RESPONSE OF OLKARIA EAST FIELD RESERVOIR TO PRODUCTION

Transcription

1 PROCEEDINGS, Thirty-Seventh Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 3 - February 1, 212 SGP-TR-194 RESPONSE OF OLKARIA EAST FIELD RESERVOIR TO PRODUCTION James M. Mariaria Kenya Electricity Generating Company P.O. Box Naivasha, Rift Valley, 2117, Kenya 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, X 1 and X 2. 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 2 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 14 MWe power plant by 213 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 195s 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 7 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, X 1 and X 2, 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 197s that led to the establishment of three geothermal power units, each comprising of 15Mwe, between 1981 and 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 12 km from Nairobi, covering an area of about 24 km 2 (figure 1). The geothermal field has been subdivided 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 MW e capacity, Olkaria II with 15 MW e capacity, Olkaria III with 48 MW e capacity and Oserian with 4 MW e. 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.

2 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 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 The third unit was commissioned in March 1985 raising the total installed capacity of the plant to 45 MWe. 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 14 MWe Olkaria I units 4&5 additional units. 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 1 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 FIGURE 5: Steam output trends from Olkaria East Field from 1981 to 21 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 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

3 FLOW (T/HR) ENTHALPY (KJ/KG) 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 The wells had been drilled to depths ranging from 9 m to 1685 m except that had been drilled to 2484 m. As fluid extraction continued during production, some of the wells that had been drilled to depths between 9 m to 12 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 (, 28, 29 and 3), two in 1996 (OW- 31 and 33) and another two ( and 34) in 21. was also deepened from 9 m to 22 m in Total steam available has been in excess since then (Table 1). STEAM SUPPLY STATUS AT OLKARIA I Olkaria I power plant uses an estimated 45 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) 1 Installed capacity (Mwe) 45 Steam demand (t/hr) 45 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) 3 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 3 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-3 being used as a hot and cold reinjection well while OW-6 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-2 This well was monitored in September 21 and was producing 18.9 t/h steam, 12 t/h brine and 1952kJ/Kg enthalpy Graph 1 Steam Water/brine Mass Enthalpy YEAR Figure 6: Output from OW-2 In August 29 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 24, 25 and 26 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 198s to From 1998 to 25, steam, brine and enthalpy have remained relatively constant

4 NORTHINGS (M) NORTHINGS (M) FLOW (T/HR) ENTHALPY (KJ/KG) (Figure 6).This was attributed to the effects of hot re-injection in well OW-3. 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-6 (Wanyonyi 211). However, cold reinjection effects from OW-6 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 From 198s to 1996 this well experienced an increase in brine output and decline in steam output resulting in decrease in enthalpy. 7 3 ENTHALPY CHANGES The enthalpy contour plots shows that the center of the field around wells, OW-18, OW-2 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-22 and seem to be receiving some cold inflow and this depicts their low enthalpies. The enthalpy contour plots for 21 (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 21 to present, pressure drawdown has been extending from the center to other parts of the field especially to the west and north Steam Water/Brine Mass Enthalpy OW-24 OW OW-31 OW-8 OW Fig 7: Output from 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 From 26 to present, there was a steady increase of steam and brine output OW EASTINGS (M) OW-15 OW-16 Figure 8.1: Enthalpy contour plot for the year 21 OW-13 OW-22 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 OW-31 OW-8 OW-2 OW-24 OW-2 OW-15 OW-16 OW EASTINGS (M) Figure 8.2: Enthalpy contour plot for the year 29 OW-13 OW-22

5 The year 21 is a good reference point since all wells currently producing were already connected to the steam gathering system by 21. A contour plot of enthalpy changes from 21 to 21 (Figure 8.3) shows that enthalpy has increased more around the center of the field, around, and the southeastern part of the field around. From 21, enthalpy decline is observed around wells OW-15, probably due to incursion of cooler fluids at depth OW-31 OW-3 OW-4 OW-8 OW-7 OW-2 OW-24 OW-6 OW-2 OW-12 OW-15 OW-16 OW-18 Figure 8.3: Enthalpy changes from 21 to 21 OW-14 OW-17 OW OW TEMPERATURE AND PRESSURE CONTOURS TEMPERATURE CONTOURS Figure 9.1: Temperature distribution at m. a. s. l M ) ( S G N H I T R O N EASTINGS (M) Figure 8.4: Enthalpy contour plot For the year 21 Figure 9.2: Temperature distribution at 5 m. a. s. l.

6 Depth (m) Figure 9.3: Temperature distribution at -5 m.a.s.l. PRESSURE CONTOURS 993 Figure 1.2: Pressure distribution at m. a. s. l Figure 1.3: Pressure distribution at -5 m. a. s. l. The well has experienced minimum cooling at 11m depths and at the well s bottom. 2 OW B Figure 1.1: Pressure distribution at 5 m. a. s. l. DOWNHOLE MEASUREMENTS Press Press Pres Temp Temp Temp Temp Well OW-8 The water rest level has remained at 85m from 198s to present. The well has had a progressive cooling since Over the last 2 years, maximum cooling has taken place at 13m. The well has cooled by about 5 C within the water column since 21. The pressure decline over the last 2 years has been minimum, i.e., a maximum of 9 bars within the liquid phase and cooling of about 18 C at about 13 m depth Press Temp Press Pressure (Bara) and Temperature ( C) Figure11: Downhole profiles in OW-8

7 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. Resource Optimization Study of the Greater Olkaria Geothermal Fields. 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. 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 Pad in readiness to drill A 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. Figure 12: Part of the wellhead equipment at 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 (2Hp) 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 14Mwe Olkaria I expansion and 14Mwe 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 213. 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 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. deepening showed tremendous results in the wells steam output. Currently, deep vertical and directional drilling to depths of 3m 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., (211): Steam Availability and Development plans at Olkaria, Kenya. Proceedings, Kenya Geothermal Conference Nairobi, November 21-22, 211 Ofwona, C. O., (21): Resource assessment of Olkaria I geothermal field, Kenya. Proceedings, World Geothermal Congress, Bali, Indonesia (21) Ouma, Peter (211): Proposal to carry out additional drilling to increase generation capacity at Olkaria Geothermal within the KenGen geothermal license area by 56MWe KenGen Internal Reports 211 Wanyonyi, Eliud (211): Report on Assessment of Reservoir and Steam Status of Olkaria East Production Field, March 211 (KenGen Internal Reports)