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Independent Review of Underground Coal Gasification - Report

Published: 6 Oct 2016
Part of:
Business, industry and innovation

An independent examination of the issues and evidence surrounding Underground Coal Gasification.

239 page PDF


239 page PDF


Independent Review of Underground Coal Gasification - Report
2. Geology

239 page PDF


2. Geology

2.0 This chapter sets the fundamental starting point for consideration of the potential UCG resource. Presentation of the issues was aided by inputs from interviews with Dr Alison Monaghan at BGS and Professors Haszeldene and Shipton ( Annex 2) as well as senior staff at the Coal Authority and consideration of the literature.

2.1 As indicated in the Introduction above, in 2004 BGS (Jones, N.S. et al) produced a study of the UK coal resource with potential for application of new exploitation technologies and it identified the broad nature of a significant coal resource across Scotland. That report and discussions with BGS provide the basis of this chapter.

2.2 This resource lies in three main components:

2.2.1 the largest province by far - Carboniferous age coals across the Midland Valley of Scotland - from east Fife to Machrihanish

2.2.2 Carboniferous coals around Canonbie in the Solway area, and

2.2.3 in Jurassic bituminous coals around and offshore Brora in East Sutherland.

2.3 The most substantial seams exist in the Midland Valley, in the Ayrshire and Douglas Coalfields in the west, in the Central and Clackmannan Coalfields in central Scotland and the Fife and Lothian Coalfields in the east. These latter two are the best known and the richest prospects in terms of knowledge, thickness and likely quality and accessibility, largely from prior deep coal prospecting and workings around the margins of and under the Firth of Forth. But from the estuary in the east to Machrihanish in the west the geological markers can be connected and relatively speaking a lot is known about these coals.

2.4 The BGS 2004 report assessed the coals area by area for potential under conventional surface (opencast) and underground mining, coal mine methane, abandoned mine methane, coalbed methane, underground carbon sequestration and underground coal gasification.

2.5 Jones et al address UCG processes and potential and the criteria for its delineation and mapping. The quotations which follow in this section are
'Reproduced from Jones N S, Holloway S, Creedy D, P, Garner K, Smith N J P, Browne, M A E & Durucan S. 2004. UK Coal Resource for New Exploitation Technologies. Final Report. British Geological Survey Commissioned Report CR/04/015N with permission of BGS/ DECC. The full report is available for download from'

2.6 "Underground Coal Gasification describes the process by which various combinations of air, oxygen, hydrogen and steam are injected into one or more in-situ coal seams to initiate partial combustion. The process generally involves the drilling of at least 2 boreholes, one to act as the gasifier and one to collect the product gases. The injectant reacts with the coal, which produces heat and drives off gases (hydrogen, carbon monoxide and methane), which are subsequently recovered through a production well. The basic chemical processes and the calorific value ( CV) of the gas produced are similar to conventional industrial gasification processes, although the final gas composition is somewhat different. Compared to CBM, UCG generally produces a gas of medium CV with a heating value of about 30% that of CBM. If air, rather than oxygen, is used as a partial oxidant then a lower CV product gas is produced with a heating value of about 10% that of CBM. The other difference is that with UCG typically 75% of the energy value of the affected coal is produced as useful energy at surface, whereas with CBM it is much lower. "

2.7 The criteria for UCG potential in the geology are,

  • "Seams of 2m thickness or greater
  • Seams at depths between 600 and 1200m from the surface
  • 500m or more of horizontal and vertical separation from underground coal workings and current coal mining licences
  • Greater than 100m vertical separation from major aquifers, and
  • Greater than 100m vertical separation from major overlying unconformities"

2.8 Underground coal gasification can take place either under shallow, low pressure conditions or at depth, under high pressure. The latest UCG projects all try to work close to the hydrostatic pressure to minimise pollution spread, and so shallow schemes (100-200m) like Chinchilla operate closer to atmospheric pressure (~10 bar) than those at greater depth such as the European trial (El Tremedal, Spain) (~60 bar). Shallow operations have lower drilling costs but the disadvantage is the potential for environmental pollution and a lower CV gas. High pressure encourages methane production and cavity growth.

2.9 "For this generic study, a minimum depth of 600m has been assumed to lessen the environmental impact at surface, in terms of hydrogeology, subsidence and gas escape. This does not rule out shallow UCG for specific sites in the UK, where the local strata and hydrogeological conditions can support operations in seams closer to the surface than 600m. The 1200m depth represents the normal limit for mining in the UK, and the same figure was used for UCG on the basis of drilling costs and working pressure at surface. More work might establish that UCG can go deeper, and there are advantages in terms of energy produced in doing so.

2.10 "A seam thickness of 2m or greater has been chosen for economic reasons - greater thickness means more coal for gasification. It has also been suggested in the European studies that UCG reactions in thin seams are not generally sustainable, although the Soviets have reported that seams down to 1m in thickness can be gasified.
Other factors that are important in any UCG scheme, but were not used in the mapping process were:

  • Impermeable layers of strata surrounding the target coal seam
  • Seam bedding dip between 5° and 30°
  • Absence of any major faults in the area
  • Low values for sulphur content, ash content and swelling index
  • Environmental and hydrogeological conditions
  • Proximity to users
  • Licence conditions that might be imposed by Regulatory and Planning Authorities

2.11 "To define UCG areas, the borehole database was interrogated to identify boreholes which contained coals in excess of 2m in thickness at depths between 600 and 1200m from surface. The 600 and 1200m lines, drawn on each map, mark the lower and upper limits of each UCG resource area. It can be seen from Figure 4 (p152 in Jones et al, 2004 - a block diagram of dipping coals which illustrates how the criteria for UCG and carbon dioxide sequestration are applied) that the maximum possible resource area is defined where the 600m line intersects the top of the coal-bearing strata and the 1200m line intersects the base of the coal-bearing strata. Coal seams that met these criteria, but were less than 100m below the base of the Permian, were excluded. Boreholes that met the criteria were plotted on a base map together with the extent of underground workings and existing mining licences. The resource area could then be defined. Three resource subdivisions could be identified: good, unverifiable and poor. These are represented on the maps (in the report) as different colour(ed) zones. Good areas meet the criteria as defined above. Unverifiable areas represent regions where the UCG potential is unknown. This may be related either to the absence of borehole data, or to the lack of deep penetrating boreholes ( i.e. >600m) within an area. Poor zones represent areas where coals are present at the required depths, but do not meet the thickness criteria."

2.12 Interestingly the description of UCG processes also addresses the question of CO 2 Sequestration. It states "Because carbon dioxide ( CO 2) sequestration requires that CO 2 remains in place for very long time periods, areas of coal suitable for mining or underground gasification are not suitable for CO 2 sequestration."

2.13 "Areas considered to contain coal resources potentially suitable for CO 2 sequestration by adsorption onto coal fall into two categories:

  • Areas of unminable coal seams (defined on the maps by areas where coal seams are at depths >1200m and >500m from mine workings), and
  • Areas where coal seams are at depths of <1200 m, but CO 2 sequestration might take place in association with underground coal gasification or coalbed methane production

2.14 The former are regarded as primary areas for CO 2 sequestration and are identified on the maps. The latter are regarded as secondary opportunities and are not marked on the maps. Because this is an immature technology, no implication as to the methodology for CO 2 sequestration is made. Figure 4 (see in chapter Introduction and Background) indicates that this area is at a maximum if it is defined at the position where the 1200m line cuts the base of the coal-bearing strata. This also creates an overlap zone between the area suitable for UCG and that of the potential CO 2 sequestration area. Hence the position where the 1200m line intersects the base of the coal-bearing strata is marked on the maps."

2.15 The Report goes on to consider risk and uncertainty issues,

"There are a number of geological factors that are important for the coal technologies and can have an impact on the exploitation of the resources; these can be viewed as risks. Many of these are described by Creedy et al. (2001). There are also areas of uncertainty regarding the accuracy of the resource assessment and mapping process. These are detailed in the following section:

2.16 " Underground Coal Gasification

  • Heavy faulting
  • Overburden composition and potential leakage of produced gases/by-products into aquifers
  • Groundwater quenching the reaction
  • Subsidence
  • Seam thickness variability
  • Coal conditions inductive to lateral cavity growth
  • Fugitive emissions or migration of potentially harmful combustion products"

2.17 It also identifies under key uncertainties, mapping processes, data availability, data reliability and the mapped and real presence of faulting and hydrogeologic issues. It goes on,

"it is clear that borehole availability plays a major factor in the determination of UCG resources and uncertainty exists as to whether all resources have been identified. In order to minimise this risk, boreholes were selected at regularly spaced intervals where possible. Where resources were identified further boreholes were selected to try and produce the best possible definition of the resource area. In the deeper parts of coalfield this was not always possible due to restrictions on borehole availability. Uncertainty also exists regarding continuity of seams between boreholes related to, for example, faulting. Only detailed site specific studies can address these issues."

2.18 Jones et al describe how they calculated UCG potential. Using the areas mapped, "Two volume calculations were performed. Firstly the minimum volume of coal available for gasification was calculated, using the equation below:

Min. vol. of coal suitable for gasification (10 6m 3) = 'good' area (m 2) x 2(m)

This calculation was made assuming that the only minimum thickness of coal ( i.e. a 2m thick seam) was available for gasification across the area.

2.19 The second calculation involved taking an average of the total thickness of coal per borehole in the areas with good UCG potential and multiplying this average figure by the area of the good polygon.

Ave. vol. of coal = suitable for gasification (10 6m 3 ) 'good' area(m 2) x average of the total
thickness of coal per borehole that meet the criteria (m)

2.20 It is difficult to determine accurate resource figures due to the limitations of the borehole dataset, particularly the fact that boreholes do not generally penetrate through the entire thickness of coal-bearing strata. In these instances it is not known whether there are coals present at greater depths that may meet the criteria. Although not truly accurate, this second calculation probably gives us a more typical idea of the volume of coal available for gasification than by applying a minimum value. The figures derived from these two calculations are given in Table 7 of the BGS report.

2.21 The minimum total volume of coal suitable for UCG in the UK is nearly 5,700 x 10 6m 3 (~7 Btonnes), whereas the total volume of coal figure derived using the average coal thickness meeting the criteria per area is nearly 12,911 x 10 6m 3 (~17 Btonnes) (Table 7 again). This represents a resource of 289 years based on the current UK coal consumption of 58 Mtonnes per year (at 2004; now somewhat lower/longer).

2.22 An extract from Table 7 in Jones et al, is reproduced and edited below,

Area Av. thickness of coal meeting UCG criteria (m) Area of Resource ( Min vol of coal available for gasifn (ass.2m seam) (M cu.m) Vol of coal available for gasifn using av thickness of coal across area (M cu.m)
Canonbie 3.9 3.89 7.78 15.2
Ayrshire 2.36 6 12 14.2
Douglas 7.5 1.3 2.6 9.8
Clackmannan 2.6 22.9 45.8 59.5
Fife 3.1 3.8 7.6 11.8
Lothian 3.8 5.6 11.2 21.3

2.23 Jones et al also set out some details for each of the main Scottish UCG areas as follows:

"Ayrshire Coalfield

Conditions suitable for UCG are generally limited in this area due to the extensive nature of previous underground mining activity. However, areas with good potential for UCG have been identified. The largest area in the Ayrshire Coalfield is between Mauchline and Ochiltree. Seams proved to exceed 2m thick at the correct depths in areas not associated with old mine workings are restricted to two boreholes: Kingencleuch No 1 (Hurlford Main 2.52m at 937m) and Drumfork Farm Bore (Lugar Main 2m at 717m). These two coals are from the Middle Coal Measures. A large area of unverifiable UCG potential exists to the west of the area of good potential. Boreholes are present in this area but typically do not penetrate to depths much in excess of 600m and no thick coals have been recorded. Hence this is an area that may have potential.

Douglas Coalfield

In the Douglas Coalfield only the Callow Knowe, Douglas Bh.76 Diamond and Eggerton boreholes proved coals suitable for UCG. In the latter borehole the Manson Coal in the Passage Formation was about 8.12m thick, corrected to 4.66m for a dip of 55° at a depth of only 260m. However, this is in a mined area so has been discounted. In the Callow Knowe Borehole this seam was 2m thick at a depth of 871m. In the Douglas Bh.76 Diamond the Ponfeigh Gas (2.41m, at 623m), from the Upper Limestone Formation, and the Wee Drum (5.54m at 781m) and the Skaterigg coals (3.28m at 792m) from the Limestone Coal Formation are all considered suitable for UCG.

Central and Clackmannan Coalfields

There are three areas considered suitable for UCG in the Clackmannan and Central coalfields. These are to the north-west of Falkirk, and two areas along the Firth of Forth. Coals that meet the criteria are the Upper Hirst (Upper Limestone Formation), Bannockburn, Wester Main, Kelty Main and No.1 and 2 Jersey coals, Glassee and Mynheer from the Limestone Coal Formation. Good prospects occur northwards from Grangemouth to the area of the former Longannet Colliery and north and west of Stenhousemuir. It is possible that the good areas extend further to the south-west, into the area between Falkirk and Cumbernauld. However, there are few deep boreholes hence this area is marked as unverifiable.

Fife Coalfield

In Fife there are two small areas that meet the criteria for UCG, one onshore and one offshore. The onshore area occurs between Glenrothes and Methil, whereas the offshore area lies along the western flank of the Leven Syncline. Coals meeting the criteria include the Upper Limestone Formation Craig Coal, and the Upper Cardenden Smithy, Lochgelly Splint, Cowdenbeath Jewel and Cowdenbeath Five Foot from the Limestone Coal Formation. To the north and east of this good UCG prospect is a large area of unverifiable UCG. Here there are no boreholes greater than 600m in depth.

Lothian Coalfield

In Lothian the coal-bearing strata are limited to a narrow synclinal area between Musselburgh and Penicuik. The extensive former underground coal mining restricts the areas available for UCG exploitation. However, small areas with potential exist immediately offshore from Musselburgh and to the south-east of Edinburgh. Seams that meet the criteria include the Lower Coal Measures Musselburgh Fifteen Foot and Seven Foot and, from the Limestone Coal Formation, the Great, Gillespie and Blackchapel."

And finally, the report identifies,

"Leven Syncline

The only areas where the Westphalian Coal Measures reach depths >1200m and therefore have potential for CO 2 sequestration in unminable coals, is in the centre of the Leven Syncline beneath the Firth of Forth. Further potential may exist in the Limestone Coal Formation in the Leven Syncline."

2.24 Coal Authority (2009) describes the policy position for their licensing of UCG. The 1994 Coal Industry Act empowers the Coal Authority to license UCG activity, starting with conditional licences normally for three to five years for exploratory work both on and offshore.

2.25 Seven such licences have been issued in Scotland. Six still active although under discussion for ownership in late 2014. Now (late August 2016), 2 licences held by Cluff Natural Resources remain in effect to July/August 2018. Coal authority guidance and classifications frame UCG operations and require engagement of the applicant with DECC, MoD and relevant other bodies - in this case SEPA, HSE, Crown Estate, Marine Scotland and the local authority. This is addressed further in the chapter on Regulation.

2.26 Further relevant issues are also addressed in the next chapter on Technology and Operations.

2.27 Several related geological issues should ideally be taken into account in considering the viability of UCG operations and their hazard profile. These are not considered here in detail but merit fuller analysis.

  • A Issues of interaction with prior mine workings.
    Good planning and controlled combustion would seek to avoid dubious nearby structures. Separation criteria already exist but these are "rules of thumb" and would need to be tailored to issues of local structure and gas and liquid movements. Ruling out gas connection with adjacent voids or differing pressure environments or where gas presence could accentuate panel burns would be expected to be factored in to operations.
  • B Issues of post-combustion response
    Gas and liquid connectivity from pore space level to transit along faults and fracture zones to movement along a hydrological gradient may all occur. Hydrostatic pressure at depth may accentuate some and constrain other effects. Late combustion products, tars etc., might be expected to be retained in cavity and might slowly become mobilised in the groundwater. Depth, hydrostatic pressure and low transmission potential would inhibit this being significant.
  • C Post Combustion gas storage
    Use of coal seams where UCG has taken place in a single or set of panels has been suggested for CCS but largely discounted as non-viable in the short term and an additional hazard without further integrity and structural analysis given the disruptive effects of combustion, flexure and hydrostatic responses.
  • D Burn-out
    What happens when cavities (combustion chambers and panels) collapse (B&C) - a fully burned out seam of 2m thickness across a front of a number of metres and along a seam of tens of even hundreds of metres might be expected even with hydrostatic encouragement to close or groundwater incursion to limit closure would create a flexure or collapse of centimetres to meters extent. These would be expected to have some seismic impact, potentially, though not certainly, gradually. Surface impact of this is unclear but potentially low.
  • E Seismicity generally
    Largely unknown impacts for UCG although some impacts have been reported from shallow sites in Australia. Base level data and modelling would be advisable.
  • F Hydrogeology/Groundwater impacts
    This area is addressed under Regulation and Environmental Impacts but in addition, baseline assessment of groundwater chemistry is discussed by Ó Dochartaigh et al (2011) and the data were used to inform BGS and SEPA approaches to groundwater ( GW) assessments and characterisation of GW generally.

2.28 Uncertainty

Several authors and stakeholders have referred to what is well known about the geology of the Midland Valley of Scotland, one of the very best known areas of coal geology worldwide. The work of BGS and its predecessors, and the Coal Authority ( CA), as well as commercial contractors and BGS contract work for licence applicants and bore hole operators etc. has added to the work produced by mining engineers and owners over two hundred years and more. It is also clearly one of the more complex geologies found, by comparison with some locations where UCG has been developed and tested in South Africa, Australia, Russia/Uzbekistan or North America.

2.29 The plate edge location of Scotland over a large part of geologic time has resulted in significant collisions and stretching of the crust producing faulting and fracturing of the main coal units and the surrounding geology. Although the depths involved in the case of the major coal seams meet CA criteria, there are issues of uncertainty that are relevant to how exploitation could progress.

2.30 Borehole coverage and mine records deteriorate rapidly moving east and south from Kincardine into the Forth and similarly west from the edge of the Leven syncline as well as at greater depth and further offshore. Fault heave magnitude and direction are less certainly known and quanta and trajectories are plotted literally with dotted lines and question marks. More drilling would clearly help fill data gaps.

2.31 The way in which the geological location where UCG combustion takes place is by definition remote from the surface and from easy access and hence difficult to model or monitor accurately. The pressure at depth is both related to gravity and overburden mass and hydrostatic pressure affecting the void spaces in the rocks and the liquids and gases present there. Opening up a cavity and causing coal and gas to combust and then removing this creates forces of expansion and then recovery, with heating and cooling also taking place. Surrounding pressure generally would seek to fill a created void. Gas is being extracted and liquids, i.e. groundwater, and secondary gases in the geology would enter the space and re-equilibriate. Structural relaxation and flexure would also occur. Therefore, the net effect of hydrostatic impacts is somewhat unclear and groundwater quality and connectivity across the geological units cannot be certain at this point. It might be supposed, however, that, where impermeable capping or low transmissibility units are present and the water bodies are at such depths or separated from any current conceivable use, risk is minimal. Seismicity is known to occur naturally here and is associated with some UCG activities. Not enough is known locally to assess these factors. Assumptions can be made about groundwater salinity, disconnection from higher aquifers, effective aquitards, minimal gas and water movements, low seismicity, small flexures of burned out cavities, minor collapses. Certainty does not exist.

2.32 It should be stressed, however, that uncertainties can also be over-played. These issues relate to some extent to groundwater and rock extraction, and certainly to exploitation of oil and gas offshore. What we must ask and consider is what is our appetite to accept these hazard factors and what arrangements would be put in place to understand and mitigate them. Robust preparatory work to enhance knowledge, including bore work would be critical, as would the establishment of a fit-for-purpose monitoring network to assess changes in well-understood baseline conditions above and below ground during any demonstration pilot and subsequent operations.

2.33 Summary

Coal of relevance to UCG exists in significant quantities in the central of the three provinces in Scotland. For now, the coal bearing geologies of the north (Brora) and the south (Canonbie), as well as the Machrihanish component of the Midland Valley coals in the west can be set aside in practical terms. They could be exploited but would be unlikely to be a priority. Similarly the western half of the Midland Valley area is less likely to be developable for now. The FoF remains the most likely area to be considered for exploitation, is the best known, mapped and explored - a position enhanced by the additional work undertaken by Belltree (2014) and has extant licences. It has been studied in some considerable detail and has, despite being substantially fractured and interrupted in some parts, the potential to be exploited by existing technologies. The major seams within the province which meet BGS/ CA criteria for depth, thickness and quality have been initially assessed and potential operators have engaged with licensing and other regulatory bodies and sought to plan on the basis of their understanding of the resource.


Belltree Ltd (2014) UCG Potential of CNR's Kincardine Licence, Firth of Forth, Scotland (2014)

Creedy DP, Garner, K, Holloway, S, Jones, N and Ren, TX (2001) Review of Underground Coal Gasification Technological Advancements. Report No. COAL R211, DTI/Pub URN 01/1041. DTI/Crown Copyright. ( BGS, Nottingham University and Wardell Armstrong)

DTI (1999) Cleaner Coal Technologies: The Government's Plan for R& D, technology Transfer and Export Promotion, Energy Paper 67, DTI, April 1999

Jones, NS, Holloway, S Creedy D P, Garner, K, Smith NJP, Brown MAE and Durucan, S. (2004) UK Coal Resource for NEW Exploitation Technologies. Final Report. British Geological Survey ( BGS ) Commissioned Report CR/04/015N. NERC

Ó Dochartaigh, B.E.; Smedley, P.L.; MacDonald, A.M.; Darling, W.G.; Homoncik, S.. 2011 Baseline Scotland : groundwater chemistry of the Carboniferous sedimentary aquifers of the Midland Valley . British Geological Survey, 91pp. ( OR/11/021) (Unpublished) Available via NORA ( NERC Open Research Archive)