7. Assessment of Existing Baseline Data
Potential fluid flow rates in the Hill of Fare granite
Fractures in granite
In granite, fluid flow is generally concentrated in fault and fracture networks due to the very low permeability of the rock mass. These faults and fractures are typically heteregenously distributed due to the long and complicated tectonic history of many granites. Fractures can either be distributed evenly throughout the rock or they can be tightly clustered in fracture zones with larger expanses of relatively intact rock between; different granite bodies will display fracture distributions somewhere between these two extremes. Since the faults and fractures are controls on permeability their properties affect how easy it is to transmit fluid through the granite. These properties are frequency, fracture length, orientation, connectivity, mineral fill, and aperture. The combination of these influences creates significant challenges to predicting the permeability of granite at depth.
Figure 22 shows examples of fracture systems intercepted by drilling in geothermal boreholes. Observations from these examples are generally consistent, in that the fractures appear in small discrete zones surrounded by larger areas of more intact granite. In the case of the Habenero project these fracture zones could be matched up between boreholes so although each fracture system is discrete they are extensive and can operate over a wide area within the granite (Wyborn 2010). Underground excavations related to radioactive waste disposal have encountered more evenly distributed fracture distribution within granite although still with significant variation (Olkiewicz 1979) however this study was at much shallower depth, being only within 400m of the surface.
Figure 22 shows three examples of fracture systems encountered in geothermal boreholes;
- a, is from Cooper Basin in Australia (Wyborn 2010) and shows three boreholes encountering the same fracture systems at different locations. Each of these fracture systems are separated by many tens of metres of relatively intact granite.
- b, shows the cumulative number of fractures encountered at depth within the granite by one of the boreholes in the Soultz-sous-Foret geothermal project in France (Dezayes et al. 2010). Here zones of higher fracture density are separated by longer stretches of granite of lower fracture density (i.e. more intact).
- c, is from the Rosemanowes geothermal project in Cornwall, UK (Richards et al. 1994). The RH12 and RH15 boreholes show the location of fracture zones which were encountered in addition to the proportion of fluid flow rate which is attributed to each fracture zone. The %flow values indicate that a high proportion of the fluid flow was located within a relatively small proportion of the fractures.
Figure 22: Examples of Fracture Systems in granites
These localised fracture zones can create concentrated zones of fluid flow or permeability. This is demonstrated in Figure 22, c, where in borehole RH12 70% of the fluid flow comes from a single fracture zone; similarly, in RH15 43% of the fluid flow comes from a single fracture zone. Such discrete zones of fluid flow were also noted in the Swedish granite HDR experiment (Jupe et al. 1992) where two fracture zones appeared to be responsible for most of the fluid flow across hundreds of metres of the borehole. These observations are repeated by other studies not related to geothermal. Barton et al. 1995 found that hydraulically conductive fractures are a subset of the overall fracture population with many being closed to any fluid flow. The Swedish radioactive waste underground laboratory in fractured granite (Olkiewicz 1979) found a relatively more even distribution of fractures but the fluid flow was still attributed to a small subset of these fractures (Black 2007).
Information on fractures and faults in Scottish granite is sparse as few comprehensive studies have been conducted (an exception being Thomas et al. 2004). Old heat flow borehole records have been used to gain information about the fractures of the other high heat production granites in the East Grampians of Scotland. As previously mentioned, these four boreholes were each 300m deep, and were drilled into the Ballater, Bennachie, Cairngorm, and Mount Battock granites. Three 10m cores from each borehole were taken at 90m, 190m, and 290m depth. From these cores the fractures were counted and their fracture densities (fractures per metre) calculated (Figure 23).
Figure 23: Fracture Density calculated from borehole cores of the high heat production East Grampian granites
It must be borne in mind that the fracture densities shown in Figure 23 are for the first few hundred metres of depth. This upper part of granite is known as the 'active' zone and is likely to be more fractured than the deeper geothermal target zones. Each of the East Grampian granites has at least one zone of high fracture density (i.e. >2 fractures per metre). The fracture density in these zones is similar to that of the high density areas of Soultz-sous-Forĕt (Figure 22b) which has had successful fluid circulation tests after further stimulation). That these zones exist within each of the East Grampian granites is positive as it indicates that such zones could also be found in the Hill of Fare Granite, possibly providing natural permeable zones. Nevertheless, it remains uncertain how these shallow fractures will correspond to those at the depths of interest.
Fracture system on the Hill of Fare granite
The first step in characterising the fracture patterns of the Hill of Fare granite would be a surface study of exposed fractures and faults. Unfortunately, initial field work showed that it would not be possible to collect robust data on the fractures without a much more detailed study requiring significantly more time and resources than the current study afforded. This could involve using a Lidar system at abandoned quarries in which the fracture orientations and distributions could be identified and analysed for otherwise inaccessible faces.
Despite the limited exposure, basic observations were still feasible at several abandoned flooded quarries. There appears to be a general trend for faults and fractures to be contained within a single slip surface which has minimal surrounding damage. General sets of faults and fracture appear to strike either North to South or West to East.
The Hill of Fare granite was emplaced at the same time as the other high heat producing East Grampian granites (shown on Figure ES.2). The shared tectonic history between these granites suggests that the fractures of the Hill of Fare would likely be similar to those shown on Figure 22. Nevertheless, the emplacement of each granite creates unique conditions so while the other granites are analogous they cannot be assumed to be identical.
Granite geothermal systems borehole flow rates: case studies
The amount of energy that can be extracted from a geothermal system is dependent upon the flow rate from the production well. Therefore, in order to investigate the energy output from a possible geothermal system it is necessary to provide some constraints on possible flow rates through fracture systems in the Hill of Fare granite. There are insufficient site-specific data to constrain likely flow rates, as explained above. Consequently, we have used several case studies to inform what fluid flow rates have been achieved in previous geothermal demonstrations and operational schemes in fractured granite. The granites in these schemes will have already undergone an appraisal process like that presented here, and thus will already have been targeted for areas that are likely to be amenable to geothermal development. These case studies cannot be considered close analogues to the Hill of Fare, but are considered as examples should the Hill of Fare granite proceed to a more detailed pilot borehole appraisal in future.
In some cases granites can produce suitably high flow rates without stimulation (Manning et al. 2007). However, in most cases, hydraulic stimulation has been used to increase the permeability of the fractured granite. Following such stimulation, Evans et al. (2005) and Jupe et al. (1992) both reported increases in permeability of several orders of magnitude, while Chabora (2012) reported an increase in productivity of 10 times. Low-magnitude induced seismicity can be anticipated during such stimulation, though never at levels likely to cause any problems (Majer et al. 2007).
Each of the case studies listed below will have distinct fault/fracture distributions. However consistent trends across the case studies will inform upper bounds to fluid flow rates that could be expected from a successful geothermal scheme in the Hill of Fare granite.
- Cooper Basin, Australia
The 1MWe Habanero Pilot plant is run by Geodynamics Ltd in the Cooper Basin in Australia. The plant targets natural occurring fractures, which are then hydraulically stimulated, in a granite body that is buried beneath several kilometres of sedimentary rock. The plant operated a trial at 4km depth where geothermal fluids were produced at 215⁰C at flow rates of 19 L/s. The planned flow rate for a production borehole in a full scale commercial plant is 70-100 L/s.
During the stimulation testing in the Cooper Basin granite, fluids were injected into the granite at flow rates of between 10-40 L/s (Baisch et al 2006). The tests also found that a single fracture within the granite could host flow rates of up to 30 L/s (Wyborn 2010).
- Soultz-sous-Foret, France
Soultz-sous-Foret ( SSF) in France has been the location of the main European test site for Enhanced Geothermal Systems ( EGS). A four month long trial circulated water between two boreholes (Gerard et al 2002) achieving a stable flow rate of 25 L/s.
During hydraulic stimulation tests at SSF fluids were successfully injected into the fractured granite at flow rates of 5 - 40 L/s.
- Rosemanowes, U.K.
This experimental EGS reservoir was created in the south-west of the UK in 1985. Stimulation of the fractures successfully linked an injection and production well with a separation of 133m. Testing continued for 300 days of circulation and achieved an average flow rate of 8.5 L/s (Richards et al. 1994), though ranging up to 20-35 L/s at various stages of testing. The planned commercial production flow rate for the scheme was 50 L/s, but the project never progressed beyond test phase.
- Fjallbacka, Swe d en
Injection tests were carried out at an EGS test site in a jointed granite in Sweden (Jupe et al. 1992). Most of the fluid flow occurred at two highly-fractured hydraulically-conductive zones at 452m and 472m depth. A total of 400m 3 of fluid was injected at flow rates of between 20-30 L/s.
- Weardale, U.K.
An exploratory borehole targeted a fault known as the "Slitt Vein" in the Weardale granite (Manning et al. 2007). The granite is buried beneath several hundred metres of Carboniferous sedimentary rocks. A pumping test was carried out upon interception with the Slitt Vein to test the permeability of the fault and fracture system. Groundwater was pumped at 17 L/s for 24 hours with only a 1m drop in head suggesting a highly permeable active fracture system that could be sustained at far greater rates as part of a doublet well system (Younger and Manning 2010), with no need for stimulation. This underlines the value of deliberately targeting large faults that are inferred to be appropriately aligned with the present-day crustal stress field.
- Fracture oil reservoirs
Fractured basement rock (i.e. hard, crystalline rock lacking matrix permeability) is an uncommon but established target for oil and gas exploration. These hydrocarbon reservoirs are analogous to the fluid flow regime of a geothermal target in fractured granite. Experience in the hydrocarbon reservoirs suggest individual fault/fracture zones are capable of providing flow rates of between 1.5-10 L/s without any reported stimulation (Li et al. 2004, Tandom et al. 1999, Donofrio 1998). Individual boreholes within these reservoirs have experienced flow rates of up to 37 L/s when multiple fault/fracture zones are intercepted (Tandom et al. 1999).
Constraining fluid flow rates for the Hill of Fare granites
The case studies above consistently show test circulation or injection fluid flow rates in the tens of litres per second. The test circulations ranged from a low average of 8.5 L/s at Rosemanowes, UK to 25 L/s at SSF, France, although Rosemanowes circulation did range up to 35l/s for a limited time. These test circulations indicated how much fluid it is feasible to circulate in a fractured granite if favourable conditions are found, such as those in Weardale, U.K where flow rates of up to 17 L/s were achieved with minimal head change in a pumping test (Younger and Manning 2010), without any stimulation of the fracture network. Additionally, the case studies of hydraulic stimulation of fractures show flow rates during injection that are similar to the above case studies for fluid circulation. The examples shown here indicate that a successful borehole-doublet in fracture granite would achieve flow rates of tens of litres per second.
Three flow rate scenarios were chosen with values informed from the case studies above (Table 6). These scenarios are not informed from any geological information from the Hill of Fare but are designed to be representative of a generic geothermal scheme in a fractured granite. As such they are not predictions of flow rates that could occur in the Hill of Fare granite, but will be used in subsequent upstream modelling to investigate how these hypothetical scenarios impact upon feasibility of the geothermal scheme.
Table 6: Assumed flow rate scenarios for a borehole-doublet scheme in fractured granite
Flow rate (l/s)