High-resolution stable carbon isotope monitoring indicates variable flow dynamic patterns in a deep saline aquifer at the Ketzin pilot site (Germany)

摘要

Stable isotopes of injected CO_2 act as useful tracers in carbon capture and storage (CCS) because the CO_2 itself is the carrier of the tracer signal and remains unaffected by sorption or partitioning effects. At the Ketzin pilot site (Germany), carbon stable isotope composition (δ~(13)C) of injected CO_2 at the injection well was analyzed over a time period of 4 months. Occurring isotope variances resulted from the injection of CO_2 from two different sources (an oil refinery and a natural gas-reservoir). The two gases differed in their carbon isotope composition by more than 27‰. In order to find identifiable patterns of these variances in the reservoir, more than 250 CO_2-samples were collected and analyzed for their carbon isotope ratios at an observation well 100 m distant from the injection well. An isotope ratio mass spectrometer connected to a modified Thermo Gasbench system allowed quick and cost effective isotope analyses of a high number of CO_2 gas specimens. CO_2 gas from the oil refinery (δ~(13)C = -30.9‰, source A) was most frequently injected and dominated the reservoir δ~(13)C values at the injection site. Sporadic injection of the CO_2 from the natural gas-reservoir (δ~(13)C = -3.5‰, source B) caused isotope shifts of up to +5‰ at the injection well. These variances provided a potential ideal tracer for CO_2 migration behavior. Based on these findings, tracer input signals that were injected during the last 2 years of injection could be reconstructed with the aid of an isotope mixing model and CO_2 delivery schedules. However, in contrast to the injection well, δ~(13)C values at the observation well showed no variances and a constant value of -28.5‰ was measured at 600 m depth. This is in disagreement with signals that would be expected if the input signals from the injection would arrive at the observation well. The lack of isotope signals at the observation well suggests that parts of the reservoir are filled with CO_2 that is immobilized.