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Feasibility Report of Fortissat Community Minewater Geothermal Energy District Heating Network

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Chapter 3: Geothermal Supply

3.1 Introduction

This chapter introduces the basic principles of minewater geothermal energy and provides an overview of the mine systems in the study area. It identifies the coal seams most likely to provide the best resource for minewater geothermal energy, and provides preliminary desk based assessment of the temperature and volume to provide estimates of the heat potential (within the mine) and instantaneous geothermal potential (for a district heating network). Kingshill Colliery is one of Scotland's largest historical mine systems and the mine seams partly underlie the southern area of Hartwood Home Farm. This was identified as by far the largest geothermal resource in the area of interest, and has been the main focus of this study. This chapter also reviews the mine records to identify targets for the drilling, considerations for the drilling techniques, and considerations for optimising performance of the operational system, including considerations of minewater chemistry and thermal breakthrough (where the cooler minewater returned to the mine after heat has been extracted decreases the source temperature). This assessment forms the evidence base for a geothermal systems option appraisal which considers the pros and cons of the various mine systems in the study area to identify two preferred options.

3.2 Minewater Geothermal Energy - Basic Principles

Conceptually, three interconnected elements are required to allow geothermal energy to be exploited from minewater (Figure 3.1, next page):

1) A heat source or heat reservoir. To be exploitable, a heat reservoir typically needs to contain permeable pathways (e.g. mine workings or natural permeability) and a substantial volume of mobile groundwater that can be pumped, via a drilled borehole, to a heat exchange system at the surface.

2) A heat transfer or energy conversion system. For example, a heat exchanger coupled to an array of electrically-powered heat pumps. These can recover heat from the mine-water and increase the temperature to levels capable of serving a District Heat Network (DHN) which supplies heat consumers.

3) A disposal system. The "thermally spent" minewater then needs to be disposed of responsibly. It can be transported via a buried pipe to a separate re-injection borehole, which returns the water to the mine system (a "zero net abstraction system"). Alternatively, the thermally spent minewater could be treated, passively and at modest cost, before being discharged to a natural watercourse. This would avoid the cost of one or more expensive injection boreholes.

Figure 3.1: Schematic diagram of a generic mine-water geothermal energy doublet system (not to scale). PWL and SWL are pumping water level and static water level respectively. PLEASE NOTE: the mined seams displayed in this diagram do not represent the actual fluid flow pathway between the injection and production wells, and are for illustrative purposes only. Also note that in some designs a plate heat exchanger may be installed at the production well head to transfer heat into a clean loop which transfers heat to the heat pumps, to avoid corrosion of the heat pumps.

Figure 3.1: Schematic diagram of a generic mine-water geothermal energy doublet system (not to scale)

3.3 The Resource - The Mine System

In this project, digitised information from mine abandonment plans has been used to build a new 3D computer model of the mine workings below and adjacent to the James Hutton Institute's Hartwood Home Farm. The project study area also considered mine workings within the project area of interest, an area approximating the Fortissat Ward of North Lanarkshire Council (Drawing 1.1).

The 3D model provides the necessary information to make an estimate of the extent and volume of mine workings available (Section 3.2.2) and the potential geothermal resource they represent (Section 3.2.3).

3.3.1 Mine Geometry

Digital scans of mine workings from coal mine abandonment plans held by the BGS were used to create Geographic Information System (GIS) shape-files of the positions of the disused mine workings, shafts and adits in and around the project area. Stone drivages (underground tunnels linking across different seams), roadways, spot height and contour data, where recorded, were also digitised.

The shape-files were then imported into MoveTM software for production of the 3D model of the subsurface geometry of the underground coal workings. The eight coal seams considered (in stratigraphic order from youngest to oldest) were:

  • Upper Drumgray Coal (UDC) - shallowest
  • Middle Drumgray Coal (MDC)
  • Lower Drumgray Coal (LDC)
  • Shotts Gas Coal (SGA)
  • Mill Coal (MILL)
  • Armadale Main Coal (ARM)
  • Woodmuir Smithy Coal (WRSM)
  • Wilsontown Main Coal (WNMA) - deepest

Mine depths are expressed relative to sea level - Ordnance Datum (OD). The Woodmuir Smithy (WRSM) and Wilsontown Main (WNMA) seams are substantially deeper (below -200 m OD, Drawings 3.1 - 3.3) than all the other worked seams, which occur above sea level (+0m OD, Drawings 3.4 - 3.9) and typically within several tens of metres of the surface.

Shallow mineworkings occur under the Hartwood Farm site to the west and south-west of Shotts/Dykehead. Most workings are less than 100 m beneath the ground surface. In addition to those listed above, seams worked within the Scottish Lower Coal Measures Formation include the Kiltongue Coal and Airdrie Virtuewell Coal. The Lower Drumgray (Shotts Smithy) Coal was worked more extensively between Salsburgh and Shotts/Dykehead. The workings were mostly less than 100 m below the surface. Scottish Middle Coal Measures Formation coals were worked in surface coal mines to the south and west of Shotts Prison.

Additional data imported into the 3D model included a digital terrain model (DTM), and the Hartwood Home Farm study area (Drawing 1.1), provided by the James Hutton Research Institute (JHI). The maximum and minimum depths (relative to OD) were calculated from the model for each seam, as well as the estimated volume of mine workings. It is likely that the worked coals across the area will vary in thickness, and therefore an average value of 1 m (taken from boreholes) was chosen for volume calculations. The results are summarised in Table 3.1. The extent of each modelled seam with elevation (OD) is shown in Appendix A3.1 in stratigraphic order. Drawing 3.10 illustrates an outline of all the 3D modelled surfaces, and Drawing 3.11 presents a 3D image of the modelled seams.

Within the wider Fortissat study area (Drawing 1.1), the following areas of mine workings were not modelled:

1. Small area in the WNMA to the south of Spoutcross along the southern boundary

2. Small area in the WRSM to the north-east of Harthill along the north-eastern boundary

3. Three areas in the LDC:

a. in the north-western corner around South Lanridge Farm

b. in the north between Kirk of Shotts and Shotts Prison

c. 1 - 2 km to the north-east of Shotts

4. Small area in the UDC around Fernieshaw along the western boundary

With the exception of the WRSM workings near Harthill, which reach depths of around 500 m, these unmodelled workings are shallow (100 m or less).

Table 3.1: Summary of mined workings within coal seams within and adjacent to the study area, and associated mined volume and elevation (OD) ranges. Note: in several cases, the locations of maximum and (especially) minimum depths lie outside the Hartwood Home Farm study area.

Coal Seam

Full Name

Minimum depth (m, relative to OD)

Maximum depth (m, relative to OD)

Potential total mined volume m3 (assuming 1 m thick workings)

UDC

Upper Drumgray Coal

238

103.7

1392517

MDC

Middle Drumgray Coal

228.6

131.1

566714

LDC

Lower Drumgray Coal

213.9

75.5

1168779

SGA

Shotts Gas Coal

210.6

196.9

17040

MILL

Mill Coal

175.7

110.8

87032

ARM

Armadale Main Coal

168.2

127.4

139456

WRSM

Woodmuir Smithy Coal

79.9

-276.8

10725782

WNMA

Wilson Town Main Coal

-9.7

-304.2

5902271

3.3.2 Mine-Water Temperature and Volume

Based on an estimated geothermal potential in the mined regions of the Midland Valley of 5 x 108 kWh/km2 (Gillespie et al., 2013), a previous feasibility study estimated a potential resource of 3.5 x 108 kWh below the original 0.7 km2 Hartwood Home Farm study area. In the light of the 3D seam model, this has now been re-evaluated. The re-evaluation will focus on the Wilsontown Main (WNMA) and Woodmuir Smithy (WRSM) seams (Drawing 3.1, 3.2 and 3.3) as these are both the most aerially extensive and deepest (presumed warmest) seams. The workings of these two seams underlie an area of 10.7 km2 (Table 3.1) and were formerly worked via Kingshill No. 1 colliery. The WRSM directly overlies the WNMA with about 30 m separation, and the two are believed to be hydraulically interlinked via the shaft of the Kingshill Colliery and also via stone drivages that are likely to have remained open after mine closure (Drawing 3.3). The coal seams dip down to the north: thus, the maximum worked depth is found in the northern part of the WNMA workings (-304.2 m OD - Table 3.1) just on the southern edge of the Hartwood Home Farm study area. The geothermal heat potential of the mines depends on the available mine-water volume and temperature.

Minewater Temperature

Temperature typically increases with depth in the earth - the geothermal gradient. This is approximately 30.5°C/km across onshore Scotland. Minewater temperatures in Scotland range from 12 to 21°C, with a mean of 17°C (Gillespie et al., 2013), but a comparison of temperature with depth in mine workings does not show a clear geothermal gradient (Figure 3.1). This probably reflects the very dynamic nature of water circulation within many mine systems.

Figure 3.2: Geothermal gradient from bottom-hole temperature-depth measurements in collieries in the Midland Valley.

Figure 3.2:  Geothermal gradient from bottom-hole temperature-depth measurements in collieries in the Midland Valley.

We do not know the water temperature in the WRSM or WNMA workings: this can only be resolved through drilling and testing a well. Thus, our estimates of geothermal potential are based on three possible scenarios:

  • Worst case: a temperature of 13.9°C, based on the recorded temperature of water discharging from a pipe intercepting the abandoned Kingshill No. 1 shaft (Appendix A3.2 and A3.4). It is felt that this is likely to be an underestimate, as the currently discharging water may have been cooled by interaction with the groundwater in shallower, overlying strata.
  • Intermediate case: a temperature of 17°C, based on Burley et al.'s (1984) recorded temperature of 17°C from 549 m bgl (below -309 m OD) at the nearby Polkemmet Colliery, and also reflecting the mean minewater temperature for Scotland.
  • Best case: a temperature of 19.2°C, based on the temperature of water pumped from the shaft of Polkemmet Colliery, which is of similar depth and is broadly analogous to Kingshill Colliery (Ó Dochartaigh, pers. comm., 2015 - see also Appendix A3.2).

Reservoir Volume

The estimated geothermal potential depends not only on the reservoir temperature, but also on the volume of warm rock and water that can be intercepted by a geothermal system. A number of assumptions are possible and these are all documented in Appendix A3.1

  • Assumption 1: only the heat contained in the minewater within the worked seams is available.
  • Assumption 2: the heat contained both in the water and mineral matrix in the worked seams is available.
  • Assumption 3: the heat contained in the worked seams and any adjacent aquifer (of assumed porosity 10%) is available.

The volume of the mine reservoir has been calculated based on each of these three assumptions and has been modified by a so-called "recovery factor". This is based on the observation that a geothermal doublet comprising an abstraction and a recharge well will preferentially access the heat contained in the strata between the two wells and may not be able to efficiently access the entire volume of mine voids.

The estimates (Appendix 3.1) are based on a calculated combined mined volume of the WNMA and WRSM (and interconnecting stone drivages) of 16.92 million m3 (Table 3.1). Because minewater discharges are observed at +200 m OD at Kingshill No. 1 Colliery and at +183 m OD at Redmire Crescent, Allanton (North Lanarkshire Council, 1999), the workings are assumed to be completely flooded. Because the WNMA and WRSM seams were worked by longwall techniques, they can be assumed to have collapsed and become filled with porous goaf. Thus, the volume of the mine workings needs to be modified to take into account this collapse (e.g. Gillespie et al., 2013) to give a final water-filled void space:

  • A minimum estimate of volume assumes a void space of 20% in areas that have been longwall mined in addition to open stone drivages with a radius of 2m, 3.62 million m3.
  • One could further reasonably assume that the surrounding rock formations (silty sandstone) will have been fractured due to mining activity, such that the void space is likely to be closer to 30%, 5.28 million m3.
  • A maximum volume estimate would assume that open roadways and drifts would increase porosity further, but the proportion of these is not known. A void volume of 35% is tentatively estimated if a number of these roadways and drifts were open, 6.12 million m3.

3.3.3 Estimated Heat Potential (WRSM and WNMA Seams)

The calculations performed by the British Geological Survey which form the basis of Section 3.2.2. have effectively resulted in 9 estimates of the geothermal potential, based on:

  • Three assumptions about the nature of the mine reservoir.
  • Three estimates of abstracted minewater temperature (best, intermediate and worst case. In all cases, the reinjection temperature is (somewhat arbitrarily) set to 7°C. In the case of Assumptions 1 and 2, these temperatures are coupled to maximum (35%), intermediate (30%) and minimum (20%) assumptions regarding the porosity of the mined strata. In the case of Assumption 3, the temperatures are coupled to best (500 m), intermediate (320 m), and worst case (120 m) assumptions regarding the thickness of the adjacent aquifer.

Details of how the results were calculated are presented in Appendix A3.1, with the results summarised in Table 3.2:

Table 3.2: Minimum, intermediate and maximum geothermal potential (in kWh) of the water stored in the in the WNMA and WRSM mines. Reinjection temperature of 7°C is assumed in all cases.

Reservoir model

Model 1

Minewater only

Model 2

Minewater & rock

Model 3

Mine and aquifer

Recovery factor

None

0.33

0.42

Mine-water temperature

13.9 °C

2.90 x 107 kWh

3.78 x 107

1.42 x 109

17 °C

6.13 x 107

4.83 x 107

5.49 x 109

19.2 °C

8.67 x 107

5.69 x 107

1.05 x 1010

The assessments made in Table 3.2 that neglect the aquifer system (i.e. Assumptions 1 and 2), assume that the minewater within the workings is isolated and does not interact with adjacent aquifer strata or the surface. However, evidence indicates that there is likely to be a good hydraulic recharge of the mine-water system; Younger and Adams (1999) suggest a complete flooding of the mine within 15 years of the closure of the Kingshill No. 1 colliery; the Carboniferous sedimentary aquifers in Scotland are expected to have a reasonably high transmissivity (10 to 1000 m2/day) even if they are not mined (Ó Dochartaigh et al., 2015). It might be hoped that this recharge would also help to sustain temperatures somewhat as natural groundwater through-flows might mix with reinjection fluids in the mine. It is therefore likely that the estimates associated with Assumptions 1 and 2 in Table 3.2 are too low.

There is thus a significant degree of uncertainty surrounding the estimates of geothermal potential in the WRSM and WNMA mined seams. However, if there is a good connection between the mine system and the surrounding aquifer, the minewater geothermal potential beneath the Hartwood Home Farm study area might be an order of magnitude higher than initially estimated, between 1.42 x 109 and 5.49 x 109 kWh. This corresponds to the initial feasibility study estimate of 5 x 108 kWh/km2; the total geothermal potential in this study is predominantly due to the increased mined area being considered. However, these estimates rely strongly on a good hydraulic continuity with the whole aquifer: if this is not the case then the geothermal potential could be up to two orders of magnitude lower.

To put these estimates in context, a resource of 1.42 x 109 kWh represents:

  • 258 years of extraction at a rate of 0.63 MW (5.5 x 106 kWh per year)
  • 71 years of heat extraction at a rate of 2.3 MW (2 x 107 kWh per year) - representing an abstraction of 50 L/s water at 18°C, with reinjection at 7°C (see Appendix 3.2), which is considered feasible given the historic pumping rates at Kingshill and Polkemmet Collieries.

3.3.4 Instantaneous Geothermal Potential

The results presented in the sections above estimate the total geothermal heat resource associated with the Wilsontown Main (WNMA) and Woodmuir Smithy (WRSM) mine workings in kWh. The assumed reinjection temperature Mathematical Equation of 7 °C and three possible minewater temperatures Mathematical Symbol of 19.2, 17 and 13.9 °C produce temperature differentials Mathematical Equation of 12.2, 10 and 6.9 °C respectively. The calculations can thus be modified relatively simply to accommodate a range of other assumptions about the minewater and reinjection temperatures.

At any given point in time, however, a production well will be producing a quantity Mathematical Symbol of water (L/s) at a temperature Mathematical Symbol. The water will pass through a heat exchanger (coupled to the evaporator of a heat pump) and a quantity Mathematical Symbolof heat will be extracted from the water (kW or kJ/s). The water will leave the heat exchanger at a temperature Mathematical Equation and will then be reinjected or otherwise discharged. The instantaneous rate of heat available is given by:

Mathematical Equation

As the volumetric heat capacity of water (Cwat) is 4.18 kJ/L/°C, the amount of heat available for every 10 L/s of water is 41.8 kW for every 1 °C temperature change across the heat exchanger.

Over the long term, it is possible (especially in a geothermal well doublet) that the temperature Mathematical Symbol of the abstracted minewater will decrease, thus also decreasing the efficiency of heat extraction. The actual sustainability of the system will depend to a large degree on the flow pathways and thermal breakthrough times, which are dependent both on the mine network and the setup of the geothermal system. These aspects will need to be modelled in detail at the next stage of project design if the minewater is to be reinjected.

3.4 Geology, Hydrogeology and Drilling

The deep Woodmuir Smithy (WRSM) and Wilsontown Main (WNMA) seams in the southern part of the study area were worked by longwall methods from Kingshill No. 1 Colliery, which comprised two shafts. The workings of Kingshill No. 1 colliery are thought to interlink into Kingshill No. 3 colliery further south and into other collieries further up-dip towards the south, via stone drivages and shafts. The situation is directly analogous to that at Polkemmet Colliery, whose hydrogeology and water chemistry are much better known, some 10 km to the NE.

3.4.1 Kingshill No.1 Colliery Background

Kingshill Colliery No. 1 (also known as Allanton Colliery) was opened in 1919 and closed in 1968. It was abandoned in 1975. The two shafts are recorded as being 344 m (No. 1 shaft) and 371 m (No. 2 shaft) deep. According to the mine abandonment plan, however, the shafts are given as 378 and 376 m, respectively. The colliery has now been demolished. According to the British Geological Survey "Wellmaster" database, Kingshill No. 1 Colliery was typically dewatered at an average continuous rate of 41.7 L/s when working, with maximum rates of 98.5 L/s.

Although Kingshill Colliery No.1 was not closed until 1968, from 1951 onwards a significant proportion of the output from the mines was brought to the surface from Kingshill Colliery No. 3, further to the south. This remained open until 1974 and was abandoned in 1975.

Further detail on the Kingshill Colliery No. 1 background, including references, are provided in Appendix A3.2, with detail on the existing passive minewater treatment system included in Appendix A3.5.

3.4.2 Mining Techniques and Hydrogeological Targets

The Woodmuir Smithy (WRSM) and Wilsontown Main (WNMA) seams at Kingshill No. 1 were worked by advancing longwall techniques. Since the 1950's, most underground mining has used longwall methods, as opposed to stoop and room mining which was common until the mid-20th century.

In longwall mining the coal seam is worked between two parallel access roadways. Following mining, the roof strata are allowed to collapse and the void becomes filled with broken overburden (called goaf). The coal panel between the roadways is generally between 100 to 250m wide and it has been estimated that about 20% of the mined void would remain after mining induced subsidence has occurred.

Mining profoundly alters the hydrogeology of targeted areas. Many shafts, roadways and other linear access structures were built to last and may be likely to remain as open voids in the subsurface today. When these structures are below the water table, they act as extremely permeable, interconnected channels for groundwater flow.

The location of longwall mining areas and access structures dictates where groundwater could be abstracted from and subsequently determines where drilling should occur. Within the seams worked at Kingshill Colliery there are three types of target that need to be considered before drilling a borehole:

Main access roadways

Main access roadways and drifts are most likely to remain open. They were typically constructed of spaced steel arches, with the roof supported by transverse concrete or timber beams between them.

Maingate and tailgate roadways

Maingate and tailgate roadways were those which flanked and provided access to longwall faces. It cannot be guaranteed that such access roadways are still open - supports may have been removed after the longwall face was worked out. It is more likely that maingate and tailgate roadways would have been left open in the case of advancing longwall faces (as in the case of Kingshill No. 1) than in the case of retreating longwall faces.

Goaf (collapsed longwall workings) and fractured strata above them

After the roadways, the worked out seam is likely to remain the most permeable zone, even though it contains goaf. The goaf horizon will be dominated by intergranular flow, while fracture flow will dominate in the disturbed zone above. The potential advantage of drilling into goaf is that one avoids the direct 1-dimensional flow pathways that roadways represent; thus heat breakthrough in a well doublet is more likely to be slower. The main disadvantage is the limited capacity for abstracting large yields. While enhanced permeability could reasonably be expected in the goaf and in the fractured strata for several tens of metres above the worked seams, one should not automatically expect that the permeability would be large enough to sustain abstractions of several tens of L/s. Reinjection of such quantities of water into a porous goaf (as opposed to an open roadway) would be even more problematic.

3.4.3 Hydraulic Risks and Optimising Performance

Targets for Drilling

For a potential production well, requiring large volumes of water, the preferred drilling target would be a main access roadway. If the disposal of thermally spent water involves reinjection wells, these should also preferably target main access roadways. In theory, injection wells are the hydraulic opposite of production wells. However, in reality, injection wells typically exhibit somewhat lower performance than production wells, such that more than one injection well may be required for each production well. If the natural mine-water levels are near the surface or even artesian - which is likely for the Hartwood and Allanton area - then we may have to inject water under excess pressure, which implies specialist construction and grouting techniques and pressure-testing.

Alternatively, it may be possible to avoid reinjecting the thermally spent water. The spent water could be treated by a passive minewater treatment facility, prior to discharge to a surface watercourse. The advantage of this would be avoiding the need to drill and maintain costly injection wells, and avoiding the risk of thermal feedback. It might also provide the added benefit of lowering minewater levels regionally and mitigating some of the negative impacts of surface mine-water discharge in the Allanton area. The main disadvantages would be the cost of constructing and operating a treatment plant, and the fact that pumping could gradually lower the water levels in the mine system, increasing pumping costs over time. The benefits of this design option over a reinjection doublet is discussed further in Chapter 5, and the potential design of any treatment plant is considered in Appendix A3.5.

Reduced system performance

There are a number of reasons why performance of an injection well may deteriorate over time. Injection wells are more prone to clogging if particles are present in the water and they can promote bacterial biofilm growth on the well screen or borehole wall. Any contact between water and atmospheric oxygen prior to or during reinjection may increase the risk of iron and manganese oxi-hydroxide precipitation. Therefore, the operation of a production-injection doublet system must be a pressurised and sealed system to minimise contact between water and atmosphere to reduce the risk of chemical clogging. Because of these risks, more than one injection well may be required for each production well.

Thermal feedback

Injecting thermally spent groundwater too close to where it is abstracted it can cause a short circuiting effect - thermal feedback or thermal breakthrough - where the cool reinjected water simply flows directly back to the production well causing a decrease in source temperature. This can compromise the efficiency of the system and even its long-term sustainability. Thus, care should be taken that the flow pathway between the production and injection well(s) should be as indirect and "diffuse" as possible, in order to decrease the speed, magnitude and risk of thermal breakthrough.

To evaluate the risk of thermal breakthrough, various modelling approaches can be adopted. In this study, two simple, analytical models have been applied to each considered geothermal well doublet option to delimit two extreme possibilities (see Appendix A3.2):

  • a 1-dimensional tunnel model which assumes a single mined roadway connection.
  • a 2-dimensional porous medium model which assumes a well doublet in a conventional porous aquifer.

The real behaviour of a well doublet in a mine is likely to fall between these two extremes - but there is a risk that it could approximate more closely to the 1-dimensional scenario, which predicts very rapid thermal breakthrough.

3.4.4 Minewater Chemistry

The chemistry of minewater can be very unlike normal groundwater, largely because it has been exposed to sulphide minerals (and their secondary oxidation products) in the worked strata. The hydrochemistry of the water from Kingshill (and nearby Polkemmet) Colliery is discussed in depth in Appendix A3.2. The chemistry of the minewater is important for the sustainable operation of the geothermal system (avoiding problems with clogging, scaling or corrosion) and is also critical if the water is to be treated prior to disposal to a surface watercourse. Although the minewater chemistry can only be determined by pumping and sampling an existing shaft or exploratory borehole, it seems possible that water pumped from the deep (Woodmuir Smithy or Wilsontown Main) workings interconnected with Kingshill No. 1 Colliery may:

  • Contain in the region of 10-70 mg/L iron, a few mg/L manganese and in excess of 1000 mg/L sulphate. The pH is likely to be circum-neutral .
  • Be chemically reducing and anoxic, and may contain ammoniacal nitrogen and hydrogen sulphide.
  • Deteriorate with initial pumping, as shallower minewater is drawn down-dip from the south, but should then improve slowly over the course of decades, as the system is flushed of pyrite weathering products.

Recent field and laboratory analyses

Manual field readings of water / air temperature, pH, redox potential (Eh) and electrical conductivity (EC) have been taken from the Kingshill minewater surface discharge between 3rd November 2015 and 26th January 2016. In addition, three duplicate pairs of samples (six samples total) have been collected on 3rd November 2015, 2nd December 2015 and 6th January 2016, respectively. The sampling point was an emergence at 55.7952°N 3.8276°W (NGR 285514 657276) prior to entry into a council-run system of treatment ponds. The raw data from the field and laboratory measurements are provided in Appendix A3.3, and a full analysis of these measurements is provided in Appendix A3.4.

3.4.5 Drilling Techniques & Submersible Pump Installation

The drilling of a geothermal well to depths potentially in excess of 300 m into the Wilsontown Main or Woodmuir Smithy coal seam workings is not a trivial undertaking. It will require a rotary drilling rig capable of drilling with sufficient verticality to encounter and penetrate a specific roadway of width c. 5 m. Potential contractors will not only have to demonstrate that they have the experience and equipment capable of constructing such a borehole, but also that they:

  • have experience of drilling, and managing drilling fluids, in deep, mined Coal Measures strata;
  • are able to manage any issues arising from encountering methane risks while drilling;
  • are able to diligently manage potential artesian conditions and to ensure excellent grout integrity;
  • are capable of responsibly managing potentially contaminated mine-water returns during the drilling process.

The drilling method will likely be a rotary method, which may involve a combination of drilling fluids. The crucial element will be ensuring sufficient precision and verticality to encounter a narrow mine roadway target at in excess of 300 m depth.

Casing materials will be selected to be compatible with expected minewater chemistry (see Appendices 3.2 - 3.4).

A carefully considered grouting program will be required, with sufficient annular clearance behind the casing to emplace a low permeability grout seal. Especially if artesian conditions, or excess pressure in a re-injection borehole, are anticipated (this will depend on the configuration of the design option selected - see Section 3.4) the grout integrity and strength will be especially important and will need to be demonstrated.

In addition to the technical requirements as detailed above, the drilling methodology will need to satisfy the relevant environmental legislation. Details on this and the licensing requirements and process are contained in Appendix A.2.

The production well will be equipped with an electrical submersible pump (ESP) at a depth well below the anticipated minimum pumping water level. The likely pump diameter will be either nominal 6 inch or 8 inch. The final pump selection will depend on:

  • The anticipated discharge (which will depend both on demand and anticipated sustainable minewater yield);
  • The anticipated pumping head, which cannot be finally known until a borehole has been test-pumped.

The diameter of the pump will constrain the diameter of the borehole above the level of pump emplacement (for example, an 8" pump will require a borehole diameter of at least 11-12"). Below the level of the pump, the borehole diameter can be narrower, resulting in cost savings.

Because injection boreholes do not require a production pump, they may be able to be drilled at a somewhat reduced diameter (although they will still need to be wide enough to ensure hydraulically efficient operation, the installation of one or more reinjection mains, and possibly a smaller diameter pump for back-pump/clearance purposes).

There is an argument for drilling a narrower diameter borehole as an exploration borehole, for sampling and test pumping purposes, prior to drilling a main production borehole. This adds considerable extra expense to the project, however. Depending on the option (Section 3.4 and Chapter 5) selected, such an exploration borehole might be converted to a re-injection borehole, or re-drilled to a full diameter production borehole.

3.4.6 Yield of a Mine vs Pumping Costs

When designing a system of pumping boreholes, it should be remembered that their total discharge will be limited by at least two factors:

(i) the yield of each production borehole, which will be controlled by the hydraulic properties of the rocks and mine workings in the vicinity of the borehole and also by interference with nearby production and injection boreholes;

(ii) the hydraulic resource contained in the mine itself and the rate at which the mine as a whole is replenished by recharge.

It is not necessarily the case, therefore, that because one borehole can sustain a yield of, say, 40 L/s, the mine would necessarily yield a sustainable rate of 120 L/s if three boreholes were drilled.

Furthermore, there is a relationship between pumping water level and discharge rate, both for an individual borehole and the mine as a whole. Thus, for example, if one borehole produces 40 L/s, this may draw the minewater level down 40 m. If two boreholes produce 80 L/s, the minewater level may drop 170 m. While the total production rate has doubled, the pumping costs associated with production may have quadrupled (as the energy expended in pumping water up from greater depth will have increased). The figures cited above are examples only: the exact relationship between discharge and water level is not known and can only be established by a pumping test conducted on a real borehole. This is important to bear in mind when considering the drilling of additional production boreholes to expand the capacity of a geothermal system utilising minewater from a single mine.

3.5 Option Appraisal for Geothermal System

Several locations within the Kingshill colliery for wells and surface discharge to a passive minewater treatment facility were assessed. Geo-Options 1 - 5 (Appendix A3.2 Geo-Options 1 - 5; A3.2 Annex Drawings GEO-1 - GEO-5) were based on a doublet minewater geothermal system with production and injection wells, whereas Geo-Options 6 - 8 (Appendix A3.2 Geo-Options 6 - 8; A3.2 Annex Drawings GEO-6 - GEO-8) incorporate a new passive minewater treatment facility adjacent to a production well. Many opportunities and constraints were taken into consideration to highlight the preferred options to proceed with specific system design and economics, including:

  • Proximity to heat demand, i.e. consumers
  • Proximity to existing minewater treatment site
  • Available land area for development
  • Subsurface hydraulic connections of mines and subsequent modelling of thermal breakthrough
  • Subsurface hydraulic connections across faults
  • Complexities of directional drilling
  • Analysis of artesian conditions
  • Assessment of environmentally sensitive areas for drilling
  • Analysis of surface gradients for gravity flow through a potential passive mine-water treatment facility
  • Proximity to existing gas and electricity networks
  • Proximity to access roads
  • Potential environmental benefits to local communities

3.5.1 Modelling Thermal Breakthrough

Five abstraction - heat exchange - reinjection (geothermal doublet) scenarios have been identified. A number of critical assumptions have been made to enable analytical modelling to be carried out.

For each modelled geothermal well doublet option (Appendix A3.2), a 1-dimensional direct roadway connection scenario (Rodriguez & Diaz, 2009) and a 2-dimensional porous medium well doublet scenario (Banks, 2009, 2011) were evaluated to effectively delimit possible extreme responses of the aquifer. The reality will be somewhere in between (and arguably closer to the 1D roadway model than the 2D porous medium model) and can only be simulated further by site-specific numerical models. Appendix 3.2 demonstrates the methodology and results for each scenario.

3.5.2 Options Ruled Out for Locating Production and Injection Wells / Surface Discharge

Eight options for the installation of a geothermal well system have been considered. These are referred to as Geo-Options to avoid confusion with the District Heat Network (DHN) Options (Chapter 4) and Development Options (Chapter 5). Five of these involve geothermal well doublets, while the other three envision only production wells, coupled with passive minewater treatment and discharge to a surface watercourse. The eight scenarios are detailed in Appendix A3.2, and a consideration of passive minewater treatment is found in Appendix A3.5. Six of the eight options have been ruled out as being technically, environmentally and/or economically unfavourable. The remaining two favoured options are presented in Sections 3.4.3 and 3.4.4. These Geo-Options are then taken forward and integrated with the preferred DHN-Options in Chapter 5.

3.5.3 Geo-Option 3 (doublet system)

Description: Both production and injection wells drilled (potentially c. 380 m deep) into the WRSM seam, north of the main E-W fault and in the southern part of the Hartwood Home Farm study area. Artesian head in workings could be c. 20 m (given a ground elevation of c. +180 m OD).

Geo-Option 3 is illustrated in Drawing 3.13.

Table 3.3: Flow pathway analysis for Geo-Option 3, assumes abstraction at 50 L/s and 18°C, reinjection at 7°C.

1-D Discrete roadway model

2-D Porous medium well doublet model

Pathway 1: L = 1732 m

Pathway = 1245 m (direct)

Breakthrough time = 5.0 days

Abstraction temperature after 10 years = 7.3°C

Heat extracted from rock after 10 years = 61 kW

Breakthrough time = 5924 days (assuming thickness of porous zone = 30 m)

Abstraction temperature after 10 years = 18°C

Table 3.4: Advantages and disadvantages of Geo-Option 3

Pro

Con

Potentially artesian heads i.e. low pumping costs at the production well. Artesian head may not be as strong as in the case of options 1 and 2, thus fewer problems with drilling and reinjection.

Environmental difficulties in drilling with potentially artesian conditions.

Moderately long flow pathways separated by fault.

Likely need to reinject under pressure.

Drilling sites not in environmentally sensitive river valley bottoms.

Minewater resurgence is an existing environmental problem to the south of Allanton, raising the potential sensitivity of new workings in this area which do not address this issue.

Geo-Option 3 was judged to be the optimal doublet system due (i) to it being within the Hartwood Home Farm study area, (ii) having a moderately long subsurface flow pathway, (iii) less strong artesian potential than Geo-Options 1 or 2 and (iv) being located away from the river. There is, however, a significant risk of rapid thermal breakthrough which will need to be evaluated with detailed numerical modelling at the next stage of feasibility study.

Geo-Option 3 is the option progressed as Development Option 2 (Chapter 5).

3.5.4 Geo-Option 6 (production well with passive minewater treatment facility)

Description: Single production borehole to over c. 340 m depth (depending on ground elevation), into WRSM seam near Kingshill No. 1 colliery, or even from colliery shaft if still accessible (Coal Authority believe that the colliery shafts are sealed, but at present it is unknown how and at what depth). Passive minewater treatment facility on former colliery land, then discharge to a watercourse.

Geo-Option 6 is illustrated in Drawing 3.12.

Flow pathway analysis: None (no reinjection)

Table 3.5: Advantages and disadvantages of Geo-Option 6

Pro

Con

Static water level likely to be within 10 m of surface, thus low pumping costs (although pumping head will probably be deeper).

Capital cost of constructing treatment works, and its ongoing maintenance (though if passive, ongoing costs should be modest).

Abstraction from Woodmuir Smithy seam will decrease heads, and should (at least partially) relieve uncontrolled overflows / waterlogging at Allanton. Thus, an environmental benefit and benefit to community.

Acquisition of an environmental liability (potentially polluting discharge).

Proper treatment system to be installed.

Possible ongoing liability for uncontrolled outbursts of mine-water if pumping ceases in future.

No costs associated with injection boreholes. No ongoing maintenance of reinjection boreholes.

Uncertainty regarding low measured temperature at existing discharge of 13.9 °C.

Fewer issues with thermal breakthrough, thus MW yields should be sustainable over long period. Potential heat extracted from 50 L/s (if such a yield can be sustained) with 11°C temp drop = 2.3 MW.

There is some potential for drawing in progressively cooler water down dip with time - thus reducing abstraction temperature (insufficient data to quantify this possibility at present).

Geo-Option 6 was judged to be the optimal scenario for a passive minewater treatment facility due to its location in close proximity to the historic minewater lagoons and current minewater leakage and drainage ditch; plenty of brownfield space and old access roads.

Geo-Option 6 is the option progressed as Development Option 1 (Chapter 5).

3.5.5 Comparison of Favoured Options

Of the two favoured options, Geo-Option 6 is regarded as the most technically favoured option. Despite the necessity to construct a passive minewater treatment system, it avoids the technical risks of Geo-Option 3, and provides added value to local communities (minewater treatment and potentially alleviation of minewater breakout issues).

The main risks associated with Geo-Option 3 would be related to

(i) Drilling, grouting and reinjecting into workings with a (potentially) 20 m artesian head and

(ii) The significant risk of thermal feedback along mine roadways.

Artesian heads will make drilling and grouting rather challenging. If grout seals are inadequate, there is risk of leakage of contaminated minewater up the annulus to the surface, which could be difficult to control. Any boreholes would need to have a rigorous abandonment and sealing plan, in the event that the geothermal system ceases to operate in the future.

The comparative merits of the two options are considered further in Chapter 5, integrating the option appraisal from the DHN systems analysis.