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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Hot Dry Rock Geothermal Energy Research at the Camborne School of Mines by ROGER PARKER, Project Director Camborne School of Mines Geothermal Energy Project, Rosemanowes Quarry, Herniss, Penryn, Cornwall, UK Introduction The Camborne School of Mines (CSM) Hot Dry placed. If-there is a need to incorporate specific localised Rock Geothermal Energy project in the period of 1977-89 geological structures in creating the reservoir, exploration has been concerned mainly with the technology of the costs (and the chances of a sterile operation) increase. development and characterisation of Hot Dry Rock Conversely, there is a need to avoid such structures in (HDR) reservoirs in a jointed granite. There has been no choosing the site, if they would tend to jeopardise the attempt to demonstrate the exploitation of the energy operation. extracted. The UK Department of Energy has been re- Rosemanowes Quarry was chosen because it was on sponsible for providing most of the funding, but the Com- the exposed Carnmenellis granite. and did not have a mission of the European Communities provided sig- ma-jor geological feature (such as a fault) at the surface. nificant support until 1986. The absence of sedimentary or metamorphic cover rock In Phase I (1977-1980). boreholes 300 m deep were has made installation of the comprehensive microseismic drilled in the Carnmenellis granite at Rosemanowes Quar- network cheaper and more effective. ry. near Penryn in Cornwall. It was demonstrated that it The rock is granite. with textures changing from was possible to connect the boreholes by hydraulic stimu- porphyritic to equigranular at about 2 km. The base of the lation of natural joints in the granite, and to circulate granite extends well below a depth of 9 km. In situ me- water through these joints (Batchelor, 1982). chanical properties are: Phase 2 (1980-1988) was carried out in three parts at lini-axial compressive strength: Rosemanowes. with the aim of investigating reservoir 103 MPa + 32 MPa.'km development at a depth of about 2 km, which was con- Young's modulus: 54 GPa + 4 GPa, km sidered to provide conditions reasonably representative of Poisson's ratio : 0.2 2-0.27 those expected at the greater depths required for commer- Density: 2640 kgsm' cial exploitation. Hydraulic stimulations using water and Two main vertical joint sets (northeast-southwest. a medium viscosity gel were used to create the reservoir. parallel to the trend of tin! copper lode mineralisation. and long periods of circulation of the reservoir were used and northwest-southeast, parallel to post-granite exten- to establish its hydraulic and thermal characteristics. sion and strike slip faults known as "cross-courses") have Phase 3 began in 1988, having as its main objective been identified from surface mapping. These joint sets the development in Cornwall ofa prototype of a commer- (although with a broad range ofstrikes) have been identified on BHTV logs to a depth of 2.6 km, and microseismic tem for generating electricity. For an acceptable data indicate their continuation to at least 3.5 km. lifetime. this prototype would require a reservoir 6 km deep occupying a rock volume of 300 million m: pro- The relationship of stress distribution to depth has ducing water at 200°C. at a rate of 75 I! s. been measured at Rosemanowes in considerable detail to a depth of 2.5 km, and these measurements have been complemented by measurements in local tin mines. Pine and Batchelor (1984) sunimarised the relationship for in HDR Environment at Rosemanowes situ stresses (in MPa) in the Carmenellis granite with Throughout the programme. the aim has been to depth (z. in km): produce a technology which is as widely applicable as UH= Is+ 287 possible. Return from investment in HDR exploitation is u/, = 6 + 12 7 unlikely to be high enough to justify high exploration ~y = 26 7 costs, and therefore the technology must be capable of Subsequent measurements at 2.5 km confirmed this adapting to the geological environment in which it is relationship. Geothermal Resources Council BULLETIN October 1989 Page 3 Three-dimensional heat flow models, based on exten- hydraulic stimulation and circulation. and have related it sive heat flow measurements and gravity surveys, indicate to the changes of in situ stress anisotropy with depth in an almost linear dependence of temperature on depth in Cornwall. the upper 7 km of crust over large portions of the Cornu- A third well (RH 15) was drilled in Phase 28 of the bian granite batholith. With an average surface tempera- CS M prqject (1 983-86), on a spiral trajectory to a depth of ture of 10°C. this results in a relationship for regionselose 2600 m, at which the bottom hole temperature was 100°C. to Rosemanowes: The aim was to intersect the microseismic zones which had -1- = 10 + 35 %, indicated downward growth of the reservoir from RH12 in Phase 2A. In order to achieve a good connection with whereTis the temperaturein"Catadepth of7 km(CSM, the injection well (R H 12). it was necessary to stimulate the 1989). tem from RH15, which was to be the new production In situ hydraulic properties have been measured at well. -1-0 reduce the tendency to leak-offand to increase the Rosemanowes at depths up to 2 km. before major hydrau- chance of jacking open the joints. rather than shear-slip- lic in.jections commenced. Low flow rate hydraulic tests at page, 5500 m' of an intermediate viscosity gel (50 ep) was low injection pressures indicated permeabilities between 1 in.jected into R H 15 at an average flow rate of 200 I, s. The and 10 p D at up to 0.7 MPa fluid overpressure. Then injection wellhead pressure was 14 to 15 MPa. Mieroseis- permeabilities rose to 60 p D, prior to onset of significant mic activity was much lower, and was confined to a more discontinuous beha.viour at over 5 MPa. restricted tube-shaped envelope extending vertically mainly between RH15and RH12(Parker, 1989a).Thevolume of this microseismic envelope was I million m: and sub- Phase Reservoir Creation and Characterisation 2: sequent experience with circulation of the reservoir In Phase2A(1980-83), two wells(RH1 I and RH12) created indicates that an effective reservoir rock volume of were drilled to a depth of 2 100 m entirely through granite, 5 to 10 million mi was produced by this viscous gel stimu- deviated to an angle of 30" from the vertical in the lower lation in 1985 (Figure I). section. They were separated vertically by 300 m at full The remaining part of Phase 2B (1985-86) and the depth, where a bottom hole temperature of 79°C was whole of Phase 2C (1986-88) were concerned with a con- recorded. Explosives were used to pre-treat the well to tinuous circulation of the reservoir stimulated in 1985, allow better water access from the borehole into the using a number of diagnostic methods to characterise the granite, but the joint stimulation was hydraulic, using reservoir. This work represents the longest continuous 26.000 m' of water injected into the injection well (RH 12) circulation of any HDR reservoir (Parker, 1989b). at flow rates up to 100 1 s. generating a wellhead pressure This extended circulation programme can be divided of 14 MPa (Hatchelor, 1983; CSM. 1987). into three stages: This hydraulic stimulation established a poor con- I. A gradual increase in the injection flow rate, using nection between RH 12 and RH I I and created a large periods of up to 6 weeks at each flow rate step in- stimulated region, below the two wells, whose predomi- e rea se, to a I Io w a p prox i mat e I y stead y-s t a t e c o nd i- nantl!. downward growth persisted throughout the subse- tions to be achieved at each step (Figure 2). Water quent circulation of the reservoir during Phase 2A. The losses remained fairly constant throughout, at about importance of the installation ofa comprehensive miero- 20 percent, and impedance reached a minimum of tem in the monitoring ofthese stimula- about 0.5 MPa! kg,'s at the maximum injection flow tion and circulation developments cannot be over- rate of35 1 IS.At flow rates as high as this. requiring an emphasised. injection pressure of 1 1.5 M Pa, the rate of water loss Circulation following the main stimulation gave an increased and microseismic activity indicated down- average water recovery of 3 I percent at an average injee- ward reservoir growth similar to that experienced in tion flow rate of 24 1 's.

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