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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 - 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 geothermal energy 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 coal (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).

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