Development of an Improved Methodology for Estimation of Fugitive Seam Gas Emissions from Open Cut Mining

The National Greenhouse Gas Inventory for the year 2002 (Australian Greenhouse Office) estimated the total fugitive emissions from both underground and open-cut mining to be 17,456 ktonne CO 2 equivalent which represents 3.2 percent of the total national emissions for that year. Of these emissions, ~40 % were considered to have arisen from open cut coal mining.

Based on an earlier project it was decided to pursue a reservoir approach to seam gas emissions estimation and to explore the possibility of using gas analyses from borecores to estimate overall GHGE for a particular mine. It was envisaged that this approach would provide key data toward developing an improved methodology for use by individual coal mines.

The work program consisted of detailed field measurements of in situ gas content of coal and associated strata from two open cut coal mines, one in the Hunter Valley and one in the Bowen Basin. Four surface exploration boreholes were drilled and cored in two opencut mines, Moura and Warkworth. Two of these boreholes were part of the usual exploration drilling program of the host mines, while two were drilled as part of the ACARP project. Samples were taken from all four boreholes and measured for the seam gas content and composition over the full length of the strata comprising coal seams which were considered to contribute to gas emissions from open cut coal mining.

The first drilling campaign was held during August 2003 at the Moura opencut in the Bowen Basin with two boreholes fully cored to depths of 240 to 250 m covering the A to D coal seams sequences. The second drilling campaign was held during June 2004 at the Warkworth open cut coal mine in the Hunter Valley with two surface boreholes drilled and cored to depths of 140 to 150 m. The boreholes traversed five coal seams from the Glen Munro to the Bowfield seams.

The results for Warkworth in the current project were lower than expected and in direct contrast to gas content measurements previously made at the same mine at distances of 2.8 and 6.4 km from the current holes, where significant gas was encountered. Subsequent closer investigation showed that the boreholes had been drilled within 200-300 m of a fault. It is believed that the fault had acted as an escape path for the gas contained in the coal and the low gas contents measured should be considered as the residual gas content. These results show that local geological formations can be very significant and a knowledge of the local mine geology is critical.

One of the key points investigated during this project was the concept of taking a 200g sub-sample from the core to represent the coal seam for gas content determination. The purpose of this was to investigate whether the gas data required for fugitive emissions estimates could be obtained without requiring a large proportion of the core. This issue was investigated at both the Moura and Warkworth sites but proved to be unsuccessful. This was for two main reasons;

• First, the competing demands for the coal between the coal geologist logging the core and the necessity to sub sample in a rapid manner so as not to lose too much gas before the gas content measurements could begin, proved to be incompatible.
• Second, the natural variation across coal seams due to the different coal bedding planes requires many samples to ensure representative values are obtained.

Measurements of the residual gas contents of the coal showed only small amounts present in the product coal, typically less than 10% of the in situ gas content. However, the value for the residual gas will be a strong function of the in situ gas content, coal permeability and the time delays between coal breakage, mining, preparation and export from the mine. Further more targeted work would be required to better define these values.

A preliminary concept for estimation of potential gas emission was developed which could serve as the basis for a full Tier 3 methodology for fugitive emissions. The method is based on geological information, the in situ gas content of coal and rock strata and operational issues such as the method of mining.

The concept takes into account the stratigraphy of the overburden and floor of coal seams including number, thickness of seams and surrounding strata, in situ gas contents, presence of major geological structures, pore system and the degree of water saturation in the pore system. Operational issues such as mine method which affects the depth of fracture zones is taken into account by defining a ?gas release zone' surrounding the mined coal seams.

In summary, a Tier 3 method can be developed but requires the following information;

• A detailed geological understanding of the major features of the mine which could influence gas content e.g. presence of faults and their impact
• Representative data on the gas contents of the affected coal seams, including those beneath the floor of the pit and in the ?gas release zone'
• Accurate estimates of the ?gas release zone' which will vary, in general, depending on geological factors and the mining method
• Measurements of the adsorption isotherms for the coals so that the gas pressures can be deduced
• Measurements of the porosities of the non-coal strata so that the deduced gas pressures can be used to estimate the amount of gas stored in the non-coal horizons
• Accurate estimates of the desorption of gas back through the highwall or from the floor of the pit
• Water content of the non-coal horizons