Published On: Wed, Jan 14th, 2015

Geothermally Powered Curacao – Reality or Deep Pipedream

GeoIn view of some recent press reports, several people have asked for my opinion on whether Curacao realistically has any geothermal energy potential. As reported in Amigoe (10 December and 5 January), specifically this refers to an apparent proposal received by Aqualectra to drill a very deep 7,000 metre geothermal well on the island.

Firstly, I must say, I was as surprised as probably many to read this. While I have a good working knowledge of geothermal or hydrothermal systems, these are generally associated with active volcanic regions of the earth. Indeed, when we try to imagine what ‘geothermal energy’ looks like most people think of countries like New Zealand, Iceland and the Philippines.

Likewise, the active volcanic islands of the eastern Caribbean or Lesser Antilles have attracted geothermal exploration for 40 years. Surface geothermal activity includes warm to hot (boiling) springs, fumaroles, solfataras and mud pools to varying extents on at least eleven islands, including Saba and St. Eustatius. While historic volcanic eruptions and associated geothermal activity are recorded on Montserrat, Dominica, St. Vincent, Guadeloupe, Martinique and St. Lucia.

Despite numerous pre-feasibility and feasibility studies on several islands (in particular St. Lucia, Geo2Guadeloupe, Nevis and Dominica), very little commercial production of geothermal power has resulted. There is only one operating geothermal power plant, on Guadeloupe, which has been on line for 30 years. The plant was upgraded with a second generator in 2005 and further refurbishment in 2013. However it still only meets 8% of the island’s needs, and has not been without intermittent production problems and social issues (including high H2S emissions).

At present the islands of Dominica and Nevis probably show the most promise for further commercial development, in particular the ambitious Wotten Waven Geothermal Energy Project on Dominica. Funded by the Dominican Government, French Development Agency and European Investment Bank the total projected cost stands at nearly 0.5 billion USD. But as with all such small island states, populations and hence electricity demands are not sufficient to require large additions of geothermal capacity.

Returning attention to Curacao. There are no known hot springs or any evidence of surface geothermal activity. While the island has a volcanic (diabase) core this is 90 million years old, and cooled significantly during exhumation (uplift) about 50 million years ago when it was accreted to the South American plate. There are no obvious major faults as potential hot fluid conduits, and any younger sediments form a thin uplifted rim around the island with no aquifer or shallow to even moderate level geothermal potential.

There are only two other ways to generate geothermal energy. (1) From another heat source such as uranium-rich granite at depth where radioactive decay of uranium and other radiogenic elements heat up the surrounding rocks. (2) Geothermal gradient, which is the natural rate of temperature increase found with respect to increasing depth into the earth’s interior. For most regions of the world (away from active volcanoes) this is about 25-30oC per km of greater depth.

In both these cases it is then necessary to create cracks through hydraulic fracturing (or ‘fracking’), which are then flooded with water pumped down from the surface to create a geothermal system in which hot water is then returned to the surface to drive a power plant. The former is usually referred to as a hot dry rock or HDR geothermal system. However, while granite is known on Aruba, and so could conceivably occur at depth on Curacao, it is not of the type known to contain significant amounts of uranium. It is therefore the second type being proposed for Curacao, which is called an EGS or Enhanced Geothermal System.

The fact that on Curacao it is proposed to drill to 7,000m implies a geothermal gradient of 30-35oC/km, to expect a 200-250oC fluid or steam out of the borehole from this depth. While actual scientifically measured geothermal data are not available for Curacao, they are for three relatively near-by petroleum wells drilled in 1989-90 adjacent to Aruba. These data, on the contrary, indicate a much lower average geothermal gradient of about 20oC/km.

While this value may not be conclusive, geological studies confirm Curacao and Aruba have related and not dissimilar thermal and tectonic histories. These measurements are also consistent with other published thermal data and modelling from petroleum studies, and our general understanding of the geodynamic setting of Curacao which show heat flow to be significantly lower than normal crust.

Unfortunately none of this is good news regarding geothermal prospectivity. Indeed a geothermal gradient of the order of 20oC/km would rather require a10km hole to reach rocks at 200-250oC. While I agree drilling to such depths is technically feasible (by extended reach drilling technologies), this is not common commercial practice in either oil or geothermal exploration. In fact globally there appear very few existing geothermal wells in excess of 4km.

Whereas onshore drilling of a geothermal well to EGS specifications is considerably cheaper than drilling a deep water petroleum well, still, for a 10km drill hole we are talking in the region of 40-50 million USD. And for the three well EGS scheme illustrated, drilling costs alone could be as much as 150 million USD.

It seems illogical such a deep scheme would be profitable compared to geothermally active islands in the eastern Caribbean, and globally where a geothermal gradient of 30oC/km appears to be the commercial threshold. Most working volcanic (high temperature) operations are located where average geothermal gradient is 100-150oC/km (New Zealand, Iceland), and HDR and EGS systems are generally in the range 50-80oC/km. Some lower temperature, shallower reservoir systems in Europe (Holland and Germany) operate in the range 30-60oC/km.

Even with oil at over 100 USD per barrel geothermal exploration and production has generally only occurred where subsidised by large state and institutional (read taxpayer) incentives, and at below-50 USD per barrel prices diesel-generated power is here to stay in the Caribbean.

Dr. John Wright is a retired consultant geologist with over 30 years’ experience in natural resource exploration.

Image: Enhanced Geothermal System of the type proposed for Curacao.

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