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