High CO2 in Narrabri Gas

Santos continues to claim that all gas used for electricity generation is a clean transitional fuel.

Nothing could be further from the truth for NGP gas, because at Narrabri very high levels of carbon dioxide (CO2), previously undisclosed, will be released direct into the atmosphere, a useless by-product of coal seam gas (CSG) production.  While the data from thousands of samples indicates a CO2 level of 25%-30% in the coal seems targetted in the NGP, Santos claims that 250 confidential samples, that only it has access to, indicate 5% CO2.  The EIS submitted by Santos has a 10% “assumption” of CO2 levels, considerably lower than the observed level.

The three numbers simply cannot be reconciled, and warrant a requirement by the IPCN to instruct  Santos to provide the favourable “250 samples with 5% CO2” for assessment, with the other thousand samples, by a panel of competent professionals.

In the absence of Santos providing this information, the documented and available data which show 25%-30% should form the basis for any planning decision.  This data and analysis  is made available on this site.

Mount Kaputar, a volcanic plug indicating the wide spread surrounding of underlying igneous rocks.

The sight of Mount Kaputar is a familiar one to the residents at Narrabri. It is located 50km to the east of Narrabri and the NGP location, and was an active volcano around 20 million years ago. What most would not know, is that the same volcanic rocks in the plug of Mount Kaputar also lie underneath the Pilliga and Narrabri area.  Millennia ago, these igneous rocks released large amounts of carbon dioxide, which migrated up and were captured in the overlying Maules Creek and Black Jack (Hoskissons) Coal Formations.

This has resulted in high carbon dioxide (CO2) concentrations in many wells in the proposed NGP area.  Of the many appraisal wells which were drilled, there are over a thousand coal seam gas analyses available on DIGS[1], and these confirm what experienced coal geologists would have warned on seeing the nearby extinct volcanic plug of Mount Kaputar : there is a high probability that the coal seams in the area contain high concentrations of carbon dioxide.

Data from the publicly available information confirms that the average reported CO2 content in CSG for the project is 25%-30% in the coal seams targeted by Santos.  The detailed well CO2 data can be seen here:

Figure 1 : Comparison of CO2 levels in the shallow Hoskissons (red) and deeper Maules Creek Formation (yellow) at Narrabri, contradicts Santos’ claims at the AGM that the lower coal seams have lower CO2 levels.

Under the Environmental Impact Assessment for Narrabri gas, CO2 would be extracted from the CSG and simply vented into the atmosphere, resulting in greenhouse gas (GHG) emissions estimated to be nearly 1.7 million tonnes of CO2. This would considerably increase the carbon footprint of the Narrabri project.

High CO2 levels mean any resource base for the NGP is probably overstated, a significant matter for shareholders and anyone who is looking to the Narrabri Gas Project to resolve energy shortages in NSW.  But as Santos does not carry any confirmed reserves for the project, the resource size is not defined – which is a highly unusual situation for a hydrocarbon development seeking regulatory approval.

There will be a material cost impact of extracting 25% CO2 to obtain the methane for market, alongside with a significant reduction in the resource base; all of which will considerably reduce any economic benefits from the project.

Responding to a question at this year’s AGM on whether the Santos Board was aware of high levels of CO2 in the NGP coals, Mr Spence said that “Yes we are aware of that, the assumption that 10% CO2 in the EIS is actually a conservative estimate. Typically when you put an EIS together you try and paint a fairly bad case to make sure you really kind of stress test it. The actual Narrabri exploration and appraisal data from around 250 gas samples that were taken across the area between 2014 and 2019 have an average CO2 content between them of just under 5%, not 10%.” 

Santos is asking stakeholders and the IPCN to trust that confidential testing of 250 gas samples between 2014-2019, none of which is publicly available, or capable of being corroborated, is a reliable predictor of likely CO2 content of Narrabri gas. This is in stark contrast with twenty years of data from 26 wells, and nearly 1,000 samples, which unambiguously show a 25%-30% CO2 content.

At the AGM, the Santos Chairman continued with “… the areas were we would expect elevated levels of CO2 are typically in the shallower coal seams, where we’re not really interested to be quite honest with you”.  

This is actually contradictory to the Santos project EIS, which states:

The Bohena Trough contains two well-developed coal measures, which are the primary CSG targets for the proposed development. These are: (a) The Late Permian Black Jack Group, which contains the Hoskissons Seam (between 6 to 10 metres thick, a laterally extensive seam, located at a depth of less than 700 metres) (b) The Early Permian Maules Creek Formation, which contains the Bohena coal seam (up to 22 metres thick, a laterally extensive seam, located at depths of between 600 and 1,200 metres).” [Emphasis added]

Santos’ targetted Hoskissons coal seam is the seam being mined for coal at the nearby Narrabri Underground coal mine, with an average of 90% CO2.

Inclusion of all gas sample data from 1998, when the first CSG well (Bohena 2) was drilled, until 2014 (when data became confidential), clearly shows that high levels of CO2 are prevalent in the main Santos target seams in wells over much of the project area. There are some areas with low CO2, but this cannot be said of the whole project area.   It is conceivable that current production is from low a few low CO2 wells, used to provide gas for the Narrabri power stations, and sampling of this non-representative wells would just confirm low CO2 in those wells.

As may be expected with CO2 introduced by faults into the coal strata, the distribution of CO2 and methane is impossible to predict, with significant lateral changes in CO2 content.  However, the data set which has been analysed spans the whole of PEL 238 and the proposed NGP area. 

The science of predicting when the CO2 in the gas will flow to the surface during drilling and production is still in its infancy, but high levels of CO2, will ultimately be produced, and if the project goes ahead, will be vented straight into the atmosphere – hardly the credentials for a “low carbon” transition fuel.

If, as suggested by Mr Spence, Santos will not be producing from the Hoskissons seam, then the entire EIS will need to be resubmitted and Santos will need to reduce any reserves associated with the shallower Hoskissons seam.

Even conventional reservoirs above volcanics can have high CO2 which has migrated in from the underlying igneous strata.  Researchers have observed that the magmatic CO2 is not only found in coals in Australia (for example Cooper Basin coal seams have 30% – 60% CO2 and 40% at Narrabri), but high CO2 of igneous origin (from isotope studies) is also observed in other types of reservoirs, such as the Caroline-1 well in the Otway Basin, which has been producing 90%-95% CO2 for over 40 years.  The CO2 in the Caroline Field has a volcanic source, assumed to be from the Holocene Mt Gambier volcanic chain.

The above material is updated research, and partly sourced from the following article:


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