Improperly abandoned well assets can pose risks to the environment and the health of neighbouring communities by contaminating groundwater and releasing toxic air pollutants such as naturally occurring radioactive materials, metals, and other volatile organic compounds such as benzene.
Without adequate plugging, oil and gas wells can also contribute to climate change, emitting large volumes of natural gas, including methane, a potent greenhouse gas.
Abandoned onshore coal seam and shale gas wells, even when decommissioned properly, require long-term monitoring of potential methane leaks and continued well integrity to prevent barrier failure and subsequent environmental damage.
The importance of proper well plugging and abandonment was brought into focus this year when it was reported that decommissioned gas wells from a Santos project offshore Western Australia had been leaking from the seabed for a decade.
The leaks had been detected by an underwater vehicle in 2013 at the Legendre gas field north of Dampier in WA’s Pilbara region.
Legendre ceased production in 2011 and its wells were decommissioned by the then operator Apache, with Santos taking over management in 2018.
Santos advised the regulator NOPSEMA that it was not feasible to stop the leaks, which were described as “ongoing gas seepage in the form of small bubbles”.
The company has proposed monitoring the wells for five years under an environment plan, with a spokesperson saying the company had engaged CSIRO to undertake the in-depth monitoring program.
The spokesperson said: “Findings to date show the gas is non-toxic and unlikely to present an environmental hazard.
“Santos is currently implementing a management plan that includes a regular monitoring program.”
NOPSEMA said it had asked Santos to “undertake activity to show the cause of the reported seepage and required that Santos submit an environment plan to demonstrate the appropriate management of any works undertaken”.
Western Australia has nearly 1,200 non-operating onshore oil and gas wells alongside about 700 producing wells, based on figures from the Department of Mines, Industry Regulation, and Safety.
Of the non-operating wells, 939 are plugged and abandoned while 247 are suspended or shut in (i.e., yet to be plugged and abandoned).
This towers over WA’s offshore liability, which has four producing wells, 45 suspended or shut in, and 311 plugged and abandoned.
Suspended wells are due to operators not plugging and abandoning the wells immediately after production stops – some may be returned to production, or the operator may be planning to decommission a number of wells simultaneously to reduce costs.
However, delaying the decommissioning process can lead to the equipment falling into disrepair and becoming unsafe, and smaller companies may be unable to afford decommissioning as their production revenues decline.
This can create massive costs, both monetary and environmentally, according to a study published by the Society of Petroleum Engineers, which analysed plugged and abandoned wells in the South Australian portion of the Cooper-Eromanga basin.
More than 600 wells were considered non-compliant, corresponding to 25.5 per cent of the total.
It estimated the total cost of well abandonment for the entire basin to be $654 million, and further analysis showed 80 per cent of the high-priority wells identified were gas.
Delaying clean-up can also lead to costlier decommissioning later on, as infrastructure can be corroded or damaged from harsh weather.
A paper published in Frontiers in Marine Science noted that the main ecological considerations for decommissioning were habitats and diversity, biological productivity, invasive species, ecological connectivity and the ocean environment.
The authors pointed out that there had been several studies showing clear benefits to leaving oil and gas infrastructure in situ (creating artificial habitats), while others demonstrated how these structures posed a risk to the environment.
There is also the potential for contaminants associated with oil and gas infrastructure to influence the value of infrastructure as habitat.
These include the effects of methylmercury bioaccumulation, naturally-occurring radioactive materials, contaminant distribution, and contaminant persistence and resuspension.
