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2 Geothermal Resources Council Transactions, VO~. 27, October 12-15, 2003 Re-Injection Strategy in the Olkaria North East Geothermal Field, Kenya Peter A. Ouma Kenya Electricity Generating Company Ltd., P.0 Box 785, Naivasha, Kenya Keywords Olkaria Nortl I East, Geothermal, Re-injection Strategy, Power Plant, Geothermal Wells. ABSTRACT Initial re-injection strategy was based on the Final Engineering Report for development of Olkaria North East Geothermal Field, prepared by Ewbank Preece (1993). In that report, only cold re-injection was proposed and all re-injection of waste brine was to take place in well OW-704 and a little in OW-703, by pumping through stainless steel pipes. That strategy had provided for drilling of only one other re-injection well in the vicinity of OW-704. However, KenGen reviewed that strategy and found out that it did not exhaust all the re-injection scenarios.. Therefore, further field activities, studies and tests were carried out in both Olkaria East and North East fields after which modifications were suggested. These included drilling of two additional wells namely, OW-R2 and OW-R3, carrying out of tracer, interference, and cold and hot re-injection tests. With the increased availability of information, following analysis of the additional data, revisions were made to the strategy so as to consider both hot and cold re-injection, distribution of the re-injected fluid to maximize pressure support and efficiency and careful and economic disposal of the fluid for the life of the plant. A lot more caution has been applied to avoid re-injecting cold fluid into the middle of the field in case of fast breakthrough, and pumping requirement has been minimized by taking advantage of gravity flow. Introduction Figure 1. Greater Olkaria Geothermal Area fields. 397 Olkaria North East geothermal field is one of the seven development sectors that when put together constitute the Greater Olkaria Geothermal Area (GOGA), located 120 kilometres north west of Nairobi. The other sectors are Olkaria East, Olkaria Central, Olkaria South West, Olkaria North West, Olkaria South East and Olkaria Domes geothermal fields (Figure 1). Re-injection may be defined as the process of returning fluid to the geothermal reservoir during utilization. The supposed benefits of re-injection to the reservoir are usually attributed to the maintenance of reservoir pressure and fluid in place. Theoretically, maintaining high reservoir pressures and mass in place

3 should reduce the effects of loss of deliverability and those of subsidence or fo~ation collapse. Apart from improvement of resource recovery, re-injection can also serve as an alternative way of disposing wastewater. In the strategy for Olkaria North East field re-injection is expected to help with both fluid disposal and reservoir pressure maintenance. Cold re-injection is intended primarily for disposal of wastewa~r whereas hot re-injection is geared towards reservoir pressure maintenance as well as wastewater disposal. According to the Feasibility Study for Geothermal Power Station at North East Olkaria (Ewbank Preece, 1989), re-injection of the separated geothermal water and cooling water blow down back into the field will help maintain the pressure in the field. Jn addition re-injection offers a very environme~tally acceptable means of waste disposal. A total of 943 thr of separated water and cooling water blow down will be generated from Olkaria North East field production. There was an initial proposal for implementing re-injection in Olkaria North East field that only considered cold re-injection by pumping wastewater through stainless steel pipes into two re-injection wells located to the North and North East of the field. That strategy was based on the Final Engineering Report for development of Olkaria North East field (Ewbank Preece, 1993). However, the strategy was reviewed by Kenya Electricity Generating Company Ltd. (KenGen) after which, modifications were proposed. This paper elaborates on the steps taken before proposing modi~cations to the initial strategy. Initial Strategy Ewbank Preece (1993) only proposed cold re-injection and the discharge water from the separation stations was to be piped to conditioning ponds located on each side of the main road passing through the steam field. The separated discharge water was to flow initially into a local one hour hold-up pond at each separation station in order to allow initial polymerization of silica, and then be conveyed by victaulic jointed stainless steel gravity drain pipes to two main conditioning and cooling ponds. From the main conditioning ponds the fluid was to flow to the main overflow pond and re-injection sump from which it was to be pumped mainly to the re-injection well OW-704 and a little to OW-703, through stainless steel pipes. To the latter pond periodically untreated blow down water (ph of 2.96) from the power station cooling towers was to be discharged. The strategy had provided for drilling of only one other re-injection well in the vicinity of OW-704 (Figure 2), and implied pumping waste brine over a head of about 165 m with severe cost implications. Strategy Review KenGen had some concerns about the above strategy and was of the opinion that it did not exhaust all the re-injection scenarios possible in the Olkaria North East field. Other wells were available for re-injection and yet were not considered, These wells included Figure 2. Initial re-injection system proposed for Olkaria North East field. 398

4 Figure 3. Olkaria North East Wells. 0W-X2,OW-R1,0W-201,0W-501,0W-708,0W-717,0W-723 and OW-724 (Figure 3). Some of these wells, although drilled for production, were found unsuitable after discharge tests. But fortunately, pumping tests had revealed significant injection capacities in them and could be utilized for re-injection. The pumping costs associated with the initial strategy were also evaluated and found to be very high. Therefore there was clear advantage in utilizing alternative wells that could receive water by gravity flow. Furthermore, injection of cold water in the middle of the field was considered too risky and hence if any re-injection was to be done in the middle of the production field it was preferred to be hot. Cold re-injection was considered appropriate at the periphery or outside the production area. Thus wells had to be identified suitable for either hot or cold re-injection. That led to additional studies, activities and tests being conducted both in the Olkaria East and Olkaria North East fields, in order to obtain more information needed for re-evaluation of the re-injection strategy. Inventory of the available injection capacity in Olkaria North East was done after which two more wells namely, OW-R2 and OW-R3 were drilled at chosen locations specifically for re-injection and pumping tests carried out to determine their injection capacities. Tracer and interference tests, hot and cold re-injection Table 1. lnjection Capacity in Olkaria 1 North I East Field. I Drilled Depth WellNo. (m) (Thr) OW-704 2,000 1,445 OW-703 OW-R2 OW-R3 OW-708 OW-717 OW-501 OW-723 2,198 2,201 2,200 2,200 2,101 1,800 2, IO0 1,697 OW-724 2, OW7201 2, trials were also performed in Olkaria East and Olkaria North East fields. lnjection Capacity Inventory Some of the wells drilled in Olkaria North East field for production were after discharge testing found unsuitable for connection to the power plant. The reasons were either low discharge - wellhead pressure or cyclic output characteristics. However, injection capacities obtained during pumping tests at completion of the wells were reasonable and it became prudent to consider them for 1 Injection Capacity re-injection (Haukwa, 1985,1987,1988; Kabira, 1989; Kagiri, ; Waruingi, 1983). These wells included OW-20l,OW-50 1, OW-703, OW-704, OW-708,OW-7 17,OW-723 and OW-724 (Table 1). Some of the wells which had reasonable injection capacities, were found unsuitable because of their location. The aim was to disperse the re-injected fluid across the geothermal field and take advantage of gravity flow as much as possible. Therefore only OW-703 and OW-708 were found suitable for hot re-injection and OW-201 and OW-501 for cold re-injection. But the combined injection capacity of OW-703 and OW-708 could not cope with the amount of hot brine reinjection requirement (Table 2, overleaf).

5 If the initial strategy was adopted the cost of pumping would be more than that for drilling ten production wells (Ouma, 1992). It therefore became economically attractive to drill two more wells at suitable locations in the field to take advantage of gravity flow and for effective dispersion of the re-injection fluid. These wells are OW-R2 and OW-R3, drilled at low elevations to the North West and South of Olkaria North East field respectively. They were sited at those locations to receive hot brine from any production well in the field by gravity flow. Upon completion, pumping tests were carried out on the two wells and sufficient injection capacity obtained (Table 3). Table 2. Brine and blow down. Station Water output OW-706 OW OW OW-7 12 OW-7 13 OW-7 14 OW OW-7 18 OW OW-72 1 OW-725 OW-727 OW-728 Blow down Total Table 3. Injection Capacity of OW-R2 and OW-R3. Well No. OW-R2 2,201 OW-R3 2,200 The amount of blow down expected from the power plant cooling towers is only 114 t/hr and can all be re-injected into OW Olkaria Central field where OW-201 is located is the inferred convergence of the outflow from Olkaria North East and Olkaria West geothermal systems (Ewbank Preece, 1989). That makes it a good candidate for cold re-injection because it is located in the outflow of Olkaria North East reservoir and fast breakthrough is unlikely. The power plant is at an elevation below that of OW- 201, therefore, re-injection into this well can mostly be achieved through pumping. There are other wells, which were drilled for production in Olkaria Central but after testing proved to be unproductive. One of these wells namely, OW-204 has been selected for re-injection of blow down alongside OW-201. Re-injection and Tracer Tests Combined re-injection and tracer tests have been conducted in wells OW-704 and OW-R3. During the tests conducted in OW-704, tracer returns were observed in all the monitoring wells namely, OW-714, OW-7 16 and OW-M2 (Karingithi, 1996). However, during the tests conducted in OW-R3, tracer returns were only observed in wells OW29 and OW-30 even though tracer returns were monitored in most production wells in Olkaria East field including make-up wells OW-32 and OW-34, which are drilled about 500m and goom, respectively from OW-R3 (Karingithi, 1994,1995). Interference rests These tests were carried out concurrently with the re-injection and tracer tests in OW-704. Wells OW-714, OW-716 and OW-725 were put on discharge while Pruett pressure monitoring devices were installed in observation wells OW-707,OW-723 and OW-724. During the tests, pressure draw down was registered in wells OW-707 and OW-724. Superposition of the pressure draw down response in the monitored wells suggested that the wells were in com~unic~tion with a pressure support bound^ (Kagiri, 1994). Review Findings Several alternative re-injection wells beside OW-704 and OW-703 became apparent and most of the alternative re-injection wells are located at lower elevations than the production wells hence for them re-injection is possible by gravity flow. Injection and tracer tests revealed that OW-704 is in direct communication with the production reservoir and thermal breakthrough would be likely during the life of Olkaria I1 power station. It therefore became important to consider a re-injection system which is diversified and flexible enough such that in case of severe thermal breakthrough due to re-injection into any of the wells, it should be possible to re-organize the system without interrupting power plant operation. Thus in identifying re-injection wells for this field, the general strategy should not be to find wells with high injectivity but to distribute the injection so as to maximize fluid pressure support as well as efficiently and economically dispose of the geothermal waste water. In that way thermal breakthrough is unlikely to adversely affect the production wells besides maintaining an environmentally cleaner atmosphere free from steam cloud that develops when condensate flashes. Hot re-injection takes place under pressure and hence there will be another advantage of elimination of corrosion by avoiding Oxygen contamination of the waste brine before re-injection. It was estimated that the cost of pumping waste brine for reinjection in OW-704, as the initial strategy had suggested, would cost more than drilling ten (10) production wells, problems of maintenance not withstanding (Ouma, 1992). A lot of savings were identified in the cost of re-injection by putting the production wells into groups and addressing their re-injection requirements individually. This could allow utilization of gravity flow where possible. For instance, bringing waste brine from OW-705, OW- 714,0W-716 and OW-725 down to the conditioning ponds near OW-715 and then pumping it back to OW-703 after flashing to atmosphere did not appear reasonable. Gathering all this waste brine at a higher elevation and directing it to OW-703 by gravity as hot re-injection could make saving in pumping cost and pipe work. OW-703 being in the production area of the field it is safer with hot rather than cold re-injection. 400

6 Taking all possible re-injection wells into account, it was POSsible to decide which wells could be utilized as the strategy was modified to include both hot and cold re-injection. After taking inventory of the re-injection capacity available in Olkaria North East field and assessing the pumping cost implications only OW- 703 and OW-708 were found suitable as hot re-injection wells. However, their injection capacities could not cope with the requirement for hot brine re-injection. Therefore, it became attractive to drill OW-R2 and OW-R3 specifically for re-injection and fortunately, pump tests revealed good injection capacities. OW-201 was found suitable as a cold re-injection well being of good injection capacity and located at the outflow of the Olkaria North East reservoir. Recommendations In order to effect the dispersion of re-injection fluid and take advantage of gravity flow, production wells were grouped. The total amount of brine from each group was quantified and reinjection well identified for each group. Blowdown from the Power station cooling towers was to be re-injected into OW-201 or OW-204 located at higher elevation than the Power Station. Thus re-injection of cooling tower blow down was to be effected by pumping. Olkaria North East fields in the buffer zone. The current wells in this category are OW-7 13, OW-7 I8,0W-7 19,0W-720,0W-726 and OW-728. The separated brine from OW-713 combine with that from OW-7 18, OW-7 19 and OW-726 separated at OW-7 19 and OW-720 and OW-728 separated at OW-728 and directed under pressure to OW-R3 for hot re-injection. This arrangement would handle about 277 t/hr of waste brine. Croup 4 This group consists of wells OW-701,OW-707,0W-715 and OW-727, which are located close to either side of the main road leading to Olkaria East field. The separated brine from OW-701 and OW-727 separated at OW-727 is combined with that from OW-707 and OW-7 15 separated at OW-7 15 and directed under pressure to OW-708 for hot re-injection. This arrangement would handle about 213 t/hr of waste brine. Blow Down Blow down from the power station cooling towers is to be pumped through HDPE pipes to OW-201. Back-up well for cold re-injection is to be OW-204. This arrangement would be able to handle 114 t/hr of water. Grouping of Wells Croup 1 These are production wells at high elevation to the eastern side of the main road leading to Olkaria East field and can easily gravitate into OW-703 and OW-R3. These wells are OW-705,OW-714,0W-716 and OW-725. Waste brine from OW-714 and OW-716 separated at OW-714 would be combined with that from OW-705 and OW-725 separated at OW-725 and directed under pressure to either OW-703 or OW-R3 for hot re-injection. This arrangement would handle about 182 t/hr of waste brine. Group 2 These are production wells also at high elevation but to the western side of the main road leading to Olkaria East field and can easily gravitate into OW- R2. These wells are OW-706, OW-709, OW-710, ~i~~~~ OW-711, OW-712 and OW-721. Waste brine from OW-72l,OW-709 would be combined with that from OW-706 and OW-71 1 separated at OW-706 and that from OW- 712 and OW-710 and directed under pressure to OW-R2 for hot re-injection. This arrangement would be able to handle about 157 t/hr of hot brine. Croup 3 These are production wells, which, although on high elevation, can more easily drain to Olkaria East field than to Olkaria North East field, and include any future replacement for OlkariaEast and 4. well groups Current Strategy Both hot and cold re-injection has been considered. Production wells have been grouped and the separated hot brine from the groups is to be directed, under pressure to a dispersed set of wells at lower elevations for hot re-injection taking advantage of gravity flow. Pumping will only be necessary for cold re-injection of condensate from power plant cooling tower blow down. The re-injection piping cost has been reduced because instead of utilizing stainless steel pipes, hot brine will take place through carbon steel pipes and cold re-injection through heavy-duty plastic pipes

7 (HDPE). Condensate from the cooling tower blow down will be treated to keep the PH near neutral (-6) and hence there will be little corrosion of the casings in the cold re-injection wells. Four (4) wells will be used for hot re-injection and two others for cold re-injection. The hot re-injection pipelines are interconnected and the hot brine re-injection is dispersed across the field such that risk of thermal breakthrough is small. Furthermore problems of thermal breakthrough will be easier to handle if they occur. Cold re-injection is being directed to OW-201 and OW- 204 instead of OW-704 as was suggested in the Final Engineering Report. These wells unlike OW-704 are located in the outflow of Olkaria North East reservoir and hence cold water is re-injected away from productive part of the field. A programme using tracers will be put in place to monitor the production wells very closely when re-injection starts. This will ensure that any effects on the producing wells are picked up in good time so that appropriate corrective measures are taken. Acknowledgement The author is very grateful to colleagues at Okaria Geothermal Project who critically read the draft and offered suggestion. Not to forget the Reservoir Engineering and Geochemistry crews who helped with collection of the data used. References Ewbank Preece, Feasibility Study for a geothermal power station at north East Olkaria. A report prepared for KPC. Ewbank Preece, Olkaria North East geothermal power station, Final Engineering report. Areport prepared for KPC. Haukwa, C.B., Completion tests on OW-501. WC Internal Report, PP 5. Haukwa, C.B., Intermediate, completion tests and heating of OW-704. KPC Internal Report, pp 6. Haukwa, C.B., Intermediate, completion tests, hating and first discharge of OW-703. WC Internal Report, pp 6. Kabira, L.M., Completion and heating tests on OW-708 KPC. KPC Internal Report, pp 3. Kagiri, D. N., Completion and Recovery report for reinjection wellsow-r2 and OW-R3. KPC Internal Report, pp 5. Kagiri, D.N., Intermediate, completion and recovery tests on OW-717. KPC Internal Report, pp 3. Kagiri D.N., Intermediate, completion and recovery tests on OW-723. KPC Internal Report, pp 3. Kagiri D.N., Intermediate, completion and recovery tests on OW-724. KPC Internal Report, pp 3. Kagiri D.N., Interpretation of interference test data from Olkaria North East field. KPC Internal Report, pp 13. Karingithi, C.W., Well OW-R3 Tracer and injection report. KPC Internal Report, pp 10.. Karingithi C.W., 1995). Olkaria North East potassium iodide tracer injection tests. KPC Internal Report, pp 22. Karingithi C.W Results of Injection and Tracer in Olkaria North East field in Kenya. KPC Internal Report, pp 10. Ouma P.A., Steam gathering system for the North east Olkaria Geothermal Field, Kenya, preliminary Design. UNU Geothermal Training Programme Report No. 9. Waruingi S. N., Completion Tests on OW-201. KPC Internal Report, PP