Isotopic investigation of venting CO2 from a subsurface source

Abstract

With this research, measuring the clumped signal on δ47 for CO2 samples obtained from naturally leaking subsurface sources in central Italy is investigated for its viability to identify the possible temperature and hence depth of the source of origin by applying a geothermal gradient. The δ47 clumped signal can be linked to specific temperature ranges, which can be transformed to depth by applying a geothermal gradient to the calculated temperature range, solving for depth. This research investigates methods of cryogenic CO2 purification processing to measure the δ47 clumped isotopic signal of a sampled CO2 gas. On a dual inlet mass spectrometer (IRMS). Whether CO2’s δ47 clumped signal can be used to derive the depth of the subsurface source of origin, depends on the upwards CO2 migration flux through a permeable, porous succession vs. the chemical equilibration reaction speed between CO2 and (ground)water. The D47 Clumped signal is deemed to be independent of variable groundwater geochemistry (Affek, 2013), and in systems containing CO2 vapor and liquid water, dissolution of carbon dioxide is the rate-limiting step (Clog et al., 2015). Using Δ47 as an indicator of the source of depth might be held to a certain limited, since the equilibration speed with water (reactions 2 & 3) could be very high considering the observed trend of increasing equilibration reaction speeds. For the purpose of mitigating hazardous conditions concerning onshore CCS by using Δ47 as an indicator to monitor for subsurface CO2 leakage, is doubtful to be a reliable application. This hypothesis is made on the extrapolation of the observed trend of Clog et al., (2015)’s results concerning an increasing isotopologue exchange rate for Δ47 at increasing temperatures in low temperature ranges. This hypothesis could not be confirmed nor denied by the performed IRMS measurements for δ47 of the sampled CO2 gases.