Montserrat geothermal development project

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The small Caribbean island of Montserrat (Fig. 1) is most well-known for the devastating eruption of the Soufrière Hills Volcano, which began in 1995 (Kokelaar, 2002) and which continues to the present time.[1] This eruption has caused great social and economic damage and rendered two-thirds of the island uninhabitable. The island is currently rebuilding; hoping to revive its economy and return to prosperity. In common with the most of the Eastern Caribbean, Montserrat has high electricity generation costs of up to US$ 0.50 per kWh (PKF, 2012; giz and CARICOM Energy Programme, 2016), due to the use of diesel-powered generation plants.[2][3] These high costs use up valuable foreign exchange and reduce the competitiveness of local industries. The government of Montserrat (GoM) with the assistance of the Department for International Development (DFID) is currently pursuing the development of geothermal energy to bring down the cost of electricity generation.

Figure 1: Location of Montserrat and the geothermal project area (figure copied from EGS (2014)).

Due to the push of high energy prices and the pull of promising geology, the idea of using geothermal energy as a power source in Montserrat has been around since at least the 1970s.[4] There have been several geothermal scoping studies in Montserrat to evaluate the feasibility of geothermal development on the island.[5][6][7] The last exploration study preceded the drilling of the first two deep geothermal wells on the island in 2013 and a third which began drilling in 2016.[8] EGS’ surface exploration programme included geochemical, geological and geophysical components. The geophysical survey consisted of magnetotelluric (MT) and Time Domain Electromagnetic (TDEM) surveys.[9] Figure 1 shows the geothermal prospect area.

Using exploration and other available data a 3D geophysical conceptual model of the reservoir was developed by Ryan et al. (2013) (Figure 2).[10] The model incorporated a 2D magnetotelluric resistivity model, a p-wave velocity (Vp) tomography model from an active seismic experiment (Shalev et al., 2010), and natural earthquake hypocenters.[11] This model was consistent with the existence of a high-temperature geothermal system as indicated by the low resistivity layer associated with a hydrothermally altered clay cap.[12] Ryan et al. (2013) interpreted the low P-velocity anomaly sitting beneath the low resistivity layer to be the high-temperature region of the geothermal reservoir.[10] Utilising both geophysical data and temperature logs from the first two wells (Ryan and Shalev, 2014) suggested that seismic velocity anomalies in the reservoir were controlled by temperature-dependent hydrothermal alteration of the reservoir rocks.[13]

Figure 2: 3D conceptual model of the Montserrat reservoir (copied from Ryan et al. (2013)).[10]

After wells MON-1 and MON-2 were drilled temperature log data was used to refine and develop the geophysical conceptual model. Ryan et al. (2014) utilised the temperature log data and the tomographic model to develop a three-dimensional temperature model of the reservoir.[13] This model was based on the correlation between logged well temperatures and the normalised seismic velocity anomalies at wells MON-1 and MON-2.[13] The modelling approach rests on the conjecture that the seismic velocity anomaly observed in the high-temperature reservoir is primarily caused by temperature-dependent hydrothermal alteration. Figure 3 shows both planar and cross-sectional views through the temperature model.

Figure 3a: Two Images of the three-dimensional temperature model developed by Ryan et al. (2014) a) planar view at 2000 m depth (the magenta dots indicate the approximate locations of wells MON-1 and MON-2; the light blue dot indicates the location of a hot spring).[13]
Figure 3b: The black dotted line indicates the line of cross-section depicted in b) which shows a N-S cross-sectional view through the model (the black, red and green lines indicate the approximate locations of MON-1, MON-2 and MON-3 respectively). The colour bars indicate temperatures in °C.

Wells MON-1 and MON-2 were sited and drilled in 2013 (EGS, 2014)using information from the conceptual models developed by (Ryan et al., 2013) as well as other geological information, such as structural analysis, and logistical considerations such as ease of access etc.[10][14] Well MON-1 began drilling on 17 March 2013 and was drilled vertically to a depth of 2298 m. Well MON-2 began drilling on 19 May 2013 and drilled vertically to a depth of 2870 m. After completion, the wells were flow tested. The wells were able to sustain flow rates of a liquid-dominated geofluid of ~34 Kg/s with enthalpy of approximately 1000 KJ/Kg.[15] Preliminary estimates indicate that the wells would be able to generate the equivalent of ~2 MWe (Nicolas Reyes pers. comm.). The peak load on the island is 2.1 MWe.[16][17] Following the completion and testing of wells MON-1 and MON-2 GoM decided that it was necessary to expand the production and reinjection capacity of the wellfield. They therefore made a decision to drill an additional well to provide adequate capacity. Well MON-3 was sited with the help of the three-dimensional temperature model developed by Ryan and Shalev (2014).[13] The final well location was also determined by logistical concerns such as availability of land and volcanic risk. Well MON-3 began drilling on 22 September 2016. Estimates of reservoir temperatures in excess of 230 °C have been made during drilling but the well has not yet been completed.

Figure 4: The approximate locations of wells MON-1, MON-2 and MON-3 (image courtesy of Bastien Poux).

Three approximately 7 m long, 4-inch diameter, cores were recovered from MON-3 at depths of 1296, 1511 and 1919 m (below sea level). Along with mineralogical and geochemical work, a series of petrological and petrophysical studies have been planned. The aim of the second set of studies is to investigate the conjectured relationship between hydrothermal alteration mineral assemblage and seismic velocity. Measurements of P and S-wave velocities along with other petrophysical parameters will be made using a triaxial apparatus capable of operating at reservoir pressures and temperatures. These parameters will then be correlated with the hydrothermal alteration petrology of the samples to help determine what, if any, relationship there is between them.

Although not yet complete, the Montserrat geothermal project has already had some success. Two wells have been drilled and successfully flow tested proving the existence of an exploitable high temperature geothermal reservoir and the island is on its way to becoming one of the few nations with a fully renewable electricity generation system. In addition, work on this project has produced a new possible tool for geothermal exploration: seismic tomography as a means of determining subsurface temperatures in high-temperature geothermal reservoirs in three-dimensions. Conceivably, greater constraint on the temperature field will allow for a better understanding of the permeability distribution of the reservoir. This technique, based on seismic tomography, could become a useful tool in geothermal development improving the targeting of productive wells and reducing the significant drilling risk in geothermal projects.

References

  1. Kokelaar, B. P., 2002, Setting, chronology and consequences of the eruption of Soufrière Hills Volcano, Montserrat (1995-1999), in T. H. Druitt and B. P. Kokelaar, eds., The eruption of the Soufrière Hills Volcano, Montserrat, from 1995 to 1999.: Geological Society, 21, 1-43.
  2. PKF, 2012, Feasibility Study into Montserrat Geothermal Energy Final Report: PKF Accountants & business advisers.
  3. giz, and CARICOM Energy Programme, 2016, The Power to change: The Montserrat Energy Policy 2016-2030: Ministry of Communication, Works, Energy and Labour, Government of Montserrat.
  4. Wright, E. P., K. H. Murray, and A. H. Bath, 1976, Draft Report on Geothermal Investigations in Montserrat: Unpublished report, Natural Environment Research Council Institute of Geological Sciences, U. K.
  5. Geotermica Italiana, 1991, Exploration for Geothermal Resources in the Eastern Caribbean,: United Nations Dept. of Technical Cooperation for Development.
  6. Principe, C., 2008, Geothermal potential in Montserrat: Scoping survey report: Instituto di Geoscienze e Georisorse, CNR
  7. Younger, P. L., 2010, Reconnaissance assessment of the prospects for development of high-enthalpy geothermal energy resources, Montserrat: Quarterly Journal of Engineering Geology and Hydrogeology, 43, 11-22.
  8. EGS, 2010, Final Report Geothermal Exploration in Montserrat, Caribbean: EGS Inc.
  9. Ryan, G. A., S. A. Onacha, E. Shalev, and P. E. Malin, 2009, Imaging the Montserrat geothermal prospect using Magnetotelluric (MT) and Time Domain Electromagnetic induction (TDEM) measurements,: Institute of Earth Science and Engineering.
  10. 10.0 10.1 10.2 10.3 Ryan, G. A., J. R. Peacock, E. Shalev, and J. Rugis, 2013, Montserrat geothermal system: a 3D conceptual model.
  11. Shalev, E., C. L. Kenedi, P. Malin, V. Voight, V. Miller, D. Hidayat, R. S. J. Sparks, T. Minshull, M. Paulatto, L. Brown, and G. Mattioli, 2010, Three-dimensional seismic velocity tomography of Montserrat from the SEA-CALIPSO offshore/onshore experiment.
  12. Anderson, E., D. Crosby, and G. Ussher, 2000, Bulls-Eye - Simple Resistivity Imaging to Reliably Locate the Geothermal Reservoir: Presented at the World Geothermal Congress 2000, 909-914.
  13. 13.0 13.1 13.2 13.3 13.4 Ryan, G. A., and E. Shalev, 2014, Seismic Velocity/Temperature Correlations and a Possible New Geothermometer: Insights from Exploration of a High-Temperature Geothermal System on Montserrat, West Indies: Energies, 7, 6689-6720.
  14. EGS, 2014, Well completion report MON-1 and MON-2: Government of Montserrat, Public Works Department, Woodlands, Montserrat.
  15. Brophy, P., B. Poux, G. Suemnicht, P. Hirtz, and G. Ryan, Year, Preliminary Results of Deep Geothermal Drilling and Testing on the Island of Montserrat: Thirty-Ninth Workshop on Geothermal Reservoir Engineering, 11.
  16. PKF, 2012, Feasibility Study into Montserrat Geothermal Energy Final Report: PKF Accountants & business advisers.
  17. giz, and CARICOM Energy Programme, 2016, The Power to change: The Montserrat Energy Policy 2016-2030: Ministry of Communication, Works, Energy and Labour, Government of Montserrat.

Corresponding author

  • Dr Graham Ryan, Seismic Research Center, University of the West Indies, St Augustine, Trinidad

External links

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Montserrat geothermal development project
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