In Danish chalk field, infrastructure used in fossil fuel extraction can be given a 2nd-life for CO2 storage. Yet, only if risks of mechanical reservoir softening upon CO2 injection are manageable.
In order to quantify and critically evaluate the risk of unmanageable reservoir compaction upon CO2 injection, we take a fundamental approach by addressing to which extent chemical rock-fluid interaction would mechanically soften Danish chalk reservoirs.
From basic scientific viewpoints, there are unresolved questions on how the CO2 affects chalk when mixed with formation waters. Mixing CO2 and water forms a weak acid that dissolves calcite minerals until a new buffered equilibrium is met.
Dissolution of the chalk framework could thus reduce its stiffness and strength. Whether chemical dissolution plays a significant mechanical role depends upon how chalk grains are held together (cementation of grain contacts), and how much calcite would dissolve in a given injection scenario - especially whether calcite cement in grain contacts would diminish or even grow.
The latter process would strengthen the frame, although it could potentially also clog pore throats. The overall importance of dissolution must be evaluated to determine if this is a real risk to the reservoir. Further, these chalks possess a large porosity but low permeability due to the small mineral grains and corresponding high specific surface area.
Thus, surface effects are much more important in chalks than in, e.g., sandstones and need investigation. Therefore, geomechanical tests are planned where finite amounts of CO2 are injected into reservoir rock samples, with controlled pore fluid composition and temperature.
Fluid will be sampled to evaluate the amount of dissolution, while mechanical stiffness and strength are estimated before, during, and after CO2 exposure.
DHRTC funded project
Geo, DTU, GEUS collaboration
Frederik Ditlevsen, Geo, Project lead
Tobias Orlander, DTU
Hanne Holmslykke, GEUS