HYDROGEOLOGY OF THE WEYBURN CO2 PROJECT AREA: BRINE DISTRIBUTION AND PRESENT-DAY FLUID FLOW

The Weyburn CO2 Project is a pioneer EOR/sequestration operation currently injecting CO2 into the Mississippian Midale Beds where carbonates and evaporites form a giant oil and gas reservoir in Saskatchewan, Canada. Hydrogeology and hydrochemistry comprise a key component of the sequestration performance prediction because the migration of CO2 may be strongly affected by the flow of formation waters. Thus, a hydrogeological analysis was completed for the entire Phanerozoic section to identify fluid driving forces, determine flow directions and rates, and locate possible conduits for cross-formational flow of formation waters and potential leakage pathways for CO2.

Formation pressures and water chemistries were mapped over an area of 45,000 km2 for 18 aquifers above, below, and including the Midale aquifer. Hydraulic heads indicate dominantly updip flow of formation waters from SW to NE across the study area. Formation water salinities range from 5 to over 350 g/L resulting in significant density variations within and between aquifers. A novel approach was used to estimate the effects of variable density fluids on flow directions in these gently dipping aquifers. Locally intense density-dependent flows are predicted in brine-bearing areas and the resulting modifications on fluid driving force directions are assessed.

The implications for CO2 sequestration at Weyburn are that: 1) Brine distributions indicate competent sealing aquitards present throughout the section and immediately above the Midale reservoir, 2) Natural hydrodynamics in the Midale aquifer will not enhance nor degrade geological sequestration of CO2, 3) Present-day, altered hydrodynamics may, however, significantly enhance trapping of injected CO2. The presence of brines and an interesting inter- and intra-aquifer variability in Total Dissolved Solids exert significant control on fluid flow patterns in the area.