Site assessment update at Weyburn-Midale CO2 sequestration project, Saskatchewan, Canada: New results at an active CO2 sequestration site.

Abstract The Weyburn Field, operated by Cenovus Energy, currently contains the largest amount of anthropogenic CO 2 injected and geologically stored in the world, with over 16 million tonnes of CO 2 sequestered as of June 2010. The IEA GHG Weyburn-Midale CO 2 Monitoring and Storage Project is in its Final Stage of research and is focussed on building on the results developed in Phase 1 of this study to provide a further understanding of parameters required to develop, implement and regulate carbon storage sites. One aspect of geological characterization of the storage site in this phase of research is to further develop the geological model used in Phase 1 by adding more wells and including several geological units not incorporated into the geological model of Phase 1. Additionally, improvements into the quantification of regional flow directions in and around the active CO 2 injection site by using hydrologic data (pressure tests and hydrochemistry analyses) not included in the original model. These stratigraphic, pressure, hydrochemical and temperature data of flow units in a 1865 km 2 area around the Weyburn Field will better define fluid movement in the injection site and assist with long-term modeling of the fate of injected CO 2 . Stratigraphic data from more than 900 wells are included in the current geological model including 200 newly picked wells. The Final Phase geological model includes: 1) an “altered zone” of anhydrite and dolostone at the up dip edge of the Weyburn-Midale reservoir that forms the caprock to the reservoir subjacent the regional seal formed by the Watrous Formation; 2) the Frobisher Evaporite, a variably thick anhydrite unit present at the base of the reservoir beneath the northern portion of the field; and 3) the Oungre Evaporite, an anhydrite/dolomite unit within the Ratcliffe beds present above the majority of the reservoir, all of which were not included in the Phase 1 model. Adding these units into the model required closely delineating the zero edges using isopach values, and then stacking the isopach thicknesses to proportionately fill the 3D grid while maintaining their complex morphology and accurately representing the hydrogeological flow units above and below the Midale aquifer. Core derived porosity and permeability was included in the model for the Midale and Frobisher aquifer units only as beyond these beds very little core exist. Pressure and hydrochemistry maps for the targeted aquifers (Midale, Frobisher and Ratcliffe) contain straddle tests and new wells drilled since Phase I was completed. Hydrogeological results from this study include; 1) Hydrochemistry maps which indicate large variations in total dissolved solids (TDS) within the target aquifers. The observed chemical and density variations have an influence on fluid movement; 2) Porosity and permeability models for the Midale, Frobisher, and Ratcliffe which will be used in conjunction will structure, chemistry and pressure maps help to elucidate fluid movement within the aquifers for long-term CO 2 storage modeling. The permeability model for the Midale aquifer is composed of data from DSTs and core to examine the geologic variations and preferential flow paths within the injection aquifer; 3) Driving force ratio (DFR) maps forces acting on fluid movement within an aquifer and incorporate aquifer structure, TDS, temperature and pressure.