Methane Emissions from Abandoned Boreholes in South Eastern Australia

Abstract:

The Surat Basin in south-eastern Queensland is one of Australia’s main coal bed methane production areas. It has also been subject to coal exploration over many years and consequently there are thousands of abandoned exploration boreholes throughout the region. Here, we present some results of field measurements aimed at locating leaking legacy exploration boreholes in the Surat Basin and to quantify their emission rates. We also discuss emission measurements made on abandoned CBM wells in Queensland and NSW that have been decommissioned according to modern practices.

Leaking boreholes were located using a Picarro 2301 CH₄ analyser mounted in a vehicle that was driven through gas fields in the Surat Basin. Where surface emissions were indicated by elevated ambient CH₄ levels, the emission rate was measured using soil flux chambers at each site. For comparison, soil gas flux measurements were also made on natural surfaces and agricultural land throughout the study areas. Ten borehole sources were located during the surveys, yielding emission rates from less than 0.1 kg CH₄ day^-1 to more than 100 kg CH₄ day^-1. A number of other known exploration borehole sites were examined which had no detectable CH₄ emissions. Plugged and abandoned CBM wells showed no CH₄ emissions except in two cases where emission rates of about 0.07 g CH₄ day^-1 were detected, which were comparable to natural wetland CH₄ emissions.

Preliminary results suggest that modern decommissioning practices appear to be effective in preventing CH₄ leakage from CBM abandoned wells. However, legacy coal exploration boreholes may represent a significant source of CH₄ in the Surat Basin, although the proportion of these holes leaking CH₄ is yet to be determined. Moreover, it is not yet clear if emissions from boreholes are affected by changes in groundwater induced by water extraction associated with gas production and agriculture. This is an area requiring further research.