executive summary

As part of a data room organized by the Dominicans government, Storengy was asked by the BEE to study the basement part of a geothermal power plant construction project. The island of Dominica (73 000 people) is a republic within the Commonwealth, independent since 1978. Its capital Roseau is counting approximately 24 000 people. It is located exactly halfway between the islands Guadeloupe 41 km north and Martinique 40 km south.
It relates to evaluate the potential of Wotten Waven geothermal field (initially presented with a capacity of 120 MWe in 2012 before the three exploration wells and 90 MWe in 2013) and estimate if it can provide enough energy for electricity production not only for home market but also for export to Guadeloupe and / or Martinique.This geothermal field is located in the southern part of the Dominica island approximately at ten kilometers east of Roseau, in an area known for its geothermal surface manifestations including mineralized hot springs, fumaroles, sulfur deposits and phreatic explosion craters (whose "Boiling Lake", one of the largest known in the world).
The main conclusions of the analysis and integration of data provided in data room, but also from additional bibliographic research carried out in support, are the following:
Geological context and conceptual model of the geothermal system
Three exploration wells (WW-01 to 03) were drilled in 2012 at depths of 1200 to 1600 m. They helped prove the existence of a deep geothermal reservoir and provide important information on its thickness, its temperature and permeability (injectivity-productivity). * Temperatures measured on wells indicate that the heat generating Wotten Waven geothermalisme is likely related to a magma chamber located under the Micotrin volcano who last erupted about 27 000 years ago.

* The geothermal reservoir is composed of volcanic rocks of various natures: andesitic flows and intrusions or dacitic and pyroclastic deposits (ignimbrites, gap, pumice, ashes). These hot rocks seem to present low porosity but are affected by numerous fractures (confirmed by the index field as well as good wells permeability from injection and Production tests) in which fluids warm up and circulate creating convection loops.

* Maximum temperature (not stabilized) measured in the reservoir is 247 °C in WW-03 and the pressure is 90 bar. Under these conditions, the water contained in the reservoir is in the form of a liquid under pressure without steam fraction in depth conditions (vaporization pressure in the order of 40 bars). This specificity and the reservoir temperature allow us to classify the Wotten Waven site in the category of high enthalpy fields with, qualitatively, a medium to low potential for generating electricity.
Based on the results from the well, the position of surface manifestations and interpretations of geological and geochemical structure, the extension of the geothermal reservoir is estimated at around 12 km ² but only 10 km ² would be usable in drilling.
Furthermore, there is still much uncertainty about this base case because, for reasons of accessibility on the surface, 3 exploration wells are located in the western half of the reservoir.
Indeed, most of the interest area is covered with dense rainforest and its quarter East is located in a nature reserve. Strictly, only 3 km ² are actually proved by wells; the exploitables 10 km ² being considered "proved + probable" in our deterministic estimation or or "P50" in our probabilistic estimation. The "proved + probable + possible" or "P10" area is approximately 20 km ².

* The total thickness of the reservoir is unknown because it has not been entirely drilled due to technical reasons (drilling in total loss in the fractured reservoir, instability of the walls of the hole and not powerful drilling machine). It is at least 800 m and our estimate is based on an average value of 1000 m (+/-200 m) considered prudent. * The most valuable fraction (hottest) of the reservoir is located around the Micotrin and probably in the eastern part still under-explored.
Capacity of the site in terms of electricity production * Potential for net electricity production of Wotten Waven geothermal field is estimated in our base case to 50 MWe over 30 years. * As there is no low production unit on-site used to acquire production data for a sufficiently long duration in order to consider the implementation and calibration of a detailed numerical model, we are clearly in the context of an evaluation of upstream resource with a very high level of uncertainty. Several independent approaches have been proposed to restrict as the much as possible uncertainties: * Empirical method with a developable power factor per km2: it simply consists in evaluating the resource of a prospect from the surface by applying a multiplication factor purely empirical and based on feedbacks from current projects. The most recent synthesis of current projetcs indicates a factor of 6.5 for MWe/km2 for the Wotten Waven site which results in a first estimation of 65 MWe. This method is considered optimistic because it averaged varied nature projects (particularly those with vapor in depth conditions whose potential is the best) * Energy balance method: it is based on an evaluation of the energy resource in place then the recoverable fraction by wells and convertible into electricity. The implementation of this method with the parameters recommended by the profession leads to a capacity of 40 MWe for the Wotten Waven site. This method is considered conservative because the energy balance does not include a supply of heat from the heat source during the operating period. * “Analogues” method: it is an internal method that was developed in this study. It is based on the identification of existing sites closest to the prospect studied from a multi-criteria selection on the parameters most influencing the capacity. A statistical analysis of these "analogues" is used to evaluate a representative capacity of the site. For Wotten Waven, this approach has identified 15 analogue sites and leads to a capacity of 50 MWe.This evaluation is considered realistic. * A probabilistic analysis conducted in this study produced the following results:
P90: 25 MWe; P50: 50 MWe; P10: 80 MWe.

* The capacity achieved in this analysis is less than that resulting from reports ELC (consulting firm) : P90 = 57 MWe ; P50 = 91 MWe ; P10 = 123 MWe. * The difference is mainly due to the optimistic value of recovery factor (RF) used by ELC. While the profession recommends based on feedback from existing projects, using an average value of 12% (with a probabilistic range between 5 and 20%), ELC company used a fixed value of 30% (in 2012) and 27.5% (in 2013) very overvalued.

Development Plan * As originally presented in terms of large-scale development for a capacity of 120 MWe based on 50 wells (30 producers and 20 injectors) * For a capacity of 50 MWe, the proposed development plan is based on the nominal operating of 14 producing wells and 8 injection wells drilled from 7 platforms (3-4 deviated or vertical wells per platform). It was planned during the 30 years of operation to drill 50% more wells than the nominal needs to take into account the possibility of dry wells or low productivity (the edge of the reservoir being unclear) but also to cover the hazards related to structural aging in hostile environment (temperature, corrosion, deposits, seismicity) and their renewal. The development requires for the total duration of the project a total of 33 wells for 50 Mwe (1.5 MWe / well) compared to 50 wells to 120 MWe (2.4 MWe / well) as originally submitted.
Other uncertainties
Samples of geothermal fluids in exploratoring wells were found unrepresentative. A Reliable chemical analysis of water and gas of WW-03, but incomplete, indicates that the composition of the reservoir water implies: * In production, precipitation of calcite at the bottom of the well to the surface, * In reinjection, a risk of precipitation of amorphous silica from 140 ° C (limiting the temperature of the reinjection water, without the use of inhibitor), * generally risk of corrosion which could not be evaluated adequately.

During the exploration phase, production tests too short and adapted monitoring lack of (interference between wells) have failed to understand the heterogeneity of the reservoir on a small scale from a dynamic point of view (flowing) nor adequately describe the productivity curves.
In the development phase, high temperature reservoirs like Wotten Waven require both a pressure maintenance by a reinjection of produced fluids while avoiding the colder fluid coming in too fast to producers. In the absence of a more detailed study on the nature and density of fracturing within the reservoir using static and dynamic data, the development scheme (positioning of producers and injection wells) still very uncertain at this stage of evaluation.
Additional exploration work is needed to better delineate the lateral extent and thickness of the reservoir. This work involves the acquisition of additional geophysical measurements but also some drilling delineation wells (estimated in the order of 10-15 Meuros).

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