Hill of Banchory geothermal energy project: feasibility report

5. Provisional Borehole Design

A future geothermal system serving Hill of Banchory will require a minimum of two deep boreholes - one for abstraction, the other for reinjection. As it is advantageous to be able to reverse the polarity of the system to combat clogging in reinjection boreholes, the basic design for both boreholes will be the same. The outline design presented here is based on direct experience over the last dozen years in the drilling of deep geothermal boreholes in northern England (see Manning et al. 2007), informed by wider insights from the geothermal sector worldwide.

Deep geothermal boreholes are essentially extremely deep water wells, and their design follows the same principles as commonplace shallow water wells, in that:

• The borehole must be engineered to stand open, without ingress of unwanted fine particles etc., and
• In the zone from which water production is desired, any means of holding the borehole open must allow for ingress of groundwater, while still preventing ingress of fine particles etc.

The key to achieving these two requirements is an effective borehole lining system, in which 'casing' (i.e. unperforated steel pipe) is installed from surface to the upper limit of the targeted production zone, with 'screen' (i.e. perforated pipe) being installed below, throughout the intended production zone. Where a rock is very strong (as is the case with granite) the screen may be dispensed with; this was done, for instance at Eastgate in Weardale, and the well was found to be standing open 6 years later without any problem. Dispensing with long runs of pipework saves on both costs and embedded carbon emissions, but a final decision on that can only be taken when the behaviour of the rock during drilling has been assessed in detail. Precautionary screening of deep sections can be achieved at lower cost (and lower notional emission) by re-using casing that has been remaindered from the offshore hydrocarbon industry.

Borehole lining requires that the installed casing be fixed in place to prevent unwanted migration of fluid outside the open lumen of the borehole. To this end, the annulus between casing and drilled borehole wall is typically plugged with 'grout', which may be a simple Portland cement or some more elaborate composite, e.g. including bentonite as an aid to impermeabilisation. In geothermal wells, a simple cement grout usually suffices, as this performs well at elevated temperatures, actually increasing in strength with temperature. A further feature of geothermal wells is that cement is typically installed through the full length of the cased zone (in the hydrocarbon sector, sometimes only a limited interval of a few hundred metres may be fully cemented). This is because the anticipated differential thermal expansion of steel casing and cement grout is best managed by minimising accommodation space; full grouting of successive concentric casings also serves a useful insulation purpose.

Figure 13: Schematic profile of drilled and cased diameters and cement runs for the range of possible geologies (i.e. allowing for a section through the Dalradian) that could be drilled at Banchory

Image: Cluff Geothermal

Of course, as the screened section must be open to allow water to enter the borehole, no cement is installed in the annulus between the screen and the borehole wall; this is either left empty (most common in geothermal boreholes) or else packed with a coarse, permeable sand / gravel filter pack. Design of a borehole essentially proceeds from bottom-up: the first thing to establish is the final screened diameter. A common choice is to select a standard steel pipe diameter of 9 ⁵/ ₈" (note that borehole components are still quoted in old units, because of US dominance of the sector). This in turn means that the drilled hole needs to be wide enough to receive pipe of this diameter; a 12" drilled diameter should usually provide enough clearance. This screened section is normally flush jointed (screw fit) continuously to join the run of unperforated pipe above the production zone all the way to surface, forming the pipe through which geothermal fluid will be induced to flow by pumping at a higher level. This innermost, narrowest string of pipe (perforated near the base, unperforated above) is often termed the "production casing". Depending on the characteristics of the overburden, one or more outer (larger diameter) casings may also be required, to exclude from the borehole any caving or fines-releasing strata. The number and diameter of such casings would depend on whether future boreholes at Banchory were commenced on the granite outcrop, or on adjoining Dalradian outcrop. If the borehole commences on granite, then a single outer casing might be all that is required, from surface down to about 20 - 30m until the hole is well established in sound granite. To allow for response to unanticipated conditions, this 'surface casing' is usually drilled and lined at a generously large diameter (the short length means that the costs are not excessive despite this). Drilling at 26" and casing at 20" is anticipated, and this would be fully cemented back to surface. If starting on the Dalradian, a secondary casing run would also be installed to prevent ingress of fine particulates (e.g. muscovite grains). This would typically be drilled at 17 ¹/ ₂" and lined with casing of 13 ³/ ₈", again cemented to surface. The production casing would then be drilled at 12", as already explained, and completed at 9 ⁵/ ₈". It is also possible to extend the productive zine by adding a narrower (e.g. 7") lower section to the hole, intersecting more fractures at higher temperature without committing to the full costs of completing at 9 ⁵/ ₈". Some of these options are summarised on Figure 13. What is not shown on the Figure is the possibility that the lower zones of the borehole be directionally drilled, to access the granite from a remote site starting on Dalradian outcrop, and / or to maximise intersections of sub-vertical fractures oriented favourably in relation to the local maximum compressive crustal stress azimuth to maximise the likelihood of tapping sufficient permeability. This detail would need to be established during a future project (see Chapter 19).

While it is typical for oil wells at these depths to be drilled using downhole motors and polycrystalline diamond compact ( PDC) bits, the high strength of granite typically demands use of special tri-cone 'rock-roller' bits and / or down-the-hole hammer, actuated either by compressed air or water. In granite, we can normally achieve adequate flushing of cuttings from the hole with air, water or foam, rather than the bentonite mud suspensions favoured in the hydrocarbon industry, thus avoiding the risk of clogging permeable fractures with mud cake.

Although drilling in granite is slower than in sedimentary strata, casing is generally swifter, so that the overall programme is not greatly attenuated. An indicative programme of about 6 weeks per well would be anticipated, allowing for preparation of the site (installing any liner required by SEPA, plus a hard core platform to receive the rig. The drilling compound would be enclosed with fencing to prevent casual access by livestock or passers-by.