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Paper Instructions PROCEEDINGS, TOUGH Symposium 2012 Lawrence Berkeley National Laboratory, Berkeley, California, September 17-19, 2012 MODELING THE OHAAKI GEOTHERMAL STSYEM E.K. Clearwater¹, M.J. O’Sullivan¹, K.Brockbank², W. I. Mannington² ¹Department of Engineering Science, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand ²Wairakei Power Station, Contact Energy, State Highway 1, Private Bag 2001, Taupo 3352, New Zealand e-mail: [email protected] ABSTRACT reservoir by convection of water and steam. Ohaaki has a base temperature in excess of The Ohaaki geothermal system lies within the 300ºC and a large gas (CO₂) content. More Taupo Volcanic Zone (TVZ) in the North Island of New Zealand. Since the early 1980s, a series information on the system can be found in of numerical models of the Ohaaki geothermal Hedenquist (1990). system have been developed at the University of Auckland in collaboration with Contact Energy The basement of the Ohaaki system is a pre- and its predecessors. The simulator used is volcanic greywacke, which down-faults to the AUTOUGH2 (adapted from TOUGH2) and the north-west. This is overlain by a volcano-clastic extra capabilities of this simulator, plus the use sequence interspersed with dacitic and rhyolitic of PyTOUGH (a library of Python scripts), have volcanic domes and flows, with complex been invaluable. permeability and porosity distributions. The two main production reservoirs are the Wairoa Natural state simulations are used to compare formation at depths of 400 to 1200 m field wide, the model results to the temperature data for the and the Tahorakuri formation on the West Bank pre-exploitation state of the reservoir. Then below depths of 1500 m. Details on the structure production history simulations are carried out, and stratigraphy of the field are outlined in Wood et al. (2001) and Rae et al. (2007). and model results for pressure, temperature, CO2 flow, and enthalpy are compared to data from OHAAKI POWER STATION the well testing period, the recovery period, and the production period. Drilling commenced at Ohaaki in 1965, with a total of 44 wells drilled between 1966 and 1984. Maintenance and improvement of the model is There was an extended period of well testing part of an ongoing effort to represent the Ohaaki and recovery up to 1988, when the Ohaaki system more accurately, enabling the model to Geothermal Power station was commissioned be utilized as a tool for reservoir management [Lee and Bacon (2000), Clotworthy et al. and to predict the future behavior of the resource (1995)]. There are now over 65 wells drilled in under various scenarios. the area. THE OHAAKI SYSTEM The plant commissioned in 1988 was 116 MWe The Ohaaki geothermal system lies on the and during the first 5 years of production, eastern margin of the Taupo Volcanic Zone generation was maintained at ~100 MWe. In (TVZ). The Waikato River bisects the system, 1993, the available steam began to decline. A dividing it into the West and East Bank areas deep drilling program was undertaken in 1995, (see Figure 1). There are two separate upflow which identified high temperatures and zones for each of the East and West Banks. permeability in the deep volcanic formations Ohaaki (along with the other systems within the underlying the West Bank [Lee and Bacon TVZ) is a high-temperature liquid-dominated (2000)]. This was relatively successful; hydrothermal convective system. The driving however, steam supply continued to decline. A force of such a system is convection of water second deep drilling program also focused on driven by density differences. Heat and mass are the West Bank was undertaken in 2005–2007 transported through the permeable rock of the (Rae et al., 2007), allowing generation output to be maintained at about 60MWe. - 1 - RESERVOIR MODEL The grid has been rotated so that columns align with the dominant faulting direction – NW-SE. Grid structure The model consists of 23 layers, each with 992 Over time, a deeper understanding of the elements, plus one atmosphere block, leading to reservoir has been developed based on data a total number of elements of 22817. The grid is gathered by various geoscience techniques and roughly a square covering 16 km by 15 km. from deeper drilling. Also over time, Figure 1 shows a plan view of the model grid. computational power has increased, and so it has been possible to make the reservoir model more and more refined. Earlier grid structures for the Ohaaki model have been described in Blakely et al. (1983), Newson and O'Sullivan (2001), 6294000 B41B37 B55 Zarrouk et al. (2004), and Zarrouk and B12 B3 O'Sullivan (2006). The reservoir model B38 B4 B58B57B56B18 B52B53 B32 discussed here is the “2011” model described in B48B54 B11B33 B15 B17 B61B60B59 B22 B9 B8 Clearwater et al. (2011). B2 B1 B21 B20 B19 B26 B31 B65B64 B23 The size of the model grid is chosen to include B5 B13 6292000 B24 as much area as is needed for recharge to the B34 B14 system, and as much depth as is feasible in terms B46 B43 B47 B35 B44 B6 B25 of the limitations of the equation of state for B36 B45 B10 B42 H2O/CO2 available in TOUGH2. The deepest B39B40 B28 B62 B51 B49 B27 well drilled at Ohaaki reaches a depth of almost B7 B16 2.6 km, and the base of the model is set at 3 km B30 B29 depth. 6290000 2798000 2800000 Figure 2. Close up of the model grid. The yellow line is the resistivity boundary, the red dots show well- head locations and the blue line is the Waikato River. The West bank lies on the North-West side of the river, the East bank on the South-East side. The greatest refinement of the grid occurs within the reservoir resistivity boundary. The aim is to allow each well to be placed in a separate block and to avoid 5-sided blocks or connections that are not orthogonal. The gradual expansion of block size goes from 250 m by 250 m in the central borefield, to 1 km by 1 km at the outer boundary of the model. A close-up view of the grid is shown in Figure 2. The layer structure of the model is shown in Figure 3. The top four layers are 100 m thick. Figure 1. Plan view of the Ohaaki reservoir model There is then a refinement down to a layer of 20 grid. The blue line is the Waikato River, the yellow m to give a good resolution of the Ohaaki line the resistivity boundary, and the red dots show Rhyolite, which acts as a fluid pathway. After well-head locations. that, the layers increase again to a maximum thickness of 250 m. - 2 - 300 1 250 200 A background conductive heat flux of 120 2 150 100 3 50 mW/m² is used— typical of the values found 0 4 -50 throughout the TVZ. This heat flux is increased -100 5 -125 -150 6 -170 in blocks close to the main reservoir, -210-190 7 -200 -250 8 -230 -300 9 -275 representing the greater heat flow anomaly 10 -350 -400 11 -450 associated with Ohaaki. The mass inflow at the -500 12 -550 base of the model represents the upwelling fluid -600 13 -650 -700 near the base of the convective plume which has 14 -762.5 mRSL centre Layer -825 not been captured within the model. The CO₂ is 15 -887.5 -950 injected at an average mass fraction of 2.5%— 16 -1037.5 which is representative of the amounts found in -1125 wells at Ohaaki. 17 -1212.5 -1300 18 -1400 A summary of the total heat, mass, and CO₂ -1500 injected into the base of the model is shown in 19 -1600 -1700 Table 1. The total amount of carbon dioxide 20 -1825 injected gives an average flowing mass fraction of 2.5%. A total heat input of 119 MW is Elevationofinterfaces layer mRSL -1950 applied to the model, which is close to the 21 -2075 natural heat flow of around 100 MW (Allis, -2200 1980), but as there is large uncertainty around 22 -2325 this value, the model is reasonable. -2450 23 -2575 Table 1. Total flow into the base of the model. Figure 3. Vertical layer structure and layer names. Enthalpy Temperature Total (kJ/kg) (°C) Boundary Conditions Mass 1430 314.4 68.32 kg/s Due to the large area of the model (and also Heat - - 39.64 MW from observing pressure changes at the CO₂ 1430 310.51 1.7 kg/s boundary blocks), the model is thought to be large enough to capture all pressure changes created within the borefield and within the Populating grid blocks whole of the hydrothermal convective regime, Geoscience data is invaluable in helping to and so all side boundaries are treated as closed. decide what rock properties should be assigned This is also a reasonable approximation for to each element in the model. Recent Ohaaki, because of the high CO2 content and collaboration with ARANZ and GNS Science large boiling zone. The pressure changes in the and the use of the LEAPFROG geological reservoir get buffered by the expansion and modeling software has introduced an automated contraction of the boiling zone, and hence do not way of extracting the geological model spread to the edges of the model. rocktypes and applying them to the elements in our TOUGH2 model. New TOUGH2 simulation The top layer of the model follows the surface of input files can be created within the software the water table. The temperature and pressure of and results can be visualised (e.g.
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