Injection of CO2 into the Cardium Formation, central Pembina Field, Alberta was initiated in 2004 in two inverted 5-spot waterflood patterns. The purpose of this flood was to assess the economic viability of enhanced oil recovery (EOR) using CO2. To study CO2 behaviour within the reservoir and assess long-term risks of CO2 leakage and storage potential, a geological and hydrogeological model was developed for the Cardium Formation, and for both the underlying and overlying strata. The injection zone, Cardium Formation, was sub-divided into four permeable and three impermeable reservoir units. The underlying and overlying strata were subdivided based on their geological potential to contain CO2 over the long-term (5,000 years).
Permeable reservoir units in the Cardium Formation include the lower, middle and upper sandstones, and the conglomerate. Most of the storage capacity of the Cardium Formation is contained within the three sandstone beds, which exhibit average porosities between 14.8 percent and 16.4 percent. Permeability in the sandstones increases predictably with increasing porosity, but is variable from sandstone to sandstone. The lower sandstone has the lowest average permeability (9.5 md), which is about half the average permeability of the middle and upper sandstones (21.4 and 19.8 md, respectively). There is no relation between porosity and permeability in the conglomerate. The conglomerate on average is 50 percent more permeable (33 md) than the middle and upper sandstones; although, permeability may exceed one Darcy in individual beds. The higher permeability of the conglomerate results in it acting as a “thief” zone for injected CO2. This tendency is further compounded by both the endemic use of hydraulic fractures (which propagate vertically) to stimulate oil production, and the positive buoyancy of CO2 relative to water.
The Cardium Formation at Pembina occurs within the middle of the 650 m thick Colorado Group shale. The effectiveness and integrity of the overlying shale succession, as a seal for the Cardium Formation, is vital to the long-term storage of CO2. Leakage through this shale is very unlikely as it is thicker than 300 m and separates two aquifers with significantly different regional pressure regimes (the Cardium Formation was originally over-pressured whereas the overlying Wapiti Formation is sub-hydrostatically pressured).