Acid Fluid Utilization andGeochemical Modeling
Example of Chloride Type of Acid Fluidin Onikobe Geothermal Power Plant

10 Dec. 2018

GNS Science – JOGMEC Geothermal Workshop

Norifumi Todaka
Nittetsu Mining Consultants Co., Ltd.
1

Location of Onikobe Geothermal
Power Station

Onikobe Caldera

Takahinata-yama（Quaternary Volcano）
Onikobe P/Smodified from J-POWER’s data2

Subsurface temperature

Temperature (deg.C)

Down-flow
Altitude (m)Up-flow

GO-11Heatconduction

3

Temperature distribution at -500m asl(J-POWER, 1987)

Well temperature profiles(J-POWER, 1987)

Chemistry of separated watersfrom Onikobe production wells

4Acid water

Neutral water

pH-2.8 - 3.8

6.7 - 7.8

H2 Smg/l<0.5<0.5 - 2.1

T-CO2mg/l<10<10Clmg/l

4300 - 5650

2160 - 4840

SO4mg/l22.2 - 50.0

14.7 - 23.9

Na

mg/l

1850 - 2130

1100 - 2170

Kmg/l

345 - 435

198 - 355

Ca

mg/l

474 - 721

179 - 650

Mg

mg/l

25.1 - 58.3

0.31 - 5.51

Fe

mg/l

97.8 - 371

0.05 - 0.32

Almg/l0.04 - 0.43

0.15 - 0.21

SiO2mg/l

527 - 725

613 - 695

Bmg/l

62.4 - 80.9

43.5 - 78.2

As

mg/l

0.58 - 1.77

0.47 - 0.58

Compiled from Todaka and Tezuka (2002) and Todaka et al. (2004)

Classification of acid fluid based onformation mechanism
Magmaticu “Excess” Cl: Onikobe (max. 68.9 meq/L), Kakkonda (max.
26.3 meq/L, WD1-a: 2110 meq/L), Tatun (21.5-70.8 meq/L)
ü HCl(g) à HCl(aq)“Excess” Cl is defined as chloride equivalent concentrations significantly greaterthan the sum of equivalent concentrations of cations other than H+.
by Truesdell (1991)

u Non “Excess” Cl: Miravalles, Shiramizugoe
ü 4SO2(g) + 4H2O à 3SO42- + 6H+ + H2S(aq)Non Magmatic(acidificationin reservoir)u Non “Excess” Cl:Ø Due to dissolution of alunite and native sulfur (Fournier’sHypothesis: Miravalles, Hatchobaru, Shiramizugoe
ü 4S(s) + 4H2O à SO4 + 2H+ + 3H2S(aq)ü KAl3(OH)6(SO4)2 à 6H+ + K+ + 2SO42- + 3AlO2Ø Due to mixing of acid sulfate fluid inflowing into reservoir:Palimpinon, Bacon-Manito, Tiwi, Hatchobaru
Ø Due to formation of sulfide minerals: Yamagawa, Mahanagdong
ü ZnCl42- + H2S(aq) à ZnS(s) + 4Cl- + 2H+
ü PbCl3- + H2S(aq) à PbS(s) + 3Cl- +2H+
Ø Due to mixing of acid and neutral waters: Onikobe, Kakkonda

5

Currently there is not a general consensus on origin of many acid geothermalwaters.

Geochemical characteristicsof acid waters (1)300

6Wide pH range due to influence ofreinjection water

5.5

250

WD1-a

1000*SO4/Cl (eq/eq)

5200

Kakkonda

150

pH

4.5

Shiramizugoe

100

50

OnikobeTatun30Onikobe
WD1-a 5

Shiramizugoe

2.5

Miravalles

0Hatchobaru

3.5

Hatchobaru

Kakkonda

Tatun

4101520

2

Miravalles050

SO4 (meq/L)
Miravalles

Shiramizugoe

Hatchobaru

Tatun

150

100250300

OnikobeKakkonda

1000*SO4/Cl (eq/eq)
OnikobeKakkonda

Miravalles

Shiramizugoe

Hatchobaru

Sumikawa acid fluids (SO4-Cl type) are plotted out of the range.

6200

J-POWER's proprietary data (Onikobe) is plotted in these diagrams.

Tatun

OnikobeGeochemical characteristicsof acid waters (2)NaNaCl"Excess" Chloride

granite

10KKClaverage crustal rockbasaltCl/2◆: Onikobe ◆: Tatun ◆: Kakkonda ＋: Kakkonda (#14) ◆: Kakkonda (WD1-a)●: Sumikawa-Ohnuma ●: Miravalles ●: Shiramizugoe ●: Hatchobaru
×: HCl dissolved rock

7

J-POWER's proprietary data (Onikobe) is plotted in this diagram.

Feed zone locations ofacid/neutral wells in Onikobe

#127
#135

10%

19.3%

#128
10.5%
Neutral wells#130
Acidic wells

#134

10.4%

18.5%

#131
#133

17.8%

13.4%

8

Percentage of total flow rate of acidic water and neutral wateron Janualy 2001Akasaka et al. (2001)

Todaka et al. (2004)

Distribution of acid and neutralfluids
WestEast

Elevation mASL

500

Al0S
Kan-500

Kgt

Gr-1000

Flow rate
Production well (neutral fluid)
Produciton well (acidc fluid)
Reinjection well

9050t/h
100t/h150t/hadded acid well (GO-11) in the figure of Akasaka
et al. (2001)

500m

Relationship between acid fluid
and alteration minerals
ü Rocks of acid feed zones are strongly alteredand are composed of quartz, kaolinite,pyrophyllite and pyrite.
ü Based on a locative relationship between theacid fluid zone and the acid alteration zone, it
is determined that acid fluid might be
upwelling along the fault. Acid fluid
distribution areas are locally understood.
ü Strongly altered rock with no relic minerals
would have little ability of neutralizing acidwater.

quartz*smectitechlorite
illitekaolinite
pyrophyllite
epidoteprehnitepyritelaumontite
wairakaite
calciteplagioclase*
Temperature (℃) 100*: primary mineral is included.

W

Onikobe P/S

150

200

250

modified from Todaka et al. (2003)

EPyrophyllite zone

10J-POWER (1987)

Casing design of acid well (1)In the center of the Onikobe field
Previous
Present design
design11

In the case of deep acid fluid production, duplex stainless casing in the shallow
zone and openhole in the deep zone (8-1/2 inch) are designed to prevent
corrosion, resulting in extension of production life.

Casing design of acid well (2)In the western side of the Onikobe field
Previous
Present design
design12

In the case of two feed zones of acid and neutral fluids, deep cemented
production casing is designed to prevent acid water inflow, resulting in lesscorrosion and scaling.

Smectite scaling in someproduction wells
Mn-rich smectite scale coexisting withgalena and sphalerite precipitated insidesome production wells. Fluid production
stopped before long. Clean-out wascarried out to produce the fluid again.
Mixed
fluid
Acidicfluid

Smectite
scale

5mm

Microscope
Neutralfluid13

Fluid mixing in a well

Caliper log

Chemical structure of smectite scale(Ca0.17Na0.01K0.01)(Al0.18 Mg1.74Mn0.85Fe0.03)(Si3.87 Al0.13) O10(OH)2
Todaka et al. (2003)

Smectite scaling due to mixing ofacid and neutral waters in a well20

ü It was hypothesized that it would be supersaturatedwith respect to smectite and some sulfides due tomixing of acid and neutral fluids in a wellbore.

8tetrahedrite

7

Supersaturation
region

pH

106

pH

Saturation Index

155galena

5smectite

04sphalerite

-5
0
100:0

Acid

20
80:20

40
60:40

60
40:60

80
20:80

3100
0:100

NeutralMixing Ratio (well 130 : well 134)
Ajima et al. (2000)

ü To test such a conceptual model and thegeochemical behavior by mixing of two waters,
simulation of fluid-fluid mixing by various mixing
ratios of two fluids was carried out using the code
CHILLER.
ü In the calculation, saturation index of smectite*
becomes higher by mixing. Some sulfide minerals
are predicted to precipitate in a similar region assmectite. It is consistent with the field data.
ü The result shows that smectite scale would
precipitate due to mixing of acid and neutral fluids.
ü To prevent scaling as well as corrosion, J-POWER
has modified casing design and has made up a wellwith only neutral fluid feed zones 20 years ago. It isthe same strategy as the Tiwi geothermal field inPhilippines.
*: Thermodynamic data of Mn-rich smectite was calculated by theempirical method of Tardy and Garrels (1974).

14

Silica scaling due to mixing of acid andneutral waters in reinjection well

ü Mixed waters of pH6 indicate thehighest silica scaling rate.
ü pH control of higher than 4.5 wasineffective in silica scale inhibition in
the experiment.
ü The results indicate that a separate
injection system without mixing ofacid and neutral waters should bemade to prevent the silica scaling inreinjection well.
ü pH modification with acid water fromproduction well has not been
performed in the Onikobe field to date.

Scaling rate (mg/cm2・week)ü Field experiment was carried out totest a kind of pH modification
technique by mixing of acid water(#131) and neutral water (#129).

102

101
129
号井#129100
Original water
Mixed water

10-1

#131
131
号井10-2

3

4

5

6

7

8

pH
pH
Experiments of mixing of acid and neutral
waters after separator

15

J-POWER’s proprietary dataCurrent casing designs

16

modified from J-POWER’s proprietary data

Geochemical changes in mixing
front of acid and neutral waters
500m/100blocks

Andesite
Mass source

Initial condition- acidic water- 270 ºC- 1mm/day- neutral water- 270 ºC
- 130bars
Schematic representation of 1-D flow model

mixing frontsmectite

17

Todaka et al. (2003)

Neutralization of acid water
Geochemical evolutions along fracture(porous model, TOUGHREACT)

Geochemical evolutions along fracture(MINC model, TOUGHREACT).

Todaka et al. (2004)
Todaka et al. (2005)18

Summaries
u Two types of fluids are encountered in the Onikobe geothermal reservoir: one isneutral and the other is acid. Acid water is around pH3 in the ambient condition.
u According to Truesdell and Nakanishi (2005), the acid water from the Onikobe wellis chloride “excess” water, which is considered to be magmatic origin.
u The Onikobe acid water has low SO4/Cl ratio and higher Cl concentration than theneutral water. SO4 concentration of the acid water is slightly higher than that of theneutral waters. However, the difference is very small, compared to Miravalles,
Shiramizugoe and Hatchobaru etc. High Fe2+ ion of acid water would originate
from casing.
u Acid water is distributed at the deep part (>1300m depth) in the upflow zone and at
the intermediate depth (500-1000m depth) in the lateral flow zone. It ishypothesized that acid fluid might be upwelling along a fault zone from magma.
u For acid fluid utilization, duplex stainless casing and deep cemented production
casing are designed to prevent corrosion and scaling.
u Separate injection system for either acid or neutral fluid (without mixing of acid andneutral waters) is adopted to prevent silica scaling in reinjection well.
u J-POWER now starts to replace power plant (14.9MW) in Onikobe and makes thesame strategy for acid fluid utilization. In the new Onikobe power plant, J-POWER
plans to set duplex stainless pipes (25Cr) for production casing from surface tocasing shoe.
19

Thank you for your attention.
Temperature（℃）
Reinjection block

Production block

pHReinjection block

Production block

After 30-year exploitation

Reactive transport simulation of production and reinjection in the Onikobe fieldusing TOUGHREACT (Todaka et al., 2004, JGR, Vol.109)

Acknowledgement:
20J-POWER kindly provided the unpublished Onikobe data for this presentation.
I thank the company for permission of the proprietary data disclosure.