This story was originally published in The West Australian on 27 September 2017 with the headline "Gorgon’s next big challenge." © Peter Milne.
Chevron’s Gorgon project has been producing LNG from all three trains since February, but one task remains: to start injecting carbon dioxide underground. Without the $2 billion CO2 injection effort, Gorgon can emit more CO2 than all of Collie’s coal-fired power stations.
In the 28 years from discovery in 1981 to project go-ahead in 2009 Gorgon gained the tag within the oil and gas industry of being deep, dirty, distant and dry.
The depth was 220m of water, the distance was 130km from the mainland, dry meant the gas had little valuable condensate, and the dirt was the 14 per cent of carbon dioxide in the gas from the Gorgon field, one of two that feeds the project.
In the end, reducing CO2 emissions shaped the whole project.
It resulted in construction on Barrow Island with limited land area, little infrastructure, strict environmental and quarantine conditions, and the need to transport almost everything in by sea. However, Barrow Island had one advantage, a sandstone formation called the Dupuy, 2500m below the island, that could store CO2.
Gorgon will emit 6.1 million tonnes of CO2 a year and inject a further 3.4 million tonnes a year under Barrow Island, according to Gorgon’s Greenhouse Gas Abatement Plan. The total CO2 produced by Gorgon is greater than the 9.1 million tonnes that is emitted from Collie’s coalfired power stations.
Injecting 80 per cent of the CO2 from the reservoirs will make Gorgon the second biggest CO2 storage project in the world.
The cost was estimated to be $2 billion in 2010 before the cost of the whole Gorgon project increased by 46 per cent. Separating the CO2 from the reservoir gas was required to avoid the CO2 causing corrosion and freezing into solid dry ice in the LNG plant, so was not an additional cost. Gorgon has a long long-term target to store 95 per cent of the reservoir CO2, or almost four million tonnes a year.
Gorgon will not separate and store the CO2 from the turbines that generate power and liquefy the gas. The only two projects that store C02 from combustion, in Canada and the US, are on a smaller scale than Gorgon and recoup the expense by injecting the CO2 into depleted oil reservoirs to increase production.
Only four of the 17 operating carbon storage projects do not use the CO2 for increased oil production
The first step in storing the CO2 underground it is to compress it to a pressure sufficient to force it down the injection wells and then deep into the sandstone and siltstone of the Dupuy Formation.
When compressed the CO2 is neither a true gas or liquid, but is kept at a so-called super-critical phase by controlling the temperature and pressure of the gas. The supercritical CO2 is easier to drive into the Dupuy, because it flows like gas but requires less storage volume as it is dense like liquid. The 7km pipeline from the compressors at the LNG plant to the injection wells was at risk as CO2 mixed with water is highly corrosive. The configuration of the compressors ensured the water is entrapped in the CO2 flow and doesn’t become so-called free water where it can cause corrosion.
Nine wells have been drilled to inject the CO2 into the Dupuy. Up to nine more may be required, depending on the performance of the injection, to keep the gas pressure at the bottom of the wells low enough to avoid fracturing the formation and disturbing the planned flow of over seven tonnes of CO2 a minute into the sandstone.
Once underground the CO2 sticks to the surface of rock grains or dissolves into the water between them.
As more CO2 is stored underground, the pressure in the Dupuy could increase and reduce the flow through each injection well.
This pressure increase is managed by extracting water from one side of the Dupuy at the same CO2 is injected into the other.
Several kilometres west of the injection wells are four wells with electric submersible pumps that can lift over 12,000 tonnes of water a day to the surface. This water is then reinjected into a formation above the CO2.
A Chevron spokeswoman said: “Construction of the Gorgon Carbon Dioxide Injection Project is in the final stages which includes final testing and commissioning activities. This is expected to take some time to ensure the system is ready for safe and reliable operations.”