Determination of Calcite Scaling Potential in OW-903 and OW-914 of the Olkaria Domes field, Kenya

Transcription

1 PROCEEDINGS, Fortieth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 26-28, 2015 SGP-TR-204 Determination of Calcite Scaling Potential in OW-903 and OW-914 of the Olkaria Domes field, Kenya Ruth Wamalwa, KenGen 785, Naivasha Keywords: Geochemistry, simulations, calcite scaling ABSTRACT Non condensible gas is a major potential factor likely to affect the reservoir pressure in the Olkaria geothermal reservoir which has been commissioned by Kenya Electricity Generating Company LTD (KenGen) as the Olkaria IV Geothermal Power Project. Under static conditions, the pressure within the production zone ranges from 120 to 160 bars while reservoir temperatures range from 200 to 320oC at 2000 to 2400m. Based on gas pressures estimated from applying Henry s law, approximately 60 bar (at 0.02 kg NCG/kg reservoir brine) to 100 bar (at 0.05 kg NCG/kg reservoir brine) is dissolved non condensible gas pressure. The non condensible gas is over 90% carbon dioxide. The reservoir fluids at these wells are bicarbonate waters and thus likelihood to produce calcite scale in the wellbores at the depth of gas breakout during production. The gas breakout pressures (or bubble point) i.e. the pressure below which the fluid will begin to transform from 100% liquid to twophase. Gas breakout pressure is the sum of the gas pressure and water pressure at the reservoir temperature. These values can be estimated using Henry s Law and the steam tables. In the deep reservoir of Olkaria, bubble points are estimated to be between 80 bar (at 0.02 kg NCG/kg reservoir brine) and 100 bar (at 0.05 kg NCG/kg reservoir brine). At the flow rates (<220 tph) that the dynamic surveys are run, measured pressure in the well falls below this gas breakout pressure between 900 and 1200 m. Wellbore simulation is used to estimate the depth of gas beak out at higher flow rates. However, since the gas breakout occurs at greater depths at higher flow rates, it is important to estimate the depth of gas breakout at multiple mass flow rates to manage the potential effect of scaling in the feed zone as well as the depth of scale inhibitor injection. 1. INTRODUCTION The Olkaria geothermal resource is located in the Kenya Rift valley, about 120 km from Nairobi. Geothermal activity is widespread in many parts of the Kenyan rift and 14 major geothermal prospects have been identified. The Olkaria geothermal field is inside a major volcanic complex that has been cut by N-S trending normal rifting faults. It is characterized by numerous volcanic rhyolitic) domes, some of which form a ring structure, which has been interpreted as indicating the presence of a buried volcanic caldera. Olkaria is surrounded by further geothermal prospects, such as Suswa, Longonot and Eburru. Exploration of the Olkaria geothermal resource started in 1956 with deep drilling commencing in A feasibility study in 1976 indicated that development of the geothermal resource was feasible and consequently a 30 MWe power plant was constructed (Ouma, 2010). Three power plants are currently installed in the field and producing electricity; Olkaria I with 45 MWe capacity, Olkaria II with 105 MWe capacity and Olkaria III with 48 MWe capacity. The first two are operated by KenGen while the third is operated by Orpower4 Inc. The Olkaria I power plant consists of 3 units commissioned between 1981 and 1985 while Olkaria II, which also has 3 units, was commissioned between 2003 and The Olkaria III power plant was commissioned in two phases between 2000 and In addition the geothermal resources of the NW part of the Olkaria area are utilized both for direct heat and small scale electricity generation by the Oserian flower farm. The parts of the Olkaria geothermal field being utilized or under development have been subdivided into sectors that include Olkaria East (Olkaria I), Olkaria Northeast (Olkaria II), Olkaria West (Olkaria III) and Olkaria Domes (Olkaria IV). KenGen s present estimation of the possible generating capacity of their 204 km2 total concession area in Olkaria indicates that it may sustain as much as an additional 840 MWe long-term generation (Ouma, 2011). Of these 280 MWe are soon entering the implementation phase, a 140 MWe expansion of Olkaria I and a 140 MWe installation in Olkaria IV. As result of intensive production drilling in progress since 2007 steam availability corresponding to more than 300 MWe has been confirmed in the Olkaria East and Olkaria Domes sectors. Therefore a capacity of about 540 MWe still remains untapped, according to KenGen s estimates, which is the motivation to carry out the present Optimization Study. 2. RESERVOIR CONDITIONS The Olkaria geothermal reservoir is a layered reservoir. The surface geology comprise of Volcanic Complex dominated by comendites and pyroclasts fall deposits (pumice fall and ash deposits) mainly from Olkaria and Longonot volcanoes. The subsurface geology of the Olkaria geothermal field can be divided into six broad lithostratigraphic groups based on age, tectono-stratigraphy, and lithology, as revealed by data from more than eighty deep wells in the geothermal area (Omenda, 2000). 1

2 The Mau Tuffs, trachytes, basalts and ignimbrites are considered to be the oldest rocks encountered in the Olkaria area and are common on the surface in the area west of Olkaria Hill, but are absent in the East field due to an east high-dipping high-angle normal fault traversing Olkaria Hill. The plateau trachytes are of Pleistocene age and dominantly composed of trachytic lavas with minor basalts, tuffs and rhyolites. This formation hosts the geothermal reservoir for the East and Olkaria Domes fields. Olkaria basalt consists of basaltic flows, minor pyroclastic deposits and trachytes. It is believed to form the cap-rock for the Olkaria geothermal system (Haukwa, 1984). The Olkaria basalt formation underlies the Upper Olkaria volcanics and is composed of numerous thin basaltic flows separated by thin layers of tuffs, minor trachytes, and occasional rhyolites. The formation has been penetrated by nearly all wells in the East and North East Olkaria fields. However, this formation is conspicuously absent in the West field. According to hydrothermal studies (Browne, 1984, Muchemi, 1992) and reservoir modelling (Ambusso and Ouma, 1991) Olkaria basaltic formation is considered to act as cap-rock for the Olkaria geothermal system, as indicated by sharp temperature increases below the formation. The Upper Olkaria volcanics consists of comendite lavas and their pyroclastic equivalents, ashes from Suswa and Longonot volcanoes and minor trachytes and basalts (Thompson and Dodson, 1963). These rocks occur from the surface down to a depth of about 500m. Comendites are the dominant rock in this formation. The youngest lava of the Upper Olkaria formation is the Ololbutot comendite, which has been dated at 180±50 yrs (Clarke et al., 1990). The crater for this young lava is structurally controlled along the N-S Ololbutot fault. 3. DEPTH OF GAS BREAKOUT OR BUBBLE POINT Estimated Gas+Water Pressure and Dynamic Pressure Surveys Gas breakout or two-phase conditions occurs at the depth at which the gas pressure plus water pressure exceeds the total pressure (bubble point depth). Pgas can be estimated using Henry s Law and the minimum water pressure can be estimated using steam tables as follows: Pgas = Xgas * KH Pliq = Pwater@sat T Ptot = Pgas + Pliq where KH = Henry s law constant at the reservoir temperature and Xgas is the mole fraction of gas in the reservoir. OW-913 OW-914 Reservoir Temperature (oc) Non condensible Gas (NCG) kg/kg Mole Fraction NCG, Xg Henry s Law Constant, Kh Pgas (bar) Pliq (bar) Ptotal (bar) Depth of Ptotal@low flow (m) Low (survey) flow (tonnes per hour, tph) 2

3 Casing Depth (m) Depth of major entry (m) The estimated depths under dynamic conditions are shown in Figures 1 and 2. Considering that dynamic surveys occur below the maximum or production flow rate, these depths are minimum gas breakout depths. The depth of gas breakout at full production flows is obtained by wellbore simulation. Figure 1: Gas Breakout Depth during the dynamic Survey of Well OW

4 Figure 2: Gas Breakout Depth during the dynamic Survey of Well OW WELLBORE SIMULATION OF GAS BREAKOUT PRESSURE A steady-flow wellbore model (Garg et al., 2004) was used to model the dynamic pressure and temperature profiles in wells X and Y, and thereby to constrain the values of various parameters in the wellbore model. The dynamic pressure and temperature profiles were obtained at relatively low discharge rates. The constrained wellbore models were then employed to forecast the response of the wells under various discharge rates. 4. CONCLUSIONS The depth of gas breakout (bubble) depth in high gas wells can be estimated using the measured down hole pressures from dynamic surveys and calculated total pressure where the total pressure is equivalent to the liquid water pressure plus the gas pressure. However, this estimation is limited to the flow conditions of the dynamic survey, specifically the mass flow rate Using wellbore simulation, the gas breakout depth can be estimated at multiple flow rates. Results imply that for all practical discharge rates of Well X, the bubble point will remain within the cased and cemented portion of the borehole and above the main fluid entry. Therefore scale can be mitigated with practical set depth for the capillary tubing. However, results for Well Y imply that for all practical discharge rates, the bubble point is located within the slotted 7 inch liner. For a discharge rate of about 360 tph, the two-phase zone will extend to the feed zone depth, and risk of scaling the feed zone occurs. Evaluation of gas breakout or bubble points is a critical reservoir management tool for high gas liquid-dominated geothermal resources. 4

5 REFERENCES Ambusso, W.J. and Ouma, P.A., 1991: Thermodynamic and permeability structure of Olkaria North East field: Olkaria fault. Geothermal Resource Council Transactions, 15, Haukwa, C.B., 1984: Recent measurements within Olkaria East and West fields. Kenya Power Co., internal report, 13 pp. Browne, P.R.L., 1984: Subsurface stratigraphy and hydrothermal alteration of the eastern section of the Olkaria geothermal field, Kenya. Proceedings of the 6 th New Zealand Geothermal workshop, Geothermal Institute, Auckland, Muchemi, G.G., 1992: Structural map of Olkaria geothermal field showing inferred ring structures. Kenya Power Company internal report Omenda, P.A., 2000: Anatectic origin for Comendite in Olkaria geothermal field, Kenya Rift; Geochemical evidence for syenitic protholith. African Journal of Science and Technology, Science and Engineering series, 1, 39-47PP Thompson, A.O., and Dodson, R.G., 1963: Geology of the Naivasha area. Geological Survey of Kenya, Kenya, report 55pp. Clarke, M.C.G., Woodhall, D.G., Allen, D., and Darling G., 1990: Geological, volcanological and hydrogeological controls on the occurrence of geothermal activity in the area surrounding Lake Naivasha, Kenya, with coloured 1: geological maps. Ministry of Energy, Nairobi, 138 pp. Garg, S.K., Pritchett, J.W., Alexander, J.H. (2004). A new liquid hold-up correlation for geothermal wells. Geothermics 33,