Chemical-mechanical coupling observed for depleted oil reservoirs subjected to long-term CO₂-exposure - A case study of the Werkendam natural CO₂ analogue field

Geological storage of CO₂ is one of the most promising technologies to rapidly reduce anthropogenic emissions of carbon dioxide. In order to ensure storage integrity, it is important to understand the effect of long-term CO₂/brine/rock interactions on the mechanical behaviour of a storage complex. As most of these reactions are too slow to reproduce on laboratory timescales, we studied a natural CO₂ analogue reservoir (the Röt Fringe Sandstone, Werkendam field, the Netherlands; 125-135 Ma of CO₂-exposure) and its unreacted counterpart. We focused on CO₂-induced mineralogical and porosity-permeability changes, and their effect on mechanical behaviour of both intact rock and simulated fault gouge. Overall, CO₂-exposure did not lead to drastic mineralogical changes. The CO₂-exposed material shows a stronger dependence of permeability on porosity, which is attributed to differences in diagenesis (closed-system diagenesis and hydrocarbon emplacement) taking place before CO₂ charging. The limited extent of reaction was in part the result of bitumen coatings protecting specific mineral phases from reaction. In local, mm-sized zones displaying significant anhydrite cement dissolution, enhanced porosity was observed. For most of the reservoir the long-term mechanical behaviour after CO₂-exposure could be described by the behaviour of the unreacted sandstone, while these more 'porous' zones had a lower rock strength. In addition, CO₂-exposure did not affect the fault friction behaviour, and slip is expected to result in stable sliding. Simple stress path calculations predict that reservoir failure due to depletion and injection is unlikely, even for the 'porous' zones, nor will fault reactivation occur for realistic injection scenarios.