Institution of Mining and Metallurgy
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$ 0 ! f-1 , & l p £ > INSTITUTION OF MINING AND METALLURGY SPECIAL GENERAL MEETING MINERAL EXPLOITATION IN CORNWALL 4th - 6th October, 1978 ST. IVES, CORNWALL Preliminary Studies of Dry Rock Geothermal Exploitation in South West England By: A. S. Batchelor C. M. Pearson Release for Announcement In Energy Research Abstract* DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Preliminary Studies of Dry Rock Geothermal Exploitation in South West England Authorss Anthony S. Batchelor, B.Sc., Ph.D., C.Eng., MIMM Project Director, Geothermal Exploitation Project, Camborne School of Mines. C« Mark Pearson, B.Sc., A.C.S.M., Geothermal Technical Officer, Camborne School of Mines. jjjp Copyright retained by the authors who permit the free reproduction of this material provided the source is acknowledged. -2- Introduction Mining in Cornwall has always been associated with unusually high rock temperatures at quite shallow depth and this has meant that consider able quantities of heat have to be rejected from the mines in the form of warm water and air. Henwood (1) reports that the adit outflow from the United Downs area was 38m3/min at 28°C or the equivalent of 47MWt and recent measurements at South Crofty Mine show that over 6MWt is still being re jected from the mine water alone. The objectives of the authors’ current research programme are directed towards the development of a method of exploiting the heat as a commercial product in its own right; with the possible bonus of the in-situ leaching of low grade minerals. The work reported within this paper is based on the results of our initial studies and it serves to highlight the principal problems that are facing the development of artificial geothermal exploitation. Relevant Geological Features of South West England The counties of Devon and Cornwall have been mapped extensively by inumerable authors during the last 200 years because of the intense mineralisation associated with the granites. Bromley (2) has recently summarised the majority of the geological and geophysical data appertain ing to its structure although recent re-interpretations (Unpub) of the gravity surveys show that a granite depth of 15Km under Dartmoor increasing to 20Km under Land’s End forms a credible model. The megastructure of the batholith is now assumed to be a 6000Km^ intrusion into Devonian and Carboniferous sediments whose undulating ridge is now exposed as 5 major plutons, forming the elevated moorlands. There fore, the volume of rock must be greater than 102000Km3 and the thermal content of the rock which is at a temperature greater than.50°C is in excess of 3.4'x 10^-^MJ or 1.25 million million tons of coal. An extraction ratio of only 0.0167. would yield the thermal energy equivalent of 100 million tons of coal or one year's U.K. coal production. A Summary of the Thermal Data Temperatures as high as 55°C were recorded in the mines during the last century but it was not until the late 1960's that the systematic study of the local heat flow commenced. Tammemagi et al (3,4) presented the results of this work which clearly identifies heat flows of 100-130mW/m2 -3~ in and near the granites, Wheildon et al (5) have conclusively shown that the high heat flows are not only connected to the known mineralised areas but extend right across at least two of the exposed plutons, Carnmenellis and Bodmin. The temperature gradients presented in (5) are in the range 27-34°C/Km with a mean of 29.8°C/Km. Speculative Evaluation of the Heat Content The previous two sections present the field information that a possible thermal deposit may exist; in the same way that geophysics may indicate the possible discovery of an orebody. The target in this case is a suffi ciently large body of rock at elevated temperatures and the- only proof that those temperatures exist would come from deep exploratory holes. The temperature is clearly the equivalent of the grade of an orebody but in this case the value of the commodity should improve with depth. Unlike an orebody it is possible to predict the temperature changes with depth by postulating a model based on the physics of heat flow. Batchelor (6) has proposed a local model where the heat flow through the rock is conductive and there is an exponentially decreasing radioactive heating component. This leads to the following description of temperature with depth-s T - (( Qa - qs.D) .Z + D2.q .(1 - exp(-Z/D))) .1+Ta K Where s- T = Temperature, °C Z = depth, m Qg = measured surface heat•flow, 0.1 W/m2 average qs = surface heat generation from radioactive elements, 4 x 10~6 W/m3 K = Thermal Conductivity, 3.3 W/mK Ta = Year Average Surface Temperature 10°C D = A constant in the range 5-12000m Figure One shows the predicted temperature profile for depths to 6Km, the current feasible drilling depth. It can be seen that the slightly decreasing geothermal gradient can be averaged at 27°C/Km with little loss in accuracy. The magnitude of the accessible heat between any two depths is then -4- simply derived from-: Q = jf.Cp.G.(z2 - Z p 2 where-: Q = Heat Content, J/m^ Cp = Specific Heat, 822J/kg°C y = Mass density, 2618 kg/m3 G - Geothermal Gradient, 0.027°C/m For example, the heat content of the granite at temperatures above 150°C but within the reach of a drill, i.e. between 5200m and 9000m, is 1.56 x 10^2 j/m2 or 58 tons of coal equivalent per square metre. This latter figure represents a coal seam, 38m thick. It is, however, more reasonable to consider extractions between 2 and 4km at temperatures in excess of 64° C but below 12Q°C applied to industrial and space heating, and this reduces the coal equivalent to 13 tons/m2 or an 8m thick seam. If this model is reasonable then the whole batholith contains the equivalent of 78000 million tons of coal or ten times the reserves of the new Selby coal field. However, .an extraction ratio of 5 % would be required to match the output from Selby if the coal can be won with a 507. overall extraction ratio and, additionally, coal can be utilised far more efficiently. The.exploratory work is continuing under the guidance and control of Mr. J. Wheildon of Imperial College and it is hoped that the remainder of the region will be mapped by the end of 1979. The Theory of Exploitation The heat is contained in the mass of the granite and can only be liberated by cooling the rock. The natural hot springs of South Crofty are formed by freshwater circulating through fissures and extracting the heat over many thousands of square metres. The seawater content of this water (<17., Alderton et al (7)) indicates that the horizontal permeability is extremely low. Natural extractions of this nature and at sites like The Geysers occur because of the rare coincidence of three features: (1) Rock at elevated temperature, with (2) Sufficient permeability and (3) Water recharge to make up the fluid lost by venting. The latter two features must be added or enhanced by the engineer seeking to exploit the heat within the rocks by creating artificial reservoirs. The most satisfactory method of extracting the heat is the creation of a zone of broken rock so that water can permeate it with ease. In con cept it would be similar to a zone of block caving. Unfortunately a simple calculation shows that even a modest 50MWt plant running at 150°C would need over 14 tons/min to be broken and cooled from 200°C. This represents a rock moving exercise of over 7 million tons/year to maintain the output" and"make~ a profit out of a commodity that is only worth £3 million p.a. There seems little option but to generate series of permeable fissures that can extract heat from within the fissured zone and generate a sufficiently large local cooling area to remove heat from the surrounding intact mass. The Shape of the Exploitation Zone and Extraction Ratios A minimum volume of rock within the fracture zone is enclosed by a plane, disc like geometry while the minimum area of heat transfer for a given rock volume is obviously spherical. The choice between them is not easily quantified as can be seen from the following example. Consider a proposed 50MWt extraction system emplaced in rock at 200°C. If the demand temperature is 120°C then the system can be considered ex hausted when this temperature is reached. It is possible to determine the size of the necessary planar fracture and an equivalent cylindrical zone to produce a well life of twenty years using simplified models. (i) The Planar Crack Carslaw et al (8) show that-? Q = AT.K. / Z L Y * a X k.t) where Q = Constant Rate of Heat .Extraction, Watts AT = Fall in temperature, °C K = Thermal Conductivity, 3.3W/mK for Granite k = Thermal Diffusitivity, 1.6 x 10"^ m^/s for granite t = time, secs A = Area of heat flow, m^, N.B. both sides of the crack The above example needs an area of 5.4 million square metres or a fracture of approximately 1 km in radius. (ii) The Cylindrical Zone. Radius - Height 6- Hodgkinson (9) shows that:- k.t/R2 AT.K (erf ( _L . ) . (1 - exp(-l/A>i))).du (Q/rfL) ARu where L = length of cylinder in metres other notation as above Solution of this equation is non-trivial but it shows that a zone 360m long and in radius is needed to support this demand.