The side cores were drilled with a conventional workover drill rig Cabot Franks 500 with a mud circulation rate of 200 L min⁻¹. The main functions of the drill mud is to provide hydrostatic pressure in the bore hole, to cool the drill bit while drilling and to extract drill cuttings as well as all gases entering the bore hole at depth. For the extraction of the gases dissolved in the drill mud a conventional gas separator with a free outlet was applied (Fig. 2). This gas trap did not allow for the quantification of absolute gas amounts entering the borehole. While the drilling mud was equilibrated with atmospheric air before being pumped down into the borehole, the outlet mud showed a corresponding atmospheric gas composition if gas had not entered the borehole from the geological formation.

Figure 2Experimental setup of the mud gas extraction system and the field laboratory during drilling the side tracks at Ktzi 203.

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The liberated gases were pumped to a nearby field laboratory and analyzed qualitatively and quantitatively around the clock in real time using a gas mass spectrometer (Omnistar, Pfeiffer Vacuum, time resolution of measurement 1 min, detection limit approximately 1 ppmV). Using these devices the composition of the drill mud gas phase was continuously analyzed for the volumetric quantity of H₂, He, CO₂, Ar, N₂, O₂, CH₄. A Los Gathos CO₂-isotope analyzer was employed in the field to continuously analyze the ${}^{\mathrm{12}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}{/}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ isotopic ratio of the mud gas phase at one minute intervals. LGR's CO₂-isotope analyzer provided measurements of ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ with a standard deviation σ of ±1 ‰. For the correction of the internal instrument drift a standard gas (ambient air; ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ $=-\mathrm{8}$ ‰) was measured automatically every 6 hours using a multiport inlet unit.

The core barrels retrieved at the surface were cut into 1 m sections and then shrink-wrapped with plastic foil to collect potentially discharging gas from the rock material within the first hour. After a degassing period of at least two hours, an injection needle mounted to the mass spectrometer inlet capillary was inserted through the wrapping foil. This procedure allowed for the direct determination of the trapped core gas with a mass spectrometer. Furthermore, core gas samples were collected for ¹³C∕¹²C-analyses of the CO₂. For this purpose, a further needle was inserted through the wrapping. This needle was connected to a hose that linked via another needle to a borosilicate glass vial (i.e. Exetainer^®) with a butyl-rubber septum. The gas was then transferred to the Exetainer using a syringe inserted into the same Exetainer via the septum. In this way 40 mL of sampling gas were sucked through the 12 mL vial resulting in a 3-times replacement with sample gas.

The analyses of the Exetainer gas were performed at the GFZ laboratories using a system consisting of a gas chromatograph (GC 6890N, Agilent Technology) connected to a GC-C/TC III combustion device coupled via open split to a MAT 253 mass spectrometer (ThermoFisher Scientific). The gas samples were injected with a gas-tight syringe into the programmable temperature vaporization inlet (PTV, Agilent Technology) of the GC which separated the gases on a PoraPlot column. Helium carrier gas transferred the CO₂ to the mass spectrometer for determination of the carbon isotope ratios. The standard deviation σ for the isotopic measurements was ±0.5 ‰. The ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ were reported in ‰ vs. VPDB (Vienna Pee Dee Belemnite).

Analyses of the core and mud gas phase from the side track drilling in 2017 revealed atmospheric concentrations of O₂, N₂, Ar, He, but higher concentrations for CO₂. Both side tracks overlap at the depth section between 643 and 654.5 m. In this section the CO₂ concentrations for both side tracks showed similar concentration-depth trends but at different concentration levels. The CO₂ concentration in side track-a was generally about 2500 ppmV higher than in side track-b (Fig. 3).

Samples from the muddy cap rock section showed CO₂ concentrations of about 500 ppmV. This rose drastically to values of up to 12 000 ppmV at the lithological transition to the reservoir sandstone of the Stuttgart formation due to the presence of injected CO₂ in the pore space and decreased again to about 1000 ppmV in a muddy to silty layer (648–655 m) with lower porosity. In the sandstone section below 655 m (side track-a) CO₂ concentrations of only up to 11 000 ppmV were measured. Therefore, the mud gas CO₂ values of 2017 were significantly lower than the concentrations from core gas collected during drilling of the main hole in 2012 with maximum CO₂ values of up to 99 vol. %. But generally both data series revealed the same depth trend.

Figure 3Lithological profile and measured CO₂ concentration in the mud gas of side track-a and -b as well as its ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ from mud and core gas (side track-b). For comparison the CO₂ concentration and the ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ of core gas from 2012 is given. Squares, dots and stars represent analyses of single samples. Lines are continuous measurements at five-minute intervals.

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Only isotope analyses of the core and the mud gas from side track-b were used for a comparison with previous core gas data presented by Barth et al. (2015) for the coring of Ktzi 203 main hole in 2012 (both using potash drill mud). Core and mud gas samples from the cap rock showed isotope values ranging from −25 to −35 and −10 to −20 ‰, respectively. The isotope values of the CO₂ in the reservoir below a depth of 630 m were significantly different varying by around −45 ‰ in the core gas and by −28 ‰ in the mud gas phase. Generally, the isotope data of 2017 showed the same depth-trend as of core gas detected in 2012. Merely the variation range was different in 2012 from −18.6 ‰ in the cap rock to −36.6 ‰ in the reservoir (Barth et al., 2015).

Due to the applied methods and the unknown total amount of drill core degassing only semi-quantitative statements on occurring CO₂ are possible. The detected CO₂-concentrations-depth trend and the ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ signatures during the side track drilling clearly indicate the injected CO₂ in the reservoir horizon. As in 2012, the CO₂ concentration drastically increases at the lithological intersection between the cap rock and the reservoir sandstone. The highest CO₂ concentrations were measured in the top layer of the reservoir sandstones where most of the CO₂ dispersed during the injection process (Kiessling et al., 2010). As the density of CO₂ under the prevailing conditions is significantly lower than that of water, an upward migration of free CO₂ within the reservoir formation can be expected.

Differences in the CO₂ concentration of mud gas from side track-a and side track-b can be explained by the use of different drilling muds. Track-a was drilled with “Sole-fluid” containing mainly Na⁺ (97 g L⁻¹) and Cl⁻ (146 g L⁻¹) at a pH of 7, whereas side track-b was drilled with a potash-drilling mud (K⁺ = 56 g L⁻¹, CO${}_{\mathrm{3}}^{\mathrm{2}-}$ 45 g L⁻¹) at a pH between 10 and 11. As the CO₂ is controlled by H₃O⁺ activity of the applied drilling mud and by carbonate precipitation, where with decreasing pH of the mud the CO₂ content in the gas phase increases (Zimmer and Erzinger, 1995), generally higher CO₂ concentrations were expected at side track-a. The more depleted ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ of the core gas compared to mud gas can be explained by a strong influence of the alkaline potash drill mud with a ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ of −50 ‰ as described by Nowak et al. (2013). The CO₂ in the cores was trapped together with the drill mud in plastic foil for several hours whereas the mud gas had contact with the drill mud for several minutes only during the lag time. As the drill mud gas was less influenced by the drilling mud, we assume that these data better represent the pristine isotope composition of the CO₂ in the geological formations.

The isotope values from the cap rock samples of −10 to −20 ‰ cover the range of typical Triassic formation fluids in the North German Basin (Möller et al., 2008) and give no indication of an infiltration of injected CO₂ with an average ${\mathit{\delta }}^{\mathrm{13}}{\mathrm{C}}_{\mathrm{CO}{}_{\mathrm{2}}}$ of $-\mathrm{30.0}\pm \mathrm{0.5}$ ‰ (Barth et al., 2015). In contrast, the isotope values from the reservoir samples with on average −28 ‰ indicate the presence of the injected CO₂ in the sandstone formation.

The CO₂-concentrations and isotope signatures of the core and mud gas from 2017 show very similar depth profiles as compared with data from 2012. This result indicates that during the period of five years no significant movement of the CO₂ in the formation had occurred. The low CO₂ concentration in the cap rock and its isotopic composition suggests that the penetration of injected CO₂ into the Weser formation was insignificant.

The data will be accessible after publication in the data repository of the GFZ at: http://gfzpublic.gfz-potsdam.de/pubman/.

Andreas Jurczyk (Helmholtz-Centre Potsdam – GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany), Christian Kujawa (Helmholtz-Centre Potsdam – GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany), Cornelia Schmidt-Hattenberger (Helmholtz-Centre Potsdam – GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany), Stefan Lüth (Helmholtz-Centre Potsdam – GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany), Fabian Möller (Helmholtz-Centre Potsdam – GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany)

The authors declare that they have no conflict of interest.

This article is part of the special issue “European Geosciences Union General Assembly 2018, EGU Division Energy, Resources & Environment (ERE)”. It is a result of the EGU General Assembly 2018, Vienna, Austria, 8–13 April 2018.