•6th African Rift Geothermal Conference

Edwin Wafula

•31st October -6th November 2016

United Nations Conference Centre, Addis Ababa

INTERPRETATION OF GEOCHEMICAL DATA BASED ON DIFFERENT MODELS OF RESERVOIR CHARACTERISTICS:

A CASE FOR OLKARIA

2

Conclusion

Mineral saturation

Geothermometry

Geochemical modelling

Geology

Introduction

Contents

3

Introduction

One of geothermal

prospects in

Kenyan Rift

Olkaria divided by

7 sectors

Development by

KenGen/ Orpower

Olkaria produces

~677 MWe

4

Geology

From Otieno, 2016 (Modified from Clarke et al.,

1990)

5

Subsurface Geology

Upper volcanics: pyroclasts

Olkaria basalts: Minor basalts, minor tuffs

Plateau trachytes: trachytes, minor basalt, rhyolites

Mau tuffs: Tuffs, minor trachytes, rhyolites

Pre- Mau volcanics: Basalts, trachytes, tuffs

Proterozoic metamorphic basement system

6

Hydrothermal mineralogy

•Smectite -zeolite

•Smectite-chlorite- illite

0-550m bgl

•Epidote- chlorite-illite

550-1400m

bgl

•Actinolite-epidote-chlorite-illite

1400-3000m

bgl

• Epidote, garnet,chlorite,

• Actinolite • Albite,

chalcedony, quartz, adularia,

• Biotite, calcite, flourite, pyrite, illite

7

Geochemistry

• Study of origin of geothermal fluids and

reactions

• Fluid rock interaction & equilibrium concept

• Study chemical reactions based on activity

products and equilibrium constants

• Predict mineral reactions and saturation

states.

8

Aquifer composition

Cl serves as an

indicator of upflow

150-600 ppm in

Domes

200-600 ppm in NE-

field

More variations in E-

field

150-1400ppm. Deep

E-field wells have

SE wells have

Cl > 1000 ppm (800

series)

9

Geochemical modelling

• Single phase liquid aquifer

• No initial vapour • Calculated

enthalpy corresponding to aquifer temperature

Model 1

• Two phase

vapour and liquid

aquifer

• Initial steam

fraction

• Measured

enthalpy is in

excess due to

phase separation

Model 2

10

Aquifer composition

Well Enth1 pH1 Cl1 CO21v

X1 Enth2 pH2

Cl2 CO22v X2

15 1047 6.9 535 0 0 2140 7.8 509 2762 0.6

16 981 6.8 497 0 0 1534 7.8 491 4138 0.3

709 1216 6.4 523 0 0 1921 8.4 518 3479 0.5

714 1170 7.7 535 0 0 1303 7.7 497 12686 0.1

11

Geothermometry

• Silica, Na/K, H2S geothermometers applied

• The silica geothermometer 𝑞𝑢𝑎𝑟𝑡𝑧 + 2𝐻2𝑂 =𝐻4𝑆𝑖𝑂4

• The tNa/K 𝑙𝑜𝑤 − 𝑎𝑙𝑏𝑖𝑡𝑒 + 𝐾+ = 𝑚𝑖𝑐𝑟𝑜𝑐𝑙𝑖𝑛𝑒 +𝑁𝑎+

• The tH2S 14𝑝𝑦𝑟 + 1

2𝑝𝑦𝑟𝑟 + 𝐻2𝑂𝑙 =

1

4𝑚𝑎𝑔 + 𝐻2𝑆𝑎𝑞

12

Geothermometry

Well Tmsd tNa/k1 tNa/K

2 tqtz1 tqtz

2 tH2S1 tH2S

2

15 242 244 244 253 247 241 192

16 228 226 227 241 240 232 184

29 241 240 240 255 244 253 198

719 241 268 241 276 252 248 202

13

Geothermometry

14

Fluid rock interaction

• Fluid rock interaction results in dissolution of rock

• Aquifer fluid composition is controlled by

interaction with mineral

• Minerals either undergo dissolution or

precipitation

• Model used in aquifer fluid evaluation influences

state of saturation

• Prediction of mineral formation could be done

15

Mineral Saturation

16

Mineral Saturation

17

Gas equilibria

H2 H2S

18

Gas equilibria

CO2

19

Conclusions

• Aquifers could be liquid only or two phase

vapour and liquid

• Model used determines the initial

composition of components

• Na/K ,Tqtz temperature match the measured

temperature, not H2S

• Reservoir initially single phase liquid ,phase

separation by depressurization boiling

• Mineral saturation state depends on initial

aquifer conditions

20

THE END