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HOT DRY ROCK - A EUROPEAN PERSPECTIVE

J.D. Garnish

Energy Technology Support Unit AERE Harwell, Oxon, England

ABSTRACT permeability but natural hydrothermal circulation in existing fractures. For the purposes of this review HDR Research into hot dry rock technology is being pursued research is defined narrowly as work directed towards the actively in several European countries. All these projects creation of a heat transfer zone in otherwise impermeable employ variations of the basic hydrofracturing approach. rock, and the extraction of useful heat by the circulation of Following proof of the concept at Los Alamos, the UK fluid through that zone. project has concentrated on understanding the rock Within the relatively small community of geothermal mechanics of reservoir creation. The most important outcome has been recognition of the importance of the specialists, there has been understandable scepticism of such activities. Most geothermal development is aimed at in-situ stress field and its effect on the mechanism of the commercial exploitation of natural resources in a fracture growth. The emphasis is now being placed on climate where competing fuels are still readily and cheaply reducing impedance and flow losses within a very large available. HDR technology, by contrast, is still very much at fracture system. A full-depth prototype is expected to be the research stage and is unlikely to come into its own much build in Europe within the next decade. before the turn of the century. Taking the longer view, INTRODUCTION however, the prospect of a technology that could make available a large indigenous energy source (even though it All commercial exploitation of to may not be as cheap as some available today) is one that date has relied on the presence of extractable water in the justifies a substantial research effort. It is for this reason rocks that constitute the immediate source of the heat. This that the governments of the USA, Japan and several requirement for natural permeability places a severe western European nations, as well as the Commission of restriction on the locations where geothermal energy can the European Communities, are supporting work on HDR. be exploited and, in many instances, on the grade of heat available. Unfortunately, the highest grade of resources are THE ECONOMICS OF WDR found at the plate margins, while the majority of the world’s population live in tectonically stable areas where Some very simple calculations are adequate to show aquifers deep enough to be useful occur comparatively that the thermal resource contained in the accessible rocks rarely. of the Earth’s crust is vast. The heat obtainable from If methods could be found to permit heat extraction cooling one cubic kilometre of rock by 1”C is equivalent to from rocks that are not naturally permeable, especially the the energy content of 70 000 tonnes of (Smith, 1973). crystalline basement rocks that underlie most of the land The implication of this is that the heat contained in the mass, then granites of southwest England, for example, is equivalent -the available resource would be increased by several to that of the UK’s entire known coal reserves (Batchelor, orders of magnitude, 1982a). Similar comparisons would be possible in any other -geothermal resources would become exploitable in all country. It is clear that even if only a tiny fraction of this countries and in almost all locations, and heat is in practice recoverable, .the basic resource is very -temperatures above 150”C would be accessible almost large indeed. everywhere, offering the option of electricity Evidence of a large resource would by itself be of little generation. significance, however, unless there were reason to think The problem of extracting heat from impermeable that it could be exploited economically. There are rocks has become known as the ‘Hot Dry Rock’ (HDR) indications that it may be, but the economics are hard to problem. Clearly, there is no hard and fast division between substantiate in rigorous quantitative terms in advance of ‘permeable’ and ‘impermeable’ rocks, and there are demonstration of the technology. All that is possible at conventional geothermal resources with low matrix present is to establish certain target parameters that the

329 Hot Dry Rock - A European Perspective

US on-shore oil & gas drilling

I I I I I I1 2 3 L 5 6 DEPTH, z (km)

Figure 1. Trends of drilling cost with depth technology must achieve, estimate the costs of attaining Combining versions of equations (1) and (2) those parameters and so derive a range of costs for the heat appropriate to specific case studies allows a first estimate of produced. heat costs to be derived as a function of reservoir depth and Fortunately, the problem is not entirely open-ended. geothermal gradient. In the studies published so far, the Though there is little experience of deep drilling in hard results are of the form shown in Figure 2; see, for example, rock, and costs are therefore difficult to specify with any Garnish (1976), and Armstead and Tester (1985). While precision, enough has been done to allow some estimation absolute values of costs depend on both operational and of trends, which can be compared with the extensive institutional factors (e.g. production flow rate, interest on experience ‘world-wide in deep hydrocarbon drilling. capital, etc.), the general form of these curves differs very Various algorithms have been proposed, typically of the little from one study to another. Note particularly the steep form increase of unit costs at shallow depths (actually at Drilling cost A.z.exp(B.z) (1) temperatures below a threshold limit), and the rather flat minima showing comparative insensitivity of output costs where A and B are constants, and z represents depth. (See, to depth at higher temperatures. These factors suggest that for example, Milora and Tester (197QAppendix 2. Figure for the range of geothermal gradients likely to be 1 of the present paper shows some comparative data.) encountered in most basement rocks (i) HDR would not be Terms may be added to cover site preparation, surface plant cost effective below about 100°C and (ii), because costs start costs, etc., but in most cases drilling costs will be the to rise again at greater depths, that there is little to be dominant factor and could account for as much as 70 gained by drilling deeper than about G to 8 km. In practice, percent of total costs. The extra terms usually show only a such depths have in any case rarely been attained in weak dependence on z, the reservoir depth. The heat crystalline rock, so for estimating purposes a depth limit of recoverable, on the other hand, can be represented by a about 7 km is usually assumed. function of the form: A maximum output temperature in any particular Heat recoverable =D.G. (z - ZO)* (2) location, or the depth necessary to reach a required where D is a constant, G is the geothermal gradient and zo temperature, can now be calculated and the costs estimated. represents the depth at which the minimum temperature Hence, we can calculate how much heat must be sold or useful for a particular operation can be reached (Garnish, electricity produced during the reservoir lifetime for the 1976). scheme to be economic. From this, and the required mode

330 J.D. Garnish

of use of the heat (and thus the reject temperature), the can be created in granite with the parameters given above necessary production flow rate can be specified. The and exploited by a single pair of boreholes, then this could remaining parameters - effective heat transfer and result in electricity generating costs in the range 3 to 5 surface and reservoir volume for a life time commensurate p/kWh (ETSU, 1982). If the same technologycould then be with the economic calculations - are then easily derived. applied to basement rocks beneath a centre of population, For conditions in northern Europe, the following additional sales of heat from a co-generation plant might parameters would seem to be the minimum necessary for substantially improve these figures. The major uncertainty an HDR scheme capable of supplying energy at costs (perhaps as high as 50 percent) lies in the estimation of the competitive with conventional fuels: costs of drilling to 6000 m in granite (see Figure 1). Another approach to reducing overall costs is to use Flow rate 50-75 l/s (800-1200gpm) ) the initial capital more effectively; this is the approach Effective surface area 2 x loGmz ) (3) Effective rock volume 2 x m3 adopted in US studies based on extrapolation of data from lo8 ) the Los Alamos HDR project (Murphy and others, 1984). These studies assume a more highly developed technology, Such a scheme would permit recovery of 30 to 50 MWt of with a regular pattern of multiple boreholes exploiting a heat over 20-plus years at 150 to 200°C, allowing planar reservoir to produce a scheme with an output of 75 generation of 3 to 5 MWe. MWe. In this way, a smaller number of holes is required for One further factor needs to be taken into account, the a given output so the initial cost per unit is reduced. work required to circulate fluid through the reservoir. Estimated costs from this work are in the range 4 to 5 Clearly there must be an upper limit to this if the pumping Q/kWh. At the present stage of development of the power is not to form too large a fraction of the useful technology, as the authors admit, the realism of this output; an important aspect of current research is to reduce approach-with its assumptions about the regularity of reservoir impedance to about 0.1 MPa/l/s (1 psi/gpm) underground structures-must be open to question, but it and hence restrict pumping power to around 10 percent of does illustrate the order of costs that might be achieved in the recoverable electrical output. favourable circumstances. It will be clear, therefore, that any statment of the All that can be said with certainty about the economics economics of HDR systems must at present be couched in of HDR at the present time is that there is a prospect of terms of ". . . if these parameters can be attained, then the recovering heat (and generating electricity) economically economics will be . . ." and few absolute statements are provided the technology can be developed to prodace possible. Calculations for the UK suggest that, if a reservoir reservoirs with the required parameters and life times. The main thrust of research has therefore been directed towards the problem of reservoir creation.

THE BASIS OF HDR RESEARCH Serious research into HDR began only in the early 1970s, at Los Alamos, New Mexico, in the USA. That work, and all the other projects that have developed as a result, centers around the concept of using hydraulic pressure to create an interlinked fracture system in an impermeable rock (hydrofracturing) and thereby to link an input and output borehole. It is perhaps worth noting that in 1978/9 six countries (Federal Republic of Germany, Japan, Sweden, Switzerland, UK and USA) collaborated in an IEA study known as MAGES (Man Made Geothermal Energy Systems). This was intended as a 'brainstorming session' to see whether any other approaches were worth trying. The conclusion was that, though other methods might be valid in the future, for the time being hydrofracturing represented the best way forward (IEA, 1980). During the early years of the Los Alamos project the process of hydraulic stimulation, which is widely used in the oil industry, had been visualised as the creation of an artificial fracture in a homogeneous rock mass by the application of hydraulic pressure, the fracture being created by tensile failure of the intact rock and subsequently held open by 'jacking' forces due to the water pressure. Basic 3 I 5 6 7 theory suggests that the fracture will open against the DEPTH (km) minimum prinicpal stress (Le. it will parallel the direction of maximum stress) and, as the greatest and least stresses in Figure 2. Generalized trends of unit output costs with depth rocks at depth are usually horizontal, the fracture will grow

331 Hot Dry Rock - A European Perspective

as a vertically oriented disc - the classic ’penny-shaped’ it might be possible to re-open natural fractures lying in frac. Furthermore, since the compressive stress increases directions other than that of the maximum stress and hence with depth at a rate greater than the hydrostatic gradient, to develop several different flow paths between wells the fracture would be expected to grow upwards. These (Cornet, 1981). This is theoretically possible, and can be early ideas assumed the creation of a single artificial planar demonstrated in the laboratory, but there is considerable fracture. scepticism whether fractures can be made to propagate in The Los Alamos project team opted to work directly less preferred directions in a real basement rock over the on a full-scale, full-depth system in granite - first at 3000 distances of several hundred metres implied in a full-scale m (200°C) and later at 4500 m (>325”C). Though the team HDR scheme. The field work, at Le Mayet de Montagne in was able to show that the basic concept was valid, by driving central France, involved connecting a pair of 200 m wells a 60 kWe turbogenerator for a couple of months from heat about 30 m apart and then linking in a third well at the apex derived from thegranite in their 3000 m (Phase I) reservoir of an equilateral triangle (Cornet and others, 1983). This and by extracting heat for periods of up to a year, significant was at first interpreted as showing the intersection of two problems remained to be solved - particularly those of vertical fractures in a T-shape and was taken as a obtaining adequate heat-transfer surface and a low demonstration of the validity of the concept, but it is now impedance. The deeper reservoir has given rise to even clear that at this shallow depth the linking fracture is greater difficulties; at the time of writing, one hole is being sub-horizontal. The concept thus remains unproven, and sidetracked in an attempt to establish a good connection. plans for a full-scale HDR project at 4500 m Though the work at Los Alamos has been widely reported (ENERGEROC) have been shelved. and is well-known, the related work in Europe is perhaps less widely understood and for that reason forms the main THE CSM PROJECT topic of this contribution. The UK project originated from a re-analysis of the early results from Los Alamos. Batchelor, of Camborne HDR RESEARCH IN EUROPE School of Mines (CSM), recognized that these results The European projects owe much to the pioneering implied a much more complex picture of reservoir creation studies at Los Alamos but have so far been undertaken on a than is suggested by the simple theory, and the project more modest scale. Experiments on the nature of under his direction at Rosemanowes Quarry in Cornwall hydrofracturing in granite, the properties of the fractures has arisen from these ideas. For convenience, it is referred so formed and methods of characterizing those fractures to as the CSM project. It is designed to test the concept that have been mounted at depths of 200 to 300 m in the UK, hydraulic injection opens up existing fractures and planes France and in the Federal Republic of Germany (FRG). In of weakness (‘joints’) and can thereby create suitable heat the UK, work has progressed to the creation of a research exchange surfaces in a massive granite. It has successfully reservoir at intermediate depth, 2000 m, while created a very large reservoir but has raised a number of hydrofracture research in a single borehole has been carried fundamental questions about the nature of the flow-paths out in FRG at a depth of 3300 m. created at depth and the feasibility of controlling reservoir These European ventures have arisen within national conditions. The rock stress regime, now recognized as programmes, and have been co-ordinated only via scientific being the controlling parameter in reservoir development, contacts and discussions. The Commission of the European is believed to be representative of that underlying much of Communities (CEC) has supported much of this work and Europe and, probably, most tectonically stable continental has provided an invaluable forum for such liaison. areas. In view of its recognized importance, it is worth Work in FRG has taken place at two sites: a test of describing some of this work in more detail. fracture formation and characterization at about 250 m Phase I of the CSM experiment (1977-80) was a depth in the Falkenberg grainte (Rummel and small-scale field trial aimed at linking four boreholes at a Kappelmeyer, 1983), and a much larger scale operation at depth of 250 to 300 m. The earlier work at Los Alamos had Urach in the Swabian Alps. The latter involved a single already shown that boreholes could be linked by borehole through 1600 m of sedimentary overburden and hydraylically stimulated fractures (at 3000 m, 200°C) 1700 m into fractured gneissic basement, with a bottom thoughthe impedance to flow was unacceptably high. Much hole temperature of 143°C. A series of hydrofracturing of this impedance was concentrated in the zones tests were performed, largely as a means of gaining immediately surrounding the boreholes. One objective of experience of the process, and these resulted in a the CSM Phase I was to show that the use of controlled connection via inclined fractures between two zones in the explosive stimulation could destroy the stress borehole. Though consideration has been given to the concentration at the wellbore and initiate a number of small .installation of a co-axial completion to permit circulation, fractures (ca. 1 m long), which could then be stimulated the effective heat transfer surface is very small and it seems hydraulically to provide a low-impedance link between the unlikely that this will lead to a practical HDR operation. boreholes by way of the natural joint system (Batchelor and Full details of the work have been published by Haenel others, 1980). (1982). This work was successful in all its aims. The four The French have been following an interesting boreholes were linked over a horizontal distance of 40 m, variation on the basic theme. Cornet, of 1’Institut de and explosive stimulation was shown to reduce the system Physique du Globe in Paris, has been attempting to test in impedance by a factor of 50 relative to that achieved (both the field the idea that by careful control of pressure and flow at CSM and Los Alamos) by hydraulic fracturing alone.

332 J.D. Garnish

The problems in the second category (failure of logging tools and drilling equipment; difficulties with cables, well completions and packers; etc.) are peripheral to the main HDR problem. Though they are severe, solutions to many of these problems are already being devised in other programmes (e.g. for high-enthalpy natural aquifers). The view was taken in the UK that such problems are secondary to the main one - that of whether or not the natural joint system could be stimulated to an adequate extent - and that the next stage of the work should avoid high temperatures. It was decided, therefore, that Phase I1 should be sited at an intermediate depth (ca. 2000 m) where the temperature would be only about 80°C. This would also keep access costs (drilling, etc.) to an acceptable level while permitting operation in what should be a representative stress regime. The penalty, of course, is that temperature- dependent aspects such as geochemistry or thermal drawdown effects cannot be evaluated directly. With this exception, Phase I1 is aimed at the construction and operation of a reservoir with characteristics that, if reproduced at depths where the temperature is 200" Cy would result in a commercially viable system. These characteristics were set out on page 331. LOWER HEMISPHERE POLAR MUAL AREA STEREONET Phase IIa lasted from 1980 to 1983. During this period, two wells were drilled to a vertical depth of ca. 2000 m. The / lower portions were deviated to 30 degrees from the vertical with the upper well (RH11) in the same plane as, and 350 m vertically above, the lower one (RH12). As at Figure 3. Orientations of joint sets, well directions and principal stresses Los Alamos, this configuration was chosen to maximize the number of vertical joints intersected while keeping drilling problems (and costs) within acceptable limits (Batchelor, The lowest impedance value achieved was close to that 1982b). Deviation was restricted to 30 degrees in view of thought to be necessary for a commercially viable system the rapid increase in drilling problems experienced at Los (0.1 MPa/ l/s). Alamos when the inclination built above this value. In several senses, however, the conditions of this As theory suggests that a hydraulic fracture will tend experiment were unrepresentative of those to be to rise as it propagates, the plan was to fracture the lower encountered in a full-depth system. At depths greater than well and allow the stimulated zone so generated in the 400 to 500 m the minimum principal stress will normally vertical joints to rise and intersect the upper well. be horizontal (and hence fractures will open in a vertical Prior to the drilling phase no data were available on plane), but at shallower depths the relative magnitude of stress magnitudes or directions at depth in the Cornish vertical and horizontal stresses is site-specific and granite, though it was assumed that the directions were unpredictable. In Cornwall at 300 m the direction of likely to coincide with those of the natural fractures. These minimum stress is vertical; consequently, the fractures that had been mapped from surface exposures (see Figure 3). As were opened in this shallow experiment were essentially the north northwest-south southeast trending joint set (Set horizontal. It is now recognized that the behaviour of such 1) gives rise to hot water flows in the local mines, it was fractures is fundamentally different from those at depth assumed that this set was subject to the lesser constraint (mode of opening, consequent water losses, etc.) but the and should be more readily stimulated. To cater for the results of Phase I gave sufficient confidence in the possibility that the orthogonal set might also be stimulated, experimental procedures to permit a start to be made on however, the well direction was chosen to cross both joint Phase 11. sets (see Figure 3). Phase I1 of the CSM project, which is still under way, is Subsequent stress measurements in-hole, supported a 7-year programme aimed at the creation and by later measurements in the mines (Pine, Tunbridge and management of a full-scale reservoir. Drawing on Kwakwa, 1983; Pine, Ledingham and Merrifield, 1983) experience gained at Los Alamos, however, it clear that the showed that the stresses are anisotropic (see Figure 4).This problems encountered there could be separated into two was expected, though the degree of anisotropy seems categories: unusually high. A more important finding was that the -those arising from the properties of the reservoir rocks direction of maximum principal stress is offset about 20 to ('rock mechanics'), and 30 degrees west from the Set 1 joint direction (Figure 3) -those arising as a result of the high in-hole temperatures and that the azimuth chosen for the wells coincides almost (200°C in the first reservoir, >325"C in the second). exactly with that of the stress. The lack of coincidence

333 Hot Dry Rock - A European Perspective

connection had been established, the reservoir was developed by pumping at rates of 10 to 50 I/s, with Overburden stress uv - Hydrofracrurin wellhead pressures generally less than 10 MPa (Batchelor, Min. horir. ski u -.a 1983). MOR. twrir. srress 0: ---a Overcoring Development of the reservoir was followed primarily Averoge s1resses by microseismic monitoring - listening to and locating the 2 I standard dev. - 400 - noises emitted by the fracture surfaces. Development of a fully computerized monitoring net, consisting of four sets of surface-mounted accelerometers and a string of 600 - hydrophones downhole in RH1 1, permitted on-line data capture, analysis and location. By this means, some 30,000 locatable microseismic events were recorded during the 800 - various stages of reservoir development, of which nearly EY 5000 were locatable with an accuracy generally better than I *20 m except at the greatest depths. The envelope defined b- 1000 - by the microseismic events eventually encompassed a 0 volume of GOO million m3, while tracer tests and the lack of J a observable thermal drawdown pointed to an effective 0 \ swept surface area of 1 to 5 million m2(CSM, 1984). In F 1200 - a \ W those terms, the experiment appeared to have met its > objectives, but a number of problems and unexpected o! I \ .\ effects were encountered. 1400 - I\ \ During the stimulation that took palce between November 1982 and March 1983, the structure that was generated grew to substantial proportions. It was tightly I600 - confined by the stress field to a pseudo-planar structure several hundred metres thick, and its major axis, as expected, was orientated vertically. Some upward growth I800 - certainly occurred, allowing a connection between the wells, but unexpectedly the microseismic mapping showed \ that the bulk of the structure grew downwards. It eventually ibi 1 rI- I I reached a depth of km, 2.5 km below the wells. Figure 20 40 60 80 loo 120 4.5 5 shows a composite view of the microseismic locations. STRESS (MPa) Though the downward growth could not be explained Figure 4. Magnitudes of in-situ stresses in the granite by current theories of hydraulic stimulation, an anlysis by the CSM group of the reaction to be expected when jointed between stress directions and joint sets has been decisive in rock with anisotropic in-situ stresses is subjected to high the manner of reservoir development, as we shall see, and pressure fluid injections showed how either upward or the coincidence of well and stress directions has influenced downward vertical growth could occur (Pine and Batchelor, the degree of connection between the wells. The open 1984). This analysis showed that shearing of the joints sections of both holes were investigated first by low- would occur in preference to, and at lower overpressures pressure hydraulic tests. A number of significantly than, the tensile ('jacking') mechanism assumed in

, fractured zones were identified, but it was decided to conventional analyses. This shearing would act on the joint initiate a new fractured zone about 100 m long in the tight set subject to the greater shear stress. Subsequent analysis lower section of the injection hole (RH 12). Within hours of the nature of the microseismic signals confirmed that the

' of the start of hydraulic stimulation the percentage of flow source involved a shearing mechanism along the expected leaving the well via the shot zone increased from 18 to 51 joint planes, and it appears from the measured in-situ percent (CSM, 1982). It was also shown that the explosive stresses that conditions at Rosemanowes are ideal for stimulation had significantly reduced the inlet impedance downward rather than upward growth at this depth. so that no further pre-conditioning was required. This This finding, if it proves to be generally applicable, work achieved the first objective of the experiment. will be fundamenta! to future thinking on HDR reservoir From early high-pressure tests and the stress development, because shear stimulation will require lower measurements, it was concluded that wellhead pressures in overpressures and will result in different degrees of joint excess of 14 MPa would be required to jack open the opening and differentdirections of growth from those that fracture system, and that injection flow rates of more than would be predicted on the basis of the earlier assumed 90 I/s would be needed to maintain this pressure. Injection jacking mode. Batchelor (personal communication, 1984) into RH12 was begun at about this rate and a connection has suggested that in an anisotropic stress regime the between the wells was established after about 12 hours. The situation where the directions of principal stresses have production well (RH11) flowed-to surface 12 'hours after rotated some 20 to 30 degrees with respect to the natural the start of pumping, by which time a total of nearly 10,000 fractures may represent a lower energy configuration and m3 (2 million gallons) had been injected. Once a that many sites may thus show this condition. In this

334 J.D.Garnish

GROUND LEVEL IEPTH BELOW I BELOW URFACE SEA LEVEL KPTH I- iURFACE

1404 n 140411

2000n 2000n

40001. VIEWING DIRECTION 4000r

i. L'

Figure 5. Elevations of the microseismic zone context it is interesting to note that analysis of the the significance of this ordering is likely to be crucial to microseismic signals from the Los Alamos Phase I1 further development of a successful reservoir. Hitherto, the reservoirs, though in a decidedly a typical stress regime presence of seismicity has always been taken as an indicator close to a magma chamber, shows that fracture growth in of flow paths within the reservoir. With thegreater volume these reservoirs also occurred by shearing rather than of data now available, however, it is clear that in some cases tensile jacking (Cash and others, 1983). It should be (e.g. the very deep events) the seismicity can be indicating emphasised, however, that while the shearing mechanism only hydraulic continuity but not necessarily mass transfer, may be ubiquitous the direction of growth may be upward while other zones, which have been proved to carry large or downward depending on the gradient of shear stress flows (e.g. that around the upper open-hole section of with depth. In Cornwall, for example, it is estimated that RH1 1), have remained aseismic throughout the upward growth would occur below about 4090 m. experiment. As seismic mapping is the only tool so far The existence of this shearing mechanism has available for long range monitoring of the reservoir, these important implications for the management of reservoir differences must be understood. structure. Because it occurs at such low overpressures (ca. 4 During circulation over a period of nearly a year, a MPa above hydrostatic rather than theca. 10 MPa expected total of more than 340 000 m3 of water were injected into for tensile jacking), the pressures required for circulation the granite at Rosemanowes, but only about 100 000 m3 even through a low-impedance reservoir may be sufficient were recovered. The remainder was stored or lost within to precipitate uncontrolled growth. Accordingly, other the zone of stimulated fractures. Though the average methods of reservoir operation may be necessary, perhaps recovery rate was less than 30 percent, however, the even using downhole pumps or controlled flashing to keep instantaneous recovery was only weakly dependent on the reservoir pressures below hydrostatic. injected flow rate. While very high flows (>90 l/s) can be A further finding, illustrated in Figure 5, is that a recovered from the production well during short-term marked degree of ordering - of as-yet unknown origin - venting, sustainable production rates have remained fairly occurs within the overall structure. An understanding of constant in the range 6 to 10 l/s for injection flow rates

335 Hot Dry Rock - A European Perspective

varying from 10 to 50 l/s (wellhead pressures 7 to 10 MPa) can be achieved at intermediate depth. Discussions have or even more. The implication is that, while the inlet and been held between teams in France and Germany, within outlet impedances have both been reduced to small values, the UK and with the European Commission, and it is to be there remains another mechanism (not previously hoped that such discussions could lead within the next 2 or encountered at Los Alamos) that is limiting flow through 3 years to a Europe-wide integrated programme rather the reservoir and preventing the attainment of the project than a continuing series of discrete projects. If this work objectives on impedance and flow rate. In fact, in view of and (or) Los Alamos is successful, a commerical-scale the weak correlation between inlet pressures (and flows) prototype could be in commission by the mid-1790s and and outlet flows, the concept of an overall ‘impedance’is no HDR could start to fulfill some of its promise in the first longer particularly helpful. Nevertheless, it was clear that quarter of the next century. unexpected effects were occurring in the reservoir, that the flow losses were too great for a commercially viable operation and that the overall pumping powers required ACKNOWLDEGEMENTS for circulation were too high. As a result, it has not been The author would like to thank his colleagues in possible to get within a factor of ten of the sustainable Europe, whose work is acknowledged above, and Dr. J.K. target production figure of 75 l/s. Linacre for helpful discussions during the preparation of Detailed examination of the microseismic map this review. The views expressed, however, are entirely (Figure 5) suggested that each well was associated with a those of the author, and do not necessarily represent those large subvertical reservoir structure, the interconnection of of either the UK Department of Energy or the European which was poor. This model was consistent with the Commission. findings from detailed hydraulic testing. To improve this connection by pumping alone would have entailed attempting to open orthogonal (Set 2) joints against almost REFERENCES the maximum principal stress and, as experience had Armstead, H.C.H., and Tester, J.W., 1985, “Heat Mining” (in press). shown, this would lead preferentially to runaway shearing growth of the reservoir structures in the Set 1 joints with Batchelor, A.S., 1982a, The creation of hot dry rock systems by combined explosive and hyraulic fracturing: Proceedings of the lnternaional attendant water losses. Conference on Geothermal Energy, Florence, v. 2, p. 321-342. it Accordingly, was decided that the way forward Batchelor, A.S., 1982b, The stimulation of a hot dry rock geothermal would require a third well, crossing the reservoir structure reservoir in the Cornubian granite, England: Proceedings of the beneath the two existing wells. This well would have two Eighth Workshop on Geothermal Reservoir Engineering, Stanford, primary objectives: CA, 14-16 December, p. 237. (i) to reduce the overall system impedance to the point Batchelor, AS., 1983, Hot dry rock reservoir stimulation in the UK: An where circulation experiments could be performed at extended summary: Extended Summaries of the Third International Seminar on the Results of EC Geothermal Research, Munich, 29 useful rates, and Nov-1 Dec 1893, EUR 8853 EN, p.693. (ii) to penetrate and sample within the reservoir both Batchelor, AS., Pearson, C.M., and Halladay, N.P., 1980, The aseismic and intensely seismic regions, in an attempt to enhancement of the permeability of granite by explosive and improve understanding of seismicity as a reservoir hydraulic fracturing: in AS. Strub and P. Ungemach (eds.), development tool. Proceedings of the Second International Seminar on the Results of EC Geothermal Research, Strasbourg,4-6 March 1980, publ. as EUR This third well (RH15) was drilled during October to 6862 by D. Reidel Publishing Co., 1009 p. December 1784, following a right-handed helical path from Cash, D., Homuth, E.F., Keppler, H., Pearson, C.,and Sasaki S., 1983,Fault surface to a true depth of 2650 m (ca. 2800 m along the plane solutions for microearthquakes induced at the Fenton Hill hot hole) and crossing the reservoir orthogonally some 350 m dry rock geothermal site: Implications for the state of stress near a below RH12. Its inclination over the lower section is 25 to Quaternarylvolcanic centre: Geophysical Research ktters, v. 10,no. 30 degrees and its azimuth changes from N.130” W. near 12, p. 1141-1144. surface to N.20” E. through the reservoir. Preliminary Cornet, F.H., 1981, Etude sur la fracturation des fractures hydrauliques hydraulic testing suggests that at the time of writing a dans les roches: EUR 7287 FR. connection has been established between RH12 and RH15. Cornet, F.H., Hosanski,J.M., Bernaudat,F.,and Ixdoux, E., 1983,Shallow This has to be characterized more fully by pressure and flow depth experimentation on the concept of energy extraction from testing, however, before any decision can be taken on what- deep hot dry rocks: in Nemat-Nasser, S., Abe, H., and Hirakawa, S. (eds.), Hydraulic Fracturing and Geothermal Energy: The Hague, further stimulation may be required. Martinus Nijhoff Publishers. CSM, 1982, Camborne School of Mines Geothermal Project, Newsletter No. 13. FUTURE DEVELOPMEN.TS CSM, 1984, Camborne School of Mines Geothermal Project, Newsletter No. 15. By late 1985 results should be becoming available, both ETSU, 1982, Strategic review of the technologies: from the CSM work and from the redrilling at Los Alamos, ETSU/Rl3, publ. HMSO (London). which will permit decisions on the way forward. In Europe Garnish, J.D., 1976, Geothermal energy: The case for research in the in particular plans are already being. made for one or more United Kingdom: publ. HMSO (London). “full scale” systems (i.e. capable of attaining temperatures Haenel, R. (ed.), 1982, The Urach Geothermal Project: Stuttgart, E. up to 200OC) should the CSM project prove that the targets Schweizerbart’sche Verlagbuchhandlung (Nagele u. Obermiller).

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IEA, 1980, MAGES: Summary report of the programme of research and Pine, R.J., Ledingham, P., and Merrifield, C.M., 1983, In-situ stress development on Man-Made Geothermal Energy Systems: measurements in the Carnmenellis granite: 11. Hydrofracture tests at International Energy Agency, Paris. Rosemanowes Quarry to depths of 2000 m: International Journal of Milora, S.L., and Tester, J.W., 1976, Geothermal Energy as a Source of Rock Mechanics and Mining Sciences and Geomechanical Abstracts, Electric Power: Cambridge, The MIT Press. v. 20, no. 2, p. 63-72. Murphy, H.D., Drake, R.H., Tester, J.W., and Zyvoloski, G.A., 1984, Pine, R.J. and Batchelor, AS., 1984, Downward growth of hydraulic Economics of a conceptual 75MW hot dry rock geothermal electric stimulation by shearing in jointed rock: International Journal of Rock : Los Alamos National Laboratory Report No. Mechanics and Mining Sciences and Geomechanical Abstracts, v. 21, LA-UR-83-2258. no. 5, p. 249-263. Pine, R.J., Tunbridge, L.W.,Kwakwa, K., 1983, In-situ stress Rummel, F., and Kappelmeyer, O., 1983, The Falkenberg geothermal measurements in the Carnmenellis granite: I. Overcoring tests at frac-project: Concepts and experimental results, in Nemat-Nasser, South Crofty mine at a depth of 790 m: International Journal of Rock S., Abe, H., and Hirakawa, S. (eds.), Hydraulic Fracturing and Mechanics and Mining Sciences and Geomechanical Abstracts, v. 20, Geothermal Energy: The Hague, Martinus Nijhoff Publishers. no. 2, p. 51-62 Smith, M.C., 1973, Geothermal Energy: Los Alamos Scientific Laboratory Report No. LA-5289-MS.

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