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INSTITUTION OF MINING AND METALLURGY

SPECIAL GENERAL MEETING

MINERAL EXPLOITATION IN

4th - 6th October, 1978

ST. IVES, CORNWALL

Preliminary Studies of Dry Rock Geothermal Exploitation in South West

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

Authorss Anthony S. Batchelor, B.Sc., Ph.D., C.Eng., MIMM Project Director, Geothermal Exploitation Project, 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 .

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. Now the total heat extracted over twenty years at 50MWt is 3.15 x 10*® MJ (1.16 Mtce) yet the total heat available from within the cylinder is only 1.58 x 10*® MJ so at least 507. of the extracted heat has come from outside the fissured zone. It is possible to calculate the distance at which a small, 2% or 1°C, temperature drop occurred after twenty years by using the finite cylinder solution from (9)-;

t _R i t - A T = ____ 1 r . dr I A.*. Cp.k

2 *2 exp ,

where y = Rock density, 2618 kg/m^ for granite Cp = Specific Heat of Granite, 822J/kg.K r = distance from centre plane b = half length of cylinder This leads to the solution that A T = 1°C at r = 550m approximate; implying that an 857. extraction can be achieved.

Alternatively the distance to the 1°C temperature drop for the planar crack is given by-: (Hodgkinson(9))

T )

which implies a z value of less than 150m. The extraction ratio in this case would be 187.. Intuitively the effective difference between the two seems correct since -7- much more granite is exposed to cooling fluid in the fractured volume rather than the plane fissure. The problems arise when the engineering of such systems is considered. Engineering The Artificial Reservoir

In order to create the highly permeable fractured volume required to form the reservoir between two or more wells, energy needs to be expended on the intervening rock mass. Initially this will form a number of multi­ directional fractures which can subsequently be further extended and interact with any natural discontinuities in the area.

The main techniques used for the fracturing of rock in the mining and oil well industries are explosive methods and hydraulic fracturing. Haimson (10) has shown that hydraulic fracturing will produce a single vertical planar fracture aligned normal to the direction of least principal, compressive stress. Therefore, this would be very suitable for creating and extending a large planar fracture between two wells, but,,alone, is of no use for the creation of a mult­ iple fractured reservoir.

The maximum crack extension from a hole using explosives is in the order of forty times the hole diameter (Kutter and Fairhurst (11), Pearson (12)), thus the size of any reservoir which can be initiated solely by explosives is limited. Large explosive charges also have the disadvantage in that their effect on the initial radial fracture zone induced by the seismic wave is to crush it when the gas pressure builds up. This leaves a highly deformed cylindrical mass of rock surrounding the hole, which is largely impermeable. Musa (13) has shown that the energy required to extend a fracture in granite 2 is only lOOJ/in , many times less than that required to .dilate it. For example, 2 laboratory tests on the Carnmenellis Granite have shown that over lAOKJ/m is required to dilate a fracture to 5mm. Clearly to form artificially a highly fractured reservoir, a number of fractures need to be induced initially by a high power source, followed by a low power source to extend the fractures further.

If the temperature of the rock is high enough it can be thermally shocked with cold water to form the initial cracks. If this is not possible, as at Rosmanowas, small explosive charges can be used to produce similar radial fractures around a hole which can then be further extended under hydraulic pressure. There is considerable evidence that the Los Alamos system has indeed induced sizable thermal shock zones at the boreholes.

Fractures extended from a borehole will always tend to align themselves normal to the direction of minimum compressive stress. Thus, in order to -8- establish in which direction the reservoir will preferentially develop, the

relative components of the horizontal stress field at the depth of the reser­ voir need to be carefully considered. Any favourably orientated natural fractures could also be advantageous when activated.

The only evidence of structural movement in S.W England due to a non- uniform horizontal stress field is along a few isolated faults (Taylor (14)), noteably the NW-SE trending Sticklepath Fault in Devon. However, evidence from the local mines that near vertical stopes have stayed open over many tens of years would suggest that a strong stress anisotropy does not exist.

As an indication of the direction of the horizontal stress field, the natural joint sets can be considered. At Rosmanowas Quarry, two primary orthogonol sets exist, the earlier formed set being, NW-SE trending, and the later developed set being NE-SW trending. These natural joint systems exist at depth and can be seen in the local mines, at 600m depth. The spacing between individual open joints is in the order of tens of metres at that depth compared with tens of centimetres on surface.

Therefore, the first three 300m holes are being drilled on a triangular grid, two sides of which are parallel to the two dominant joint sets. Preferential activation of fractures between any two holes will indicate the direction of maximum compressive stress and the direction in which the fractured reservoir could be extended.

The Identification of Fractures

A large number of different techniques exist in order to establish the' existence of fractures in the rock mass surrounding a borehole. The majority of them are concerned with running geophysical logging tools down a hole, to establish at what depths it is intersected by fractures.

The following table lists the different logs which have been used in work at the Carwynnen Site:-

Tool Effect Verdict

Caliper, 4 arm Sharp spikes on the log Possible fracture

3-D Sonic Characteristic 'V' Definite fracture pattern with 30cm and 90cm tool Natural Gamma Increase in count, Fracture or vein Spike Spectral Gamma Uranium Spike only Natural Fracture Uranium, Thorium Vein Potassium contd.

Tool Effect Verdict

Density, Reduction spike Fracture Gamma Gamma Increase spike Vein

T.V. Camera Visible Definite fracture

35 mm Camera Visible (stereo overlap Definite fracture enables dip and direc­ tion to be determined)

Straddle-packer Higher volume accept­ Possible fracture ance for a given pressure

Although the above listed logs enable the fractures that intersect the hole to be identified, it gives no indication of their extent, or any quan­ titative analysis of the surrounding fractured reservoir.

In cases where two or more holes exist, seismic tools can be used to map the intervening fracture pattern. However, in order to establish the physical characteristics of the rock mass and fractures surrounding a hole, water pumping tests need to be carried out.

Pumping Tests

Water injection tests are carried out in order to determine the extent of fracturing in a rock mass surrounding a hole, primarily by considering the change of pressure and quantity with time. The length of hole to be tested is sealed off with hydraulically inflated packers, then water is pumped through the top packer into the surrounding rock mass.

Two types of test are carried out, namely flow tests in which either the flowrate or pressure is kept constant during pumping and the other is re­ corded with respect to time; or venting tests in which water is pumped into the adjacent fracture reservoir until a pre-determined pressure is reached, when the over pressure is removed and the volume of water vented up the hole is recorded.

Single-rate and multi-rate constant flow tests have been carried out at both the Carwynnen and Rosmanowas test sites. The rise in pressure during the test is plotted on a chart recorder and at the end of each test when pumping is stopped, the pressure decay graph is also recorded. The use of - 10-

these tests lies in the analysis of the recorded graphs and has so far been mainly concerned with establishing the nature of the flow of water away from the holes. For example, whether flow has been radial into the surrounding infinite rock mass or whether it has been through a fracture dominated re­ servoir.

The equation for radial constant flow through an infinite, homogenous, isotropic rock mass is (after Mathews & Russell(15)).

p = q M In ( 4kt ) o 4-Kkh jB>* 1^,2

p = Over Pressure (atm) o q = Flowrate (cm^/s) = Viscosity (cp) k = Permeability (darcy) h = Length of Hole (cm) If = Euler's Constant t Time (s) JB = Mean Compressibility of Rock and Water S Hole radius (cm) rw

If the flow from the hole is primarily radial, then a plot of the rise in over pressure against the logarithm of time will be linear. However, in all the tests carried out at Carwynnen and Rosmanowas this has not been so, Figure Two being a typical plot. Indeed, the first tests carried out at Carwynnen (Pearson (12)) showed that the flow into the surrounding rock from a 36m long borehole which had been cement grouted and re-drilled was only 3 2.2 cm /s at a constant over pressure of 6 atmospheres. This has been ex­ plained by laboratory permeability tests, giving the value of the rock fabric permeability as being as low as 0.01 pd, and the fact that the borehole surface area that is pressurised is relatively small.

The equation governing linear constant one dimensional flow into a semi­ infinite medium is (after Carslaw et al (8)).

where 2A = Total Fracture Surface Area This equation can be re-written as either - 11-

log p k log t + log £ - (3) o A

or A ( k p ) % = (t)% - (4) ...... P o

If flow is linear, then equation (3) shows that a plot of log Pq againstlog t will be a straight line of gradient 0.5 with an intercept equivalent to the function ______

Likewise from equation (4), a plot of the rise in over pressure against the square root of time will be a linear plot. The gradient will give the L value of V"t/P> and since the function q(>x /tT) is a constant, the other function (AVk ) of equation (4) can be investigated. This is referred to as the "rock property" function and is at present being investigated as both the rock permeability and compressibility, and the area of the fracture change with an increase in pressure.

Figure Three is a plot of the pressure rise against the square root of time from the second part of a two rate pumping test carried out on Hole Rl. It is typical of the plots obtained from all the flow tests carried out at Carwynnen and Rosmanowas, namely that the overall trend is linear, with marked deviations occurring when the pressure rise is first much less and then greater than the average. This is thought to be due to the opening and pressurisation of fractures further away from the hole as the test progresses. These deviations are noticeably less pronounced the longer the test is con­ tinued although the longest single-rate test carried out has been for two hours duration. The meaning and repeatability of these phenomena is being investigated since an understanding of their significance is of paramount importance if such tests are going to be used to give quantitative analysis of either artificially created or naturally fractured reservoirs.

The venting tests that have been carried out show that for a unit increase in pressure there is a unit increase in the recovered volume of water. This has since been confirmed on the Los Alamos Site where it has been found that as the pressure of the injected water is increased, so a proportionally greater area of the natural fracture system is opened up. On removal of the overpressure the opened fractures collapse and a volume of water indicative of the fracture area is vented.

It is anticipated that further research into the analysis of all the aspects of pumping tests will lead to a better understanding of the charac- - 12-

terlsties of the rock mass surrounding each borehole. Much useful information has already been gained, although a higher degree of certainty and accuracy on the interpretation of data will only result after much more field work has been carried out.

The Current Research Programme

The use of explosives to create a small fractured reservoir between two boreholes has been successfully investigated at Carwynnen Quarry on the north western edge of the Carnmenellis Granite. (Pearson (12)).

Present research is being concentrated at Rosmanowas Quarry where an artificially induced fractured reservoir is to be created between a number of 300m deep holes. The work can be divided into the following chronological divisions.

(1) The measurement of the undisturbed hydraulic properties of the rock.

(2) The creation of a number of short radial cracks around each hole.

(3) The re-measurement of the hydraulic properties of the rock

(4) The extension of the radial fractures to produce a highly fractured reservoir between the holes.

(5) The re-measurement of the hydraulic rock properties, with particular reference to the swept area of rock available for heat transfer.

The present status of the project is that the first of the holes has been drilled and the in-situ rock properties are being investigated It is hoped to report on the first series of tests at the meeting. Conclusions There is a mounting body of evidence to show that South West England is a possible dry geothermal resource of. enormous capacity. Los Alamos have demonstrated that a planar fracture extraction zone can be emplaced and ex­ tract heat at low power ratings (5MWt). However, much higher ratios are theoretically possible from fractured volumes of rock and the authors' activities are directed towards generating a multiple fractured reservoir involving as many activated natural fractures as possible. The principal outstanding problem is that an unambiguous interpretation of down-hole reservoir geometry from pump tests has proven elusive. It is hoped that future tests by ourselves may add a little to the intensive efforts by many groups to solve this problem. - 13-

Acknowledgement 3

Funding for this stage of the work has been from the Science Research Council and the authors extend their thanks for this support. Two local firms, Penryn Granite Ltd and GompAir Construction and Mining Ltd. have also given invaluable support to the development of the project and its very existence is due almost entirely to their assistance.

Many members of the School’s staff have given time and considerable effort towards the work and we are particularly grateful to Dr. P. Hackett Mr. G. Shrimpton, Dr. A. Bromley, Mr. M. Waller, Mr. J. Lloyd, Mr. N. Tregembo and Mr. J. Mortimore for their help.

Dr. J. Garnish and Mr. D. A. Gray of the E.T.S.D. and the I.G.S. re­ spectively have been extremely helpful in the development of the programme and we express our thanks to them.

All views expressed in the paper are entirely the responsibility of the authors and no other party. References

1 Henwood, W, "On Temperatures in Mines", Trans. Roy. Geol. Soc. Cornwall, Vol.5.

2 Bromley, A.V., 1976, "Granites in Mobile Belts, the Tectonic Setting of the Cornubian Batholith." Jnl. Camborne Sch. Mines. Vol 76, pf.40

3 Tammemagi, H, Wheildon, J; -1974 "Terrestrial Heat Flow and Heat Generation in South West England", Geophys. J.R. astr. Soc., V38, pf 83-94

4 Tammemagi, H, Wheildon, J . , 1977 "Further Data on the South West Heat Flow Anomaly", Geophys. J. R. astr. Soc., V49, pf. 531-539

5 Wheildon, J, Francis, M, Thomas-Betts, A; 1977 "Investigation of the South West Thermal Anomaly Zone", Seminar on Geothermal Energy, Brussels, Dec. 77, EUR 5920, pf 175

6 Batchelor, A.S., 1978; "The Granites of South West England and their Engineering Properties Related to Possible Dry Rock Geothermal Exploitation" Nordic Symposium on Geothermal Energy, Gothenburg, May 29-31st, 1978

7 Alderton, D, Sheppard, S., 1977, "Chemistry and Origin of Thermal Waters from South West England", Trans. I.M.M. Sec. B, Vol 86, pf B191

8 Carslaw & Jaeger "Conduction of Heat in Solids", Clarendon Press, Oxford, 1959

9 Hodgkinson, D, UKAEA, Theoretical Physics, Unpub. Note. AERE RWMP (76) M27 and AERE - R8763, Harwell

10 Haimson, B. "Hydraulic Fracturing in Porous and Nonporous rock and its potential for determining in-situ stresses at great depth", University of Minnesota PhD Thesis 1968

11 Kutter H. K. & Fairhurst C. "On the fracture process in blasting". Int. Jnl. Rock Mech. Min. Sc. Vol 8 ppl81-202, 1974

12 Pearson C.M. "Study of fracture properties of granite, applied to geo­ thermal extraction". Final year thesis, Camborne School of Mines, 1977

13 Musa N. "Study of the Post-failure behaviour of rock using a servo controlled testing machine". Final year thesis, Camborne Sch. Mines 1977.

14 Taylor R. (I.G.S. Exeter) - personal ‘communication, 1978

15 Mathews, C.S. & Russell D.G. "Pressure buildup and flow tests in wells". Monograph of the SPE of AIME, 1967. DEPTH in m etres 00 _ 6000 00 _ 4000 5000 00 _ 3000 2000 1000 __ _ an nli Gaie rm urn Gohscl Measurements Geophysical Current from Granite enellis Carnm rdce Tmprtr Prfl Truh The Through rofile P perature Tem Predicted IUE ONE FIGURE Averaged G radient, 27»6° /km C 27»6° radient, G Averaged 0 0 0RC ) ^ ^ c) y i T A R p E T ROC* |0 40 20 a a o B oie using rofile P Current lim it of field data field of it lim Current Predicted , , f , r

vrg Surface Average ufc Ha Gnrto®41“ W/m^ 4x10“^ Generation® Heat Surface hr l odciiy 3. mk /m W .3 3 * Conductivity al Therm ufc Ha Fo = .IW'^m^ = Flow Heat Surface e eaue lO^C = perature Tem

r I I I I = = 6000m 180

: WELLHEAD PRESSURE CPSID 20 26 32 36 44 50 ■■ .2 - t ------i - 1.4 ------1 ------1.6 1------1 ------1.6 1 ------1 ------2.0 1 ------1 ------1 Oi TIME LOGia ELED RSUEAANT O TIME LOG AGAINST PRESSURE WELLHEAD 2.2 ------1 ------IUE 2 FIGURE 2.4 1 — ---- 1 ------2.6 - h 2.6 10 3.0 H ----- 1 ----- h- WELLHEAD PRESSURE CPSID j QAE OT F TIME OF ROOT SQUARE ELEDPESR AANT QAE OT F TIME OF ROOT SQUARE AGAINST PRESSURE WELLHEAD FIGURE 3 FIGURE