The Hontomin Technology Development Plant (TDP) for CO₂ geological storage, located close to the city of Burgos (Spain), is currently the only active onshore injection site in the European Union. It is managed by Fundacion Ciudad de la Energía (CIUDEN) and it has been recognised by the European Parliament as a key test facility.

The principal reservoir/seal pair is formed by Lower Jurassic carbonate rocks (limestones and dolostones) sealed by marls and black shales. The rocks at around 1,500 m depth take the form of a structural dome, where the main seal is the Marly Lias and Pozazal Formations and the reservoir is the Sopena Formation. The reservoir has a high level of fracturing and it is compartmentalised into geological blocks, but this does not affect the seal integrity.

Hontomin Technology Development Plant

One injection well (HI) and one observation well (HA) form part of the TDP, with the facilities for CO2 injection and water conditioning connected to HI. The vertical wells have been drilled to depths of up to 1,600 m on the flank of the domed reservoir, with a distance of 50 m between them at surface. Both wells have been equipped for deep monitoring (pressure/ temperature sensors, reservoir water sampler, electrical resistivity tomography, distributed temperature system, distributed acoustic sensor and hydrophone array).

Thirty one passive seismic stations comprise a monitoring network covering an area of 18 km2. Eight wells form part of the hydrogeological monitoring network, three of which have been specifically drilled for the project into the Utrillas Formation overlying the top seal, where freshwater aquifers are located (wells drilled to depths of 150 - 400 m). These wells are equipped with instrumentation to remotely check groundwater chemistry and level to confirm that CO2 has not migrated out of the storage reservoir into shallow formations.

Different monitoring campaigns were conducted on and around the TDP in order to characterise the reservoir/seal pair (e.g. 2D- 3D reflection seismic survey, vertical seismic profile in the injection well, controlled source electromagnetic, microgravimetry, soil gas monitoring and hydrogeological studies).

The challenge at Hontomin was to manage the low injectivity of the reservoir. Laboratory and field tests were designed and performed to overcome this issue. Finally, a CO2 injection strategy, utilising liquid phase CO2 was implemented, and the operational efficiency and the effects of impurities in the CO2 stream were analysed. The injection strategy has been patented under Spanish regulations (Spanish Patent 201500151).

CO2 injection in liquid phase. Operational monitoring: CO2 Well Head Pressure (dark blue line), CO2 flow (green line), Bottom Hole Pressure (light blue line) and brine flow (brown line).

Hontomin TDP is now ready for larger scale injection (10, 000 t) and will host field activities planned in ENOS work package 1 ”Ensuring safe storage operations”. Research will include injecting CO2 at larger scale, updating the dynamic model, passive seismic monitoring, geochemical/geophysical monitoring and risk management.