HYDROGEOLOGICAL INVESTIGATIONS AT THE STORAGE CAVERN FOR HEATED WATER AT AVESTA

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1 Hydrogeology in the Service of Man, Mémoires of the 18th Congress of the International Association of Hydrogeologists, Cambridge, HYDROGEOLOGICAL IVESIGAIOS A HE SORAGE CAVER FOR HEAED WAER A AVESA CL. AXELSSO 1, A. CARLSED 2, J. JOHSO 2, L. KARLQVIS 1, Y. LIU 1,. OLSSO 1 and L. SÂRBLAD 2 ^Uppsala Geosystem Ltd, Uppsala, Sweden ^Geological Survey of Sweden, Uppsala, Sweden ABSRAC he hydrogeological investigations for the Avesta Project have been carried out over a long period. hey were started more than six months prior to the start of construction to obtain the basic information on the groundwater system on which subsequent effects of construction and operation are superimposed. Groundwater conditions were evaluated as regards their behaviour during the construction stage and as regards the influence of the groundwater on the efficiency of the storage of energy in the Avesta rock cavern. he flow of groundwater gives rise to an additional convective energy loss, which will be of the same order of magnitude as the heat loss by conduction through the rock. Most of the convective heat loss is due to water flowing from the pressurized rock cavern to the pressure sinks formed by the adjacent underground spaces. IRODUCIO he hydrogeological investigations at the Avesta hot water store constitute one of the programmes of investigation for the Avesta Project. hese investigations have been carried out because the groundwater is of major significance in both the construction of the rock cavern and in the final performance of the cavern as a heat store. he works follow the basic programme prepared in conjunction with the start of the Avesta Project (Fogdestam et al, 1981). It has been found practical to divide the work into three separate stages, each of which describes different stages in the construction and operation of the store. he reason for making this division is that the hydrogeological/hydraulic conditions are completely different during these three stages. he time schedule stages is: I Initial stage December 1979 to the start of construction in August 1980 II Construction stage August 1980 to May 1982 Ilia Operating stage, long May 1982 to December 1983 term heating cycles 111b Operating stage, January 1984 to December 1984 weekly heating cycles 104

2 his article is a brief description of the work carried out and contains some of the results that have been obtained during the course of the investigations. SIGIFICACE OF GROUDWAER I SORIG HEA I A CAVER he properties of the groundwater system are of great significance to the storage of heat in an unlined rock cavern. Its influence on the rock and the influence of the rock on the groundwater are different at different phases of the storage project. he natural dynamics of the groundwater are disturbed during the construction of the store as a result of the reduction in pressure used to keep the underground spaces dry. his results in a gradual lowering of the pressure level of the groundwater in the vicinity of the rock cavern, until a given steady state is reached, which is dependent on the hydrogeological properties of the bedrock, in combination with the hydrometeorological nature of the area. Blasting during excavation of the cavern results in a redistribution of the initial stresses in the rock around the structure. his altered state of stress, in turn, affects the fissure system of the rock, the waterbearing units, with a net influence that is dependent on the geometry of the installation in relation to the fissure system and the original stress field. he overall effect of the stress redistribution is a tendency to close existing fissures, thereby reducing the total groundwater flow within the area affected by the stress change. Charging of the rock cavern by filling it with heated water results in successive raising of the temperature of the rock surrounding the store to a pseudostationary level. he temperature of the rock will oscillate within certain given boundary values, which will depend on the depth and distance at which the temperature is measured from the store, the thermal properties of the rock and temperature range and storage cycle of the store. he rise in temperature affects the groundwater system by changing the viscosity and density of the water and by introducing thermoinduced stresses changes that also affect the total groundwater flow. he total effect of these stress and temperature changes in the direct vicinity of the rock store is to change the groundwater flow in relation to what would be expected on the basis of knowledge of the original hydraulic properties of the rock. he conductive energy transport from the store to the surrounding rock causes energy losses that affect the thermal efficiency of the store. he natural groundwater flow also contributes to further losses by addition of a convective loss term to the conductive one. Quantitatively, the convective transport increases with increasing water flow in the rock. When hightemperature storage is considered (temperatures above 100 C), it is necessary to pressurize the water in the store to prevent it boiling away. In this context, the groundwater pressure is used to achieve the necessary gauge pressure. It is therefore important that the groundwater reaches and retains an adequately high pressure level. he above problems and questions have formed the basis for the 105

3 programme of hydrogeological measurements in Avesta. he investigations were planned and arranged so as to shed light on the above factors and so that the results of the investigation would provide an optimal yield. MEASUREME PROGRAMME Various measurements and recordings have been made during the course of the programme. However, the aims and procedures were slightly different at the different stages. he following are the main measurements made: * recording of the groundwater pressure level * recording of the groundwater temperature * recording of the precipitation within the area of the installation * recording of the quantities of water leaking into the rock cavern and research tunnel * recording of the temperature of the water leaking in * sampling and analysis of the water leaking in Recording of the groundwater pressure and temperature has been carried out manually and automatically. he manual measurements have been performed during the entire measurement programme, whereas the automatic recordings were only started during the final phase of the construction stage. Different drillholes have been used for these measurements drilled both from ground level and from the specially constructed research tunnel. he manual measurements were made at varying intervals, depending on the phase of the project. One aim was to make as much use as possible of the preinvestigation holes drilled for the construction of the installation. he effect of this was that a large number of holes were used in the beginning, but the number was reduced successively as the rock works affected the holes. able 1 shows the drillholes used and the measurements made during the various stages of the work, and Figure 1 shows the locations of the holes. ABLE 1 Details of the drillholes and measurements made during different stages of the investigation programme. indicates groundwater level measurement and, temperature. Drillhole Diameter Length Measurements o. mm m Initial stage Operating stage Dl D2 D3 D4 D5 D6 DB1 Fol Fo2 Fo3 Fo5 Fo6 Fo

4 Fo3 B < Fo5 V ^ I > i _ A < > < > < > < > Fi^. I. r/ze locutions and direction of the drillholes used. A shows the holes drilled from ground level and B shows those drilled from the research tunnel. he manual measurements were made at a maximum interval of 7 days. o obtain a shorter interval between measurements during the operating stage, a large number of pressure and temperature transducers were installed in different drillholes. hese transducers were read 107

5 four times a day by an automatic data collection system developed for the thermal investigations. he installed transducers were located in sections of drillhole bounded by systems of sleeve packers (Figure 2). Immediately after installation of these transducers, it was found that leakage had occurred in certain transducers, so that all transducers were removed and replaced, after which they performed satisfactorily. Precipitation in the area was measured using a recording precipitation gauge. Fig. 2. Diagrammatic sketch of drillhole installations for pressure and temperature transducers. HYDROGEOLOGICAL CODIIOS Hydrogeology and geology Hydraulic tests in conjunction with the geological preinvestigation (Moberg, 1980) showed that the average hydraulic conductivity of the rock was about 3 10" 7 m/s. his value applies to both the more homogeneous rock and the minor fissure zones encountered in drillholes. he zones containing greater quantities of water are normally less extensive, but may give rise to hydraulic conductivity values exceeding 1 10~6 m/s. 108

6 75 % o loss 1.7 Fissures/metre 1.8KJ 7 m/s 40 % o loss H 2.8 Fissures/metre 4.4 K} 7 m/s Fig. 3. he relative anisotropy of the rook in relation to the orientation of the rook oavern. he investigations show that the rock is highly anisotropic, with higher conductivity across the cavern than along it. he fissure pattern in the area forms a fissure system consisting of two main fissure groups: A. A dominating, subvertical fissure group, with a direction of strike perpendicular to the longitudial axis of the cavern. B. A less frequent, subvertical fissure group parallel to the longitudinal axis of the cavern. Of these two fissure groups, the first (A) is of the greater significance for the.groundwater flow in the area. Figure 3 shows the hydraulic conductivity and fissure frequency distribution obtained from the waterloss measurements, which agree well with the fissure systems in the area. Water balance he groundwater reservoir is supplied with water by infiltration through the ground during times of precipitation. he net precipitation, which is the quantity of water that is potentially available for saturation of the unsaturated zones and for production of groundwater, is the actual precipitation, less the total amount that evaporates. he formation of groundwater is small during the period of intensive vegetation growth, when the precipitation is often not enough to maintain evaporation. Instead, there is a gradual drying out of the soil layer and the groundwater reservoir is only filled after heavy rainfall. During the winter, the water reservoir is covered by snow, so that very little groundwater is formed. When the snow melts, large quantities of stored precipitation are released and either contribute to the formation of groundwater or flow away as surface water. he maximum amount of groundwater formed during a year cannot exceed the net precipitation, which is the actual amount of precipitation less the amount that evaporates. Limits to infiltration at any one time are therefore either the availability of water, the net precipitation, or the ability of the 109

7 ground to accept the water. But measurements of the Infiltration capacity of various soils show that the permeability of most of them is sufficient to permit infiltration of almost all surplus precipitation. Basically, it is only in conjunction with rapid snow melt that the availability of water exceeds the infiltration capacity. It is therefore possible to draw the conclusion that the net precipitation normally constitutes a good measure of the amount of groundwater formed. he water balance has been calculated for the area of the Avesta installation. he net precipitation varies between 50 and 300 mm a year. GROUDWAER PRESSURE LEVELS AD FLOWS Pressure levels he investigations were started late in 1979, that is about 8 months before construction was started. he aim of this early start was to obtain reference material on undisturbed conditions in the area. All subsequent effects would be dependent on these conditions, and the recordings made during the construction and operating stage constitute the sum of the original conditions and later disturbances. Figure 4 shows the variations in groundwater level in drillhole Fo2 for the period of measurement up to the start of construction. he measurements at Avesta show low winter values during he water level rises to a maximum in conjunction with the melting of the snow. During the summer that followed, the water level dropped slowly until construction work was started, when the rate of lowering was accentuated. he natural groundwater system will always be affected during underground construction work. his influence consists of a changed flow pattern and lowering of the groundwater pressure level. he magnitude of the change depends on the installation and the hydrogeological properties of the area. Groundwater is supplied to the area by precipitation and runoff from surrounding areas. his water leaks into the installation through the fissures and fissure zones in the rock. During blasting of the rock cavern, the hydraulic properties of the rock are affected, partly by measures taken to seal the rock, and partly, indirectly, by the change in the state of stress of the rock. A marked lowering of the groundwater was noted in all drillholes during the construction stage. One example of this is shown in Figure 4. he maximum recorded lowering of the groundwater table amounted to 10 m. here was a general tendency towards greater lowering in the longitudinal direction of the maximum hydraulic conductivity, which is perpendicular to the longitudinal axis of the cavern. Drillholes above the cavern exhibit a remarkably small lowering of the groundwater level, which implies that the vertical conductivity is less than the horizontal. he peak values of lowering shown in Figure 4 are probably an effect of changes in the bedrock. hese marked peaks in the lowering of the groundwater table are of short duration and are followed by a rise in the level of the table and resumption of a more gentle rate of groundwater lowering. During underground construction work below the groundwater table, drainage pumping in the installation will lower the water pressure 110

8 in relation to that of the surrounding groundwater. his will cause the groundwater to flow towards the installation and cause a lowering of the groundwater in the area surrounding the installation. mas.i r in tl t V A h ^\ A. W \ v^ A / \ / " m.a.s.l I Fig. 4. Groundwater variations in drillhole Fo2. he amount of water leakage and the size of the area affected by drainage are dependent on the hydraulic properties of the rock; the formation of new groundwater, the depth of the installation, etc. Affected areas that form discharge areas under normal conditions will, as a result of the altered hydraulic conditions, change into recharge areas. Groundwater formation and drainage will again balance one another and all groundwater formed within the affected area will flow towards the installation. Figure 5 shows the estimated distance of influence as a function of the hydraulic conductivity of the rock, the groundwater formed in the area and the slope of the groundwater table for an installation with a drainage base 50 m below the original level of the groundwater table. In the case of the Avesta installation, the distance influenced will be about 259 m. hese calculations are based on a twodimensional calculation model, which implies that the actual influence distance will be slightly less, about 200 m, as a result of the real, threedimensional flow pattern. Based on these calculations, leakage into the cavern may be estimated to amount to between 30 and 60 1/min. he groundwater lowering model used as a basis for these calculations of the influence distance and leakage into the cavern is incorrect, in that it does not take into account the actual pressure distribution around the installation. In reality, as a result of relatively low hydraulic conductivity in combination with large formation of groundwater, the bedrock will contain groundwater, even above the installation. he lowering of the groundwater will therefore be noticeable as a depression of the groundwater table above the installation. he depth of the depression is related to the relationship between the groundwater recharge and the hydraulic conductivity, in accordance with Figure 6. hat this is the case is clearly shown by the reported level recordings. Ill

9 Hydraulic conductivity, m/s 500 «00 Influence distance,m Fig. 5. he influence distance of the rock installation as a function of the conductivity of the rock, slope of the groundwater table, and groundwater recharge (P) for an installation with a drainage base situated 50 m below the original groundwater level. umerical FEM calculations have been made for the installation, in addition to the analytical calculations, with the aim of describing the groundwater conditions during the various stages. hese calculations show that the construction stage causes leakage of about 10 1/min into the cavern, which is slightly lower than that reported above but higher than the 4 1/min that was measured. his suggests that the hydraulic conductivity assumed for the rock is twice the correct value. During the construction stage, water will leak out of the rock cavern as a result of a marked flow to the research tunnel due to pressurization of the cavern. 112

10 ' ' **' "V" "V V "'*' H' ^ H"* '*' >*' '*' V Depression 2 h Hydraulic conductivity rn/s Fig. 6. Lowering of the groundwater above the installation as a function of groundwater recharge and hydraulic conductivity. he dashed curve indicates the groundwater recharge calculated for the Avesta area. his leakage is expected to amount to a total of about 12 1/min which, taking the actual, lower permeability of the rock, corresponds to about 7 1/min. Corresponding leakage into the research tunnel is of the same order of magnitude, and of this, about 75 % may be regarded as direct flow from the cavern to the research tunnel. During the operating stage, the cavern is refilled with water, which causes the groundwater level to rise. he time for filling with water coincides with the melting of snow, which accentuates the rise in pressure. Water is supplied to the rock, both from the surface and from the cavern. During this stage, the cavern acts as a boundary condition, with a more or less constant pressure. he heat storage cycles are visible in the pressure graph, especially during the weekly storage cycles, when a marked oscillation of the groundwater pressure was recorded from all transducers. An example of this is shown in Fig

11 m.a.s.l. m.a.s.l i y w ^ jfftlmdata Ml*t té *«Sx ,/ I i Fig. 7. Pressure conditions recorded at transducer PI, in drillhole Fob. Observe the marked rise in pressure during filling of the cavern, starting during the spring of 1982, and the strongly oscillating pressure during weekly storage cycles in EMPERAURE CODIIOS AD HEA LOSSES he groundwater temperature has been recorded ever since the initial stage. he aim of these measurements was to obtain a picture of the variations in temperature in the area with time and with depth below ground level. Figure 8 shows the seasonal variation in drillhole Fol at two levels one close to the ground level and one at relatively great depth. his shows a clear seasonal variation in the groundwater system, a variation that is controlled by variations in the air temperature. he variation is greatest near the ground level and diminishes with increasing depth. A marked phase displacement was recorded. he measurement near ground level exhibits its peak during December, which is 5 months after the maximum air temperature. his phase displacement increases with increasing depth. During the construction stage, the rate of turnover of the water increases, which causes a rise in temperature and a reduction in the phase displacement. he groundwater flow in the area gives rise to the addition of a convective loss to the conductive heat loss as a result of conduction through the rock. Here the groundwater system causes groundwater at a normal temperature to flow towards the installation and become heated. On the downstream side, on the other hand, a corresponding flow of hot water away from the installation results in in

12 C C t x x * \ 1980 HHJ«X X X X X \ / \ 1983 A ^. Af woocasos*^ M*M«X>M»0«"it'' 'Fig. 8. Variations in temperature in drillhole Fol measured at different depth below ground level (4.0 m and 13.0 m below the top of the pipe). a heat loss, the magnitude of which is dependent on the magnitude of the groundwater flow. In the case of the Avesta accumulator, there are the additional effects of a strong flow from, the waterfilled cavern to the drained underground spaces of the installation (research tunnel and accumulator control room) and of pressurization of the cavern to counteract the inflow of groundwater containing minerals (with the aim of protecting the heat exchangers). On the basis of the assessments of water flowing through the system and to the research tunnel, the convective loss may be evaluated as being about 5 L of the stationary conductive loss. he effect of this is that during the initial stage storage losses caused by water flows will be negligible, but as the conductive transport of heat saturates the rock with heat, the convective loss will become increasingly significant. However, it may be said that, on the whole, this loss is of less significance to heat storage in unlined rock caverns. But one condition is that the rock cavern be located in rock of good quality and with low hydraulic conductivity. 115

13 ACKOWLEDGEMES Special thanks are extended to Project Leader Dr J. Martna of the Swedish State Power Board, and to other members of the Board who have participated in various phases of the work. During the course of the Avesta Project, many colleagues have participated actively in the work and without their assistance it would not have been possible to complete the work. We herewith extend our thanks to all of them. REFERECES Fogdestam, B., Olsson,. and Sârnblad, L., 1981: Hydrogeology (part of a description of the installation and research programme). he Avesta Project 1981:1. Moberg, M., 1980: Rock investigations for the hot water accumulator at Avesta. Swedish State Power Board. 116