Goteborg since 1984, when the test site at Fjallbacka was opened . .... the heat exchange area becomes small. This may be a ... field experiments were initiated at the test site at Fjall- backa. ...... hydraulic test data and focal mechanism of induced seismi- city. ... ring water circulation at the hot dry rock geothermal test.
);,I(TIJ)~
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]]]~ID]~®1f JF~(Q) JJID] (QJ~ DEPARTMENT OF GEOLOGY CHALMERS UNIVERSITY OF TECHNOLOGY AND UNIVERSITY OF GOTEBORG
CIRCULATION EXPERIMENTS IN 1989 AT FJALLBACKA HDR TEST SITE
Thomas Eliasson Ulf Sundquist Thomas Wallroth
Publ. Fj-10 Goteborg 1990
CODEN: CTH/HDR/P-90/10
CODEN: CTH/HDR/P-90/10
CIRCULATION EXPERIMENTS IN 1989 AT FJALLBACKA HDR TEST SITE
Thomas Eliasson Ulf Sundquist Thomas Wallroth
ISSN 1101-0SSx
Publ. Fj-10 Goteborg 1990
This work was supported by the National Energy Administration under contract 656 082-2 HDR-Fjallbacka. Further copies are available from Geothermal Energy Project, Department of Geology, Chalmers University of Technology, s-412 96 Goteborg, Sweden .
I
PREFACE The hot dry rock research, financed by the National Energy Administration, has been conducted by the Department of Geology at Chalmers University of Technology and University of Goteborg since 1984, when the test site at Fjallbacka was opened . The circulation experiments presented in this report were carried out in spring 1989 . Professor Ulf Lindblom at the Department of Geotechnical Engineering at Chalmers was the project manager during the field experiments . The microseismic monitoring and analysis was conducted by Swedish National Defence Research Institute (FOA). Professor Ragnar Slunga is greatly acknowledged for this support . The local authorities at Tanum are thanked for facilitating the field work at Fjallbacka .
CONTENTS SUHHARY . . .. . ....... . ... . ... . . . . . . .. . ..... . .... . .. . .. . 1 1 INTRODUCTION .. . .. .. .. . . . .... ..... . . . .. .... . ....... . 2 2 BACKGROUND .. . .. .. .. . . .. . . . . .. . . . .. .. .. .. .. . .. .. . .. . 4 3 TEST SET-UP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. 1 3. 2 3.3 3. 4 3 .5
Downhole equipment . .... ..... . .. . .. . . ... . ..... Pump and water supply . . . . . .. . ........ . .... . .. Data acquisition . ... .... . .. ... .. . ...... .. . . .. Safety system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microseismic equipment . .. . . . . ..... . . . . ... . .. .
5 6 6 8 9
4 TEST PROCEDURE AND RESULTS ... . . .. . . ... ... .. . . ..... 10 4 . 1 Hydraulics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . 2 Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. 3 Hydrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . 3 . 1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . 3 . 2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . 3 . 3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . 4 Microseismics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 12 14 14 14 15 21
5 ANALYSES . ...... ........ . ......... . . ........ ... . . .. 23 5. 1 Hydraulic analysis .... . . ... . ..... . . .... . . .. . 5.1 .1 T1: pressure build-up . .. . ...... . .. .. . . 5 . 1.2 T2 : pressure shut-in .. . . . ... . .. .. . .... 5.1 . 3 T3: pressure shut-in . .. . . . . . . . . . .. . . .. 5. 1. 4 T4 : pressure build-up . . . . ....... . . . .. . 5 . 1 . 5 T5 : pressure shut-in . . . . .. .. . . . . . . .. .. 5.1 . 6 T6 : pressure shut-in .. . . ... . . . .. ... . .. 5 . 1.7 Hydraulic impedance . . ... . ... . . . . . ... .. 5. 1 . 8 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 . 9 Hydraulic characterization . . ..... .... . 5 . 2 Interpretation of hydrochemical data ...... . . 5.2 . 1 Hydrochemistry . .. ... . .. . . ....... . ..... 5 . 2.3 Water-rock interactions . . ....... . . ... . 5.2.3 Summary of hydrochemical data ... . ..... 5 . 3 Analysis of induced microseismicity . .. . .... .
23 24 27 28 29 31 32 33 33 38 41 41 45 46 48
6 CONCLUSIONS . . .. . . . .. . . ...... .. . ... . . .. . . .. . ....... 53 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
/
1
SUHHARY
In 1984, Hot Dry Rock experiments started at the test site in Fjallbacka . By downhole percussion drilling the 500 m deep well Fjb1 was drilled . An HDR reservoir was generated at 450-460 m depth by massive hydraulic injections. The progressive displacement of the pressurised fluid was followed by passive microseismic monitoring. A second well, Fjb3, was drilled inclined into the stimulated fracture zone at a lateral distance of 100 m from Fjb1 and a roughly horizontal hyd-raulic contact between the wells was achieved . A stimulation carried out in the well Fjb3 at reservoir depth improved the flow capacity. In this report the results of the circulation test carried out in 1989 between Fjb3 and Fjb1 are presented. The total pumping time was 846 hours and a total volume of 5600 m3 of water was injected. Flow rate was kept constant at 1.8 1/s and the maximum injection pressure was 5 MPa. The recovery rate was continuously increasing during the circulation, to a maximum of 51\ of the injection flow rate. Hydraulic analysis has shown that highly conductive fractures exists near both wells . Altough the flow path between the wells probably consists of a fracture network, the HDR reservoir can hydraulically be defined as one single fracture interlinking the wells. The circulation test displayed that this fracture has a pressure dependent behaviour . A higher overpressure yielded increased fracture radius and fracture aperture. When the applied pressure exceeded 4-4.5 MPa a M high conductivity• fracture was extended to the recovery well . The hydrochemical data of the recovered water showed that dilution due to mixing of injected water and pre-circulation reservoir fluid was dominating, with no significant water-rock reactions taking place. The temperature of the recovery water increased throughout the test. Microseismic activity was monitored during the circulation experiment . The hydraulically induced events, located west and south of the injection well at a maximum lateral distance of 400 m, are believed to represent the major leak-off direction . The onset of seismicity at rather low injection pressures was explained using a simple peak shear strength model. Forthcoming field experiments are aimed at studying the heat exchange area and possible channelling effects. Furthermore, ·the optimum injection pressure for the actual reservoir will be determined and leak-off in relation to applied pressure and induced microseismicity will be studied .
2
1 INTRODUCTION The concept of Hot Dry Rock (HDR) geothermal energy production is based on a closed-loop circulation of water between two deep boreholes in a rock mass with low natural permeability. A high permeability connection, the HDR reservoir, is created between the wells by high-pressure hydraulic injections . Field experiments in different countries have demonstrated that the mechanism of the reservoir growth is gover ned by the interaction between the in situ rock stresses and the natural fracture pattern . The creation of artificial permeability (hydraulic stimulation) must be carefully conducted, following a design based on foreknowledge of the natural conditions in the r ock mass . The heat from the rock is extracted through conduction from the rock to the circulating water. From a commercial aspect, this energy resource must be considered as non-renewable, since the heat is extracted by cooling the affected rock volume. The conditions required to make the heat production commercially useful are : (1) High rock temperature at the reservoir depth . (2) Sufficient rock volume to supply heat over the life of the reservoir . (3) Large heat transfer area . (4) Low flow resistance between the wells . (5} Small fluid losses to non-productive regions . (6) Long-term hydro-mechanical stability. The first condition is a function of the local geology. The heat in crustal rocks originates from two sources; conducted from the mantle below and from radioactive decay of uranium, thorium and potassium within the crust . The geothermal gradients (temperature change with depth) at different locations in Sweden are generally between 10-20°C/km. These relatively low temperature gradients imply that the most economic concept of HDR technique in Sweden is to limit the drilling depth and to use a heat pump to rise the temperature for domestic heating purposes . The conditions (2) through (4) are dependent on the geometry and extent of the fracture sets within the reservoir and the fracture aperture distributions . Field experience from the deep HDR projects in the USA and England have suggested that the major flow occurs along sub-parallel fractures favourably orient ed in relati on to the earth stresses. The resistance to flow within the fractures can be reduced only by increasing the fracture apertures . Large dilations of the fractures can be achieved by keeping the pressure inside the fractures above the minimum in situ stress . This is, however, not r elistic because of large pumping costs and the risk of uncontrollable growth of the reservoir .
3
Hydraulically stimulated fractures can be kept open on reduction of hydraulic pressure by rock fragments from the walls of the fractures (self-propping) or by injected sand . Furthermore, increased permanent fracture apertures may be a result of induced shearing displacements . In cases of pronounced channelling , with flow concentrated to high conductive zones, the heat exchange area becomes small . This may be a considerable problem when proppants are injected to reduce flow resistance . Even if the HDR reservoir is created in a low permeability rock mass, there will always be some measure of permeability at the boundaries, giving rise to water losses . In a closedloop circulation, some make-up water would have to be supplied continuously to compensate these losses . Some of the apparent water losses can be considered as long-term storage . However, water losses to non- productive regions of the rock mass will reduce the system efficiency by increasing relative pumping costs and by wasting heat. Downhole pumping in the production well may improve the efficiency of the system . To minimise the losses at the reservoir boundaries , the reservoir pressure should be kept below the critical pressure required to induce shearing of joints intersecting the fractures being active in the circulation . Swedish HDR research has been carried out by the Department of Geology at Chalmers University of Technology since 1984, when field experiments were initiated at the test site at Fjallbacka. In this report an open-loop circulation experi ment commencing on 27 April 1989 and lasting for almost 40 days, is presented . It consisted of a continuous injecti on of 1.8 1/s into Fjb3 with Fjb1 on production and was aimed at investigating the hydraulic performance of the reservoir. Microseismic activity was recorded and injection and production temperatures were measured. Moreover, hydrochemical sampling and analyses enabled control of the chemical processes occurring in the reservoir .
4
2 BACKGROUND An HDR geothermal energy research reservoir has been developed in the Bohus granite on the Swedish west coast. At the research site close to the village of Fjallbacka, a 500 m deep well (Fjb1) was drilled (figure 1. 1) and a high-pressure hydraulic stimulation was performed at about 450 m depth (Sundquist, 1985; Eliasson et al, 1988 a,b) .
Fjb 3
u N
511 0
5
10
IS 20
25 30 m
Figure 1. 1 Plan view of the wells at the research site at Fjallbacka .
Based on the distribution of microseismicity, tectonic and geohydraulic information, a second 500 m deep well (Fjb3) was drilled inclined into the major seismic area at a lateral distance from Fjb1 of 100 m (Sundquist et al , 1988). The drillhole intersected the stimulated fracture zone and a roughly horizontal hydraulic contact between the wells was achieved . A short-term circulation test at a low flow rate (0.3-0.5 1/s) revealed a high hydraulic impedance in the system. To increase the permeability and reduce the skin around Fjb3, a smallvolume hydraulic stimulation was performed (Sundquist et al, 1988).
5
3 TEST SET-UP
The test equipment for the circulation experiment consisted of downhole arrangements for isolating the conductive zone and a surface installation of pump and water supply. The monitoring system comprised pressure, flow rate, temperature and water chemistry sensors. The operation of the pump and packer system was continuously controlled by a safety system . 3. 1 DOHNHOLE EQUIPMENT
In the injection well, Fjb3, a straddle packer with a 31 m long injection interval (between 449 and 480 mdepth) was used; see figure 3. 1. The packer system was isolating the fractures stimulated in March 1988 and some additional major open fractures.
Fjb1
Figure 3.1 Packer positions during circulation test 1989 . Major conductive fractures are shown schematically.
6
Between the upper packer and the wellhead at ground surface a 2\" EUE tubing was used as injection string . The straddle packer was inflated with water through a high-pressure hose . The equipment used has been described in detail by Eliasson et al. (1988b) and Sundquist et al. (1988). In the production well Fjb1 a single hydraulic packer sealed off the well at a depth of 350 m. A 32 mm standpipe transfe~ red the produced fluid to the surface.
3.2 PUHP AND WATER SUPPLY Community water was supplied from a fire hydrant through a pipe into a 60 m3 buffer tank. The rate was controlled to keep a sufficient water level in the tank. Before entering the pump, the water was filtered twice. The pumping equipment consisted of a plunger pump with a maximum operating pressure of 10 MPa at 3 . 7 1/s flow rate. The pump was driven by a 45 kW electric motor . Both units were mounted on a steel frame. The power transmission was a V-belt drive, allowing three different flow rates. The maximum pressure could be controlled manually by an overflow regulator valve. A safety system was used to ensure that the pump stopped in case of emergency . In cases of shorter power failures, the pump was automatically restarted (see section 3.4) .
3.3 DATA ACQUISITION The layout of the circulation system is schematically illustrated in figure 3. 2. Pressure and temperature measurements were carried out both downhole and at surface. The recovery flow rate was measured at surface both by a turbine flow meter and manually. In the injection well Fjb3, the downhole pressure and temperature were measured by a surface read-out gauge installed inside the injection string at 447 m depth; see figure 3.3. The temperature was measured by a Pt100 resistance element and the pressure by a semiconductor gauge. A specially designed transmitter circuit board inside a 40 mm casing, transferred current signals via a 4 conductor wire . The wire was connected to the probe by a wirehead with a weakpoint allowing a maximum tensile force of 10 kN .
7
INJECTION
WELL
FJ83
RECOVERY
WEll FJ81
P4
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7
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9
Figure 3 .2 Measuring and safety system used during the circulation test in 1989.
. STUFF ING
BOX
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Figure 3 . 3 The Fjb3 downhole measuring probe and stuffing box at wellhead.
8
The w~ r e exit at wellhead was in a specially des i gned stuffingbox ; see figure 3. 3 . The box had a rubber element for preventing leak-off along the wire. In the production well Fjb1, downhole pressure and temperature transmitters were installed in a casing mounted directly above the packer element. The signals were transferred via a 4 conductor wire, which also was used for handling the packer system . The different sensors at surface was of transmitter type and, together with the downhole equipment, connected to a control panel with analog instrumentation . The signals were continuously read by a data logger and stored on tape or floppy disc . The recovery water was hydrochemically moni tored on site both manually and with a separate memory logger system. Water samples were sent to a laboratory for extensive analysis. On site water temperature, pH, conductivity and Fe content were measured manually . The memory logger measured pH, Eh and H2 S.
3. 4 SAFETY SYSTEH
A safety system was installed to control the following situations : 1. Insufficient feed of water to pump , caused by break in feeder line between fire hydrant and pump or filter clogged due to contaminated water . This was controlled by pressure transducer P1, see figure 3.2. 2 . Break in high pressure line between pump and wellhead. This was controlled by pressure transducer P2. 3. Packer system out of operation; by-pass of injected water. This was controlled by pressure transducer P2 . 4. Packer movements due to high pressures below bottom packer , causing the string to move upwards . This was controlled by current line E1.
9
The signals from the transducers were connected to an on/off regulator which was in serial connection with the E1 sensor. If any of the sensors gave "off" signal, the pump was shut down . Nonreturn valves at wellhead ensured pump shut-downs to be analysed as shut-in tests . If the safety system itself failed, the pump was shutdown automatically.
3.5 HICROSEISHIC EQUIPHENT During the test, induced microseismicity was monitored using a set of nine vertical 10 Hz geophones distributed on rock outcrops within a distance of 450 m from the injection well; see figure 3 . 4 . The sampling rate was 1000Hz. These seismic measurements were carried out by the National Defence Research Institute (FOA). During the first part of the test (27 April - 16 May) a triggering system was used which recorded 6 seconds of data digitally at each event detection . Small seismic events were not detected at night, when the signals were distorted by longwave transmissions. This digital equipment was not available during the second part of the circulation, but an analog recording was made on magnetic tape between 16 May and 7 June . The analog system was operated 60- 70\ of this time period . 500-
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Figure 3 . 4 Geophone locations during the circulation test in 198 9 .
10
4 TEST PROCEllJRE AND RESULTS 4. 1 HYDRAULICS The open-loop circulation test between Fjb3 and Fjb1 was carried out between 27 April and 6 June 1989 with a few interruptions due to pump service and a failure in the community water supply system. The total pumping time was 846 hours and a total volume of 5500 m3 of water was injected during the experiment . The injection flow rate was kept constant at 1. 83 1/s . Injection pressure and flow rates are displayed in figure 4. 1. INJECTION PRESSURE FJBJ 11
1: 1
0'~--~------.---r---r--.,--,---.---.--~---.--,
0
1000
1100
1200
1000
1100
1200
1000
1100
1200
INJECTION FLOW RATE FJ83
PRODUCTION FLOW RATE FJ81
0
1100
Figure 4.1 Pressure and flow rate data from the circulation test. T1-T6 denote test periods hydraulically analysed (see section 5. 1J.
11 The first phase of the test was continuous, except for a short stop after 20 h, from the evening of 27 April to the evening of 1 May (100 h), when pumping was stopped and the injection well was shut in to permit pump service . After 14 hours of shut-in time, the wellhead was opened and water flowed back. A downhole pressure and temperature probe was installed inside the tubing at 447 m depth just above the upper packer. During this pumping phase the injection pressure increased continuously at a successively declining rate. The wellhead pressure at the time of shut-in was 4.6 MPa. The produced flow rate from Fjb1 increased continuously but non-linearly with time to a maximum value of 0 . 58 1/s . When the injection was resumed on 3 May (138 h) the wellhead pressure increased rather quickly, reaching 4.3 MPa after 48 hours . Afterwards, the increase was much slower, giving an approximately constant pressure of 5.0 MPa within 20 days . The produced flow rate increased slowly during the whole phase reaching 0.93 1/s on 30 May. The increase rate was almost 0.02 1/s per day the last days before pumping stopped. The maximum recovery rate corresponds to 51 % of the injection rate. The pump was shut down four times during the second pumping phase, on 10 May (315 h) because of a minor electronic failure and on 16, 18 and 25 May (454 h, 503 h and 678 h) because of pump service . All four stops lasted for less than one hour each. Meanwhile, the well was shut in and the stops did not affect the injection pressure or recovery flow rate more than marginally. During pumping, the pressure below the single packer in Fjb1 increased as a result of the increasing friction losses in the production hose . This pressure build-up, in combination with a small leakage in the packer inflation system, caused the packer to slide upwards in the well. On 13 May the packer lost its sealing ability and the well started to produce through the annulus. The production rate was hereby reduced because of the occurrence of several permeable fractures in the well. This packer was replaced by a new single packer at 322 m depth on.23 May and a better controlled production flow was achieved. On 30 May (789 h) the community water used for the test became heavily contaminated and it was decided to shut down the pump and shut in the well. On 2 June (856 h) water quality was once again satisfying and the test continued for another four days. The production rate was reduced by 0.42 1/s during the stop, but increased _again as the injection went on to a maximum of 0.87 1/s.
12
At noon on 6 June the experiment was ended and the injection well was shut in . The pressure decline following the shut-in was recorded and the production flow was monitored for a week . The shut-in pressure as well as the produced flow rate declined slowly.
4.2 TEMPERATURES During the experiment, temperature was measured continuously _ downhole in the injection well Fjb3 as well as in the production well Fjb1. However, this latter temperature probe did only work until 13 May, when the packer slided upwards and the wire was damaged (see section 4 . 1}. Temperatures monitored during the period 4 May- 13 May are shown in figure 4.2.
TEMPERAnJRES MAY 4- MAY 13
20 18 18
(j'17 I 18
ffi
-----------~~-BQ..C!600 mg/1) and Mn (>400 mg/1) was recorded in the injected fresh water. Vented water The conductivity of the water vented from the injection well, was measurered at 119 h and 123 h giving conductivity values
16
of 2. 0 mS/cm and 3.0 mS/cm, respectively. These values are plotted in figure 4.4 . No water samples were collected from the vented water . Redox conditions in production water Figure 4. 3 illustrates the change in Eh in the return water during the circulation experiment . At the start of the circulation the produced water was highly reducing . When venting the injection well at 119-123 h there was a rapid decrease in Eh in the produced fluid . During the pumping period 140-400 h there was an increase in the Eh with time . However, during the period 160-200 h, a break in this trend was noticed with a negative slope of the curve at 180-200 h. The subsequent period was characterised by a less steep slope in the Eh curve approaching the value of the injected water. When resuming the Eh monitoring after the packer failure period (see figure 4 . 3), distinctly more reduced water was produced. The period was characterised by a rather moderate slope in the Eh trend. During the beginning of the test a large release of H2 s from the production fluid was observed . The maximum concentration in the water during this period was about 2-3 mg/1. Concentrations as high as these were not recorded during the previous circulation experiments . The bicarbonate concentration was also high in the return fluid during the beginning of the experiment . ::J08.8
einj.water
z
i-100.8
Packer failure In F')b1
a: I
~-200.8
~ -::J08.8
nw£ Figure 4 . 3
~
600
800
1000
(HOURS) START: 27 APR. 1111 11:00
Plot of r e dox level (Eh) in production water versus time during the circulation .
17
Conductivity of production water In figure 4.4 the conductivity of the production fluid is plotted. Also shown is the conductivity of the injection and vented water . During the first pumping period (138 h) pumping period the general trend shows a diminishing conductivity, whereas, after the two pump stops at 786 and 954 h the level of conductivity started to rise. As in the Eh plot, an anomaly was noticed at 160-200 h with an increase in salinity of the produced water. It is noticable is that the slope in the conductivity trend at 618 h and onwards was rather flat.
ELECTRIC CONDUCTIVITY
0
• vented water injected "
e
---
--:.-·
• N
---•
---------r-----I I I
r - -1 I I
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Figure 4.4
1100
1200
Plot of electric conductivity versus time for the production water during the open-loop circulation . The 30 mS/cm conductivity of the indigenous water plots above the figure.
18 Chemistry of production water In figure 4.5 the concentration of some major and trace constituents in the return fluid is plotted against experiment time. The vertical lines in the plots show pump stops and starts (c . f . figure 4.1). The general trends identified were similar to those observed in the Eh and conductivity plots. Water samples were not taken during the first 16 h of injection . This implied that the initial (100h /
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Figure 5. 17
10 20 Conduct I v I t y ( 1 S/ c 1
30 )
Plot of Mn versus conductivity for production, injection and indigenous waters.
5. 2 . 3 SUHHARY OF HYDROCHEHICAL DATA
The hydrochemical trends in the production fluid were principally mixing trends between injected water and pre-circulation reservoir fluid, see figure 5 . 17. Basically the production fluid became progressively more diluted as the injection proceeded. Changes of chemical trends could generally be correlated with changes in the hydraulic conditions in the reservoir. As injection pressure increased , more distally located fractures with water of different composition became a part of the flow connection between the wells (new parts of the fracture reservoir was activated) . The indigenous relict saline water was highly diluted by fresh water during the gel/water stimulation in Fjb1. The precirculation and post-gel stimulation reservoir fluid was composed of stagnant weakly saline and with low Eh.
47
The chemistry of the injection and production fluid indicate that a change from calcite dissolution to calcite saturation took place in the reservoir .
1. 83 I /s 1 ax 5 MP a
f,,i;\r.l\~··m\ m\1\\\
: \11\B---1\ :·., ·;
....... ... .. .. . ····· ····· ····· ..... ····· · ·· ··· ···· ·· ···· ···· ··· ·· ..... ·· ··· ·· ···· ····· ···· ··· ··
::::: : ::::::::::::::::
:::::::::::::::::::::: :::::: ::::: :::::::::::
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Figure 5 . 18 Schematic illustration of the hydraulic situa tion during the circulation experiment
48
5.3 ANALYSIS OF INDUCED HICROSEISHICITY From the plan view of miroseismic events (figure 5.19) it can be concluded that rather few events were located in the interwell region. The overall distribution indicates that a great part of the lost fluid during the circulation flowed long distances in southern and western directions and was hereby not recovered. Desirable shear stimulation in the area around the wells seems to have been of minor importance. In figure 5.19 the locations of the induced seismic events during the circulation test is shown together with the seismicity monitored in connection with the hydraulic stimulation in 1986 (Eliasson et al . , 1988b).
NORTH
~ICROSEISMIC LOCATIONS
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