Response of Olkaria East Field Reservoir to Production

PROCEEDINGS, Thirty-Seventh Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 30 - February 1, 2012 SGP-TR-194

RESPONSE OF OLKARIA EAST FIELD RESERVOIR TO PRODUCTION

James M. Mariaria

Kenya Electricity Generating Company P.O. Box 785-20117 Naivasha, Rift Valley, 20117, Kenya E-mail: [email protected]

ABSTRACT Kenya has been exploiting the geothermal energy The surveys were carried out by the United Nations resource for the last 27 years. Geophysical and Development Programme (UNDP) in collaboration geological surveys were conducted between L. with the Government of Kenya and East African Bogoria and Olkaria and the latter identified as the Power and Lighting Company Ltd. It is estimated that most potential prospective area. Exploration of the the Kenya Rift has a potential of greater than 7000 resource commenced in 1956 that show the drilling MWe of Geothermal Power. The greater Olkaria of two exploratory wells, X1 and X2. The Olkaria region was sub-divided into seven segments majorly field was segmented into seven sectors to facilitate for easier development of the geothermal field. Two easier utilization. Extensive drilling operations exploratory wells, X1 and X2, were drilled in Olkaria started in Olkaria East Field that led to the (1956).The wells were not impressive and this establishment of three units in June 1981, November necessitated scientific review of data to atleast 1982 and March 1984, each unit producing 15MWe. determine the exact location of the resource. The Greater Olkaria Field is being exploited and Production drilling commenced in the 1970s that led currently atleast 200 MWe is being produced from to the establishment of three geothermal power units, the field and plans are underway to increase the each comprising of 15Mwe, between 1981 and 1985. output as evidenced by the drilling operations that are The plant has an installed capacity of 45MWe. on-going currently. Olkaria East Field has been sustaining the 45MWe and drilling operations are OLKARIA GEOTHERMAL FIELD currently being carried out to establish an additional 140 MWe power plant by 2013 in the same field. Hot The Olkaria Geothermal Field is located in the and cold reinjection has also been introduced to Kenya Rift valley, Naivasha, which is about 120 km enhance reservoir recharge. It is in view of this that from Nairobi, covering an area of about 204 km2 this paper seeks to explore the response of Olkaria (figure 1). The geothermal field has been sub- East Field Reservoir to production and the effects of divided into seven segments namely Olkaria East, reinjection system. Olkaria North-East, Olkaria North-West, Olkaria South-West, Olkaria Central and Olkaria Domes INTRODUCTION (figure 2). Four power plants are currently installed and producing electricity in the field, Olkaria I with Kenya was the first African country to explore and 45 MW capacity, Olkaria II with 105 MW develop the geothermal resource. The geothermal e e capacity, Olkaria III with 48 MW capacity and resources in Kenya are mainly located in the Great Rift e Oserian with 4 MWe. The first two are operated by Valley region. Geological and Geophysical surveys KenGen while the third and fourth are operated by were conducted in the 1950s between L. Bogoria and Independent Power Producers namely, Orpower4 Olkaria and the latter was predicted to have massive Inc. (Ormat) and Oserian Development Company geothermal resource. respectively.

currently connected to Olkaria I power plant and supplying a total of 782 t/h of steam and 263 t/h of brine.

FIGURE 4: Location of wells drilled in OEF

Another 6 wells have been retired due to decline in Figure 1: Map showing Kenya Geothermal sites their steam and pressure production levels. Two of the retired wells, OW-3 and OW-6, are currently OLKARIA EAST PRODUCTION FIELD used for hot and cold injection on trial basis. Additional wells have been drilled while others are Currently 45MWe is being generated by Olkaria I currently being drilled and others have been sited to geothermal power station. The first Geothermal be drilled thereafter in this field for the 140 MWe power unit at Olkaria I was a 15 MWe generating unit Olkaria I units 4&5 additional units. commissioned in June 1981 and the second 15 MWe commissioned in November 1982. The third unit was commissioned in March 1985 raising the total installed capacity of the plant to 45 MWe.

FIGURE 5: Steam output trends from Olkaria East Field from 1981 to 2010 Figure 2: Greater Olkaria Sectors

These wells will be connected during construction The turbines are 4-stage single flow running with an of the Olkaria I units 4&5 power plant. The wells inlet steam pressure of 5 bars absolute at a connected to Olkaria I power plant have performed saturation temperature of 152°C and a steam well since the plant was commissioned in 1981. consumption of 10 tonnes per hour for each After commissioning of Olkaria I unit III in 1985, megawatt hour produced. The plant has had an the field experienced some output decline. To average availability and load factor of 98 per cent mitigate, make up wells were drilled and connected since commissioning. The power generated is to the steam gathering system. The makeup wells connected to the national grid via a 132 kV restored the plant’s rated output. The steam output transmission line. So far atleast 55 wells have been trend is as shown in Figure 5 above. Some decline drilled in the Olkaria East Field. The layout of the in production occurred in the first ten years of field and location of wells is shown in Figure 3. Out exploitation due to the depletion of the shallow of The already drilled wells, 25 wells of them are steam zone but after connection of make-up wells in 1996, there have been no more declines pressure draw-down (water-level decline) due to experienced. Only eight make-up wells were drilled long term exploitation of the resource. The natural and total steam available has been in excess since rate of recharge (replenishment by rainfall) would connection of the make-up wells. not be commensurate to the rate of extraction resulting in pressure drawdown. Thus reinjection OLKARIA EAST RESERVOIR RESPONSE has been considered as an artificial means of water recharge to the reservoir, to aid in extracting more Twenty three (23) wells had been drilled and of the thermal energy stored in the reservoir connected to the steam supply system when system and to reduce land subsidence caused by Olkaria I 45Mwe had been installed in March over extraction of geothermal fluids. 1985. The wells had been drilled to depths ranging Some operational dangers and problems are from 900 m to 1685 m except OW-19 that had associated with reinjection. These include the been drilled to 2484 m. As fluid extraction possible cooling of production wells, often continued during production, some of the wells because of short-circuiting or cold-front that had been drilled to depths between 900 m to breakthrough, and scaling in surface equipment 1200 m declined in output and had to be isolated and injection wells because of the precipitation of from the steam supply system. In mitigating these chemicals in the water. declining wells productivity, new make-up wells

were drilled to restore the rated plant’s generating REINJECTION IN OLKARIA EAST FIELD capacity. Four make-up wells were connected in

1995 (OW-27, 28, 29 and 30), two in 1996 (OW- Both hot and cold reinjection systems have been 31 and 33) and another two (OW-32 and 34) in experimented in the field with OW-03 being used 2001. OW-5 was also deepened from 900 m to as a hot and cold reinjection well while OW-06 is 2200 m in 1998. Total steam available has been in a cold reinjection well, utilizing the cooling towers excess since then (Table 1). blowdown from the Olkaria I power plant. OW-12

used to reinject cold water from L. Naivasha into STEAM SUPPLY STATUS AT OLKARIA I the reservoir but it has since been stopped.

Olkaria I power plant uses an estimated 450 t/h of EFFECTS OF REINJECTION IN OEF steam at an inlet pressure of 5 bar a. The wells

connected to Olkaria I power plant have a total Well OW-02 steam output capacity of 782 t/h. This well was monitored in September 2010 and

was producing 18.9 t/h steam, 12 t/h brine and Table 1: Summary Status of Steam Supply 1952kJ/Kg enthalpy.

Design inlet pressure (bar g) 4.2 50 2500 Specific steam consumption (t/hr/Mwe) 10

Installed capacity (Mwe) 45 40 2000 Graph 1 Steam demand (t/hr) 450 Steam Water/brine Mass Steamfield output capacity (t/hr) 782 Enthalpy

30 1500)

)

H /

Excess steam output (t/hr) 332 (

(

L H

Excess steam output supplied to Olkaria I(t/hr) 32 F

T N

20 1000E Excess steam at Olkaria I (t/hr) 300 Part of the steam from wells drilled in the Olkaria

East Field is supplied to Olkaria II Power Plant. 10 500 Currently, there is an excess of about 300 t/hr of steam from Olkaria I field. A summary of steam supply status is shown in Table 1 above. 0 0 1980 1985 1990 1995 2000 2005 2010 YEAR REINJECTION Figure 6: Output from OW-2

Reinjection is when water (hot or cold) is pumped In August 2009 the well gave an average output of deep underground within the geothermal system 18.8 t/h of steam, 13.6 t/h of brine and enthalpy of itself (infield reinjection) or outside the system 1871 kJ/kg. The annual average steam outputs in (outfield reinjection). Geothermal reinjection 2004, 2005 and 2006 were 17.1 t/h, 17.5 t/h and systems are being employed in the geothermal 19.1 t/h respectively. A general trend of decrease fields as a method for waste-water disposal for in steam and brine was observed from the early environmental reasons. Recent developments have 1980s to 1995. From 1998 to 2005, steam, brine shown that it is also being used to counteract and enthalpy have remained relatively constant (Figure 6).This was attributed to the effects of hot ENTHALPY CHANGES re-injection in well OW-03. Over the last 2 years, The enthalpy contour plots shows that the center of brine has had increasing trend resulting to a decline the field around wells OW-10, OW-18, OW-20 and in enthalpy. This may be attributed to the cold OW-24 &28, 31 and 33 has the highest enthalpy. reinjection currently taking place in OW-06 The southern and eastern part of the field around (Wanyonyi 2011). However, cold reinjection effects wells OW-15, OW-16, OW-19, OW-22 and OW-26 from OW-06 have been felt of late as it has been seem to be receiving some cold inflow and this noted from an increasing trend of brine that has depicts their low enthalpies. The enthalpy contour resulted to a decline in enthalpy. plots for 2001 (FIG 7.3) is similar to the present contour plot but the enthalpy values are higher now Well OW-19 than they were five years ago. This shows that most From 1980s to 1996 this well experienced an part of the field has experienced pressure drawdown increase in brine output and decline in steam output resulting in boiling. FIG 7.4 shows that from 2001 resulting in decrease in enthalpy. to present, pressure drawdown has been extending from the center to other parts of the field especially 70 3000 to the west and north.

60 OW-32 OW-29 OW-30 2500 9902600

OW-26 9902400 50 OW-28 2000

OW-19 ) 9902200

Steam G

)

/ J

R 40 Water/Brine K

H OW-25

(

Mass Y (

1500 9902000

P OW-33 L

W Enthalpy OW-24

O H

L OW-20

) T

F 30

N (

9901800 OW-27

G N

1000 I OW-23

H OW-18

T OW-31 OW-10

20 R 9901600 O

N OW-8

500 9901400 10 OW-15

OW-5 OW-19 9901200 OW-2 OW-22 0 0 OW-16 OW-13 1980 1985 1990 1995 2000 2005 2010 9901000 OW-11 OW-21 9900800 Fig 7: Output from OW-19 199200 199400 199600 199800 200000 200200 200400 200600 200800 201000 201200 EASTINGS (M) However, from 1996, there is an increase in both Figure 8.1: Enthalpy contour plot steam and brine output resulting to an almost for the year 2010 constant enthalpy (Figure 7). This is attributed to OW-32 OW-29 the effects of cold injection conducted in well OW- 9902600 OW-30

12 from 1995 to 1996. From 2006 to present, there OW-26 9902400 was a steady increase of steam and brine output. OW-28

9902200

WELL PRODUCTION MONITORING OW-25 9902000

Bi-annual output monitoring is done in wells that OW-33 OW-24 )

M OW-20

(

are delivering steam to Olkaria I power plant for G 9901800 OW-27

N I

H OW-23 T

R OW-18

periods running from January-June and July- OW-31 OW-10 O

N 9901600 December. The main objective of the steam field OW-8 monitoring is to help observe important changes 9901400 taking place in the reservoir. These include OW-15 OW-5 OW-19 changes in reservoir temperature and pressure, 9901200 OW-2 OW-22 OW-16 OW-13 enthalpy and mass output changes as well as well’s 9901000 OW-11 cyclic behaviors. These changes could result from OW-21 9900800 reservoir boiling, over-exploitation, entry of cold 199200 199400 199600 199800 200000 200200 200400 200600 200800 201000 201200 water into the reservoir that can cause cooling, EASTINGS (M) Figure 8.2: Enthalpy contour plot wellbore scaling or direct re-injection returns in the for the year 2009 reservoir. Careful monitoring techniques help to map out thermodynamic and chemical changes before they cause adverse effects in the reservoir.

The year 2001 is a good reference point since all TEMPERATURE AND PRESSURE wells currently producing were already CONTOURS connected to the steam gathering system by 2001. A contour plot of enthalpy changes from 2001 to 2010 (Figure 8.3) shows that enthalpy TEMPERATURE CONTOURS has increased more around the center of the field, around OW-10, and the southeastern part of the OW-32 field around OW-30. From 2001, enthalpy OW-38A OW-35 decline is observed around wells OW-15, OW-23 9902500 OW-26 OW-30 probably due to incursion of cooler fluids at OW-28 OW-29 335 OW-33 330 depth. 325 OW-32 OW-29 OW-25 320 9902000 315 OW-34 310 OW-26 305 OW-41 9902400 300 OW-28 295 OW-30 OW-35A 290 OW-36A 285 9902200 OW-27 9901500 OW-23 280 OW-34 275 OW-25 270 OW-5 OW-19 9902000 OW-36 265 OW-33 OW-24 OW-37A 260 OW-20 OW-11 255 9901000 250 9901800OW-27 OW-21 OW-14 245 OW-23 240 OW-18 OW-31 OW-10 235 9901600 230 OW-12 OW-8 OW-17 225 9900500 220 215 9901400 OW-7 210 OW-15 OW-3 OW-19 9901200 OW-2 OW-22 OW-44 OW-4 OW-5 OW-16 9900000 OW-6 OW-42 OW-13 9901000 OW-11 199500 200000 200500 201000 OW-21

199400 199600 199800 200000 200200 200400 200600 200800 201000 Figure 9.1: Temperature distribution Figure 8.3: Enthalpy changes from at 0 m. a. s. l. 2001 to 2010

OW-38A OW-32 OW-30

9902600 9902500 OW-35 OW-26 OW-28 OW-35A OW-29 9902400 310 305 OW-25 300 OW-34 9902000 OW-41 295 9902200 OW-33 290 285 280 OW-27 275 9902000 OW-10 OW-23 270 M) OW-37A 265 ( 9901500 260 S G 9901800 255 N OW-36 OW-36A HI 250 T OW-19 R OW-5 245 O N 9901600 240 235 OW-11 9901000 230 OW-21 225 9901400 220 215 210 205 9901200 200 9900500 195 190 9901000 185

9900800 199200 199400 199600 199800 200000 200200 200400 200600 200800 201000 201200 9900000 OW-44 OW-42 EASTINGS (M) Figure 8.4: Enthalpy contour plot 199500 200000 200500 201000 For the year 2001 Figure 9.2: Temperature distribution at 500 m. a. s. l.

OW-38A OW-32 OW-30 OW-38A OW-32 OW-30 9902500 OW-35 OW-26 OW-26 OW-28 9902500 OW-35A OW-29 310 OW-28 OW-29 305 130 OW-35A OW-35 OW-25 300 OW-34 125 9902000 OW-41 295 OW-25 OW-33 290 120 9902000 OW-33 OW-34 OW-41 285 115 280 OW-27 275 OW-36A 110 OW-10 270 OW-27 OW-23 OW-23 105 OW-37A 265 9901500 260 100 255 9901500 OW-36 OW-36A 250 OW-37A 95 OW-5 OW-19 245 OW-36 90 240 OW-5 OW-19 235 85 OW-11 9901000 230 80 225 OW-11 OW-21 9901000 220 OW-21 75 215 210 70 205 65 200 9900500 195 60 190 9900500 55 185 50

9900000 OW-44 OW-42 OW-44 9900000 OW-42 199500 200000 200500 201000 Figure 9.3: Temperature distribution at 199500 200000 200500 201000 Figure 10.2: Pressure distribution at -500 m.a.s.l. 0 m. a. s. l

PRESSURE CONTOURS 9903000 OW-38A OW-42 9902800 OW-32 OW-38A 175 OW-30 170 9902600 OW-38 165 9902500 OW-35 OW-26 160 155 OW-28 OW-29 9902400 150 145 OW-35A 94 140 92 OW-35 OW-41 135 OW-25 90 9902200 130 88 OW-34 125 9902000 120 86 OW-35A OW-36A 115 9902000 OW-33 OW-34 84 110 OW-41 105 82 38B OW-27 100 OW-36A OW-23 80 OW-10 9901800 95 78 OW-44 90 OW-37A 76 85 9901500 74 80 72 9901600 75 OW-36 70 70 65 OW-5 OW-19 68 9901400 OW-19 66 64 OW-37A OW-5 OW-11 62 9901000 9901200 60 OW-21 199200 199400 199600 199800 200000 200200 200400 200600 200800 58 56 54 52 Figure 10.3: Pressure distribution at 50 9900500 48 -500 m. a. s. l. 46 44 The well has experienced minimum cooling at 1100m depths and at the well’s bottom. 0 9900000 OW-44 OW-42

200 199500 200000 200500 201000

400

Figure 10.1: Pressure distribution at Press 19.02.85 Press 17.10.00 500 m. a. s. l. 600 Pres 10.05.06

Temp 19.2.85 )

m Temp 17.10.02 (

DOWNHOLE MEASUREMENTS Temp 10.05.06 h

t 800 Temp 03.05.07 p Press 03.05.07 e

D Temp 19.08.08 Press 19.08.08 Well OW-08 1000 The water rest level has remained at 850m from 1980s to present. The well has had a progressive cooling 1200 since 1985. Over the last 20 years, maximum cooling 1400 has taken place at 1300m. The well has cooled by about 5°C within the water column since 2001. The 1600 pressure decline over the last 20 years has been 0 50 100 150 200 250 300 350 Pressure (Bara) and Temperature (°C) minimum, i.e., a maximum of 9 bars within the liquid phase and cooling of about 18°C at about 1300 m Figure11: Downhole profiles in OW-08 depth. Over the last five years, the wellbore pressure has Resource Optimization Study of the Greater Olkaria increased by 1-2 bars within the water column Geothermal Fields”. showing that the reservoir pressures are recovering. This well has been shut-in for a long time and therefore values obtained here are quite reliable.

ON-GOING DEVELOPMENTS IN OEF

WELLHEAD GENERATION The wellhead technology has been embraced by the organization and a pilot project is under construction. Green Energy Generation Ltd is undertaking the project. OW-37A was earmarked for the construction of a 5MWe wellhead plant. Plans are also there to increase the capacity of wellhead generation to 75MWe in the Greater Olkaria Field. Figure13: Mast Raising of KGN-Rig I on OW-11 Pad in readiness to drill OW-11A

DISCUSSIONS The EPF wells have been showing considerable supply of steam to the power station over the last two decades. The production performance has exceeded the earlier predicted performance of the field. This might be attributed to the introduction of in-field cold and hot re-injection technology and deeper drilling technology.

CONCLUSION Olkaria East Field has proven to be highly productive and more geothermal power can be Figure 12: Part of the wellhead equipment obtained from it. Deep drilling in the field has at OW-37A shown considerable improvement in the productivity

of the wells drilled. OW-5 deepening showed DRILLING OPERATIONS tremendous results in the wells steam output. Drilling activities in Olkaria I and the Greater Olkaria Currently, deep vertical and directional drilling to are currently being undertaken as evidenced by the depths of 3000m has been embraced in the Greater number of Drilling Equipments on site. Currently, Olkaria region. In Olkaria I field, OW-38B proved five rigs are on site, four hired and three owned by to be the biggest producer in Kenya and the whole the Kenya Electricity Generating Company of Africa. (KenGen). Among the three rigs that are owned by

KenGen, two of them (2000Hp) are newly acquired. REFERENCES The decision to purchase the two rigs is the company’s commitment to rapidly avail enough Bore, C.K., (2011): Steam Availability and steam for power generation and showing its Development plans at Olkaria, Kenya. Proceedings, commitment to the production of clean, reliable, Kenya Geothermal Conference Nairobi, November environmental friendly and cost- effective energy in 21-22, 2011 the national grid. Olkaria I Unit 4&5 will be installed Ofwona, C. O., (2010): Resource assessment of to utilize the steam that has and yet to be realized as Olkaria I geothermal field, Kenya. Proceedings, drilling operations are on-going. The contract has World Geothermal Congress, Bali, Indonesia (2010) already been awarded for 140Mwe Olkaria I expansion and 140Mwe Olkaria IV development with Ouma, Peter (2011): Proposal to carry out additional Sinclair Knight Merz (SKM) being the overall drilling to increase generation capacity at Olkaria consultant. The two projects will be carried out Geothermal within the KenGen geothermal license concurrently and they are expected to be area by 560MWe KenGen Internal Reports 2011 accomplished at the end of 2013. Wanyonyi, Eliud (2011): Report on Assessment of Reservoir and Steam Status of Olkaria East FIELD OPTIMIZATION STUDY Production Field, March 2011 (KenGen Internal For continued assessment of the resource, KenGen Reports) has contracted a consortium composed of Mannvit hf, ÍSOR, Vatnaskil ehf and Verkís hf from Iceland, which has been awarded the project entitled “Provision of Consultancy Services for Geothermal