Role of fracturing and regional tectonic structures on secondary porosity generation in a CO2 storage plant: Hontomin pilot-plant (Spain)

The long-term managing from the geological hazard point of view of the Hontomín onshore pilot-plant for CO₂ storage, located in Spain and recognized as the first and only key-test facility in Europe, is one of the main objectives stated in the ENOS European project. This project is led and funded by the European Network of Excellence on the Geological Storage of CO₂ (CO₂GeoNet).

The complex geological emplacement of the Hontomín Carbon capture and storage plant is considered rather relevant to analyse the impact of fracturing and both local and regional strain field on the reservoir parameters. The reservoir of Hontomín pilot-plant is formed by highly fractured Middle Jurassic dolomites with associated secondary porosity. This parameter is one of the main concerns when managing CO₂ storage and monitoring.

In order to characterize the fracture pattern and its implications on a proper CO₂ monitoring, we characterized the surface structural elements through the study area and their relationship with fractures affecting the reservoir porosity. The methodology followed in this work is mainly based on detailed geological mapping (field work complimented with orthophoto analysis), adding missing information from previous works. This analysis does not increase the cost for long-term monitoring, given that they are low-cost and the results are acquired in a few months.

The main structural trend in the study area, concerning faults with a wide range of displacement and metric to decametric folds, follows a regional E-W orientation. On the other hand, fractures show two main sets of trends, from NW-SE to NE-SW.

This fracturing pattern, considered as a conjugate fracture system, corresponds to the tectonic stress recorded in both Mesozoic and Cenozoic sedimentary successions where the Hontomín pilot-plant is placed. Riddle faults formed from a nearby regional right-lateral strike slip fault (Ubierna Fault) are the responsible structures for the fracture system affecting the area and the reservoir. Moreover, this fracturing pattern is in agreement with local to regional active tectonic field from Cenozoic times to present-day, when the Ubierna Fault recorded its maximum right-lateral displacement (15 km).

Secondary porosity within the reservoir can be produced from this fracture pattern, highly increasing the permeable migration paths for CO₂ migration after the injection. Therefore, we state that a combination between fracture analysis and structural and tectonic study, should be considered as mandatory in the monitoring phases of the CO₂ plume, during and after injection operations.