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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: jmariaria@kengen.co.ke
ABSTRACT
Kenya has been exploiting the geothermal energy
resource for the last 27 years. Geophysical and
geological surveys were conducted between L.
Bogoria and Olkaria and the latter identified as the
most potential prospective area. Exploration of the
resource commenced in 1956 that show the drilling
of two exploratory wells, X1 and X2. The Olkaria
field was segmented into seven sectors to facilitate
easier utilization. Extensive drilling operations
started in Olkaria East Field that led to the
establishment of three units in June 1981, November
1982 and March 1984, each unit producing 15MWe.
The Greater Olkaria Field is being exploited and
currently atleast 200 MWe is being produced from
the field and plans are underway to increase the
output as evidenced by the drilling operations that are
on-going currently. Olkaria East Field has been
sustaining the 45MWe and drilling operations are
currently being carried out to establish an additional
140 MWe power plant by 2013 in the same field. Hot
and cold reinjection has also been introduced to
enhance reservoir recharge. It is in view of this that
this paper seeks to explore the response of Olkaria
East Field Reservoir to production and the effects of
reinjection system.
INTRODUCTION
Kenya was the first African country to explore and
develop the geothermal resource. The geothermal
resources in Kenya are mainly located in the Great Rift
Valley region. Geological and Geophysical surveys
were conducted in the 1950s between L. Bogoria and
Olkaria and the latter was predicted to have massive
geothermal resource.
The surveys were carried out by the United Nations
Development Programme (UNDP) in collaboration
with the Government of Kenya and East African
Power and Lighting Company Ltd. It is estimated that
the Kenya Rift has a potential of greater than 7000
MWe of Geothermal Power. The greater Olkaria
region was sub-divided into seven segments majorly
for easier development of the geothermal field. Two
exploratory wells, X1 and X2, were drilled in Olkaria
(1956).The wells were not impressive and this
necessitated scientific review of data to atleast
determine the exact location of the resource.
Production drilling commenced in the 1970s that led
to the establishment of three geothermal power units,
each comprising of 15Mwe, between 1981 and 1985.
The plant has an installed capacity of 45MWe.
OLKARIA GEOTHERMAL FIELD
The Olkaria Geothermal Field is located in the
Kenya Rift valley, Naivasha, which is about 120 km
from Nairobi, covering an area of about 204 km2
(figure 1). The geothermal field has been sub-
divided into seven segments namely Olkaria East,
Olkaria North-East, Olkaria North-West, Olkaria
South-West, Olkaria Central and Olkaria Domes
(figure 2). Four power plants are currently installed
and producing electricity in the field, Olkaria I with
45 MWe capacity, Olkaria II with 105 MWe
capacity, Olkaria III with 48 MWe capacity and
Oserian with 4 MWe. The first two are operated by
KenGen while the third and fourth are operated by
Independent Power Producers namely, Orpower4
Inc. (Ormat) and Oserian Development Company
respectively.
Figure 1: Map showing Kenya Geothermal sites
OLKARIA EAST PRODUCTION FIELD
Currently 45MWe is being generated by Olkaria I
geothermal power station. The first Geothermal
power unit at Olkaria I was a 15 MWe generating unit
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 2: Greater Olkaria Sectors
The turbines are 4-stage single flow running with an
inlet steam pressure of 5 bars absolute at a
saturation temperature of 152°C and a steam
consumption of 10 tonnes per hour for each
megawatt hour produced. The plant has had an
average availability and load factor of 98 per cent
since commissioning. The power generated is
connected to the national grid via a 132 kV
transmission line. So far atleast 55 wells have been
drilled in the Olkaria East Field. The layout of the
field and location of wells is shown in Figure 3. Out
of The already drilled wells, 25 wells of them are
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
their steam and pressure production levels. Two of
the retired wells, OW-3 and OW-6, are currently
used for hot and cold injection on trial basis.
Additional wells have been drilled while others are
currently being drilled and others have been sited to
be drilled thereafter in this field for the 140 MWe
Olkaria I units 4&5 additional units.
FIGURE 5: Steam output trends from Olkaria
East Field from 1981 to 2010
These wells will be connected during construction
of the Olkaria I units 4&5 power plant. The wells
connected to Olkaria I power plant have performed
well since the plant was commissioned in 1981.
After commissioning of Olkaria I unit III in 1985,
the field experienced some output decline. To
mitigate, make up wells were drilled and connected
to the steam gathering system. The makeup wells
restored the plant’s rated output. The steam output
trend is as shown in Figure 5 above. Some decline
in production occurred in the first ten years of
exploitation due to the depletion of the shallow
steam zone but after connection of make-up wells
in 1996, there have been no more declines
experienced. Only eight make-up wells were drilled
and total steam available has been in excess since
connection of the make-up wells.
OLKARIA EAST RESERVOIR RESPONSE
Twenty three (23) wells had been drilled and
connected to the steam supply system when
Olkaria I 45Mwe had been installed in March
1985. The wells had been drilled to depths ranging
from 900 m to 1685 m except OW-19 that had
been drilled to 2484 m. As fluid extraction
continued during production, some of the wells
that had been drilled to depths between 900 m to
1200 m declined in output and had to be isolated
from the steam supply system. In mitigating these
declining wells productivity, new make-up wells
were drilled to restore the rated plant’s generating
capacity. Four make-up wells were connected in
1995 (OW-27, 28, 29 and 30), two in 1996 (OW-
31 and 33) and another two (OW-32 and 34) in
2001. OW-5 was also deepened from 900 m to
2200 m in 1998. Total steam available has been in
excess since then (Table 1).
STEAM SUPPLY STATUS AT OLKARIA I
Olkaria I power plant uses an estimated 450 t/h of
steam at an inlet pressure of 5 bar a. The wells
connected to Olkaria I power plant have a total
steam output capacity of 782 t/h.
Table 1: Summary Status of Steam Supply
Design inlet pressure (bar g) 4.2
Specific steam consumption (t/hr/Mwe) 10
Installed capacity (Mwe) 45
Steam demand (t/hr) 450
Steamfield output capacity (t/hr) 782
Excess steam output (t/hr) 332
Excess steam output supplied to Olkaria I(t/hr) 32
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.
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.
REINJECTION
Reinjection is when water (hot or cold) is pumped
deep underground within the geothermal system
itself (infield reinjection) or outside the system
(outfield reinjection). Geothermal reinjection
systems are being employed in the geothermal
fields as a method for waste-water disposal for
environmental reasons. Recent developments have
shown that it is also being used to counteract
pressure draw-down (water-level decline) due to
long term exploitation of the resource. The natural
rate of recharge (replenishment by rainfall) would
not be commensurate to the rate of extraction
resulting in pressure drawdown. Thus reinjection
has been considered as an artificial means of water
recharge to the reservoir, to aid in extracting more
of the thermal energy stored in the reservoir
system and to reduce land subsidence caused by
over extraction of geothermal fluids.
Some operational dangers and problems are
associated with reinjection. These include the
possible cooling of production wells, often
because of short-circuiting or cold-front
breakthrough, and scaling in surface equipment
and injection wells because of the precipitation of
chemicals in the water.
REINJECTION IN OLKARIA EAST FIELD
Both hot and cold reinjection systems have been
experimented in the field with OW-03 being used
as a hot and cold reinjection well while OW-06 is
a cold reinjection well, utilizing the cooling towers
blowdown from the Olkaria I power plant. OW-12
used to reinject cold water from L. Naivasha into
the reservoir but it has since been stopped.
EFFECTS OF REINJECTION IN OEF
Well OW-02
This well was monitored in September 2010 and
was producing 18.9 t/h steam, 12 t/h brine and
1952kJ/Kg enthalpy.
1980 1985 1990 1995 2000 2005 2010
YEAR
0
10
20
30
40
50
FL
OW
(T
/HR
)
0
500
1000
1500
2000
2500
EN
TH
AL
PY
(K
J/K
G)
Graph 1Steam
Water/brine
Mass
Enthalpy
Figure 6: Output from OW-2
In August 2009 the well gave an average output of
18.8 t/h of steam, 13.6 t/h of brine and enthalpy of
1871 kJ/kg. The annual average steam outputs in
2004, 2005 and 2006 were 17.1 t/h, 17.5 t/h and
19.1 t/h respectively. A general trend of decrease
in steam and brine was observed from the early
1980s to 1995. From 1998 to 2005, steam, brine
and enthalpy have remained relatively constant
(Figure 6).This was attributed to the effects of hot
re-injection in well OW-03. Over the last 2 years,
brine has had increasing trend resulting to a decline
in enthalpy. This may be attributed to the cold
reinjection currently taking place in OW-06
(Wanyonyi 2011). However, cold reinjection effects
from OW-06 have been felt of late as it has been
noted from an increasing trend of brine that has
resulted to a decline in enthalpy.
Well OW-19
From 1980s to 1996 this well experienced an
increase in brine output and decline in steam output
resulting in decrease in enthalpy.
1980 1985 1990 1995 2000 2005 2010
0
10
20
30
40
50
60
70
FL
OW
(T
/HR
)
0
500
1000
1500
2000
2500
3000
EN
TH
AL
PY
(K
J/K
G)OW-19
Steam
Water/Brine
Mass
Enthalpy
Fig 7: Output from OW-19
However, from 1996, there is an increase in both
steam and brine output resulting to an almost
constant enthalpy (Figure 7). This is attributed to
the effects of cold injection conducted in well OW-
12 from 1995 to 1996. From 2006 to present, there
was a steady increase of steam and brine output.
WELL PRODUCTION MONITORING
Bi-annual output monitoring is done in wells that
are delivering steam to Olkaria I power plant for
periods running from January-June and July-
December. The main objective of the steam field
monitoring is to help observe important changes
taking place in the reservoir. These include
changes in reservoir temperature and pressure,
enthalpy and mass output changes as well as well’s
cyclic behaviors. These changes could result from
reservoir boiling, over-exploitation, entry of cold
water into the reservoir that can cause cooling,
wellbore scaling or direct re-injection returns in the
reservoir. Careful monitoring techniques help to
map out thermodynamic and chemical changes
before they cause adverse effects in the reservoir.
ENTHALPY CHANGES
The enthalpy contour plots shows that the center of
the field around wells OW-10, OW-18, OW-20 and
OW-24 &28, 31 and 33 has the highest enthalpy.
The southern and eastern part of the field around
wells OW-15, OW-16, OW-19, OW-22 and OW-26
seem to be receiving some cold inflow and this
depicts their low enthalpies. The enthalpy contour
plots for 2001 (FIG 7.3) is similar to the present
contour plot but the enthalpy values are higher now
than they were five years ago. This shows that most
part of the field has experienced pressure drawdown
resulting in boiling. FIG 7.4 shows that from 2001
to present, pressure drawdown has been extending
from the center to other parts of the field especially
to the west and north.
Figure 8.1: Enthalpy contour plot
for the year 2010
Figure 8.2: Enthalpy contour plot
for the year 2009
199200 199400 199600 199800 200000 200200 200400 200600 200800 201000 201200
9900800
9901000
9901200
9901400
9901600
9901800
9902000
9902200
9902400
9902600
OW-2
OW-5
OW-8
OW-10
OW-11OW-13
OW-15
OW-16
OW-18
OW-19
OW-20
OW-21
OW-22
OW-23
OW-24
OW-25
OW-26
OW-27
OW-28
OW-29 OW-30
OW-31
OW-32
OW-33
199200 199400 199600 199800 200000 200200 200400 200600 200800 201000 201200
EASTINGS (M)
9900800
9901000
9901200
9901400
9901600
9901800
9902000
9902200
9902400
9902600
NO
RT
HIN
GS
(M
)
199200 199400 199600 199800 200000 200200 200400 200600 200800 201000 201200
9900800
9901000
9901200
9901400
9901600
9901800
9902000
9902200
9902400
9902600
OW-2
OW-5
OW-8
OW-10
OW-11OW-13
OW-15
OW-16
OW-18
OW-19
OW-20
OW-21
OW-22
OW-23
OW-24
OW-25
OW-26
OW-27
OW-28
OW-29 OW-30
OW-31
OW-32
OW-33
199200 199400 199600 199800 200000 200200 200400 200600 200800 201000 201200
EASTINGS (M)
9900800
9901000
9901200
9901400
9901600
9901800
9902000
9902200
9902400
9902600
NO
RT
HIN
GS
(M
)
The year 2001 is a good reference point since all
wells currently producing were already
connected to the steam gathering system by
2001. A contour plot of enthalpy changes from
2001 to 2010 (Figure 8.3) shows that enthalpy
has increased more around the center of the field,
around OW-10, and the southeastern part of the
field around OW-30. From 2001, enthalpy
decline is observed around wells OW-15, OW-23
probably due to incursion of cooler fluids at
depth.
Figure 8.3: Enthalpy changes from
2001 to 2010
Figure 8.4: Enthalpy contour plot
For the year 2001
TEMPERATURE AND PRESSURE
CONTOURS
TEMPERATURE CONTOURS
Figure 9.1: Temperature distribution
at 0 m. a. s. l.
Figure 9.2: Temperature distribution
at 500 m. a. s. l.
OW-2
OW-3
OW-4 OW-5
OW-6
OW-7
OW-8
OW-10
OW-11
OW-12
OW-13
OW-14
OW-15
OW-16
OW-17
OW-18
OW-19
OW-20
OW-21
OW-22
OW-23
OW-24
OW-25
OW-26
OW-27
OW-28
OW-29
OW-30
OW-31
OW-32
OW-33
OW-34
199400 199600 199800 200000 200200 200400 200600 200800 201000
9901000
9901200
9901400
9901600
9901800
9902000
9902200
9902400
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
210215220225230235240245250255260265270275280285290295300305310315320325330335
OW-5
OW-11
OW-19
OW-21
OW-23
OW-25
OW-26
OW-27
OW-28 OW-29OW-30
OW-32
OW-33
OW-34
OW-35
OW-35A
OW-36
OW-36A
OW-37A
OW-38A
OW-41
OW-42OW-44
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
185190195200205210215220225230235240245250255260265270275280285290295300305310
OW-5
OW-10
OW-11
OW-19
OW-21
OW-23
OW-25
OW-26
OW-27
OW-28 OW-29
OW-30OW-32
OW-33OW-34
OW-35
OW-35A
OW-36 OW-36A
OW-37A
OW-38A
OW-41
OW-42OW-44
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
199200 199400 199600 199800 200000 200200 200400 200600 200800 201000 201200 EASTINGS (M)
9900800
9901000
9901200
9901400
9901600
9901800
9902000
9902200
9902400
9902600
N O R T H I N G S ( M )
Figure 9.3: Temperature distribution at
-500 m.a.s.l.
PRESSURE CONTOURS
Figure 10.1: Pressure distribution at
500 m. a. s. l.
DOWNHOLE MEASUREMENTS
Well OW-08
The water rest level has remained at 850m from 1980s
to present. The well has had a progressive cooling
since 1985. Over the last 20 years, maximum cooling
has taken place at 1300m. The well has cooled by
about 5°C within the water column since 2001. The
pressure decline over the last 20 years has been
minimum, i.e., a maximum of 9 bars within the liquid
phase and cooling of about 18°C at about 1300 m
depth.
Figure 10.2: Pressure distribution at
0 m. a. s. l
Figure 10.3: Pressure distribution at
-500 m. a. s. l.
The well has experienced minimum cooling at
1100m depths and at the well’s bottom.
0 50 100 150 200 250 300 350
Pressure (Bara) and Temperature (°C)
1600
1400
1200
1000
800
600
400
200
0
Dep
th (
m)
Press 19.02.85
Press 17.10.00
Pres 10.05.06
Temp 19.2.85
Temp 17.10.02
Temp 10.05.06
Temp 03.05.07
Press 03.05.07
Temp 19.08.08
Press 19.08.08
Figure11: Downhole profiles in OW-08
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
185190195200205210215220225230235240245250255260265270275280285290295300305310
OW-5
OW-10
OW-11
OW-19
OW-21
OW-23
OW-25
OW-26
OW-27
OW-28 OW-29
OW-30OW-32
OW-33OW-34
OW-35
OW-35A
OW-36 OW-36A
OW-37A
OW-38A
OW-41
OW-42OW-44
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
4446485052545658606264666870727476788082848688909294
OW-5
OW-10
OW-11
OW-19
OW-21
OW-23
OW-25
OW-26
OW-27
OW-28 OW-29
OW-30
OW-32
OW-33 OW-34
OW-35
OW-35A
OW-36
OW-36A
OW-37A
OW-38A
OW-41
OW-42OW-44
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
OW-5
OW-11
OW-19
OW-21
OW-23
OW-25
OW-26
OW-27
OW-28 OW-29
OW-30OW-32
OW-33 OW-34
OW-35OW-35A
OW-36
OW-36A
OW-37A
OW-38A
OW-41
OW-42OW-44
199500 200000 200500 201000
9900000
9900500
9901000
9901500
9902000
9902500
199200 199400 199600 199800 200000 200200 200400 200600 200800
9901200
9901400
9901600
9901800
9902000
9902200
9902400
9902600
9902800
9903000
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
155
160
165
170
175
OW-5
OW-19
OW-34
OW-35
OW-35A OW-36A
OW-37A
OW-38
OW-38A
OW-41
OW-42
OW-44
38B
199200 199400 199600 199800 200000 200200 200400 200600 200800
9901200
9901400
9901600
9901800
9902000
9902200
9902400
9902600
9902800
9903000
Over the last five years, the wellbore pressure has
increased by 1-2 bars within the water column
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.
Figure 12: Part of the wellhead equipment
at OW-37A
DRILLING OPERATIONS
Drilling activities in Olkaria I and the Greater Olkaria
are currently being undertaken as evidenced by the
number of Drilling Equipments on site. Currently,
five rigs are on site, four hired and three owned by
the Kenya Electricity Generating Company
(KenGen). Among the three rigs that are owned by
KenGen, two of them (2000Hp) are newly acquired.
The decision to purchase the two rigs is the
company’s commitment to rapidly avail enough
steam for power generation and showing its
commitment to the production of clean, reliable,
environmental friendly and cost- effective energy in
the national grid. Olkaria I Unit 4&5 will be installed
to utilize the steam that has and yet to be realized as
drilling operations are on-going. The contract has
already been awarded for 140Mwe Olkaria I
expansion and 140Mwe Olkaria IV development with
Sinclair Knight Merz (SKM) being the overall
consultant. The two projects will be carried out
concurrently and they are expected to be
accomplished at the end of 2013.
FIELD OPTIMIZATION STUDY
For continued assessment of the resource, KenGen
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
Resource Optimization Study of the Greater Olkaria
Geothermal Fields”.
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
obtained from it. Deep drilling in the field has
shown considerable improvement in the productivity
of the wells drilled. OW-5 deepening showed
tremendous results in the wells steam output.
Currently, deep vertical and directional drilling to
depths of 3000m has been embraced in the Greater
Olkaria region. In Olkaria I field, OW-38B proved
to be the biggest producer in Kenya and the whole
of Africa.
REFERENCES
Bore, C.K., (2011): Steam Availability and
Development plans at Olkaria, Kenya. Proceedings,
Kenya Geothermal Conference Nairobi, November
21-22, 2011
Ofwona, C. O., (2010): Resource assessment of
Olkaria I geothermal field, Kenya. Proceedings,
World Geothermal Congress, Bali, Indonesia (2010)
Ouma, Peter (2011): Proposal to carry out additional
drilling to increase generation capacity at Olkaria
Geothermal within the KenGen geothermal license
area by 560MWe KenGen Internal Reports 2011
Wanyonyi, Eliud (2011): Report on Assessment
of Reservoir and Steam Status of Olkaria East
Production Field, March 2011 (KenGen Internal
Reports)
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