As an IEA Technology Collaboration Programme, IEAGHG has an active interest in the technology roll-out of CCS including the development of innovative concepts such as evaluating potential synergies between geothermal energy and CCS including CO2 geological storage.  There is growing interest in both technologies especially with the prospect of significant advances towards full-scale multi-million tonne CO2 storage sites.  As a former programme manager of the UK’s geothermal R&D programme I would like to give a personal perspective on the outcome of that programme and how it might relate to new opportunities including prospects for Saudi Arabia.  Recent developments in the use of CO2 in geothermal energy will be reviewed and how they might be related to Saudi Arabia.

In January 2020 KAUST hosted an excellent conference on the potential for geothermal in Saudi Arabia which highlighted the potential for Enhanced Geothermal Systems (EGS) in the Arabian Shield Area.  The conference also highlighted that linking desalination to electricity generation provides a higher cost-benefit for the combined processes.

Experience from the UK’s geothermal R&D programme, and elsewhere, showed that EGS can have limitations.  The UK’s programme did demonstrate the effectiveness of creating a heat exchange reservoir by detecting the pattern of fluid migration using sensitive seismic monitoring techniques. 

However, subsequent circulation led to a short-circuit and a fall in energy output.  Despite this set back the experience led to improvements at the European EGS site at Soultz.  There has also be a renaissance in EGS even in the UK with the development of a new EGS project that has targeted a fault zone at depths of between 5.2 and 2.4 km.  The key take-away message here is successful development of EGS, and CO2 storage sites, depends on good site characterisation and monitoring.  There is excellent potential for technology cross-over, for example microseismic monitoring and the use of robust monitoring tools for heat and pressure detection.  There are also advocates for the use of CO2 as a heat transfer medium in EGS sites. 

 

The combination of CCS, more specifically CO2 storage and geothermal energy, is an area of growing interest.  The potential for using CO2 as a heat transfer medium was investigated at the Cranfield pilot where a field demonstration was conducted on the basis of CO2 thermal properties however it was not possible to sustain. 

 

The concept of utilising geothermal energy with CCS in the power sector has also been investigated, at a theoretical level for a 521 MW coal-fired power plant in northern Nevada.  In this case geothermal energy would be used to reduce the parasitic energy demand on the capture plant by as much as 7%.  Another concept is currently being pioneered by BRGM in collaboration with a US company Partnering in Innovation who are developing a novel technique for combining geothermal energy with CO2 capture.  The CO2-DISSOLVED concept builds on mature technology.  CO2 is dissolved and then injected into a conventional low-enthalpy geothermal doublet.  Initial results suggests its economically promising but the technology is limited to relatively small-scale emission sources ~150,000 tonnes /year because of limitations imposed by the distance between injection and production wells and the salinity of brine.  Heat demand also needs to be coincident with a CO2 source.  

Within the context of Saudi Arabia the best prospects for conventional low-enthalpy geothermal energy are along the eastern margin of the country near the Arabian Gulf where the geothermal Phanerozoic sedimentary succession is thickest and formation reservoir temperatures reportedly range from 60°C - 115°C.  In this region there are a number of significant large-scale point sources of CO2 emissions.  For example desalination plants emitting an estimated 440 kT – 1.4 MT CO2 / year and a CCGT power plant emitting as much as ~7.4 Mt CO2 year.  These estimates depend on assumptions about technology performance and plant operation. 

The immediate conclusion to draw from these observations is that new technologies that combine CO2 storage and geothermal, that have yet to be proven at scale, are unlikely to be preferentially selected.  Demonstration storage sites (Quest, Decatur are ~1.0 Mtonnes/year) have clearly shown that CO2 storage is safe and effective.  Over 40 years of CO2-EOR in the USA also testifies to the maturity of subsurface storage.  

Technically and economically CO2-EOR and conventional injection in a saline aquifer are likely to be higher priorities in Saudi Arabia, however there may be locations where small-scale emission sources could be considered for combine applications.  Another important consideration is that large-scale ~7 Mt CO2/ year point source emissions like base load CCGTs will need large-scale reservoirs with excellent permeability properties (>100 darcy-meters) to ensure plume migration and effective pressure management.

The most immediate benefit from the combined experience of geothermal energy and CCS communities is learning from their development experience particularly in areas like well integrity and seismic monitoring and interpretation.  Using geothermal to reduce the heat load for desalination or the parasitic load on CO2 capture plant should also be considered.