Stratigraphy, permeability and capillary pressure analysis in the Deadwood and Winnipeg Formations (Williston Basin, Saskatchewan, Canada): insights for CCS and geothermal energy

The Cambro-Ordovician Deadwood and Winnipeg Formations have emerged as targets for CO2 storage and geothermal energy production in recent years, but wells penetrating the Cambro-Ordovician section in southern Saskatchewan are few and far apart. This thesis is centered on the first full-length core drilled through the Deadwood and Winnipeg Formations in southern Saskatchewan retrieved from DEEP Border 1 well (101/12-10-1-11W2) that was drilled by DEEP Earth Energy Production Corp. in 2018, near Estevan. The core includes the entire Deadwood and Winnipeg Formations with the exception of 14 m missing at the contact between the two formations. This core offers a valuable opportunity to improve our understanding of reservoir units of the Deadwood and Winnipeg Formations in Southern Saskatchewan. The overarching goal of the thesis is to identify the reservoir units within the Deadwood and Winnipeg Formations of Southern Saskatchewan, better constrain their petrophysical properties and heterogeneities, and their impact on fluid flows in the context of CCS and geothermal energy production. We present a comprehensive description of the DEEP Border 1 core with a total of 191.8 m of available core examined and described. The core includes the Deadwood Formation, the Black Island Member of the Winnipeg Formation, and a few meters of the Icebox Member of the Winnipeg Formation. Core description accounts for lithology, grain size and sorting, physical sedimentary structures, mud content, and ichnological data. A total of 19 facies are identified and grouped into 8 facies associations, ranging from tidal to proximal offshore and deltaic environments. Porosity and permeability analyses are used to estimate reservoir properties in the different facies and identify the best reservoir intervals. Helium porosimetry is conducted on 113 core plug samples. Prior to cutting the plugs, pressure decay permeability to nitrogen is measured on the slabbed core at the same locations from where the plugs were later extracted. Overall, reservoir quality is better in the Winnipeg Formation. The highest porosity and permeability values are observed in upper shoreface sandstones in both the Deadwood and Winnipeg Formations, especially in the sections characterized by abundant vertical trace-fossils of Skolithos; and in the lower shoreface bioturbated sandstones of the Winnipeg Formation. Six reservoir facies are selected to further characterize permeability heterogeneities and evaluate their impact on fluid flow. Permeability is measured using a minipermeameter on a centimeter-scale grid spanning the full width of slabbed core sections at the exception of a 1 cm margin left on both sides. The data is digitalized into high-resolution permeability grids, which serve as the basis for numerical modeling in MODFLOW and enable the estimation of bulk permeability, both vertically and horizontally, at the core scale. Our results indicate that bioturbation is associated with elevated permeability in the Deadwood and Winnipeg Formations. The intensity of bioturbation and grain size are determined to be the main controls on the flow rate. The bioturbated Skolithos-bearing sandstones appear to be the best reservoir horizons overall, capable of supporting the highest flow rates. Core plug samples collected for porosity are also used for capillary pressure and irreducible water saturation analysis using a high-speed centrifuge. The data obtained on 97 samples is used as input for modeling post-injection migration of CO2 in the Deadwood and Winnipeg Formations using the software Permedia, with a modified invasion percolation (MIP) approach. Several migration scenarios are simulated following the injection of 50 megatonnes of CO2 to the DEEP Border 1 well to assess the probability of distribution of CO2 in the reservoir. All scenarios show that the CO2 injected at the DEEP Border 1 well is likely to intersect with the Aquistore plume located 25 km to the north-east especially if CO2 is injected in the middle and lower perforations of the Deadwood Formation (perforations 2 and 3). To limit that risk, we recommend that CO2 injection, should it take place at the DEEP site, would target the Winnipeg sandstones. That perforation also presents the highest chance of retaining CO2 into the reservoir due to the presence of sandstones with low capillary entry pressure and low irreducible water saturation. Our analysis shows how closely the migration and retention of the CO2 in the reservoir are connected to facies distribution, highlighting the importance of facies identification and characterization before modeling.

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