Electromagnetic geophysical measurements at Petite Terre (Mayotte) in 2019 offshore.
© Pascal Tarits, Université de Brest Occidentale/IMAGIR/MAPPEM
The need
In 2017, the electricity mix of the Mayotte département included photovoltaic production to the extent of 5%, spread over more than 70 facilities on the island, and 95% diesel thermal production, provided by the two plants operated by Electricité de Mayotte. In order to diversify its sources of electricity supply, the Mayotte Departmental Authority has since 2005 been implementing a large-scale programme to develop renewable energies, including to evaluate Mayotte's geothermal potential to produce electricity.
As part of this programme, the geothermal exploration study carried out by BRGM in Mayotte between 2005 and 2008 identified the Petite Terre zone as the area with indications of a potential deep (> 1,000 m), high-temperature (> 200°C), active geothermal resource that could be used for electricity production. Following this initial study, in order to confirm the existence of a geothermal zone of interest and to locate it precisely, it was deemed necessary to implement a deep exploration programme, which included the drilling of boreholes. Given the cost and complexity of such a programme, a preliminary stage must be conducted in order to specify and size the exploration campaign as well as possible, given the objectives and uncertainties specific to Petite Terre. The first phase of this preliminary stage was carried out by BRGM with support from the Mayotte branch of the French Agency for the Ecological Transition (ADEME) from June 2018 to December 2019.
The results of phase 1
The new geoscientific data (including geophysical data)collected for this project confirmed a strong deep geothermal potential at Petite Terre, as well as the need to drill several deep exploratory boreholes (up to 2,000 m deep) in order to:
- characterise the geological formations;
- identify the different sources of incoming water, their temperature and their physical and chemical characteristics;
- determine the permeability of the reservoir;
- estimate the latter's capacity to generate electricity.
Phase 2 - 2021-2022
Before any exploratory drilling, a complementary study must be carried out to establish the best location for the boreholes and assess the chances of a successful outcome. This study, started in early 2021, will gather the geoscientific elements necessary for any decision to invest in exploration drilling. It is based on targeted geoscientific work (e.g. geological, geophysical, geochemical and hydro-thermal) on Petite Terre, as well as on the use of data acquired by the Mayotte Volcanological and Seismological Monitoring Network (REVOSIMA), set up because of the exceptional seismic-volcanic activity observed off Mayotte since May 2018, which is probably due to the presence of a high-temperature geothermal resource.
Using the results
At the end of this second study phase, depending on the results, exploratory drilling may be considered in order to confirm the presence of an exploitable geothermal resource and to estimate it quantitatively. This information will constitute one of the key elements in order to move on to the next phase of exploitation of the geothermal resource of Petite Terre to produce electricity, and thus potentially significantly increase the share of renewable energy in Mayotte's energy mix (several tens of percent of total production).
The drilling will not be carried out by BRGM.
The partners
- Mayotte Departmental Council
- ADEME Mayotte
- French Development Agency
- Electricité de Mayotte
Geothermal energy is a controllable renewable energy and a viable solution for meeting our need for energy autonomy. Thanks to BRGM’s contribution, the study confirmed the existence of deep geothermal potential on Petite Terre. These results now need to be refined through a more in-depth exploratory phase. This will confirm whether electricity can be produced from a geothermal resource and whether the proportion of renewable energy in Mayotte's energy mix can be increased.
