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Geothermal energy of reservoirs and heat sources in Japan estimated from heat flow measurements and others
Geothermics (i97 o) - SPECIALISSUE2 U. N. Symposium on the Development and Utilization of Geothermal Resources, Pisa 1970. Vol. 2, Part I
Geothermal Energy of Reservoirs and Heat Sources in Japan Estimated from Heat Flow Measurements and Others
M. HAYAKAWA * ABSTRACT The Author discusses the possibility of balance between heat and water supply and discharge of steam in a geothermal field. Moreover it is possible to c'alculate the e n e r ~ of the ~,eothermal fields in Japan taking into account the related knowledge of the energy of hot springs. After extimating the geothermal energy, some comparisons of it with the energies of earthquakes, volcanic eruptions and again hot springs and also heat flow values in Japan are presented. Future prospects for the development of geothermal power generation are examined.
Introduction
There are many hot springs and .~eothermal phenomena owing to post-volcanic activity in |at)an. and there are some wells in these places, from which the near surface temperature distributions can be measured. In such places, the value of heat flow consists of that derived not only from the usual thermal conduction, but also from the heat transportation by liquid, steam or gas. And the temperature curves show a steep slope in the shallow part and then those sloopes become gentle. Generally speaking, in lapan, in the ease of the shallow Dart, the heat flow is 30-40 HTU and in the deep part this is 4-5 HFU and the average becomes 12-13 I-IFU. These values seem to be rather lower than expected; however, taking note of the heat flow into the well through the well wall from the surrounding rocks after the completion of the well and, considering furthermo,'e the outward heat flow through the outlet of th~ well after the start of the steam jet, the victurc completely changes. For example, in the ease of Matsukawa. these values become 2.3× 103 HFU before, and 3 × 109 HFU after, respectively. Such large amounts can only be obtained in the ease of concentration of outward heat flow through the well. Now, let us consider the penetration speed of hot water into the well from the area's rock formation. This is 5 × 10"~ era/see. On the other hand, the permeability of rock core sample shows 10" to 10"~ era/see. This may indicate that the major part of the liquid which has entered the well through the well surface has been through fissures or cracks. * College of Marine Sciences and Technology, Tokai University, (formerly Geological Survey of Japan), Japan.
Then, how many years can such a steam jet continue in such areas? After making certain assumptions, the result is in the order of 10' years. By using these data, the geothermal energy of reservoirs and making certain assumptions, the energy of heat sources are being studied. The possibility of balance between supply of heat and water and discharge of steam is discussed, and it is concluded that these quantities are well balanced for the present in the case of Matsukawa. Developing these results, it is possible to calculate the energy of the geothermal fields in lapan, by taking into account the related knowledge of the energy of hot springs in lapan. After estimating the geothermal energy, some comparisons of this with the energies of earthquake occurrences, volcanic eruptions and again hot springs, and also the heat flow values in lapan, were made, and also the relations between them, including the mechanics of these, were discussed. Finally, future prospects for the development of geothermal power generation were examined. The content of this paper is divided into three parts as follows. 1) Energy estimation in one geothermal area, Matsukawa, in lapan. 2) Energy of lapanese geothermal fields and its relation to the energies of earthouake occurrences, volcanic eruptions, hot springs and heat flow values. 3) Future prospects for the development of geothermal electric power. Energy e s t i m a t i o n in one g e o t h e r m a l area, M a t s u kawa, J a p a n
There are many hot springs and geothermal phenomena owing to post volcanic activity in Iapan. The utilization of natural steam from volcanic sources for generating electric power has already been started in lapan. By taking this opportunity, near surface temperature distributions have been obtained by drilling wells. For example, some suitable geothermal places for this purpose can be noted. These are Matsukawa, Onikobe, Nasu Oshirakawa, Takenoyu and Otake (from north to south) 459
As is already well known, the value of heat flow in such places consists of that derived not only from usual thermal conduction, but also from heat transportation by liouid, steam or gas. Generally, the temperature curves in the wells show steep slopes in the shallow part and then these slopes become gentle in the deeper part. From these data, the heat flow in the case of the shallow part is 30.40 HFU and in the deep part this is 4-5 HFU and the average becomes 12-13 HFU. These heat flow values are seem to be rather lower than expected; however, taking note of the heat flow into the well from the surrounding rocks through the well wall after completion and, considering furthermore the outward heat flow through its outlet after the start of the steam jet, the picture completely changes. Let us take as an example Matsukawa. The recovery. temperature curve of one well at Matsukawa is shown by the present writer in a separated paper, for this Symposium, for the seismic study in geothermal area. By using these curves the heat flow into the well from the surrounding rocks through its wall after completion can be calculated as follows (Figure 1). Volume of water in the well ,~ r = h -- 3 X 10Tcms (r = 10 cm) Heat necessary to raise the temperature of water in the well up to 200°C:
Therefore, Q=l×
50 x 106 x 150 = 3 x 103cal/cm~ see 3600 x ~ x 15~ ~_3×I0 HFU
This is a really wonderful heat flow value. Such an unexpected amount can only be obtained in the case of concentration of outward heat flow through the well. Next, let us consider the speed of penetration of hot water into the well from the area's country rock formation through slotted casing. This is obtained by dividing the volume by the total area of slots on the surface of the well through which the liquid could penetrate into the well, consequently the speed of penetration becomes V
2~rh"
O.O5 c m l s e c =
x 0.1 + ~ :
(r=10
a X
10--2 c m / s! e c
cm)
(0.1 is the percentage of slots to well surface area). On the other hand, the values of porosity and permeability given by using rock core samples are different from those obtained above. In Table 1, you will see the values of porosity detained by using rock core samples at various depths in some well at Matsukawa. The average value of porosity becomes 1 3 % a n d the corresponding permeabilities are between 10-s - 10.9 cm/sec.
3 x I0~x200_--6x 10gcal T^m~s 1. ~
Surface area of well (inside), through which the formation heat can enter the well: 2~crh" +
~P
--
3XIO g cm2
It requires 10 days to arrive at this stage, therefore heat flow in the well from the surrounding rocks through its wall becomes: 6x 3 x I0 ~ x
I09 cal
8.6 x
=2.3
lO'cm'sec
x
_.: 2.3
10-3cal/cm~sec x
10~
HFU
This is already an enormous amount. Now, let us consider the latter case, that is the outward heat flow Q through the outlet of the well after the start of the steam jet. In this case, H. TAgEOCHI and the writer considered this problem as follows.
Porosity o/ rock cores at Matsukawa.
Depth (m)
Porosity (%)
560 - 567 597 - 603 645 - 665
18.5 - 20 11.5 - 12 19 27
702 - 710
9
Depth(m) Porosity(%) 7 1 0 - 720 772 . 778 780 - 800
4.2 9 15
These are very interesting matters, because the speed of penetration of hot water is only 109- 10s cm/sec in the case of rock samples, while the value obtained practically from the well shows 5 × 10.2 cm/ sec; this may indicate that the major part of liquid which entered the well through the well surface, has passed through fissures and cracks of the formation, and not by permeation. Then next, what is the outward speed of the steam jet itself from the well through the outlet? This can be calculated by the next simple equation.
where O
c p V r T
= cpVfT.To)/~r
2,
specificheat of steam, densityof steam, volume of steam (converted to liquid) 50 ton/hour radius of outlet of borehole f15 cm) temperature of steam (150°), To original temperature of precipitation (O°C)
460
I/ x 2000,~-500m/sec
=,.,,
where 2000 is the specific volume expansion of steam from liquid at 1 kg/cm ~- under 160°C. The observed value of the speed of steam at just outside the well is also 500 m/sec.
Assuming the existence of pure steam, these results of such high speeds seem to be rather strange by theoretical considerations. However, assuming the coexistence of pure steam and liquid, such speeds can be reasonable (O. RUMI 1967). We have already seen the enormous amount of outward heat flow from the outlet at very high speed as was explained above; then how many years can such a steam jet continue in such an area? To reply to this question, H. TAKZUCm and the writer considered this problem as follows. First, we have to consider the heat capacity of the original steam from the magma and then the heat brought out from well per unit of time (second). For the former, it can be calculated by the following equation: cpATV where c specific heat of steam, • density of steam, V volume of steam (here, 10% of the total volume of the magma, whose radius 2.2 km is assumed as the volume of steam in the magma), AT temperature difference " between the inside and the outside of magma (here, 400°C is assumed as this difference). Therefore, this value becomes as follows, 4
1 x 400 x ~ n ( 2 . 2 x 10~)~ x 0 . 1 = 1 . 7 x 10l~ cal For the latter, c p V (TwTo) - 2 X 10e cal/sec There are four wells; consequently, the total heat brought out from four wells per second becomes 8 × 10" cal/ sec. Therefore, the life of the steam becomes: 1.7 x l0 is Life of s t e a m ~ 8x106
2 x 10"sec
As one year is 3 X 107 sec, consequently, the life of steam becomes the order of 10' year. Well, then, it will be interesting to convert the heat flow energy of steam to some other energy system such as seismic or volcanic ones.
The heat brought out from one well per hour becomes c ~ V (TwTo) = 7.5 × I0 9 cal/hour As one calorie becomes 4 . 1 8 × IOS erg, therefore, the above value is converted to 3 . 2 × i0 ~7 erg/hour. In another word, this is 3.2 × 1017 × 8.8 × 10s erg/year - 3 × 10t~ erg/year It is wonderful that during one year, such a lot of energy can be discharged. From four boreholes, we have about 1022 erg/year. Now, let us consider the possibility of energy balance between supply of heated water and discharge of steam.
In this case, at Matsukawa, Discharge/year = 200 ton X 8760 h = 1.8X I(Y ton/year Geothermal area = 8 km 2 Precipitation/year = 1800 mm (_~ 1.8 m) Consequently, the ratio of discharge to total precipitation becomes discharge precipitation
1.8 x 10~ 1 - - 1.8 x 8 x 100 ~--- ~ ------0.125
This value is very interesting, because this may suggest the possibility of the circulation of water (from outside) in the underground formation by considering the value of porosity and permeability. Finally, it will be interesting to calculate the mechanism of the hydrothermal system. Can the heat raise the temperature of the water to the necessary temperature within limited times? For this, first we have to consider the amount of deposit of hot water and discharge per year (the geothermal area = 2 km × 4 km). The deposit of hot water is 2000m x 4000m x 1 0 0 m ~ 8 × l0 s ton (100 m is the effective thickness in the case of well of 1 km depth). The discharge/year = 1.8× 10~ tons. Therefore the life of steam by only the present deposits becomes 8xlos 1.8 x i0~ - 400 years Besides this, the heat supply from depth is 2 . 3 × 10.3 cal/cm 2 sec, therefore the necessary time to increase the reservoir temperature to 300°C by heat supply from depth becomes: 8 x los x lOs x 300 eal 2.3 x 10--s x 8 x 10'° cal/sec
40 years
If the porosity of the bottom surface of the reservoir is 0.1 the necessary time becomes 400 years. Anyway, it is possible to consider that the heat can raise the temperature of the water to the necessary temperature within limited times. This means a good balance between discharge and supply. Now, let us look at the situation of another geothermal field, Otake, in Kyushu. In this case the geological structure related to the geothermal hydrotherreal system is slightly different from that of Matsukawa. Namely at Otake, owing to T. N o o u c m ' s research work, in the course of drilling work, a loss of circulation water is frequently encountered at every bore, owing to its entry into porous formations of fissures. At Otake, the rocks are compact and impermeable, so the positions where escaping water occurs, are believed to 461
be cracks. Here, the word <~crack >~ is used as a composite meaning of joint, fissure, fault etc. At the horizons of cracks, temperatures measured shortly after the circulation of drilling water ceased, showed a decrease, but with the lapse of time this decrease disappeared. This is due to the heating effect of escaping water, and it is believed that most bore discharges are fed from these cracks. In the area now under development, a few cracks have been met with at every bore, regardless of depth or location. The coefficient of permeability of fresh andesite, which was measured on some boring cores, showed a value as small as 10"9 - 10"1° era/see. It however gradually increases when rocks undergo alteration and permeability is being studied now. Fine cracks are often found in altered rock. In such a case, regardless of the degree of alteration, the permeability increased as high as 10-4 era/see. Since the permeability of andesite is small, the altered zone will not be formed all at one time. It is considered that alteration is likely to take place quite close to the crack wall at first, and with the increase of permeability of this part, the adjoining part undergoes alteration in turn. In this way, the altered zone extends far and wide. Anyway, the coefficients of permeability of rock at Otake geothermal field were in the order of 10 1°10" era/see in the experiment. However, T. Nooucati noticed that the permeability obtained from the outflow from the outlets of boreholes showed the value of 10 2
cm/sec. Therefore, although the geological condition at Otake is different from that of Matsukawa, we can say the similar information obtained at Matsukawa, will be seen in the case of Otake, too. That is, a difference in the values of permeability between laboratory and field, may indicate that the major part of the liquid which entered the well has been through fissures or cracks of the formation, and not by permeation. Besides this, except for a few details, there are many similarities between Matsukawa and Otake geothermal fields; for example, the total outflow of steam from the boreholes, temperatures in boreholes and others; consequently, it is quite possible to apply the results which the writer obtained in the ease of Matsukawa to Otake geothermal field, including the values of geothermal energy. Energy of J a p a n e s e g e o t h e r m a l fields a n d Its relation on the e n e r g i e s of e a r t h q u a k e occurrences, volcanic e r u v t i o n s , h o t springs a n d h e a t flow v a l u e s
In the first half of this paragraph, the energy of the lapanese geothermal fields is explained by using the results of the previous paragraph, and in the second half, the relation of energy between geothermal fields, earthquake occurrences, volcanic eruptions, hot springs and also the heat flow values in Japan, are discussed. 462
From the previous paragraph, we can say that the geothermal energy from one small unit area of geothermal field in Japan is the order of 10" erg/year. As will be seen in the next paragraph, we have a great possibility of being able to increase the geothermal electric power of Matsukawa and Otake to several times the present amounts in a few years; therefore, we can expect that a unit of geothermal energy from one geothermal field will be in the order of 5 × 1022 erg/year. On the other hand, we will be able to take into account ten geothermal fields for practical economic use by exploitation, in the near future, although there are about 100 geothermal fields in Japan; that means that the exploitable geothermal energy for the near future in lapan, will be in the region of 5 × 1023 erg/year. This corresponds to the electric power of one million kilowatts. Of course, I might be saying too much about the amount of power generation at the present stage considering the following facts, that the total world use of the earth's natural heat as an energy source, mainly in Italy, New Zealand, Iceland, the United States USSR, Mexico and Japan, has now reached about one million kilowatts, and Japan's power from this source totals only a little over 30,000 kilowatts, and a power of 100,000 kilowatts can be reached in one or two years. However, it is not too much to say that practically we have still much more geothermal energy than that mentioned above in Japan. In the following, this matter is explained. D..E. WHITE made a natural heat flow table (D. E. WHITE 1965). By using this table, the total heat flow quantities are 2.7× 10° eal/sec, in other words this amounts to 9.5× 10 i6 cal/year. In this table, the values of Matsukawa, Otake, Onikobe and similar places are written, as unknown values, because of insufficient data from Japan at that time. In this case. 7.5 × 101~ cal/year are necessary to generate the electric power of one million kilowatts; consequently, it becomes possible to generate 13 million kilowatts by using the above obtained amounts of energy. Because (9.5× 10 TM) : (7.5× 10 ~) ~
13.
And the life of these becomes (2 × 1021 cal) : (9.5 × 10 ~6 cal/year) ~ after WroTE.
2 × 104 year
In this case, 2)< 1021 cal is the amount of stored heat energy for the present, apart from future heat energy. Now, let us consider the lapanese case. The writer wishes to apply the study by T. FUKUTOMI for this purpose (FuKUTOMI 1961). In his paper, he explained and discussed the discharging pattern of hot springs, the mixing of groundwater with hot spring water held underground from a few score meters to several hundred meters, heat energy flowing out from a hot spring locality, and the relation between hot spring phenomena
and volcanic activity by using the hot springs in Okkaido. He calculated the total heat energy Qu which is discharged from hot springs in Hokkaido, and he obtained the value of
2 × 1024 erg energy corresponds to the Gutenberg Richter's earthquake magnitude, M = 8.3, this is the energy of an enormous earthquake. 5 × 10TM cal/year energy corresponds to half of the world heat energy according to WHITE. Therefore, the writer thinks that after WmTE'S table supplied with the remaining energy values which were written as unknown so far in that table, the total heat energy be increased more. As already mentioned above, at present, at Matsukawa, 20,000 kilowatts are being generated; this corresponds to the energy of 1.2 × 1.022erg/year (M = 6.8). By taking these facts of 2 X 10z' erg/year and 102: erg/year in lapan and Matsukawa, respectively, into consideration, the total heat power from the hot springs in Japan becomes
OH = 6.3 X 109 cal/min
By using this value, he calculated the total heat flow energy from the hot springs in Japan Oj including Hokkaido, the Mainland, Shikoku and Kyushu. In that case, at first, the fact that the total numbers of hot springs in Hokkaido and in all lapan are 90 and 1113, respectively, was taken into consideration. N~mely, O j a = 6 . 3 × l0 g cal/min× ~1113 -sX
_
10TM cal/min
and also, similar calculations were made by him, by applying the fact that the numbers of gushing out sourees of hot springs on the ground surface in Hokkaido and in all Japan are 411 and 10,578, respectively, Qj~ = 6.3 x 109 cal/min x 10,578 41--~ -
16 X 101°cal/min
Namely, the total heat flow energy of hot spring areas in lapan is in the order of 10" cal/min. In other words, this is 5 X 10~6 cal/year Converting the calorie into erg, this value becomes 2 × 102` erg/year In this case, of course, the energy related to the present active volcanoes and the energy to be generated in the future as magmas, were excluded.
®
20,000 kW x
On the other hand, by applying WHITF'S results, this becomes about 6 million kilowatts. In the case of hot springs, the hot fluids are largely obtained from the shallow parts, say about 300 meters deep; therefore, if we take into account the fact that the deeper hot fluids are being utilized for geothermal electric power, (for example, to the depth of 600 m), we can expect the geothermal pov, er as follows, 5 million kW X 2 = 10 million kW. Now, at this stage, let us consider the energy of earthquake occurrences, volcanic eruptions, hot springs again, and the heat flow values in Japan. Ca. Tst;Bol made a very useful diagram (Figure 2). This diagram shows the integrated earthquake energy released in and around Japan, during 79 years from 1885 till 1963. In the abscissa, the time (in years) is
® zs X 10
2 X 1 f.;-"~ l X l 0 z2 - 4 m i l l i o n k W
®
erg 6
ET J I
I I
t
J
!
t
!
,,'yf'/
1oo-
50/
~ 4
!
0 j
,eso ,.oo ,,,o
,uo
,,,.lo,..o (gfter
H,.o ,se.
y0a,
Ch. l"sub,,i )
1) Scheme o] a w e l l / o r the calculation o/ the heat /low into it #ore the surrounding rocks. 2) Earthquake energy released in the Japan area from 1885 to 1963. 3) Volcanic areas in ]apan.
F I G S . I , 2 , 3. - -
463
showed, while in the ordinate the integrated earthquake energies are plotted. Ca. T s u s o I made many instructive conclusions. Here, the writer wishes to utilize some of these results. As you will see from this diagram, the average energy of earthquakes per year becomes 2 × 10'~ erg/year And the differences between two inclined parallel lines, shows the value of 2.5 × 102~ erg/year (M =
8.4)
this corresponds to the maximum earthquake in Japan. T. MATSUZAWAmade a so-called Matsuzawa's process for seismic activity; this is a kind of heat engine, assuming the phase transition of rocks in the upper part of the mantle (M. MATSUZAWL H. HAS~OAWA 1954). Following his idea, it i'~ necessary to supply the heat energy of 8 × 10" cal/cm-~sec from the mantle to the earth's crust, to explain T s u ~ o I ' s result. Next, let us see the energy discharged by volcanic eruptions in Japan. K. NAKAMURA of the ERI of the University of Tokyo estimated the energy by volcanic activities. (K. NAKAMURA 1965). In his paper, various energies released in relation to a volcanic activity are classified and evaluated, based mainly on YOZOVAMA'S and SUOIMU~'S works. The energies are classified into four different categories, i.e. 1) heat transferred by solid and gaseous volcanic gas (Eth), 2) energy spent in expansion of volcanic gas (mainly water) (EeL 3) heat lost by underground conduction (EeL 4) work done against gravity (Ep). Eth
is estimated at ( 1 . 4 ± 0 . 3 ) X l O ~ ° M ( e r g ) where M is the total mass of solid eruptive product in grammes. Ee is estimated at 1.5× 109M (erg) in which the kinetic energy of explosion and of ground vibrations is included. Ec is difficult to estimate in direct connection with M. Ec consists of a part of the excess value of the heat flow of volcanic belts towards non-volcanic ones. Therefore, it need not be considered in the discussion where heat flow values are calculated independently. Ep can be almost neglected when we consider the energy economy including the crust and upper mantle as a whole, because compensative work should be done for the gravity, below and around the volcano. For the references, two tables are shown below. Thus, the order of magnitude of the whole volcanic energies is expressed as (1.6 + 0 . 4 ) × IO~°M erg ( + Ec) in terms of total mass of solid volcanic product, which is the only key to the magnitude of past volcanic activities. From these, we can calculate the energy involved in volcanic eruptions. (Besides this, I. YOKOVAMA calculated the average volcanic energy value per year as 9 X 10.6 cal/cm °"see).
Next, the hot springs energy was already known as described above; and the heat flow studies have mainly been made by S. UYEDA, in Japan. (S. UVEDA, TerrestrialHeat Flow in Japan, in many recent volumes of ERI, Univ. Tokyo). By him, it has been established that the heat flow distribution is quite distinct in and around Japan. O n the whole, the high heat flow region coincides with the Cenozoic volcanic region in the area, i.e., the lapan Sea side of the Honshu (mainland of
TABLE 2. --- Estimated energies of di]/erent jorras as released during a single volcanic activity. Energy released by
Oshima volcano, lzu 1953-1954 (x 1021 erg) 1950-1951 x 102a e r g )
r::at transferred by solid product (Eths)
7.1 ~ 6.7x 10° tremors 1 x 10--: Local shocks 5 x 10--4 <7 x 10----4 1.3 ~ 2.5 x 10-1
~zround vibration (Ev) explosive ejection (Ek) work done against gravity (Ep) Energy supplied to volcanic activity
1.9
~
8 . 4 x IO a
9.6~7.5x 10° 2 x I0----4 2 x 10--.* <7.3 x 10---~ >4.2~3.5 x 10--1 >l.lxlO a
Usu volcano (1943.1945) Showa-Shinzan( x 1024 erg) 3.2x 10° > 1.8 x 10-~ 5 x 10--4 (< 3.6 x 10----~) 2.9 x 10-1
Classification and general evaluation oi various energ'es released by volcanic activities. M denotes the total mass o! eruptive product.
TABLE 3.
-
-
x M (gr) x 10a0 erg 1. Heat transferred by volcanic product a. lavas and pyroclastics b. gas (water) c. dissociation of gas from magma 2. Energy expended in the expansion of gas (water) a. kinetic energy of explosion b. ground vibration 3. Heat lost by underground conduction (a part of terrestrial heat flow) 4. Work done against gravity 464
(Eths) (Ethw) Ed) (Re) (Ek) (Ev)
1.2 --0.2 0.22 0.003 0.15 < <0.18 (0.01) (0.01)
(Ec) total
[<0.6 x 10''e cal/cm: sac] !.6+0.4 (-i-Ec) 0.1 ~10 km (1~ 100 kin)
(Ep)
Japan) arc, and the low heat flow regions lie on the Pacmc Ocean side. The pattern of heat flow distribu. tion seems to have important bearings on many other geological and geophysical aspects, of the Japanese islands. Finally, the heat flow value in Japan is 2 x 10 .6 cal/cm: see in average. It is very interesting to compare these energy values. They are as follows, natural heat flow 2 × 10.6 cal cm: sec = 1 x 10a~ erg, year hot springs energy 1.5 (2.0)x 10-~ erg/year geothermal energy 5 x 10=4 erg. year (in the case of the amount to 1 km depth) volcanic energy 10:a + 10=4 erg/year earthquake energy 0.5 × 108 cal:cm ~ sec = 2 x 10a~ erg;year There are many interesting results here. One of them is that most of the assumed energy supplied from mantle to earth's crust has not yet been accounted for so far (probably, some of it has escaped laterally). Another matter is as follows: the natural heat flow is small compared with that of geothermal energy or hot spring energy. This depends on the process differences of transporting heat from deep to shallow parts; namely, the former has been done by heat conductivity, while in the second case, the energy has been trasported directly with hot liquid or steam or gas. Likewise, we may define the relation between the energy of earthquakes and that of volcanic eruptions; namely, the earthquake occurrence energy is stored in the rocks slowly; however, in the case of volcanoes, volcanic energies can be transported from the magmatic source to the shallow part directly by the help of steam and gas. Regarding geothermal energy, this is also very interesting, if we do not use this energy, probably, during long periods of time, the heat energy might be transformed into other forms, or escape slowly laterally. However, if we can utilize the geothermal energies from depth, we can extract a lot ol energy from deeper parts. In such a case, no one can deny that there are also the possibilities to decreasing earthquake and volcanic energy by using geothermal energy.
Future prospects for the development of geothermal energy utilization in Japan. As you will have seen from the previous paragraphs geothermal energy is enormous; however, there are some restrictions due to the balance of water and heat in suitable geothermal structures and also to the chemical compositions of hot fluids. But, we can estimate the rough value to be exploited in the near future in lapan. In Japan, there have been some tests, one of them has already been mentioned in the previous paragraph. Besides this, another estimation is made as follows. We can consider six volcanic zones (Figure 3) for the present purpose as,
A B (1) 100 k m X S 0 km
! j
(2)
fi X
50
~> X 5 0
~>
(3) 200
>> X 3 0
>>
(4)
50
~> X 5 0
,~
(5) I00
>> X 5 0 >> × 5 0
>> >>
(6) 100 where
A B C D
-----
C
D
0.22 k m =
capacity
5 . 1 X 1 0 '2 tons
length of side of each volcanic zone width of side of each volcanic zone estimated depth (1.5 km) porosity ( 15 % )
Therefore, if we assume to use these amount within 500 year, we can get the geothermal electric power of: 5 × 1012 10 million kW 4.5 × 10~ -because, for the generation of I0,000 kW, it is necessary to have the steam amount of 100 t / h - 8 . 8 × I03× I02 t/y consequently, for the 500 years' use, it is necessary to have the amount of 8.8X 10zX 102X5X 102 -- 4 . 5 × 108 tons The assumption of 500 years is sufficient, because in these years, it is quite possible to reach the required amount of the water in the reservoir and to heat it to the necessary temperatures. This means, theoretically we will be able to use these energies forever. Recently, T. N o c u c m evaluated tim geothermal energy in Japan by assuming the possible magma chamber behaviour, and he obtained similar results to the ones explained above. In the near future, the writer expects the development of the use of natural steam of volcanic origin to generate electric power, and to compete with nuclear energy. Japan has increased its efforts to use geothermal energy for generating electric power for reasons that are obvious: natural shortages of fossil fuels, which, with a few exceptions, are rare throughout the four islands and limited available sources of hydroelectric power. REFERENCES FUKUTOMt T. 1961 -- Rate of discharge of heat energy from the principal hot spring localitiesin Hokkaido, Japan. Y.
Fac. Sci. Hokkaido Univ. 1,351.
M^TSffZAWA T., HAS~G^WA H. 1954 - - Feld Theorie der Erdbeben elliptisches Wellen Gebiet. Bull. Earthq. Res. lnst., Tokyo.
NAK^MORAK. 1965 - - Energies dissipated with volcanic activi~ ties - Classification and evaluation. Bull. volcan. Soc. ]apan.
RUMI O. 1967 - - Determination of thermodynamic state of water vapor in steady adiabatic flow along a vertical pipe (well). Termotecnica, 8, 407. WroTE D. E. 1965 - - Geothermal energy. Circ. U. S. geol. Surv., 519. 465
Geothermal Energy of Reservoirs and Heat Sources in Japan Estimated from Heat Flow Measurements and Others
M. HAYAKAWA * ABSTRACT The Author discusses the possibility of balance between heat and water supply and discharge of steam in a geothermal field. Moreover it is possible to c'alculate the e n e r ~ of the ~,eothermal fields in Japan taking into account the related knowledge of the energy of hot springs. After extimating the geothermal energy, some comparisons of it with the energies of earthquakes, volcanic eruptions and again hot springs and also heat flow values in Japan are presented. Future prospects for the development of geothermal power generation are examined.
Introduction
There are many hot springs and .~eothermal phenomena owing to post-volcanic activity in |at)an. and there are some wells in these places, from which the near surface temperature distributions can be measured. In such places, the value of heat flow consists of that derived not only from the usual thermal conduction, but also from the heat transportation by liquid, steam or gas. And the temperature curves show a steep slope in the shallow part and then those sloopes become gentle. Generally speaking, in lapan, in the ease of the shallow Dart, the heat flow is 30-40 HTU and in the deep part this is 4-5 HFU and the average becomes 12-13 I-IFU. These values seem to be rather lower than expected; however, taking note of the heat flow into the well through the well wall from the surrounding rocks after the completion of the well and, considering furthermo,'e the outward heat flow through the outlet of th~ well after the start of the steam jet, the victurc completely changes. For example, in the ease of Matsukawa. these values become 2.3× 103 HFU before, and 3 × 109 HFU after, respectively. Such large amounts can only be obtained in the ease of concentration of outward heat flow through the well. Now, let us consider the penetration speed of hot water into the well from the area's rock formation. This is 5 × 10"~ era/see. On the other hand, the permeability of rock core sample shows 10" to 10"~ era/see. This may indicate that the major part of the liquid which has entered the well through the well surface has been through fissures or cracks. * College of Marine Sciences and Technology, Tokai University, (formerly Geological Survey of Japan), Japan.
Then, how many years can such a steam jet continue in such areas? After making certain assumptions, the result is in the order of 10' years. By using these data, the geothermal energy of reservoirs and making certain assumptions, the energy of heat sources are being studied. The possibility of balance between supply of heat and water and discharge of steam is discussed, and it is concluded that these quantities are well balanced for the present in the case of Matsukawa. Developing these results, it is possible to calculate the energy of the geothermal fields in lapan, by taking into account the related knowledge of the energy of hot springs in lapan. After estimating the geothermal energy, some comparisons of this with the energies of earthquake occurrences, volcanic eruptions and again hot springs, and also the heat flow values in lapan, were made, and also the relations between them, including the mechanics of these, were discussed. Finally, future prospects for the development of geothermal power generation were examined. The content of this paper is divided into three parts as follows. 1) Energy estimation in one geothermal area, Matsukawa, in lapan. 2) Energy of lapanese geothermal fields and its relation to the energies of earthouake occurrences, volcanic eruptions, hot springs and heat flow values. 3) Future prospects for the development of geothermal electric power. Energy e s t i m a t i o n in one g e o t h e r m a l area, M a t s u kawa, J a p a n
There are many hot springs and geothermal phenomena owing to post volcanic activity in Iapan. The utilization of natural steam from volcanic sources for generating electric power has already been started in lapan. By taking this opportunity, near surface temperature distributions have been obtained by drilling wells. For example, some suitable geothermal places for this purpose can be noted. These are Matsukawa, Onikobe, Nasu Oshirakawa, Takenoyu and Otake (from north to south) 459
As is already well known, the value of heat flow in such places consists of that derived not only from usual thermal conduction, but also from heat transportation by liouid, steam or gas. Generally, the temperature curves in the wells show steep slopes in the shallow part and then these slopes become gentle in the deeper part. From these data, the heat flow in the case of the shallow part is 30.40 HFU and in the deep part this is 4-5 HFU and the average becomes 12-13 HFU. These heat flow values are seem to be rather lower than expected; however, taking note of the heat flow into the well from the surrounding rocks through the well wall after completion and, considering furthermore the outward heat flow through its outlet after the start of the steam jet, the picture completely changes. Let us take as an example Matsukawa. The recovery. temperature curve of one well at Matsukawa is shown by the present writer in a separated paper, for this Symposium, for the seismic study in geothermal area. By using these curves the heat flow into the well from the surrounding rocks through its wall after completion can be calculated as follows (Figure 1). Volume of water in the well ,~ r = h -- 3 X 10Tcms (r = 10 cm) Heat necessary to raise the temperature of water in the well up to 200°C:
Therefore, Q=l×
50 x 106 x 150 = 3 x 103cal/cm~ see 3600 x ~ x 15~ ~_3×I0 HFU
This is a really wonderful heat flow value. Such an unexpected amount can only be obtained in the case of concentration of outward heat flow through the well. Next, let us consider the speed of penetration of hot water into the well from the area's country rock formation through slotted casing. This is obtained by dividing the volume by the total area of slots on the surface of the well through which the liquid could penetrate into the well, consequently the speed of penetration becomes V
2~rh"
O.O5 c m l s e c =
x 0.1 + ~ :
(r=10
a X
10--2 c m / s! e c
cm)
(0.1 is the percentage of slots to well surface area). On the other hand, the values of porosity and permeability given by using rock core samples are different from those obtained above. In Table 1, you will see the values of porosity detained by using rock core samples at various depths in some well at Matsukawa. The average value of porosity becomes 1 3 % a n d the corresponding permeabilities are between 10-s - 10.9 cm/sec.
3 x I0~x200_--6x 10gcal T^m~s 1. ~
Surface area of well (inside), through which the formation heat can enter the well: 2~crh" +
~P
--
3XIO g cm2
It requires 10 days to arrive at this stage, therefore heat flow in the well from the surrounding rocks through its wall becomes: 6x 3 x I0 ~ x
I09 cal
8.6 x
=2.3
lO'cm'sec
x
_.: 2.3
10-3cal/cm~sec x
10~
HFU
This is already an enormous amount. Now, let us consider the latter case, that is the outward heat flow Q through the outlet of the well after the start of the steam jet. In this case, H. TAgEOCHI and the writer considered this problem as follows.
Porosity o/ rock cores at Matsukawa.
Depth (m)
Porosity (%)
560 - 567 597 - 603 645 - 665
18.5 - 20 11.5 - 12 19 27
702 - 710
9
Depth(m) Porosity(%) 7 1 0 - 720 772 . 778 780 - 800
4.2 9 15
These are very interesting matters, because the speed of penetration of hot water is only 109- 10s cm/sec in the case of rock samples, while the value obtained practically from the well shows 5 × 10.2 cm/ sec; this may indicate that the major part of liquid which entered the well through the well surface, has passed through fissures and cracks of the formation, and not by permeation. Then next, what is the outward speed of the steam jet itself from the well through the outlet? This can be calculated by the next simple equation.
where O
c p V r T
= cpVfT.To)/~r
2,
specificheat of steam, densityof steam, volume of steam (converted to liquid) 50 ton/hour radius of outlet of borehole f15 cm) temperature of steam (150°), To original temperature of precipitation (O°C)
460
I/ x 2000,~-500m/sec
=,.,,
where 2000 is the specific volume expansion of steam from liquid at 1 kg/cm ~- under 160°C. The observed value of the speed of steam at just outside the well is also 500 m/sec.
Assuming the existence of pure steam, these results of such high speeds seem to be rather strange by theoretical considerations. However, assuming the coexistence of pure steam and liquid, such speeds can be reasonable (O. RUMI 1967). We have already seen the enormous amount of outward heat flow from the outlet at very high speed as was explained above; then how many years can such a steam jet continue in such an area? To reply to this question, H. TAKZUCm and the writer considered this problem as follows. First, we have to consider the heat capacity of the original steam from the magma and then the heat brought out from well per unit of time (second). For the former, it can be calculated by the following equation: cpATV where c specific heat of steam, • density of steam, V volume of steam (here, 10% of the total volume of the magma, whose radius 2.2 km is assumed as the volume of steam in the magma), AT temperature difference " between the inside and the outside of magma (here, 400°C is assumed as this difference). Therefore, this value becomes as follows, 4
1 x 400 x ~ n ( 2 . 2 x 10~)~ x 0 . 1 = 1 . 7 x 10l~ cal For the latter, c p V (TwTo) - 2 X 10e cal/sec There are four wells; consequently, the total heat brought out from four wells per second becomes 8 × 10" cal/ sec. Therefore, the life of the steam becomes: 1.7 x l0 is Life of s t e a m ~ 8x106
2 x 10"sec
As one year is 3 X 107 sec, consequently, the life of steam becomes the order of 10' year. Well, then, it will be interesting to convert the heat flow energy of steam to some other energy system such as seismic or volcanic ones.
The heat brought out from one well per hour becomes c ~ V (TwTo) = 7.5 × I0 9 cal/hour As one calorie becomes 4 . 1 8 × IOS erg, therefore, the above value is converted to 3 . 2 × i0 ~7 erg/hour. In another word, this is 3.2 × 1017 × 8.8 × 10s erg/year - 3 × 10t~ erg/year It is wonderful that during one year, such a lot of energy can be discharged. From four boreholes, we have about 1022 erg/year. Now, let us consider the possibility of energy balance between supply of heated water and discharge of steam.
In this case, at Matsukawa, Discharge/year = 200 ton X 8760 h = 1.8X I(Y ton/year Geothermal area = 8 km 2 Precipitation/year = 1800 mm (_~ 1.8 m) Consequently, the ratio of discharge to total precipitation becomes discharge precipitation
1.8 x 10~ 1 - - 1.8 x 8 x 100 ~--- ~ ------0.125
This value is very interesting, because this may suggest the possibility of the circulation of water (from outside) in the underground formation by considering the value of porosity and permeability. Finally, it will be interesting to calculate the mechanism of the hydrothermal system. Can the heat raise the temperature of the water to the necessary temperature within limited times? For this, first we have to consider the amount of deposit of hot water and discharge per year (the geothermal area = 2 km × 4 km). The deposit of hot water is 2000m x 4000m x 1 0 0 m ~ 8 × l0 s ton (100 m is the effective thickness in the case of well of 1 km depth). The discharge/year = 1.8× 10~ tons. Therefore the life of steam by only the present deposits becomes 8xlos 1.8 x i0~ - 400 years Besides this, the heat supply from depth is 2 . 3 × 10.3 cal/cm 2 sec, therefore the necessary time to increase the reservoir temperature to 300°C by heat supply from depth becomes: 8 x los x lOs x 300 eal 2.3 x 10--s x 8 x 10'° cal/sec
40 years
If the porosity of the bottom surface of the reservoir is 0.1 the necessary time becomes 400 years. Anyway, it is possible to consider that the heat can raise the temperature of the water to the necessary temperature within limited times. This means a good balance between discharge and supply. Now, let us look at the situation of another geothermal field, Otake, in Kyushu. In this case the geological structure related to the geothermal hydrotherreal system is slightly different from that of Matsukawa. Namely at Otake, owing to T. N o o u c m ' s research work, in the course of drilling work, a loss of circulation water is frequently encountered at every bore, owing to its entry into porous formations of fissures. At Otake, the rocks are compact and impermeable, so the positions where escaping water occurs, are believed to 461
be cracks. Here, the word <~crack >~ is used as a composite meaning of joint, fissure, fault etc. At the horizons of cracks, temperatures measured shortly after the circulation of drilling water ceased, showed a decrease, but with the lapse of time this decrease disappeared. This is due to the heating effect of escaping water, and it is believed that most bore discharges are fed from these cracks. In the area now under development, a few cracks have been met with at every bore, regardless of depth or location. The coefficient of permeability of fresh andesite, which was measured on some boring cores, showed a value as small as 10"9 - 10"1° era/see. It however gradually increases when rocks undergo alteration and permeability is being studied now. Fine cracks are often found in altered rock. In such a case, regardless of the degree of alteration, the permeability increased as high as 10-4 era/see. Since the permeability of andesite is small, the altered zone will not be formed all at one time. It is considered that alteration is likely to take place quite close to the crack wall at first, and with the increase of permeability of this part, the adjoining part undergoes alteration in turn. In this way, the altered zone extends far and wide. Anyway, the coefficients of permeability of rock at Otake geothermal field were in the order of 10 1°10" era/see in the experiment. However, T. Nooucati noticed that the permeability obtained from the outflow from the outlets of boreholes showed the value of 10 2
cm/sec. Therefore, although the geological condition at Otake is different from that of Matsukawa, we can say the similar information obtained at Matsukawa, will be seen in the case of Otake, too. That is, a difference in the values of permeability between laboratory and field, may indicate that the major part of the liquid which entered the well has been through fissures or cracks of the formation, and not by permeation. Besides this, except for a few details, there are many similarities between Matsukawa and Otake geothermal fields; for example, the total outflow of steam from the boreholes, temperatures in boreholes and others; consequently, it is quite possible to apply the results which the writer obtained in the ease of Matsukawa to Otake geothermal field, including the values of geothermal energy. Energy of J a p a n e s e g e o t h e r m a l fields a n d Its relation on the e n e r g i e s of e a r t h q u a k e occurrences, volcanic e r u v t i o n s , h o t springs a n d h e a t flow v a l u e s
In the first half of this paragraph, the energy of the lapanese geothermal fields is explained by using the results of the previous paragraph, and in the second half, the relation of energy between geothermal fields, earthquake occurrences, volcanic eruptions, hot springs and also the heat flow values in Japan, are discussed. 462
From the previous paragraph, we can say that the geothermal energy from one small unit area of geothermal field in Japan is the order of 10" erg/year. As will be seen in the next paragraph, we have a great possibility of being able to increase the geothermal electric power of Matsukawa and Otake to several times the present amounts in a few years; therefore, we can expect that a unit of geothermal energy from one geothermal field will be in the order of 5 × 1022 erg/year. On the other hand, we will be able to take into account ten geothermal fields for practical economic use by exploitation, in the near future, although there are about 100 geothermal fields in Japan; that means that the exploitable geothermal energy for the near future in lapan, will be in the region of 5 × 1023 erg/year. This corresponds to the electric power of one million kilowatts. Of course, I might be saying too much about the amount of power generation at the present stage considering the following facts, that the total world use of the earth's natural heat as an energy source, mainly in Italy, New Zealand, Iceland, the United States USSR, Mexico and Japan, has now reached about one million kilowatts, and Japan's power from this source totals only a little over 30,000 kilowatts, and a power of 100,000 kilowatts can be reached in one or two years. However, it is not too much to say that practically we have still much more geothermal energy than that mentioned above in Japan. In the following, this matter is explained. D..E. WHITE made a natural heat flow table (D. E. WHITE 1965). By using this table, the total heat flow quantities are 2.7× 10° eal/sec, in other words this amounts to 9.5× 10 i6 cal/year. In this table, the values of Matsukawa, Otake, Onikobe and similar places are written, as unknown values, because of insufficient data from Japan at that time. In this case. 7.5 × 101~ cal/year are necessary to generate the electric power of one million kilowatts; consequently, it becomes possible to generate 13 million kilowatts by using the above obtained amounts of energy. Because (9.5× 10 TM) : (7.5× 10 ~) ~
13.
And the life of these becomes (2 × 1021 cal) : (9.5 × 10 ~6 cal/year) ~ after WroTE.
2 × 104 year
In this case, 2)< 1021 cal is the amount of stored heat energy for the present, apart from future heat energy. Now, let us consider the lapanese case. The writer wishes to apply the study by T. FUKUTOMI for this purpose (FuKUTOMI 1961). In his paper, he explained and discussed the discharging pattern of hot springs, the mixing of groundwater with hot spring water held underground from a few score meters to several hundred meters, heat energy flowing out from a hot spring locality, and the relation between hot spring phenomena
and volcanic activity by using the hot springs in Okkaido. He calculated the total heat energy Qu which is discharged from hot springs in Hokkaido, and he obtained the value of
2 × 1024 erg energy corresponds to the Gutenberg Richter's earthquake magnitude, M = 8.3, this is the energy of an enormous earthquake. 5 × 10TM cal/year energy corresponds to half of the world heat energy according to WHITE. Therefore, the writer thinks that after WmTE'S table supplied with the remaining energy values which were written as unknown so far in that table, the total heat energy be increased more. As already mentioned above, at present, at Matsukawa, 20,000 kilowatts are being generated; this corresponds to the energy of 1.2 × 1.022erg/year (M = 6.8). By taking these facts of 2 X 10z' erg/year and 102: erg/year in lapan and Matsukawa, respectively, into consideration, the total heat power from the hot springs in Japan becomes
OH = 6.3 X 109 cal/min
By using this value, he calculated the total heat flow energy from the hot springs in Japan Oj including Hokkaido, the Mainland, Shikoku and Kyushu. In that case, at first, the fact that the total numbers of hot springs in Hokkaido and in all lapan are 90 and 1113, respectively, was taken into consideration. N~mely, O j a = 6 . 3 × l0 g cal/min× ~1113 -sX
_
10TM cal/min
and also, similar calculations were made by him, by applying the fact that the numbers of gushing out sourees of hot springs on the ground surface in Hokkaido and in all Japan are 411 and 10,578, respectively, Qj~ = 6.3 x 109 cal/min x 10,578 41--~ -
16 X 101°cal/min
Namely, the total heat flow energy of hot spring areas in lapan is in the order of 10" cal/min. In other words, this is 5 X 10~6 cal/year Converting the calorie into erg, this value becomes 2 × 102` erg/year In this case, of course, the energy related to the present active volcanoes and the energy to be generated in the future as magmas, were excluded.
®
20,000 kW x
On the other hand, by applying WHITF'S results, this becomes about 6 million kilowatts. In the case of hot springs, the hot fluids are largely obtained from the shallow parts, say about 300 meters deep; therefore, if we take into account the fact that the deeper hot fluids are being utilized for geothermal electric power, (for example, to the depth of 600 m), we can expect the geothermal pov, er as follows, 5 million kW X 2 = 10 million kW. Now, at this stage, let us consider the energy of earthquake occurrences, volcanic eruptions, hot springs again, and the heat flow values in Japan. Ca. Tst;Bol made a very useful diagram (Figure 2). This diagram shows the integrated earthquake energy released in and around Japan, during 79 years from 1885 till 1963. In the abscissa, the time (in years) is
® zs X 10
2 X 1 f.;-"~ l X l 0 z2 - 4 m i l l i o n k W
®
erg 6
ET J I
I I
t
J
!
t
!
,,'yf'/
1oo-
50/
~ 4
!
0 j
,eso ,.oo ,,,o
,uo
,,,.lo,..o (gfter
H,.o ,se.
y0a,
Ch. l"sub,,i )
1) Scheme o] a w e l l / o r the calculation o/ the heat /low into it #ore the surrounding rocks. 2) Earthquake energy released in the Japan area from 1885 to 1963. 3) Volcanic areas in ]apan.
F I G S . I , 2 , 3. - -
463
showed, while in the ordinate the integrated earthquake energies are plotted. Ca. T s u s o I made many instructive conclusions. Here, the writer wishes to utilize some of these results. As you will see from this diagram, the average energy of earthquakes per year becomes 2 × 10'~ erg/year And the differences between two inclined parallel lines, shows the value of 2.5 × 102~ erg/year (M =
8.4)
this corresponds to the maximum earthquake in Japan. T. MATSUZAWAmade a so-called Matsuzawa's process for seismic activity; this is a kind of heat engine, assuming the phase transition of rocks in the upper part of the mantle (M. MATSUZAWL H. HAS~OAWA 1954). Following his idea, it i'~ necessary to supply the heat energy of 8 × 10" cal/cm-~sec from the mantle to the earth's crust, to explain T s u ~ o I ' s result. Next, let us see the energy discharged by volcanic eruptions in Japan. K. NAKAMURA of the ERI of the University of Tokyo estimated the energy by volcanic activities. (K. NAKAMURA 1965). In his paper, various energies released in relation to a volcanic activity are classified and evaluated, based mainly on YOZOVAMA'S and SUOIMU~'S works. The energies are classified into four different categories, i.e. 1) heat transferred by solid and gaseous volcanic gas (Eth), 2) energy spent in expansion of volcanic gas (mainly water) (EeL 3) heat lost by underground conduction (EeL 4) work done against gravity (Ep). Eth
is estimated at ( 1 . 4 ± 0 . 3 ) X l O ~ ° M ( e r g ) where M is the total mass of solid eruptive product in grammes. Ee is estimated at 1.5× 109M (erg) in which the kinetic energy of explosion and of ground vibrations is included. Ec is difficult to estimate in direct connection with M. Ec consists of a part of the excess value of the heat flow of volcanic belts towards non-volcanic ones. Therefore, it need not be considered in the discussion where heat flow values are calculated independently. Ep can be almost neglected when we consider the energy economy including the crust and upper mantle as a whole, because compensative work should be done for the gravity, below and around the volcano. For the references, two tables are shown below. Thus, the order of magnitude of the whole volcanic energies is expressed as (1.6 + 0 . 4 ) × IO~°M erg ( + Ec) in terms of total mass of solid volcanic product, which is the only key to the magnitude of past volcanic activities. From these, we can calculate the energy involved in volcanic eruptions. (Besides this, I. YOKOVAMA calculated the average volcanic energy value per year as 9 X 10.6 cal/cm °"see).
Next, the hot springs energy was already known as described above; and the heat flow studies have mainly been made by S. UYEDA, in Japan. (S. UVEDA, TerrestrialHeat Flow in Japan, in many recent volumes of ERI, Univ. Tokyo). By him, it has been established that the heat flow distribution is quite distinct in and around Japan. O n the whole, the high heat flow region coincides with the Cenozoic volcanic region in the area, i.e., the lapan Sea side of the Honshu (mainland of
TABLE 2. --- Estimated energies of di]/erent jorras as released during a single volcanic activity. Energy released by
Oshima volcano, lzu 1953-1954 (x 1021 erg) 1950-1951 x 102a e r g )
r::at transferred by solid product (Eths)
7.1 ~ 6.7x 10° tremors 1 x 10--: Local shocks 5 x 10--4 <7 x 10----4 1.3 ~ 2.5 x 10-1
~zround vibration (Ev) explosive ejection (Ek) work done against gravity (Ep) Energy supplied to volcanic activity
1.9
~
8 . 4 x IO a
9.6~7.5x 10° 2 x I0----4 2 x 10--.* <7.3 x 10---~ >4.2~3.5 x 10--1 >l.lxlO a
Usu volcano (1943.1945) Showa-Shinzan( x 1024 erg) 3.2x 10° > 1.8 x 10-~ 5 x 10--4 (< 3.6 x 10----~) 2.9 x 10-1
Classification and general evaluation oi various energ'es released by volcanic activities. M denotes the total mass o! eruptive product.
TABLE 3.
-
-
x M (gr) x 10a0 erg 1. Heat transferred by volcanic product a. lavas and pyroclastics b. gas (water) c. dissociation of gas from magma 2. Energy expended in the expansion of gas (water) a. kinetic energy of explosion b. ground vibration 3. Heat lost by underground conduction (a part of terrestrial heat flow) 4. Work done against gravity 464
(Eths) (Ethw) Ed) (Re) (Ek) (Ev)
1.2 --0.2 0.22 0.003 0.15 < <0.18 (0.01) (0.01)
(Ec) total
[<0.6 x 10''e cal/cm: sac] !.6+0.4 (-i-Ec) 0.1 ~10 km (1~ 100 kin)
(Ep)
Japan) arc, and the low heat flow regions lie on the Pacmc Ocean side. The pattern of heat flow distribu. tion seems to have important bearings on many other geological and geophysical aspects, of the Japanese islands. Finally, the heat flow value in Japan is 2 x 10 .6 cal/cm: see in average. It is very interesting to compare these energy values. They are as follows, natural heat flow 2 × 10.6 cal cm: sec = 1 x 10a~ erg, year hot springs energy 1.5 (2.0)x 10-~ erg/year geothermal energy 5 x 10=4 erg. year (in the case of the amount to 1 km depth) volcanic energy 10:a + 10=4 erg/year earthquake energy 0.5 × 108 cal:cm ~ sec = 2 x 10a~ erg;year There are many interesting results here. One of them is that most of the assumed energy supplied from mantle to earth's crust has not yet been accounted for so far (probably, some of it has escaped laterally). Another matter is as follows: the natural heat flow is small compared with that of geothermal energy or hot spring energy. This depends on the process differences of transporting heat from deep to shallow parts; namely, the former has been done by heat conductivity, while in the second case, the energy has been trasported directly with hot liquid or steam or gas. Likewise, we may define the relation between the energy of earthquakes and that of volcanic eruptions; namely, the earthquake occurrence energy is stored in the rocks slowly; however, in the case of volcanoes, volcanic energies can be transported from the magmatic source to the shallow part directly by the help of steam and gas. Regarding geothermal energy, this is also very interesting, if we do not use this energy, probably, during long periods of time, the heat energy might be transformed into other forms, or escape slowly laterally. However, if we can utilize the geothermal energies from depth, we can extract a lot ol energy from deeper parts. In such a case, no one can deny that there are also the possibilities to decreasing earthquake and volcanic energy by using geothermal energy.
Future prospects for the development of geothermal energy utilization in Japan. As you will have seen from the previous paragraphs geothermal energy is enormous; however, there are some restrictions due to the balance of water and heat in suitable geothermal structures and also to the chemical compositions of hot fluids. But, we can estimate the rough value to be exploited in the near future in lapan. In Japan, there have been some tests, one of them has already been mentioned in the previous paragraph. Besides this, another estimation is made as follows. We can consider six volcanic zones (Figure 3) for the present purpose as,
A B (1) 100 k m X S 0 km
! j
(2)
fi X
50
~> X 5 0
~>
(3) 200
>> X 3 0
>>
(4)
50
~> X 5 0
,~
(5) I00
>> X 5 0 >> × 5 0
>> >>
(6) 100 where
A B C D
-----
C
D
0.22 k m =
capacity
5 . 1 X 1 0 '2 tons
length of side of each volcanic zone width of side of each volcanic zone estimated depth (1.5 km) porosity ( 15 % )
Therefore, if we assume to use these amount within 500 year, we can get the geothermal electric power of: 5 × 1012 10 million kW 4.5 × 10~ -because, for the generation of I0,000 kW, it is necessary to have the steam amount of 100 t / h - 8 . 8 × I03× I02 t/y consequently, for the 500 years' use, it is necessary to have the amount of 8.8X 10zX 102X5X 102 -- 4 . 5 × 108 tons The assumption of 500 years is sufficient, because in these years, it is quite possible to reach the required amount of the water in the reservoir and to heat it to the necessary temperatures. This means, theoretically we will be able to use these energies forever. Recently, T. N o c u c m evaluated tim geothermal energy in Japan by assuming the possible magma chamber behaviour, and he obtained similar results to the ones explained above. In the near future, the writer expects the development of the use of natural steam of volcanic origin to generate electric power, and to compete with nuclear energy. Japan has increased its efforts to use geothermal energy for generating electric power for reasons that are obvious: natural shortages of fossil fuels, which, with a few exceptions, are rare throughout the four islands and limited available sources of hydroelectric power. REFERENCES FUKUTOMt T. 1961 -- Rate of discharge of heat energy from the principal hot spring localitiesin Hokkaido, Japan. Y.
Fac. Sci. Hokkaido Univ. 1,351.
M^TSffZAWA T., HAS~G^WA H. 1954 - - Feld Theorie der Erdbeben elliptisches Wellen Gebiet. Bull. Earthq. Res. lnst., Tokyo.
NAK^MORAK. 1965 - - Energies dissipated with volcanic activi~ ties - Classification and evaluation. Bull. volcan. Soc. ]apan.
RUMI O. 1967 - - Determination of thermodynamic state of water vapor in steady adiabatic flow along a vertical pipe (well). Termotecnica, 8, 407. WroTE D. E. 1965 - - Geothermal energy. Circ. U. S. geol. Surv., 519. 465