Geothermal investigations in an area of induced seismic activity, northern São Paulo State, Brazil

TECTONOPHYSICS ELSEVIER

Tectonophysics 253 (1996) 209-225

Geothermal investigations in an area of induced seismic activity, northern Paulo State, Brazil Tereza Higashi Yamabe a,* ,1, Valiya M. H a m z a b a Faculdade de Ci~ncias e Tecrmlogia Universidade Estadual Paulista, Presidente Prudente SP, Brazil b Observatdrio Nacional, CNPQ, Rio de Janeiro R J, Brazil

Received 27 April 1994; accepted 22 May 1995

Abstract

Geothermal investigations were carded out in Nuporanga, state of S5o Paulo (Brazil), where occurrence of seismic activity has been found to be closely related to opening of groundwater wells. Results of macroseismic studies show that seismic activity had its beginning in May 1977, soon after completion of drilling of the COLABA well and most of the initial seismic events are located close to it. Geothermal investigations were initiated in September 1977 on the assumption that fluid movements associated with seismic activities are capable of producing short-term time-dependent changes in the local thermal regime and in the hope that identification of such time-dependent changes would contribute to a better understanding of the nature of local seismicity. Results of thermal logs in the COLABA well reveal the existence of an unusual thermal regime with a constant temperature zone (CTZ) down to 175 m followed by a zone in which temperature rises rapidly (TGZ), in the interval of 180-204 m. Repeated thermal logs carried out over a period of three months reveal temperature drops of up to 0.8°C taking place in the TGZ immediately after periods of intense microseismic activity. Temperature measurements of pumped water also show changes of lesser magnitude, occurring in the CTZ, closely related to the frequency of seismic events. Substantial temperature changes related to periods of seismic activity were also observed in two nearby wells in Nuporanga. The available geothermal and macroseismic data have been used in the development of a simple model of the process that have triggered seismicity in Nuporanga. According to this model the COLABA well acts as a natural siphon drawing water from a perched aquifer at depths of less than 40 m and injecting it to a fault zone at about 175 m. The model allows a coherent explanation for the observed correlation between seismicity and absence of pumping. During periods in which pumping is suspended the static level of water in the well is high and the pressure exerted by the water column induces a reduction in frictional resistance at the fault zone. During pumping the water level falls to its dynamic level and consequently the pressure at the fault zone is also lower. Such pressure changes are apparently sufficient to trigger microseismic activity in Nuporanga. The observed temperature drops immediately after tremors could be attributed to the cooling effect associated with the penetration of relatively cold water from upper levels into newly opened fractures. Also the small but significant rise in temperatures during seismically quiescent periods can he considered as a result of warming up of stationary fluid bodies within fracture zones.

* Corresponding author. Present address: lnstituto AstronSmico e Geoffsico, Universidade de SSo Paulo, C.P. 9638, CEP 01065-970 S~o Paulo SP, Brazil. 0040-1951/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0040- 195 1(95)00055-0

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I'.H. Yamabe. V.M. Hamza / Tectonophysics 253 (1996) 209-225

Measures taken on the basis of this model has been successful in "'switching o f f " seismicity on at least two occasions. In view of this success we conclude that geothermal investigations can be of considerable use in understanding the nature of fluid m o v e m e n t s associated with near-surface earthquakes.

1. Introduction In the present work we report the initial results of geothermal investigations carried out in Nuporanga in the state of Sgo Paulo (Brazil) where occurrence of seismic activity has been found to be closely related to opening wells for groundwater. These investigations were carried out on the assumption that fluid movements associated with seismic activities are capable of producing transient changes in the local geothermal gradient and terrestrial heat flow. Though the importance of groundwater movement in an altering subsurface temperature regime has been known in connection with geothermal studies, very few attempts have so far been made to use this efficient technique for detecting fluid movements associated with seismic activity. We hope that the results in the present work will draw attention to the usefulness of integrated geothermal and seismological studies. Investigations on the variations in subsurface temperatures associated with seismicity seem to have been carried out mostly in the tectonically areas in Japan. Thus Notsu et al. (1980) reported coseismic temperature changes of 0.2 to I°C in a 7-m- deep well in the Usu volcanic region. Shimamura and Watanabe (1981) reported coseismic changes in temperatures of several tenths of a millidegree at depths of 100-210 m for earthquakes occurring in the Hokkaido region. Shimamura et al. (1985) suggested that the amplitude of such tele-temperature variations are related to the earthquake magnitude as well as the epicentral distance. Recently, Wakita et al. (1988), in a comparative study of short-term and intermediate-term geochemical precursors, reported temperature changes associated with the 1978 Izu Oshima-Kinkai earthquake. Their results seem to indicate precursory drop in temperatures of nearly 5°C followed by a coseismic rise of nearly 10°C. Such large-scale variations are without doubt induced by substantial fluid displacements occurring in the focal regions of earthquake activity. The nature of fluid movements associated with

shallow-focus seismic events are not known in detail but it is conceivable that displacements along fault planes within and sometimes even near the focal region would lead to alterations in local permeability and induce a redistribution of pore fluids. Such movements of pore fluids can easily induce alterations in the local geothermal field. The main objective of the present work is to present a summary of the results indicating short-term changes in the geothermal field of the Nuporanga area where subsurface fluid movements associated with opening of groundwater wells are believed to be responsible for the occurrence of seismicity. A detailed analysis of the seismological investigations will be presented elsewhere. There is abundant literature on the role of fluids in triggering seismic activity. For example there are many cases of seismicity associated with artificial water reservoirs, one of the well-known cases being that of the Koyna dam, India (Gupta et al., 1969). Bozovic (1974) presented a review of seismicity associated with artificial water reservoirs. Also many cases of seismic activity induced by fluid movements through wells have been reported. Evans (1966) and Healy et al. (1968) reported seismicity associated with forced injection of fluids into a deep disposal well near Denver, USA. Seismicity induced by injection of water has also been reported from Japan (Ohtake, 1974) and Canada (Milne and Berry, 1976). An attempt to control seismicity artificially by fluid injection has been reported by Gibbs et al. (1973) in the Rangely area, Colorado. Earthquakes related to hydraulic mining of salt deposits near fault zones in western parts of New York state were reported by Fletcher and Sykes (1977). There are, however, very few reported cases of seismic activity induced by a well drilled for oil or water. This may be because of the rare combination of specific geological conditions and in-situ stress distribution necessary for the occurrence of such a phenomenon. In order to induce seismicity the well must connect an upper perched aquifer with a rather " d r y " fracture zone below. The resulting self-injec-

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996) 209-225

tion of water through the well may increase the fluid pressure in the fracture zone, which under certain in-situ stress conditions may reach a level that permits movements along local fault planes. Berrocal et al. (1978) cited some cases of microseismic activity probably related to wells in the Paranfi basin.

2. History of seismicity of the Nuporanga region Nuporanga is situated close to the northeastern limit of the Paranfi basin at 20°44'S latitude and 47°55'W longitude. Geology of the area has been studied in some detail (Cottas, 1977). The major rock types cropping out in the region are tholeiitic flood basalts of the Serra Geral Formation and Cenozoic sedimentary formations. The flood basalts are 119 to 147 m.y. old (Amaral et al., 1966) and overlie sedimentary formations of Triassic to Jurassic age. Overlying these basaltic lava flows are the isolated pockets of Cenozoic sedimentary formations which include Bauru Group sandstone and younger unconsolidated sediments. A simplified geologic map of the area is shown in Fig. 1. Geotectonic investigations suggest that the Nuporanga region is not an active area and there are no known active faults in neighboring regions. There have been no systematic instrumental studies on the seismicity of the Nuporanga region, the nearest high-resolution seismic observatory, at that time, being about 650 km away in Brasilia. Apart from seismic activity associated with dams in the states of S~o Panlo and Minas Gerais, no seismic events have been reported from the northeastern parts of the Paranfi basin. Available historic records of the region are devoid of any mention of seismic events in the last 400 years (Berrocal et al., 1984). Nuporanga can therefore be considered as situated in an essentially aseismic region. Seismic activity seems to have started in May 1977 soon after completion of drilling a well (designated hereafter as COLABA) to a depth of 217 m in the southern part of the town of Nuporanga. Initially, during May and June the tremors were felt only in localities close to this well. By July and August the intensity and frequency of the events increased to such a level that it was felt over the whole town as well in neighboring areas. However, seismological and geothermal investiga-

211

tions could be initiated only by the end of September 1977, more than four months after the beginning of seismic activity. The seismological data are being analyzed and the results are to be published later (Yamabe and Berrocal, 1994). During the first half of December 1977 a second well (designated hereafter as FM) was drilled to a depth of about 140 m, about 1 km southwest of the COLABA well. A new surge in seismic activity occurred on December 14 and 15 soon after completion of this well. The initial seismic observations were made using a portable seismograph to record tremors but because of the high level of background urban noise existent in the area it was difficult to extract useful information from the records. Thus a macroseismic survey was conducted during the period August 22, 1977 to August 1978 to put together the history of local seismicity. Results of this survey, which involved several resident observers in the town, were used in estimating the intensities of the main events. Most of the events, felt as small subsurface explosions by observers living near the well, were estimated to have magnitudes less than two. There were, however, several larger events with intensities as large as IV on the Modified Mercalli scale. Most of the microseismic activity was located in the southern part of the town to the east of the COLABA well. There were also reports of isolated tremors from farm houses situated as much as 4 to 5 km from the town. The large number of tremors felt on December 14 and 15, 1977 caused panic among the local residents. A meeting of local and state government authorities was arranged in the following week, where it was suggested that a phase of continuous pumping of the COLABA well should be started as early as possible. This suggestion was made on the basis of inferences regarding subsurface fluid movements, drawn mainly using geothermal data available at that time (see Section 4 for a more detailed discussion). The continuous pumping was carried out for 37 days, from December 26, 1977 to February 1, 1978, and the effect on the seismicity was obvious when the number of events dropped off sharply in January. During February pumping was reduced to 4 hours/day. Seismic activity increased by the last week of February whereupon it was decided to increase pumping duration from 4 to 16 hours/day. This rate was

212

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996) 209-225

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T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996)209-225

213

PUMPING OF THE COLABA WELL

20

°oo 7 4h/day

15

10I I 5-

! I'

~1 iI o i

0

DECEMBER/77 30

JANUARY/78

60

FEBRUARY

90

MARCH

120

APRIL

150

Fig. 2. Frequency of seismic events per day noted by resident observers in Nuporanga during the period December 1977 to April 1978. The hatched area refers to the duration of pumping of the COLABA well. Note the phase lag between changes in pumping duration and swarms of microseismic events.

maintained for the next 54 days. By April 1978 continuous pumping of the FM well was started and pumping of C O L A B A was again reduced to 4 h o u r s / d a y by the end of April 1978. Seismic activity tapered off significantly after May 1978, with the two wells being pumped. The relation between pumping rates and frequency of events is shown in Fig. 2. 3. G r o u n d w a t e r wells in Nuporanga Several groundwater wells had been drilled in the neighboring areas of Nuporanga without inducing any seismic activity. Within the municipal limits of Nuporanga itself two wells had been drilled prior to the occurrence of seismic activity in 1977, one to a depth of 135 m in 1969 and another to a depth of 95 m in 1972. The first one (PR well) is located about half a kilometer from the southern town limit. Drilled

to a depth of 135 m it has been used for supplying water to the town since 1969 at the rate of 15 m 3 / h . The second one (CC well) was drilled in 1972 at the southwestern part of the town, about 2300 m west of COLABA. Drilled to a depth of 95 m, it was abandoned till 1977 because of the infiltration of surface water into the well through fissures in basaltic formations and holes in casing pipes. In 1978 this well was redrilled to a depth of 145 m and new casing pipes were installed. The C O L A B A well was drilled by a private company in 1977 at the southeastern part of the town, the total depth drilled being 217 m. As mentioned earlier seismic activity began after the drilling of this well was completed. During the first half of December 1977 a new well (FM) was drilled by the municipality of Nuporanga to a depth of 140 m at a site about 1600 m to the southwest of C O L A B A well, not far from the site of the first well, PR. The locations of these wells are shown in Fig. 3.

Fig. 1. Simplified geologic map of Nuporanga and neighboring regions in the northeastern part of S~o Paulo State, Brazil.

T.tt. Yarnabe, V.M. ttan za / Tectonopt yvicv253 (1996) 209-225

214

little additional tails of drilling cussion in this C O L A B A well

Practically no information is available on the drilling histories of the earlier wells, PR and CC. In the case of the FM well the driller's record indicates that basalt was encountered at a depth of about 20 m but

-47 ° 46'

information is available on the dehistory and lithology. Thus the dissection is limited essentially to the lithology and its drilling history.

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Fig. 3. Relative locations of wells within the urban limits (hatched area) of Nuporanga. The numbers below the well simbols refer to local altitude in meters.

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996) 209-225

The COLABA well was drilled for supplying water to a local milk refrigeration center. It passes through 16 m of alluvial deposits and then enters basalt of the Serra Geral Formation. No samples of basalt through which the well passed have been preserved. The driller's log indicated "dark to gray" basalt in the interval 16-81 m, amygdaloidal basalt from 81 to 183 m and dark basalt again from 183 to 217 m. Very little additional information about the well's lithology exists. During drilling, however, presence of a " d r y " fissure or cavity of vertical dimensions of 2-2.5 m was noticed by the driller at a depth of about 175 m. A pumping test was carried out on May 11, 1977 using a submersible pump at a depth of 92 m. During this test the flow rate was 35 m3/h and the dynamic level of water during pumping was determined to be at a depth of about 90 m, the static level being 19 m. The well is cased and cemented to a depth of 17 m to avoid seepage of near-surface waters through alluvial deposits. Two additional 24 hours pumping tests were carfled out with the aim of verifying the existence of any relation between short pumping and frequency of seismic events. None was observed. The second pumping test on October 12 was similar to the first one on May 1 l, using a submersible pump installed at a depth of 92 m. During this test sound of a "waterfall" within the well was noticed. When the pumping was stopped it took about 2 minutes for the water to rise from the dynamic level to the level of the waterfall. During the third pumping test on November 20, 1977 a smaller pump was used which pumped water at the rate of 24 ma/h. During this test the sound of the "waterfall" could be heard only for a few seconds after the cessation of pumping, indicating that this time the dynamic level of water was close to that of the waterfall itself. From these observations and from the known diameter of the well it is possible to calculate the depth of the waterfall. The calculated value is about 30 + 5 m.

4. Results of temperature measurements and its correlation with local seismicity patterns The initial geothermal work in Nuporanga consisted of making thermal logs in the COLABA and CC wells, measurements of temperatures in some

215

cisterns at the southern and eastern limits of the town and monitoring temperature of water during pumping tests. Later, measurements were extended to the PR and FM wells. Obtaining systematic measurements at regular time intervals proved to be a difficult task mainly because of the expensive logistical setup necessary for maintaining equipment and personnel for logging operations in Nuporanga, situated at a distance of nearly 400 km from $5o Panlo. Measurements were therefore made whenever it was possible to arrange facilities and resources for field trips between $5o Paulo and Nuporanga. The results of thermal logs in the four wells and of the measurements carried out during pumping tests are given separately in the following sections.

4.1. Thermal well logs Temperatures were measured using a calibrated thermistor thermometer. The probe was attached to a three-conductor logging cable and resistance measurements made at the surface using a modified Wheatstone bridge. The relative accuracy of the logging system is estimated to be better than 0.01°C but absolute accuracy in detecting temperature changes over longer periods (of as much as 8 months involved in the present study) is no better than 0.1°C. Measurements were usually made at intervals of 5 m while the probe was being lowered into the well. The results obtained for each well are discussed separately below:

4.1.1. The Colaba well The first thermal well log was made on 7 7 / 0 9 / 2 9 and subsequent ones on 7 7 / 0 9 / 3 0 , 7 7 / 1 0 / 0 6 , 7 7 / 1 0 / 0 8 , 7 7 / 1 0 / 1 5 , 7 7 / 1 1 / 1 0 and 7 7 / 1 1 / 1 1 . Logs were made to a depth of about 204 m where the probe was stopped by an obstacle. It was thus not possible to make measurements in the interval 204217 m. The vertical distribution of temperature in the well during the first, fifth and sixth log are shown in Fig. 4. Similar results obtained for the other logs are not shown in Fig. 4, to retain clarity in the presentation of data. The thermal well logs indicated the following important features: (1) presence of a constant temperature zone (CTZ) down to a depth of about 175 m;

216

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996) 209-225

indicated a "dry cavity" of 2 - 2 . 5 m in vertical dimensions. At this point it is convenient to consider the possible mechanisms responsible for the CTZ and the TGZ. Under normal conditions a constant temperature zone in a well is possible only in the

(2) a high-temperature gradient zone (TGZ) between 180 and 204 m; and (3) a gradual decrease of temperature with time in both the CTZ and the TGZ. Note that the separation between the CTZ and the TGZ occurs at the depth where the driller's log

24

TEMPERATURE 26 27 i i

25 I

(°C) 28 i

29 i

30

30 • [-]• COLABA

AL•

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•DO • [-]•

WELL

77.09.29

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AL~

LITHOLOGY Alluvial Deposits ACID

Dark Basalt

90 AL• A~

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Gray Basalt Red Yellow

A~

Amigdaloidal Basalt

Green

A~ A~ Aq •

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150 &3 • AS• Ar • AE~• AE •

180

•

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210

F i g . 4. Vertical distribution o f temperature during three logs in the C O L A B A well.

w e l l . A l s o shown is the simplified lithologic section o f the

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996) 209-225

presence of a strong heat transfer mechanism that is capable of offsetting the conductive thermal gradient established by the local geothermal heat flux. Usually this happens when there is substantial fluid flow in or around the well or if there is active convection. The second possibility is ruled out in the present case because basaltic formations are in general highly impermeable and very high thermal gradients are necessary to initiate convection in a 6" diameter well. Substantial horizontal fluid flow at the base of the CTZ can carry off vertical heat flow responsible for maintaining the geothermal gradient. This possibility is considered unlikely since the presence of the " d r y " cavity at the base of the CTZ argues against it. For the same reason the possibility of upward fluid flow starting at the base of the CTZ is also ruled out. On the other hand, pumping tests revealed the existence of a "waterfall" at a depth of about 30 m capable of supplying substantial quantities of water into the well. Therefore downward flow of water from an upper perched aquifer to the cavity zone at 175 m appears to be the most likely reason for the existence of the CTZ. In the interval 180-204 m temperature increases rapidly from around 25.3 to 29.0°C. The possibility of warm water flow at the base of the TGZ as responsible for maintaining a high gradient in this interval can be ruled out, because temperature measurements during pumping tests indicated no significant contribution of warm water from the lower parts of the well (see also the discussion in Section 4.2). The temperature gradients in this interval, a summary of which is presented in Table 1, are found to be highly variable from as low as 8 to as much as

217

288°C/km. The vertical variations are too large to be ascribed to changes in thermal conductivity or three-dimensional refraction effects. On the other hand, downward flow of cold water to the base of the CTZ can induce rapid cooling of the upper layers of the TGZ and can thus generate anomalously high gradients. Since the general shape of temperature curve in the CTZ shows that most of the fluid flow terminates at the top of the TGZ, due probably to the existence of a zone of water loss at this depth, we consider heat transfer by water flow to be the dominant mechanism responsible for this unusual temperature distribution in the COLABA well. The temporal variations in the thermal field and its relation to seismicity are of considerable interest in the present context. First we note that the magnitudes of temperature drops in the TGZ are much higher than those in the CTZ. Also there is a general tendency for the gradients in the upper part of the TGZ to decrease with time. To look at this problem in a little more detail we present in Fig. 5 temperatures measured in the TGZ at depths of 170, 180, 190 and 200 m, for the period September-November 1977. Also indicated in this figure are the main periods of seismic activity. It is clear that the data can be interpreted as indicative of significant temperature drops occurring after periods of main seismic activity. Unfortunately, the sampling rate of thermal data is poor making it difficult to extract information regarding the exact form of variation. Hence the dashed lines connecting data points in Fig. 5 should be considered only as a schematic indication of short-term temperature changes in the TGZ. The variation of interval temperature gradients also pro-

Table 1 Interval temperature gradients observed during seven logs in the lower part of the COLABA well in 1977 Temperature gradients (°C/km) Date:

09/29

09/30

10/06

10/08

10/15

11/08

11/11

22 20 98 132 128 178 108

10 76 136 104 108 148 120

12 86 106 140 108 150 193

12 96 120 120 150 94 288

8 34 126 104 124 164 240

2 14 42 50 184 283 283

0 0 8 106 206 252 45

Depth interval(m) 170-175 175-180 180-185 185-190 190-195 195-200 200-204

7\H. Yamabe, V.M. Hamza / Tectonz)physics 253 (1996) 209-225

218

COLABA

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Fig. 5. Temperature at different depths in the bottom part of the COLABA well. The broken lines connecting the data points indicate one of the possible form of temperature variations. The main periods of seismic activity, with more than twenty events per day, are indicated as stippled bars. vide additional information on the nature of changes in the thermal field. They are indicative of large-scale variation in the local vertical heat flux and can be interpreted as resulting from the operation o f an unsteady heat transfer process. 4.1.2. The CC well Three thermal logs were carried out in this well in 1977 to depths o f nearly 90 m and seven more in 1978 after the well was redrilled in January 1978 to 140 m, Disturbances induced by initial drilling activities in 1972 are considered to be absent in the logs of 1977. The only other possible source of disturbance is the one related to the rather infrequent use of this well during the period 1972-1977. However, the magnitude of this disturbance is believed to be small as the duration of pumping was only a few hours per week and the time elapsed between the last pumping activity and the first thermal log was more than a month. The first of the seven thermal logs in 1978 was, however, made soon after redrilling in January 1978. The duration of this redrilling was less than a week and the time elapsed between the cessation o f drilling and the first log was approximately

two days. Thus, one may expect to find the effect of drilling disturbances in the set o f thermal logs made in 1978. Lachenbruch and Brewer (1959) show that for shallow wells the magnitude of drilling disturbances falls to within 0.05°C for times larger than three times the drilling period and the gradients return to near equilibrium values within a few days at depths less than 300 m. According to the model of Lachenbruch and Brewer (1959) the magnitude of disturbance is larger in the upper part of shallow boreholes. In the present case examination of thermal logs in 1978 do not reveal any anomalous time-dependent behavior in the temperature distribution in the upper part o f the CC well relative to those at deeper levels and hence the presence drilling disturbance relaxation can be considered to be small and possibly within the detection limit. The results of some of the thermal logs at the CC well are shown in Fig. 6. They indicate a gradient of about 1 8 ° C / k m for the interval 2 0 - 9 0 m and a higher gradient of 3 4 - 4 7 ° C / k m for the interval 1 0 0 - 1 4 0 m. The gradient values obtained during the different logs are given in Table 2. It is clear from Fig. 6 and data presented in Table 2 that time-dependent changes in

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996) 209-225 TEMPERATURE ('C) 24

25

26

27

28

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219

temperature and thermal gradients have also taken place in this well; however, the magnitudes of such changes are much smaller compared to those observed in the COLABA well. Also the absence of a constant temperature zone suggests that heat transfer by fluid movement, if any, is not significant in and around this well. 4.1.3. The PR well Only one thermal log could be carried out in this well when the pump installed at a depth of 30 m was removed for maintenance in 1978. The time interval between cessation of pumping activity and the thermal well log was approximately four weeks. The long-term pumping activity of this well is likely to have modified the subsurface temperature distribution in the vicinity. Availability of a second thermal log would have helped in estimating the magnitude of this perturbation, but unfortunately the pump was

B, Ol=

TEMPERATURE 23

150

0

24 I

25

26

('C) 27

Fig. 6. Vertical distribution of temperature in the CC well during three logs. The second and third logs were made after the well was redrilled to a depth of 140 m.

28

PR WELL •

1

78.01,21

Table 2 Geothermal gradients in the CC well during ten logs Date of log

Depth interval (m)

Thermal gradient (°C/km)

77/10/12 7 7 / I 1/11 77/11 / 19 78/01/21

35-90 35-90 35-90 45-105 105-136 40-100 100-135 40-105 105-136

19.93 + 0.46 19.71 +0,20 20.77 + 0,53 25.15 -I- 1.34 48.83 + 1.93 20.41 4- 0.40 44.79+ 1.80 20.70 4- 0.26 43.404- 2.13 21.03 4- 0.36 44,844-9.23 17.95 -t- 0.32 36.43 4-0.68 19.29 + 0.28 44.69 4- 2.92 18.25 4- 0.73 37.90+ 2.30

78/02/28 78/03/23 78/04/20 78/05/23 78/07/06 78/08/20

40-100

100-136 40-100 100-136 30-100 100-135 40-100 100-135

50

•

A

,=,, ca

100-

150

Fig. 7. Vertical distribution of temperature in the PR well during the log on January 21, 1978.

220

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996)209-225

reinstalled before further measurements could be made. The depth distribution of temperatures obtained in the first well log is shown in Fig. 7. In the interval 6-105 m the temperature gradient is 17.07 + 0.52°C/km, not very much different from the gradient obtained for a similar depth interval in the CC well near COLABA. The long history of a pumping from 30 m depth has possibly not seriously affected the temperature distribution below 60 m. In the interval 110-135 m the gradient is considerably higher, the calculated value being 58.40 __+ 2.09°C/km. Such a change in gradient may in part be related to change in thermal conductivity of different types of basaltic rocks at different depths. In the absence of information on lithology it is difficult to estimate the magnitude of gradient change related to changes in thermal conductivity. 4.1.4. The FM well Three thermal logs were carried out in this well during 1978 and the results obtained are shown in Fig. 8. As in the case of the CC well these results are believed to be free of the drilling disturbances effects in view of the time interval of slightly over a month between the cessation of drilling and the first well log. The first two thermal logs indicate general convex-shaped temperature distributions in the intervals 20-60 and 60-140 m. However, there has been a small but significant reduction in temperature values during the period between these two well logs. The results of the third thermal log reveal a drastic reduction on the measured values along the entire length of the well, accompanied by a change in the shape of the temperature curve from convex to concave. Thus a dramatic change has taken place in the local geothermal regime within a span of less than three weeks, the magnitude and nature of which cannot be attributed to effects such as those induced by drilling activities. Turning our attention to the pattern of seismicity for the period February to March 1978 we note that February was a period of moderate seismic activity which began after the reduction of pumping rate of the COLABA well. In the time interval between the second and third thermal well logs, in March 1978, the seismic activity included two strong tremors on March 17 only five days before the third well log at the FM well. Could it be possible that these events

TEMPERATURE ('C) 25 I

24 I

26 I

27 I

28 i

FM WELL • • D

50

•

•

00

•

o•

G

78.01.21

7

78.02.28

•

78.03.22

• 0

•

0. uJ 0

100

•

U

•

[]

• []

•

150

Fig. 8. Vertical distribution of temperature in the FM well during three logs in 1978.

are responsible for the drastic changes in the subsurface thermal regime of the FM well? A definite answer to this question cannot be given in the absence of detailed thermal logs prior to and immediately after March 17. On the other hand, an affirmative response is plausible if we make the reasonable assumption that the relation between seismicity and temperature variations at the site of the FM well can be similar in nature to that observed for the nearby COLABA well (see Fig. 5). The results of the third thermal well log in FM can then be considered as an indication of fluid movement associated with seismic activity. The change in shape of the temperature distribution from convex to concave imply a change in the direction of flow of groundwater from upward to downward. However, in the absence of detailed observations during pumping tests, it is difficult to determine whether there are substantial flows within this well. Unfortunately further measurements could

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996)209-225

221

tudes of these corrections are small and opposite in sign and consequently the overall correction should not be in excess of a few tenths of a degree. Thus the aquifer temperature must be in the interval 25-26°C implying a corresponding depth range of 3 0 - 9 0 m. An important implication of this depth estimate is that most of the water come from a shallow aquifer while the contribution of warm water from greater depths, if any, is insignificant. Presented in Fig. 9 are the results of temperature measurements carried out during nine pumping tests, for the period October 1977 to August 1978. Also shown in Fig. 9 are the major periods of seismic activity allowing an easy visual examination of the pattern of seismicity in relation to variation on pumped water temperature. The most striking feature is the "saw-tooth" pattern of temperature variations and its relation to periods of seismicity. It appears that the temperature of the pumped water drops immediately after periods of seismic activity while there is a tendency for recovery of temperature during relatively quiet periods. As can be observed from Fig. 9 this cycle seems to have repeated several times. In view of the earlier discussion regarding

not be carried out in this well after the installation of a submersible pump in April 1978.

4.2. Temperature measurements during pumping tests Measurements of water temperature at the well head were carried out during pumping tests in an attempt to define the depth interval of the principal aquifer. The procedure adopted includes inserting the probe about one meter inside the pipe delivering water at the surface and monitoring temperature over periods of 1 to 2 hours. Preliminary tests indicated that after an initial period, varying from about 15 to 30 minutes, the temperature attained a stable value. This was considered as the temperature of the pumped water. During the first pumping test of the COLABA well, with a flow rate of about 36 m3/h, the temperature of water remained nearly constant in the interval 25.5-25.6°C. This value must be corrected for the heat generated by the pump system as well as for possible lateral loss of heat during upward flow through metallic pipes. Preliminary calculations, following the procedure of Hamza et al. (1986), show that for flow rates in excess of 20 m 3 / h the magni-

PUMPINGOF THE WELLS COLABA

FM

25.4

G" n25.2

',

¢3 ILl a.

? ~

¢ J I Ii

Q- 25.0

is

sl •

w ¢v"

'

,'

e,

z

iI

er" W 24.8 O. W p-

I

24.6

i

OCT/77

I

I

ii NOV

i

i DEC

,, J

I JAN/78

FEB

il i

,, MAR

5o,=,,

I

. APR

I MAI

, l

. i JUN

.

i

JUL

0

AUG

Fig. 9. Observed variations in temperature of pumped water from the COLABA well during the period October 1977 to August ]978. The broken lines connecting the data points indicate one of the possible form of temperature variations. The vertical bars at the bottom refer to the main periods of microseismic activity.

222

T.H. Yamabe, V.M. Hamza / Tectonophysics 253 (1996) 209-225

temperature profiles in the wells, the drops of the temperature values can be considered as a consequence of downward flow of cold water into newly opened fractures producing rapid cooling of the rock mass. During relatively quiet periods downward flow is much less and water that is stagnant in the fissures receives geothermal heat thereby registering a recovery of temperature to its equilibrium value.

(T - T o ) °

J

40

I

I

_ _ _

COLABA WELL

t

I,

L

7710.15

60

5. Inferences about the flow regime It is clear from the discussions in the previous sections that the unusual thermal profiles and timedependent variations in temperature observed in the wells in Nuporanga are indicative of a subsurface environment in which heat transport occurs not only by conduction but also by fluid movements. Estimates of the magnitude of such advective heat transport can be obtained from the temperature profiles for certain simple cases. Ramey (1962) presented a solution for the problem of heat transfer for fluid flow in a cylindrical conduit with radial heat losses. This method has been employed widely in the study of thermal regime of oil wells under producing conditions. Bredehoeft and Papadopulos (1965), based on an earlier work by Stallman (1963), presented a solution for the problem of simultaneous transfer of heat by conduction and advection in a homogeneous permeable medium. This method has been used extensively in the geothermal literature for evaluating fluid flow effects on the thermal regime (for example Cartwright, 1970; Mansure and Reiter, 1979; among others). Both methods assume steady-state conditions and therefore are not strictly appropriate for dealing with unsteady thermal regimes. However, for cases in which heat transfer by fluid flow is substantial (as is the case in Nuporanga), most of the short-period components of the transient effects are " s m e a r e d " out rapidly, leaving only a " t a i l " of long-period variation. Thus, steady-state solutions can be useful in extracting gross features of the thermal regime that is still undergoing slow changes. Thus in the present case the methods of Ramey and that of Bredehoeft and Papadopulos were adopted in obtaining a preliminary understanding of the subsurface flow pattern in Nuporanga. Ramey's method is appropriate for interpreting

0.10

0.00

80

A

E 100 "r I-.D. 14.1 O 120

140

A A=1000

160 &

Fig. 10. Example of fit provided by the method of Ramey (1962) for log data from the upper part of the COLABA well. The continuous lines are the theoretical curves for different values of the flow parameter (see text for details).

the thermal well logs at the COLABA well where there are strong indications that water flow is dominantly within the well. On the other hand, the method of Bredehoeft and Papadopulos is considered more appropriate for interpreting the temperature profiles in the CC well and the two first well logs in the FM well where the flow is considered to be within the formation. Both methods are essentially curve-fitting procedures in which the observed temperature profile is compared to a set of theoretical curves for different values of a parameter that determines the flow regime. An illustrative example of the use of Ramey's method is presented in Fig. 10 for the thermal log

T.H. Yamabe, V.M. Harnza / Tectonophysics 253 (1996) 209-225

f (F,Z/L)

0.0

0.0

0.4 I

0.8 '

I

Table 3 Values of flow parameters and velocities estimated using Ramey's model Well

COLABA

0.2

0.4

11='2"~~ ~

--

'

.,J 0.6--

~ Ib=7 0.8--

L=(45-25)m • L=(60--45)rn • L=(130-65)m 1.0-

223

FM

Log date

77/09/29 77/10/08 77/10/15 77/11/10 77/11/11 78/03/22

Depth interval

Flow parameter

(m)

(m)

45-165 45-145 45-170 45-140 45-180 35-135

10000 10000 5000 1500 10000 200

Velocity (m/s) 0.39 0.39 0.23 0.06 0.39 0.01

Presented in Table 3 are the final results obtained for the COLABA well using Ramey's method. As can be seen from this table the best fitting values of the flow parameter are relatively high, in the range 1000-10,000 m, implying significant rates of downward water flow in this well. The velocities calculated for this range of values of flow parameters are 0.06-0.39 m / s for the upper part of the COLABA well, above the zone of water loss at 175 m. Also included in this table is the estimate of velocity for the FM well based on the log data of March 22, 1978. The results obtained for the CC well using the method of Bredehoeft and Papadopulos are given in Table 4. In this case the calculated velocities are of the order of 10 -8 m / s , a value in the general range

Fig.11. Example of fit provided by the method of Bredehoeft and Papadopulos (1965) for the log data from the FM well. The continuous lines are the theoretical curves for different values of the flow parameter (see text for details).

obtained in the COLABA well on October 15, 1977. The continuous lines in this figure are the theoretical curves for two different values of the flow parameter that bracket the observed temperature profile. An example of the use of the method of Bredehoeft and Papadopulos is illustrated in Fig. 11 for the log data obtained on January 21, 1978 for the FM well. The continuous lines in this figure are the theoretical curves that best fit the data in different depth intervals. The velocities of fluid flow can be calculated from the values of the best fitting flow parameter, the geometry of the flow path and representative values of the heat transfer coefficients.

Table 4 Values of flow parameters and velocities calculated for the CC well using the model of Bredehoef and Papadopnlos Log date

Depth interval (m)

78/02/28

40-95 100-125 40-50 55-100

0.5 0.2 1.0 0.0

100-125

1.0

78/03/22

78/04/20

78/05/23

Flow parameter

Velocity (10 -g m / s )

1.3 0.0 -0.9

0.5 0.4 5.5 0.0 2.2 3.5 0.0 1.6

120-130

- 0.6

3.3

40-70 70-90 95-120 120-130

0.7 -0.4 0.6 -0.4

35-55 55-90 90-120

1.2 - 1.1 1.2 -2.3

224

T.H. Yamabe, V.M. Hamza / Tecwru~physics 253 (1996)209-225

often quoted for flows in permeable geologic media. However, it is nearly six orders of magnitude less than the velocities calculated for flow within the COLABA well.

6. Conclusions

The seismic activity in Nuporanga had its beginning after the drilling of the COLABA well and most of the initial seismic events were located close to it. Geothermal studies show that the COLABA well is characterized by an unusual temperature distribution not observed in any other well in the area. Analysis of the temperature distribution in the COLABA well points to the existence of a downward flow of groundwater from an upper perched aquifer to a zone of water loss, most probably related to the "dry cavity" noticed by the driller, at a depth of about 175 m. Temperatures in the zone below this cavity are changing substantially with time. Its relation to periods of seismicity indicate that significant temperature drops occur immediately after major periods of seismic activity in a similar manner to the temperature drops observed in pumped water at the well head. Substantial changes in the vertical distribution of temperature were also observed in the FM well after the seismic activity in March 1978. Thus significant changes in the geothermal regime are taking place in Nuporanga and these changes are intimately related to the pattern of local seismicity. These observations can be pieced together in molding a schematic model of the process responsible for seismicity in Nuporanga. According to this model, drilling of the COLABA well opened a channel for self-injection of water from a perched aquifer at shallow depth to a relatively dry fracture zone below, most probably connected with a dormant fault at depth. The model assumes that the static height of the water column in the COLABA well is sufficient to produce significant reduction in the sliding friction of basalt blocks, and release of latent seismic energy stored within the fracture zone. During pumping the water level falls to the dynamic level, consequently the pressure in the fault zone drops to a level probably insufficient to produce movements in the fault zone. The model thus allows a coherent explanation for the observed correlation between occur-

rence of seismicity and absence of pumping of this well. The fact that measures taken on the basis of this model have allowed seismicity to be "switched off" on at least two occasions attests to its success. In view of this we conclude that careful monitoring of temperature changes in a seismically active area can lead to identification of the focal region where fluid movements associated with stress changes are taking place. Hence once an area has been identified as prone to earthquakes, geothermal studies must be capable of pointing out specific localities where active deformation is taking place.

Acknowledgements

This work was carried out as part of a Master's Thesis project by the first author (Tereza Higashi Yamabe). We would like to thank the Faculty of Science and Technology of the Universidade Estadual de S~o Paulo and the Instituto Astronrmico e Geoffsico of the Unversidade de Sao Paulo for the support and cooperation without which it would have been difficult to carry out the field work in Nuporanga. The project received financial support from Conselho Nacional de Desenvolvimento Cientffico e Technolrgico, CNPQ (Process no. 2222.0870/79). We are thankful to Cooperativa de Laticfnios de Batatais and the Mayor of the Nuporanga municipality during 1977-1978 for permission to make temperature well logs and to the resident observer Mrs. Elvira Pedroso Lellis for her careful and detailed observations of seismic events, which had been of invaluable help in understanding the history of seismicity in Nuporanga. The present work has benefited from many fruitful discussions with our colleagues Drs. Jesus A. Berrocal, Remy D. Antezana and Marcelo Assump~go.

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