CO2 injection in liquid state as an efficient storage concept for reducing greenhouse gas emissions

Abstract

Deep geological formations have a great potential to significantly reduce carbon dioxide (CO2) emissions to the atmosphere through both geologic carbon storage and geothermal energy. Geologic carbon storage permits storing large amounts of CO2, in the order of tens of millions of tons of CO2 per storage site. However, it may be argued that the economic costs are excessively high and that a CO2 should be utilized somehow to make injection profitable. A promising solution is to use CO2 as circulating fluid to produce geothermal energy because CO2 is more efficient than water. We propose to inject CO2 in liquid state, rather than in supercritical conditions, which are the conditions at which CO2 will stay once it equilibrates with the pressure and temperature of storage formations. Liquid CO2 has a higher density than supercritical CO2, which significantly reduces the required compression energy at the wellhead because CO2 flows downwards mainly by gravity. If liquid CO2 injection is combined with production of supercritical CO2 from the storage formation a thermosiphon is created, which permits circulating CO2 at a minimum operational costs and generate carbon-free geothermal energy. The potential downside of liquid CO2 injection is that the rock around the injection well is cooled down, which generates contraction and thermal stress reduction, which eventually could reactivate fractures or even promote hydraulic fractures. We assess the geomechanical stability of the caprock as a result of this cooling and find that a stress redistribution occurs around the cooled region, which tightens the caprock. Overall, liquid CO2 injection is an energetically efficient injection concept that can permit both reducing CO2 emissions to the atmosphere and to generate geothermal energy. The targeted audience are scientists and engineers interested in geothermal energy and coupled processes occurring in the subsurface.