncertainties that scientists had about the make up of the sandstone aquifers.

“We are very confident of our results,” says Janardhanan. “On one hand our modelling was informed by field experiments where injection trials and tracer tests were done to check how things really worked. On the other hand, we used mathematical techniques to quantify how likely any undesirable outcomes were in light of the insights from these experiments. This integrated approach helped increase our confidence in the modelling results.

“The most exciting thing for me was our collaboration through GISERA with APLNG. They gave us comprehensive data on injection and pump trials that would normally cost us millions of dollars. Having the industry experts provide us with these data and insights was extraordinarily valuable.

“We also used data from other research projects to validate our models. I am confident that reinjecting water from CSG is like any other managed aquifer recharge scheme and the risks posed are minimal.”

Other GISERA water research is investigating any other possible chemical reactions from re-injected treated CSG water and is also characterising the origin and nature of groundwater in the Surat and Bowen Basins.

For example, researchers are investigating how old the groundwater is that is stored in the Surat Basin’s sandstone aquifers. This tells them how long water will take to move from where it is injected to the nearest bore.

“We are still working this out,” says Sreekanth. “But we do know that the groundwater is very old, of the order of several thousands of years and possibly close to a million years old in some of the deeper formations. This makes it clear that water can’t be replenished very quickly by natural recharge, and artificial means like injection are essential for the sustainable use of the groundwater resources.”