CARBON ISOTOPE MUD GAS DEPTH PROFILE AT THE AQUISTORE CARBON SEQUESTRATION SITE, ESTEVAN SASKATCHEWAN, CANADA

The Aquistore project is one of Canada’s first large-scale aquifer CO₂ storage projects. CO₂ will be captured at SaskPower’s Boundary Dam coal-fired power generation station and transported approximately four kilometers via underground pipeline to a newly-drilled 3400 m deep injection well. During the drilling of the 3400 m deep injection and observation wells, mud gas samples were collected at periodic intervals to create a carbon isotope depth profile prior to injection and long term CO₂storage.

Seventy six gas samples were collected from recirculating drilling mud. Samples were collected using Isotubes and the carbon isotope ratios (¹³C/¹²C) for methane, ethane, propane, and butane were measured for each sample using a Finning-MAT 252 GC-IRMS. Samples were run in duplicate or triplicate to ensure accurate results and then plotted to create a vertical compound specific carbon isotope mud gas depth profile. To ensure the sampling procedure was accurate, approximately 10% of the samples were collected at the same horizon in both wells and compared.

The isotope depth profile is complex showing many inflection points and abrupt inflections at some formational boundaries, indicating no crossformational communication. Overall δ¹³C for methane ranged from -73‰ PDB near the surface to -39‰ PDB at approximately 3200 m. Cretaceous δ¹³C ranged from ‑73‰ to -56‰, Jurassic δ¹³C from -62 to -56‰, Mississippian δ¹³C from -54 to -43‰, Devonian from -61 to -45‰, Silurian strata yielded a δ¹³C ratio of -53‰, Ordovician from -51‰ to -46‰, and Cambrian strata from -50‰ to ‑39‰.This trend shows that near surface, current subsurface gases are dominated by more negative biogenic gases while with increasing depth, more enriched thermogenic gases are present.

After injection of CO₂, in the event that gases begin to migrate to the surface, this background characterization will allow for the comparison of the leaked gas with the known gas distribution. This will allow the source to quickly be determined and therefore enhance the ability to take appropriate measures to reduce the impact of the affected area.