Refinement of the Weyburn-Midale geological and hydrogeological model: Developing a better framework to determine reservoir response to injected CO2 and subsequent CO2 movement

Abstract The Weyburn field, operated by Cenovus Energy, currently contains the world's largest amount of injected and geologically stored anthropogenic CO2, with over 20 million tonnes of CO2 sequestered as of June 2012. The IEA GHG Weyburn-Midale CO2 Monitoring and Storage Project is in its second phase of research and, in order to refine the final model, adds new geological and hydrogeological data to the already rich dataset from Phase I. One aspect of geological characterization of the storage site in this phase of research was to further refine the geological model used in Phase I by examining more wells and including additional geological units. Well density has been increased from Phase I to more than 900 wells including 200 newly picked wells to more accurately locate the zero edges of these subcropping Mississippian units. Additionally, the model has been improved to integrate geological units not included in the Phase I model, including: (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 to 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. Adding these units, with their irregular termination edges, into the Petrel model resulted in unrepresentative reservoir architecture as stacking the multiple Mississippian beds and then truncating them with the sub-Mesozoic unconformity created residual artefacts, which could not be correctly resolved in the resulting 3D grid. It was therefore necessary to delineate the zero edges by using false wells with zero isopach values, and then stack these isopach thicknesses to proportionately fill the 3D grid while maintaining their complex morphology. This resulted in an improved representation of the architecture of the hydrogeological flow units present in the Weyburn reservoir. Other challenges included the various types of data projections. This was resolved by using a consistent methodology for converting between all the coordinate systems. Pressure and hydrochemistry maps for the aquifers of interest (Midale, Frobisher and Ratcliffe) contain straddle tests and new wells drilled since Phase I was completed. Straddle drill stem tests were not included in Phase I of the project. Investigating these tests permits a better evaluation of the possibility of cross-formational flow between the Mississippian aquifers, and of the competency of inter- and intra-aquifer evaporites as potential barriers for fluid movement. Hydrogeological results from this study include: (1) hydrochemistry maps which indicate large variations in total dissolved solids (TDS) within the aquifers of interest (Ratcliffe, Midale and Frobisher) – the observed chemical and density variations have an influence on fluid movement; (2) pressure maps which illustrate unique variations between the aquifers of interest; (3) hydrochemical cross sections which display the lateral and vertical formational movement of fluids in the aquifers of interest; (4) pressure depth plots [P(d)] that demonstrate the original formation pressure present in the aquifers of interest as well as the current pressure regime present in the CO2 flood area; and (5) driving force ratio (DFR) maps that illustrate the combined forces acting on fluid movement within an aquifer and incorporate aquifer structure, TDS, temperature and pressure. The P(d) plots and the DFR maps add valuable insight regarding the fate of fluid movement in the aquifers of interest by locating regions that are in vertical communication as well as identifying areas where stagnation points or down-dip fluid movements are occurring. Additionally, the analysis of a recently completed observation well helps to gain insight into the pressure regime and fluid movement currently present in the storage unit. Results indicate that the Ratcliffe has predominantly horizontal flow and is isolated from the underlying Midale and Frobisher. These hydrogeological results can elucidate the movement of fluids in the Midale, as well as the vertical movement of fluids between the Midale, Frobisher and Ratcliffe. These new data and model were utilized by other research disciplines of the project including risk assessment and modelling of long-term storage. Results indicate that the Midale Beds at the IEA GHG CO2 Weyburn Monitoring and Storage Project are a secure location for CO2 sequestration.