Potential methane emissions from aquifer and coal seam gas groundwater extraction: Effect of open and closed sampling methods and new emission factors

Abstract

Greenhouse gas fugitive emissions contribute to climate change, of which 18 % are thought to be methane. Aquifer groundwater, and gas production water extraction are two potential fugitive emission sources. Aquifer groundwater is a vital water source composing ∼ 36 % of total water consumption worldwide. The world’s largest artesian basin, the Great Artesian Basin (GAB), Australia, has an estimated 322,327 GL/yr of groundwater extracted in Queensland for domestic, livestock, town water, irrigation and industrial use. Water is also produced with coal seam gas extraction. Dissolved methane was measured from deep and shallow GAB aquifers and coal seam gas production water via both closed and open sampling methods. The commonly used open sampling method can lose methane during the sampling process and underestimate concentrations. The resulting data were used to compare estimated methane fugitive emissions from groundwater extraction. This study compares the emissions obtained using the two sets of data from the two methods. A deep GAB aquifer, the Precipice Sandstone, has an estimated maximum methane emission of 1.89E-02 Tg/y using the closed sampling method, but only 2.28E-07 Tg/y using the open sampling method (Surat Basin, Queensland). The maximum estimated methane emission from coal seam gas (CSG) produced water in the whole state of Queensland is 1.88E-03 Tg/y using the closed method, however using the open sampling method it is only 9.56E-07 Tg/y. Considering only CSG produced water from the Surat Basin, the maximum estimated methane emission is 8.94E-04 Tg/y.

We suggest a new lower emission factor for CSG produced water based on actual dissolved gas measurements of 0.031 tonnes per ML produced water. This is an order of magnitude less than the Australian National Inventory Report CSG production water emission factor of 0.31 tonnes /ML (derived from a USA simulation study). We also suggest new, different, emission factors that could be applied to deep gassy aquifers, shallow aquifers, and aquifers containing interbedded coal. This study demonstrates that greenhouse gases from groundwater estimates can be affected by the gas concentration sampling method, of which there has still not been a standard robust and accepted method in Australia. More widespread groundwater methane sampling is urgently needed, especially for deep, old aquifers, to address the current uncertainties in emissions, along with the need for further characterization of the stable isotope signatures of the sources that may enable bottom-up attribution and allocation of gas from different sources.