Underestimated and right under our feet

Colin Brown.

Even as new shale gas licence areas are about to be announced by the UK coalition government and with increasing conferences on the topics, there are discussions about a possible national moratorium on shale gas, increasingly vocal opposition from action groups, planning applications being refused and a seemingly ever-increasing amount of ill-informed, alarmist comment.

So with all this noise about shale gas going on, has CBM somehow lost its place and potential as an indigenous UK energy resource?

Far from it. The case for CBM remains as strong as ever and its uses are still just as versatile. It can be injected directly into the gas grid, with or without ‘cleaning’. It can be burnt at source to provide electricity, either to the grid or to a specific energy user. It can be burnt directly as fuel in an industrial process. Or it can be used as a fuel source for combined heat and power (CHP).

There are many areas within the UK where unworked coal resources could prove viable for CBM. Scotland is leading the way at the 330 km2 Airth CBM project in Midland Valley, where Dart Energy is already producing natural gas to generate and export electricity into the local power network – with applications for permission for fourteen further wellsites to produce gas to supply a distribution facility and connection site and to provide gas to the national grid.

Although exploitation of CBM resources has, in the past, suffered from a lack of UK government support, tax incentives announced in the 2013 autumn statement relating to shale gas also apply to CBM production. The recovery technologies used for both have many commonalities. And as deep, directional drilling technologies have transferred from the conventional oil and gas sector to the unconventional shale gas and potential CBM markets, the commercial viability of CBM has improved.

It is also generally true that the closer an energy source is to the point of use, the more efficient that energy conversion becomes. It therefore makes sense to search for CBM resources in areas of existing industrial activity – especially those with a high demand for energy.

Looking around the UK and back at its industrial heritage, it is clear that many centres of industry are conveniently situated over major coalfields – including the Midland Valley of Scotland, the northeast, the east and west Midlands of England, and South Wales. But maybe this is hardly surprising. Previous generations understood the efficiencies of energy and material use every bit as well as we do today.

On the downside, this also means that many of the coal resources in these areas have been heavily mined in the past, with seams of coal having been worked out over large geographical areas. So does it follow that there are no resource blocks left? Not at all. Many areas often remain where coal resources were sterilised to protect overlying sensitive structures such as hospitals and power plants from surface subsidence. And indeed, there may be deeper resources, which simply were not worked because of past technical difficulties or adverse economics.

Promising potential

If such resource blocks can be found in areas such as this, they have promising potential for CBM extraction. They will lie in close proximity to potential high energy users. Exploration and production activity will take place in an area of existing industrial activity. And the geology of the potential resources – both in terms of structure and quality – should be well known.

One such area where Wardell Armstrong has carried out preliminary research and analysis is Stoke-on-Trent in north Staffordshire. By combining local knowledge, interpretation of abandoned mine records and a review of previous boreholes and other data held by the British Geological Survey, the consultancy carried out an assessment of the potential for unworked coal resources beneath the city, together with an estimate of the in situ gas resource. This revealed good potential for CBM extraction and utilisation.

Stoke-on-Trent is an iconic location that demonstrates the collaboration of mining and industry. The local abundance of coal and clay suitable for earthenware production led to the early development of the pottery industry. Canal construction, the import of china clay from Cornwall and the advent of the railways all led to industrial growth. The area became world famous with household pottery names such as Wedgwood, Royal Doulton, Dudson, Spode, Minton and, more recently, Clarice Cliff, Suzie Cooper and Emma Bridgewater. Iron and steel industries followed – again fuelled initially by local coal and ironstone. As the ground provided the resources, the local population provided the labour, the genius and the craftsmen.

More than thirty named coal seams have been identified beneath the city as a result of Wardell Armstrong’s preliminary CBM research. Six of these have an average thickness of more than 2m. The synclinal structure of the North Staffs Coalfield is such that these seams come close to the ground surface on the eastern and northwestern sides of the city and have been heavily mined.

But significantly for the potential of CBM, the coal seams are deepest along the axis of the geological syncline – the Etruria Valley – and significant sections remain unmined. This may have been a result of protection for major industrial plants, such as the former Shelton Steel Works, or because they were simply too deep to mine economically, or because they were in places on the wrong side of significant geological faults.

Deep coal mining will not currently return to Stoke-on-Trent. But the city still remains an internationally important centre for ceramics manufacture – a high energy industry with modern kilns heated by gas from the grid. These kilns cannot be turned off and any interruption of natural gas supply to ceramic producers would be catastrophic. Alternative sources of gas supply are therefore potentially attractive and important to this industry – not only in terms of potentially cheaper energy but also with an eye on critical energy security.

Could one potential source of such natural gas resources lie in the ground beneath the city in the shape of CBM from the unmined coal beneath the Etruria Valley? Wardell Armstrong’s analysis shows that it potentially might. The pattern of recorded abandoned mine workings reveals an area of broadly unworked coal running approximately north-south along the spine of Etruria Valley. Preliminary estimates put the quantity of coal in this area at in excess of 150 million t in the six identified target seams that have an average thickness of more than 2 m.

The quantity of CBM within these seams is less certain, although general experience, as well as some dedicated studies into coal seam gas content carried out at coal mines to the west and south of Etruria Valley (Silverdale and Hem Heath), does shed some light on likely content. These studies confirmed that the North Staffs coalfield is moderately gassy with gas contents ranging from about 7 m3/t at shallow depths to about 9 m³/t at deeper horizons.

Less is known about the permeabilities of the coal seams or the surrounding strata. The percentage of the in-situ gas resource that can be recovered is therefore uncertain. It is considered, however, that recoveries ranging between 10% and 25% might be achievable.

Another unknown is the past effect on the regional stress regime of the historic mining activity that surrounds the unworked panels. On the one hand, it could conceivably be beneficial in increasing strata permeability and hence gas recovery. On the other hand, it could also have been detrimental, leading to greater gas loss from the seams through natural seepage.

One other area of concern in terms of the potential viability of economic gas extraction is the hydrogeological regime. Groundwater levels have substantially recovered after the end of pumping when the mines closed in this part of the coalfield. The vast majority of nearby mine workings will therefore be saturated. It will be critical in any extraction process to make sure that this large reservoir of groundwater is not intersected, or brought into hydraulic continuity with the areas targeted for gas production.

There are of course many hurdles to clear before any exploration can take place, let alone before any potential gas that is proven to be viable can be extracted. In the very first instance, a Petroleum Exploration Development Licence (PEDL) must be obtained for the area of choice from the Department of Energy and Climate Change (DECC). If this is successful, then any CBM exploration and/or production borehole will be subject to the full range of planning law – including scrutinising and permission by the Health and Safety Executive, the Coal Authority and the Environment Agency to make sure that public safety and the environment is protected. Here again, there could be an advantage to borehole exploration in former industrial areas where this modern extraction of energy resources may perhaps find greater public acceptance.

Conclusion

It is clear from this example in the industrial heart of the UK that there is potential for the effective exploitation of CBM from these now sterilised coal resources that lie beneath industrial locations built over historic coalfields. The juxtaposition of a potential energy source and high energy industrial users makes perfect sense. As always, there are unknowns and risks to the economic recovery of these gas resources. But the potential rewards could justify further exploration and analysis to resolve these unknowns. And in a fuel-hungry world that needs to optimise the remaining reserves of coal in the most efficient and environmentally acceptable way, CBM, without a doubt, has an important part to play.

Although this article has focused on CBM resources from a UK perspective, the pattern of industrial ‘revolution’ germinating and subsequently flowering in areas rich in commodities, such as coal, iron and clay, has much wider implications. Similar opportunities will doubtlessly be there in many other countries in the world – opening up new possibilities for CBM.

Edited by Jonathan Rowland.

About the author: Colin Brown is the Chairman of Wardell Armstrong.

Read the article online at: https://www.worldcoal.com/cbm/01072015/underestimate-and-right-under-our-feet-2824/