To improve knowledge about cap rock seal capacities and, hence, allow a better definition of standards for risk assessment, a combination of laboratory investigations and subsequent numerical modelling was performed using samples from the Watrous formation, a regionally extensive cap rock that overlies the Weyburn oilfield (southern Saskatchewan, Canada), a site of current CO2 injection for enhanced oil recovery.
Geological settings at Weyburn.
The effects of CO2 exposure on the capillary and transport properties of this cap rock material were investigated by studying pore structure, porosity, and gas permeability on horizontal and vertical plugs before and after CO2 exposure. Microscan imagery before and after CO2 ageing revealed some changes of the rock material by dissolution and mechanical alteration (fissuration), whereas porosity remained unaffected and gas permeability slightly increased.
Cracking experiments were performed to assess the consequences of CO2 injection on the mechanical behaviour of the cap rocks. Then, a probabilistic model to study the formation and propagation of crack networks in the cap rock was developed. Subsequent large scale 2D-coupled hydromechanical Finite Element Model calculations for two different initial in situ stress regimes showed that the most important hydromechanical process in the cap rock, caused by the high-pressure injection of CO2, is the general reduction of the effective stresses. The reduction of effective stresses is not only important for permeability changes and hydraulic fracturing, but also for a potential initiation of shear slip.
The impact of CO2 exposure on the geochemistry of rock material was studied by reacting crushed and sieved (<2mm) material from the Watrous formation and the Kimmeridge Clay cap rock (from the South Brae oilfield, North Sea) with CO2-saturated synthetic porewaters under in situ pressure and temperature. No significant mineralogical changes were observed to the reacted samples. The main solid phase reaction upon addition of CO2 was carbonate mineral dissolution, while no evidence was found for the precipitation of secondary carbonates or dawsonite reaction products.
For more information
please contact: Yves-Michel Le Nindre
at Bureau de Recherches Geologiques et Minieres