Core-scale geophysical and hydromechanical analysis of seabed sediments affected by CO2 venting

Safe offshore Carbon Capture Utilization and Storage (CCUS) includes monitoring of the subseafloor, to identify and assess potential CO2 leaks from the geological reservoir through seal bypass structures. We simulated CO2-leaking through shallow marine sediments of the North Sea, using two gravity core samples from ∼1 and ∼2.1 m below seafloor. Both samples were subjected to brine−CO2 flow-through, with continuous monitoring of their transport, elastic and mechanical properties, using electrical resistivity, permeability, P-wave velocity and attenuation, and axial strains. We used the collected geophysical data to calibrate a resistivity-saturation model based on Archie’s law extended for clay content, and a rock physics for the elastic properties. The P-wave attributes detected the presence of CO2 in the sediment, but failed in providing accurate estimates of the CO2 saturation. Our results estimate porosities of 0.44 and 0.54, a background permeability of ∼10−15 and ∼10-17 m2, and maximum CO2 saturation of 18 % and 10 % (±5 %), for the sandier (shallower) and muddier (deeper) sample, respectively. The finer-grained sample likely suffered some degree of gas-induced fracturing, exhibiting an effective CO2 permeability increase sharper than the coarser-grained sample. Our core-scale multidisciplinary experiment contributes to improve the general interpretation of shallow sub-seafloor gas distribution and migration patterns

Combined interpretation of marine controlled source electromagnetic and reflection seismic data in the German North Sea: a case study

This paper focuses on the combined analysis and interpretation of controlled source electromagnetic (CSEM) and multichannel reflection seismic (MCS) data along one profile in the German North Sea with the goal to reduce ambiguities in interpretation. The shallow water environment of the North Sea is characterized by a complex geological development which includes rifting, several ice age cycles, a propagating shelf margin, mass-transport deposits and salt dome formation. Seismic and electromagnetic methods are sensitive to different physical properties of the seabed and therefore complement each other. We analyse the MCS data with a migration velocity tomography and an amplitude variation with offset analysis and discuss seismic velocities and densities. For true amplitude recovery the amplitude distortions are calibrated with in situ logging data. The CSEM data are analysed in 2-D, for which, for the first time, data were included that were acquired while the instrument was towed on the seafloor in addition to the stationary sites. The CSEM inversions are constrained by seismic horizons. The joint interpretation focuses on two seismic reflectors: One can be interpreted as an unconformity marking a lithological change from fresh water-bearing glacial deposits to compacted sediments below, and the other one as a layer of fine-grained deposits potentially capping patchy shallow gas occurrences. This exemplary case study shows how the combination of both methods can benefit by interpreting complex geology and eliminating ambiguous explanations

Trans-dimensional Bayesian inversion of controlled-source electromagnetic data in the German North Sea

This paper presents the first controlled-source electromagnetic survey carried out in the German North Sea with a recently developed seafloor-towed electrical dipole–dipole system, i.e., HYDRA II. Controlled-source electromagnetic data are measured, processed, and inverted in the time domain to estimate an electrical resistivity model of the sub-seafloor. The controlled-source electromagnetic survey targeted a shallow, phase-reversed, seismic reflector, which potentially indicates free gas. To compare the resistivity model to reflection seismic data and draw a combined interpretation, we apply a trans-dimensional Bayesian inversion that estimates model parameters and uncertainties, and samples probabilistically over the number of layers of the resistivity model. The controlled-source electromagnetic data errors show time-varying correlations, and we therefore apply a non-Toeplitz data covariance matrix in the inversion that is estimated from residual analysis. The geological interpretation drawn from controlled-source electromagnetic inversion results and borehole and reflection seismic data yield resistivities of ?1 ?m at the seafloor, which are typical for fine-grained marine deposits, whereas resistivities below ?20 mbsf increase to 2–4 ?m and can be related to a transition from fine-grained (Holocene age) to unsorted, coarse-grained, and compacted glacial sediments (Pleistocene age). Interface depths from controlled-source electromagnetic inversion generally match the seismic reflector related to the contrast between the different depositional environments. Resistivities decrease again at greater depths to ?1 ?m with a minimum resistivity at ?300 mbsf where a seismic reflector (that marks a major flooding surface of late Miocene age) correlates with an increased gamma-ray count, indicating an increased amount of fine-grained sediments. We suggest that the grain size may have a major impact on the electrical resistivity of the sediment with lower resistivities for fine-grained sediments. Concerning the phase-reversed seismic reflector that was targeted by the survey, controlled-source electromagnetic inversion results yield no indication for free gas below it as resistivities are generally elevated above the reflector. We suggest that the elevated resistivities are caused by an overall decrease in porosity in the glacial sediments and that the seismic reflector could be caused by an impedance contrast at a thin low-velocity layer. Controlled-source electromagnetic interface depths near the reflector are quite uncertain and variable. We conclude that the seismic interface cannot be resolved with the controlled-source electromagnetic data, but the thickness of the corresponding resistive layer follows the trend of the reflector that is inclined towards the west