Geothermal resources have been harnessed around the world in areas where their occurrence is indicated by surface manifestations such as hot springs and fumaroles, and high temperatures suitable for electricity generation are reached at relatively shallow depths (1-3 km). This type of geothermal resource is usually found in areas with recent volcanic activity, including Mexico. While the exploitation of these “conventional” geothermal resources is using fairly well-established methods and technology, a vast amount of the worldwide geothermal resources cannot be exploited with conventional technologies. Such “unconventional” resources include enhanced geothermal systems in very hot environments (hot-EGS), and super-hot geothermal systems (SHGS), where deep geothermal fluids exceed temperatures of 350°C. Both hot-EGS and SHGS have raised the interest of industry and the scientific community alike in many parts of the world. Projects to investigate and better understand such systems are a key requirement for their eventual development and exploitation. Challenges in site development for hot-EGS and for SHGS are quite different. While EGS require (soft) stimulation methods to increase flow of fluids into the borehole, super-hot resources need specific safety measures for drilling and materials to withstand the harsh conditions of the fluid, which can lead to fast corrosion and erosion in installations.
To address these challenges, two sites with “unconventional” geothermal characteristics have been selected for a cooperation between a European and a Mexican consortium in GEMex - Acoculco (hot-EGS) and Los Humeros (SHGS).
The Acoculco site, foreseen for EGS development, has been explored by two deep wells drilled by the CFE (Comisión Federal de Electricidad). These wells found hardly any fluids, but temperatures around 300°C at a depth of 2 km. The lack of fluids at depth is surprising, as this area of Mexico is subject to abundant precipitation. Nonetheless, the high temperature gradient makes it an interesting target for exploitation and the lack of a clear resource makes it an ideal region for testing our knowledge on how to constrain an area where EGS can be performed.
The Los Humeros geothermal system is currently exploited. The northern part of the area, however, is much hotter (>380°C) than the wells used for production today. The development of this part of the reservoir presents a challenge, both because of the high temperature and because of the water chemistry. In addition, the area has only been poorly characterised by geophysical and geological surveys. Therefore, our understanding required for the development and exploitation of this super-hot resource is limited. The uncertainty about the potential of this resource and the technical challenges involved with the development of such a super-hot geothermal system currently prevent its economic development.