Assessment of submarine geothermal resources and development of tools to quantify their energy potentials for environmentally sustainable development

Submarine geothermal reservoirs contain huge amounts of energy, not been used so far but recently considered for exploitation. Their energy potentials are much larger than those of onshore geothermal resources and can cover significant parts of the global future energy demands in an environmentally sustainable way. There are two types: (i) deep resources along oceanic spreading centers, where uprising magma heats fluids circulating through fissured rocks, which emerge at vents at temperatures up to 460 °C, mixing with seawater (depth: 1000-4000 m below sea level), and (ii) coastal shallow resources where geothermal fluids emerge at fractures (depth: 1-50 m). The total length of deep systems is ∼65,000 km and there are sites where pressure and temperature are high enough that they are at supercritical conditions. The first part of this paper assesses global information on submarine geothermal resources, concluding that - using mature technology from onshore geothermal and offshore hydrocarbon exploitation - submarine geothermal resources can be an economical affordable option for energy supplies at small-large scale. In the second part a - generally valid - robust, mathematical approach is developed to quantify these resources and its applicability is demonstrated using two examples. In Baja California (Mexico), the coastal submarine geothermal potential per cubic kilometer of rock of Punta Banda, Wagner Depression and Gulf of California resulted 245, 350 and 528 MWT/km3, respectively. Transforming only 1% of this energy into electricity the capacity of the Gulf of California alone is ∼26,000 MWe. The submarine geothermal system of Santorini Caldera (Greece) yielded 869 MWe (reservoir 100 km3).