GEOTERMIA A CCS I CCU

Abstract

GEOTHERMAL ENERGY VERSUS CCS AND CCU

Abstract. The issue of potential conflict of interests between CO₂ geological storage in saline aquifers (CCS - Carbon Capture and Storage) and low-enthalpy geothermal energy is often raised by opponents of the CCS in Poland and other European countries of similar geological conditions. However, contrary to those opponents, processes accompanying CO₂ injection into deep saline aquifers can be simultaneously used for both sequestration and associated production of clean energy. Sedimentary formations occurring in the Permian-Mesozoic Basin, covering the Northern Germany, Denmark, the Netherlands North Sea, eastern England and more than a half of the territory of Poland contain deep waters of variable salinity. It is possible to combine geo thermal and CCS, both in order to reduce carbon dioxide emissions and for cost-efficient heat and/or electricity generation. The first concept is the use of CO₂ in closed, unconventional geothermal systems (HDR - Hoi Dry Rock). In case of HDR fracturing is carried out in order to enhance reservoir properties of rocks at depth of at least 3 km, reaching a temperature ofminimum 95-100 °C, sufficient for
heat and electricity generation. This method combines the geo thermal energy and CO₂ injection instead of water in a closed loop. Therefore, this method should be classified mostly as CCU, subordinately as CCS. Although it does not neutralize huge amounts of CO₂ in comparison with conventional geological storage (only about 10% of injected gas is ultimately stored in the host rock), the CCU method is much desired and produces geothermal energy with much better efficiency than the classical geothennal loop using water as a medium transporting the heat - which is the main economical and ecological advantage of this method.The second concept uses sedimentary rocks of good reservoir properties, containing saline aquifers, usually less suitable for geothermal because of high corrosivity and generally weak thermal properties (at least in Poland). CO₂ is injected into the saline aquifer, and appears at depth of minimum 800 m in a phase similar to a liquid, but of density lower than brine, so it remains on lop as a plume. If most of the injected CO₂ remains in the aquifer (i.e. it is sequestered), part of it is re-circulated in a closed loop for the heat exchange or electricity generation in a turbine. At the depth of more than 800 m, in the temperature of tens of C degrees plus, the carbon dioxide transmits the heat/energy stream several times more efficiently than the water/brine medium, which makes economic sense of such an approach.