PROCEEDINGS, Thirty-Sixth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 31 - February 2, 2011 SGP-TR-191 OVERVIEW OF THE WAIRAKEI-TAUHARA SUBSIDENCE INVESTIGATION PROGRAM K. Brockbank 1, C. Bromley 2 and T. Glynn-Morris 1 1 Contact Energy Wairakei Power Station, Private Bag 1 Taupo, New Zealand 2 GNS Science Private Bag 0 Taupo, New Zealand e-mail: kerin.brockbank@contactenergy.co.nz ABSTRACT In 8 Contact Energy undertook a comprehensive research program investigating subsidence anomalies located within the Wairakei-Tauhara geothermal system. About 4 km of continuous core from 13 boreholes in the vicinity of the four known subsidence bowls was logged and preserved. The core underwent geotechnical analysis (including compressibility measurements) and petrology tests (XRD, smectite abundance, SEM). Down-hole reservoir measurements (pressure, temperature, permeability) were conducted on all 13 boreholes including staged injectivity tests, completion tests, heating runs and chemistry sampling. This paper presents an overview of the work undertaken and a summary of the results including: Improved understanding of the processes that cause subsidence. Detailed information on the shallow aquifer characteristics and how these relate to changes in the deep high temperature reservoir. How interpreted feed zones correlate to the core and other measurements made during drilling. 3D subsidence modelling using measured rock properties obtained from the core to model historical and predict future subsidence. BACKGROUND Subsidence was first identified at Wairakei shortly after production began as reservoir pressures declined in the early 1960 s. A leveling network was developed starting with ~100 survey points in 1960 and has grown to over 0 in 2011 covering the entire Wairakei-Tauhara geothermal system. There are four localized subsidence anomalies (bowls) across the Wairakei-Tauhara field: Wairakei, Spa, Rakaunui and Crown bowls (Figure 1). In the Wairakei bowl, total subsidence is ~15 m and a maximum subsidence rate of 498 mm/year was recorded in 1979, with the rates gradually declining to around 58 mm/yr in 9. The Spa and Rakaunui bowls have a similar total subsidence of 2.9 m and 2.5 m respectively. At the Crown bowl, most of the subsidence occurred between 1999 and 9, with total subsidence of 0.9 m (Bromley et al., 2010). As part of the re-permitting process for the Wairakei Power Station in 7, the Environment Court identified a need for research and analysis into the mechanisms of subsidence across the Wairakei- Tauhara geothermal field. In 8 Contact planned a comprehensive research program to investigate the four subsidence bowls. Thirteen continuously cored boreholes to depths of -800 metres were to be drilled in the vicinity of each subsidence bowl, to investigate the geotechnical properties and improve our understanding of the processes that cause subsidence. Once complete, the wells were to be used to monitor changes in reservoir conditions.
Figure 1: Map of the Wairakei-Tauhara Geothermal System. The resistivity boundary is shown by the green band. All wells are shown by green dots with the 13 boreholes drilled for the subsidence program highlighted in red. The four subsidence bowls are named and outlined in blue. OVERVIEW OF WORK UNDERTAKEN The 4 km of continuous core recovered from the subsidence wells was logged in detail onsite, then underwent extensive geotechnical analysis, including compressibility measurements, bulk rock properties and petrology tests including X-ray diffraction (XRD), smectite abundance and scanning electron microprobe (SEM) (Bromley et al., 2010). Down-hole reservoir measurements (pressure, temperature, permeability) were conducted on all wells including completion tests and heating runs. The completion tests and heating runs provided information about the overall permeability for the well and the locations and characteristics of the feed zones. Staged injectivity tests and chemistry sampling were also performed on some of the wells. The staged injectivity tests allowed permeability measurements to be taken from shallow depths that were planned to be cased out. Downhole chemistry sampling on some wells which had internal flows were used to identify the origin of the reservoir fluids. PROCESSES CAUSING SUBSIDENCE The data obtained, especially from the core, has increased the knowledge about causes and mechanisms of subsidence. Intervals of core recovered at shallow depths (< 400 m) were found to be highly compressible and of sufficient thickness to explain the locations and amplitudes of all subsidence bowls. The bowl locations are considered to be the result from zones of anomalous rock properties related to intense hydrothermal alteration of porous and permeable formations, usually where boiling has also occurred (Bromley et al., 2010). All bowls (excluding Rakaunui) are located in close proximity to historic surface discharges of geothermal fluids. The compressing layers in all bowls are associated with zones of low pressure steam. Pressure decline in these shallow steam zones, which in turn causes consolidation of the compressible layers, has been
identified as the main subsidence mechanism (Bromley et al 2010). SHALLOW AQUIFER CHARACTERISTICS The feed zone and reservoir conditions identified from the subsidence wells have provided additional detail and improved understanding of the characteristics and inter-connections of the intermediate aquifers that lie between the shallow groundwater and the deep high temperature reservoir. Waiora aquifer over the last 50 years (caused by fluid withdrawal from the Wairakei field) migrated up into the shallower aquifers allowing the fluid to boil (natural state pressures in the deep Waiora aquifers was about 20 bar higher than measured in 9). In both the shallow and intermediate aquifers there is a horizontal pressure gradient from east to west across the Tauhara field (away from Mount Tauhara) so some pressures do not lie exactly on the trend lines. In North Tauhara, some of the shallow wells (TH07, THM12 and THM17 for example) with feedzones in the Mid-Huka aquifer have pressures that fall on the Waiora pressure trend (where Mid-Huka pressures are expected). This is shown in Figure 2 by the overlap in pressure trends between elevations of ~100 and 150 masl. In these wells the Lower Huka Falls formation thins out or is replaced by a rhyolitic breccia which provides a hydrological connection to the deep reservoir and allows the deeper Waiora pressure trend to extend up to the Upper-Mid Huka contact (Rosenberg et al., 2010). Figure 2: Pressure profiles of the shallow part of the Tauhara field showing the three aquifers: Upper, Lake level/mid-huka and the deep reservoir. A generalized geological column is shown on the left of the graph (adapted from Rosenberg et al., 2010). Figure 2 shows the feed zone pressures of the Tauhara wells during 9 and associated inferred pressure profiles of the three distinct pressure regimes: the deep Waiora reservoir, intermediate Mid-Huka (Lake) aquifer and the shallow ground water (Upper Aquifer). Figure 3: Location of the vapour zone in Tauhara (Rosenberg et al., 2010). Both the steam zone and shallow liquid pressures are now being continuously monitored in several of the monitor wells. This will allow correlations between pressure change and subsidence rates to be made in the future. Between these three liquid trends two-phase conditions are found in several of the wells. This implies that the pressure drawdown of the deeper
FEED ZONES One of the challenges of reservoir engineering is understanding the characteristics of feed zones: the source of geothermal fluids in the wellbore. Feed zone characteristics are relevant for well targeting, understanding well decline and reservoir management. Following the completion of the subsidence program, new detailed structural and geological information became available about the geothermal reservoir. An investigation is currently ongoing on how feed zones, determined from completion test data, correlate with other measurements from the drilling process and core measurements including: Lithological information Rock-Quality Designation (RQD) Recovery Factors from drill core Fluid loss zones (from drilling data) Porosity Rock strength Swelling clays Figure 5 shows a graphical comparison between all the parameters listed above for the well THM17. Figure 6 shows the core photos relating to the two feed zones: a two-phase feed at 215 m and a liquid feed zone at 275 m. To date four wells have been analysed: THM13, THM16, THM17 and THM18. Lithological information For THM17 and THM18, the feed zones correlate with lithological interfaces while THM13 and THM16 feed zones are located in the mid-sections. This suggests that feed zones are not necessarily at the formation boundaries. RQD and core recovery RQD provides an indication of the number of fractures in the core over a known distance. A low RQD implies plenty of fractures and a high RQD implies intact rock. Core recovery is good in hard, intact rock but if the rock is heavily fractured, soft or unconsolidated material, the core recovery decreases. Low RQD and core recovery are generally correlated with feed zones in THM16, THM17 and THM18. Drilling fluid loss zones The feed zones in these wells correlated extremely well with the identified drilling loss zones. However, once a full drilling loss (ie. no water coming back out the well) is encountered, loss zones below this point are masked by the permeability above and are impossible to identify. Porosity Interestingly, the feed zones in these four wells are correlated with intervals of low or average porosity implying that fracture permeability dominates. Rock strength Rock strength did not seem to be correlated to the feed zones of the four wells analysed, with the feed zones being located at places of high, moderate and low rock strength. However with THM17, the 2- phase feed zone was located at the lowest rock strength in the well, while the liquid feed zone was located at the highest rock strength (Figure 5). THM16 also had high rock strength at the feed zones while both THM13 and THM18 had moderate rock strength. This could imply that rock strength may be correlated in certain wells, with the extreme values (ie. high and low) indicating the location of feed zones, due to weakening and strengthening processes associated with 2-phase and liquid feed zones respectively. Swelling clays The feed zones in all four wells were located below the smectite clay cap (known as the conductor due to its high electrical conductivity). 3D SUBSIDENCE MODELLING The core underwent thorough geotechnical analysis, in particular compressibility measurements. These measurements allowed the stiffness for different stratigraphic formations to be defined for each borehole and thus each subsidence bowl. Table 1 shows an example of the typical stiffness data used for modelling. Table 1: Formation Typical rock properties used in the subsidence model (adapted from O Sullivan et al., 2010). Average Stiffness (MPa) Minimum Stiffness (MPa) Maximum Stiffness (MPa) Oruanui 457 18 1050 Upper 325 32 1198 Huka Falls Middle 731 124 1955 Huka Falls Lower Huka Falls 785 333 1730 This data from the core analysis, along with pressure data from the reservoir model, enabled finite element modeling to be performed on each bowl to simulate
past subsidence, with the aim of predicting future subsidence. So far it has been observed that the location of feed zones does not necessarily coincide with simple models of fracture, formation, and fault-related permeability. Further analysis of the remaining subsidence wells is currently underway. The 3D subsidence model has been able to match the size, location and magnitude of subsidence at all four subsidence bowls but the timing of the development of subsidence is not accurately matched. Figure 4: Cross-section of finite element subsidence model, showing deformation of the Rakaunui bowl (O Sullivan et al., 2010). Figure 4 shows a cross-section of the model at the Rakaunui bowl. The model was able to match the size, location and magnitude of subsidence at all four bowls but the timing of the development of subsidence is not accurately matched (O Sullivan et al., 2010). The timing effect is mostly attributed to the shallow pressures of the reservoir model not being accurately matched to well data, especially for wells closest to the subsidence bowls. Further refinement of the reservoir model is needed to better match these shallow pressures. CONCLUSIONS The Wairakei-Tauhara subsidence investigation program significantly improved the understanding of the processes involved with subsidence. The main mechanism of subsidence identified in the Wairakei-Tauhara geothermal system is pressure decline in low pressure shallow steam zones which results in the compression of the intensely altered rock in which the steam lies. Two-phase conditions found between the three aquifers imply the pressure drawdown in the deep Waiora reservoir has migrated up into the shallower aquifers, resulting in boiling. The absence of the Lower Huka Falls formation in North Tauhara has resulted in deep Waiora pressures being present in the Mid-Huka aquifer. Future work that is planned: Continuous pressure monitoring in some of the wells will allow correlations between pressure change and subsidence rates to be made in future Correlating feed zone data with core and drilling measurements for the remaining subsidence wells Improvements to the subsidence model to better match the timing of the development of subsidence ACKNOWLEDGEMENTS The authors wish to acknowledge Contact Energy for permission to publish this paper. REFERENCES Bromley, C., Currie, S., Ramsay, G., Rosenberg, M., Pender, M., O Sullivan, M., Lynne, B., Lee, S., Brockbank, K., Glynn-Morris, T., Mannington, W. and Garvey, J. (2010), Tauhara Stage II Geothermal Project: Subsidence Report. Glynn-Morris, T., Bixley, P., Brockbank, K., Sepulveda, F., Winmill, R. and McLean, K. (2010), Re-evaluating feed zone locations in a high temperature geothermal system based on evidence from deep continuously cored wells, Proceedings of the New Zealand Geothermal Workshop 2010. O Sullivan, M., Yeh, A. and Clearwater, E. (2010), Three-Dimensional Model of Subsidence at Wairakei-Tauhara. Rosenberg, M., Wallin, E., Bannister, S., Bourguignon, S., Sherburn, S., Jolly, G., Mroczek, E., Milicich, S., Graham, D., Bromley, C., Reeves, R., Bixley, P., Clotworthy, A., Carey, B., Climo, M and Links, F. (2010), Tauhara Stage II Geothermal Project: Geoscience Report.
TEMPERATURE & PRESSURE CASINGS (mrf) GEOLOGY (mrf) RECOVERY AND RQD (%) FORMATION DATA (mrf) POROSITY & ALT STRENGTH (MPa) 0 50 100 150 0 50 100 150 0.20 0.30 0.40 0.03 Core Recovery RQD 214.2 Feed Zones (2-phase) Feed Zones (Liquid) Is(50) Loss Zones Diametral (Major) Is(50) Axial Loss Zones (Minor) Porosity Temperature (C) Smectite (%) Pressure (Barg) 298.2 0 10 20 0 5 10 15 Figure 5: Graphs comparing feed zone characteristics obtained from the completion test data to measurements obtained from drilling data and coring measurements. This is a close-up view of THM17 data from mrf. Starting from the left: fully heated pressure (red) and temperature (black) profiles; casing details, in this case 3.5 perforated liner; stratigraphy details upper huka falls at the top, middle huka falls and lower huka falls formation at the bottom; RQD (red) and core recovery (blue); identified feed zones from completion test data (yellow/red) and drilling loss zones (blue); porosity (red) and smectite abundance (blue); diametral (blue) and axial (red) strength (Glynn-Morris et al., 2010).
Figure 6: Core photos of the two identified feed zones in THM17.