Main Lessons from the Complex Study of Mihályi-Répcelak Natural Co2 Occurrence

Abstract

Prediction of the fate of the injected CO2 in geological formations strongly relies on geochemical modeling, laboratory and field testing. However, all of these approaches have their major drawbacks, either being much too simplistic and/or out of realistic timescales (1–100 days vs. 100–1000 years) for deep subsurface storage. Natural CO2 occurrences provide an exceptional opportunity to study long-term behavior of the scCO2-porewater-reservoir rock system. Despite their complex geologic evolution, these occurrences have been used lately to identify CO2-related reaction features as well as to test and fine-tune geochemical models. The Mihalyi-Repcelak occurrence in NW Hungary, with core material available, not only provides a general insight into subsurface reaction processes related to CO2, but represents one of the largest deep saline aquifers in the Pannonian Basin, it also behaves as a natural laboratory for this potential Late Miocene saline reservoir. The multi-layered structure of the occurrence with separated hydrodynamic units enables the simultaneous analysis and the identification of reaction features both in the reservoirs and their sealing cap rocks providing a further step in understanding the long-term performance of storage complexes.