Analyses of pre-injection reservoir data for stable carbon isotope trend predictions in CO₂ monitoring: preparing for CO₂ injection

Baseline monitoring at the proposed enhanced gas recovery site in Altmark (Germany) was carried out in combination with theoretical and laboratory investigations to describe and predict the principles of expected stable carbon isotope and dissolved inorganic carbon (DIC) trends during CO₂ injection in reservoirs. This provides fundamental data for site-specific characterisation for monitoring purposes. Baseline δ¹³C values at the Altmark site ranged between −1.8 and −11.5 ‰ and DIC values were about 2 mmol L⁻¹. These baseline values form the basis for a theoretical study on the influences of the ambient reservoir conditions on the state of geochemical and isotope equilibrium of the reservoir fluids. Transferring this theoretical study to the Altmark site enables predictions on geochemical trends during potential injection. Assuming that CO₂ would be injected at the Altmark site to pCO₂ = 100 bar and with a δ¹³C of −30 ‰, at isotopic and geochemical equilibrium, δ¹³C_DIC values would approach this end-member, and DIC concentrations of 1,000 mmol L⁻¹ would be expected. Laboratory experiments were conducted at low pCO₂ levels (4–35 bars) to mimic the approach of a CO₂ plume at a monitoring well. These results support field investigations from other sites: that δ¹³C_DIC is a sensitive tool for monitoring CO₂ migration in the subsurface and simultaneously allows quantification of geochemical trapping of CO₂.