Study of Al2O3 Sol-Gel Coatings on X20Cr13 in Artificial North German Basin Geothermal Water at 150 °C

Al2O3 has been widely used as a coating in industrial applications due to its excellent chemical and thermal resistance. Considering high temperatures and aggressive mediums exist in geothermal systems, Al2O3 can be a potential coating candidate to protect steels in geothermal applications. In this study, γ-Al2O3 was used as a coating on martensitic steels by applying AlOOH sol followed by a heat treatment at 600 °C. To evaluate the coating application process, one-, two-, and three-layer coatings were tested in the artificial North German Basin (NGB), containing 166 g/L Cl−, at 150 °C and 1 MPa for 168 h. To reveal the stability of the Al2O3 coating in NGB solution, three-layer coatings were used in exposure tests for 24, 168, 672, and 1296 h, followed by surface and cross-section characterization. SEM images show that the Al2O3 coating was stable up to 1296 h of exposure, where the outer layer mostly transformed into boehmite AlOOH with needle-like crystals dominating the surface. Closer analysis of cross-sections showed that the interface between each layer was affected in long-term exposure tests, which caused local delamination after 168 h of exposure. In separate experiments, electrochemical impedance spectroscopy (EIS) was performed at 150 °C to evaluate the changes of coatings within the first 24 h. Results showed that the most significant decrease in the impedance is within 6 h, which can be associated with the electrolyte penetration through the coating, followed by the formation of AlOOH. Here, results of both short-term EIS measurements (up to 24 h) and long-term exposure tests (up to 1296 h) are discussed.

A comprehensive model of seismic velocities for the Bay of Mecklenburg (Baltic Sea) at the North German Basin margin: implications for basin development

The geometry of sedimentary basins is normally described by the interpretation of seismic reflectors. In addition to that, rock properties of the sedimentary successions between these reflectors give further insight into the subsurface geology. Here, we present a model for the Bay of Mecklenburg, situated at the northeastern margin of the North German Basin. The model consists of eight layers; it covers seismic velocities of sediments from the Neogene down to the base of the Permian Zechstein. We use eight seismic profiles for model building and apply seismic migration velocity analysis in combination with pre-stack depth migration. The results are interval velocities down to a depth of 5000 m. A further aim of the study is to investigate the sensitivity of these indirectly deduced velocities in comparison to direct measurements within drill holes. The velocities from this study are in good agreement with earlier results from vertical seismic profiling at a nearby well. Cenozoic and Mesozoic strata within the Bay of Mecklenburg show clear depth-dependent velocity trends. A comparison of these trends with predicted compaction trends shows that burial anomalies within Lower Triassic units are significantly higher than in Upper Cretaceous units. This finding could be explained by a greater amount of erosion during Upper Jurassic/Lower Cretaceous times than during Cenozoic times. The Zechstein layer shows a decreasing interval velocity with increasing thickness. Our study demonstrates that seismic velocities deduced from surface-based measurements are of high value in areas with sparse drilling coverage.

Multiphase inversion in the Baltic sector of the North German Basin: Influence of Africa-Iberia-Europe convergence during the Late Cretaceous and Cenozoic

<p>Within the DFG project StrucFlow, we investigate the multiphase character of Late Cretaceous to Cenozoic inversion in the Baltic sector of the North German Basin based on seismic interpretation. Our analysis rests upon modern high-resolution seismic profiles in combination with data from older seismic surveys and borehole information. The resulting seismic database consists of a dense profile network with a total length of some 10.000 km. This unprecedented seismic grid allows for a detailed tectono-stratigraphic interpretation of Cretaceous and Paleogene deposits in the Baltic sector of the North German Basin. Here, basin inversion began in the Coniacian and Santonian with uplift of the Grimmen High and minor reactivation of Zechstein salt structures. Crestal faults were formed or reactivated above salt pillows in the Bays of Mecklenburg and Kiel. The onset of inversion was contemporaneous with other adjacent basins and is likewise associated with building up intraplate stress within the European foreland related to the beginning Africa-Iberia-Europe convergence. Time-isopach maps of Paleocene deposits in the study area show a slight decrease in thickness to the west. This contrasts the prevailing trend of increasing thickness towards the southwest directed basin center and indicates a changed depositional environment. In the outer eastern Glückstadt Graben, increased thicknesses and diverging strata of late Eocene and Oligocene units indicate significant remobilization of salt structures during this time. Preexisting Triassic faults above the salt pillows “Schleimünde” and “Kieler Bucht” at the eastern border of the Glückstadt Graben were reactivated and form a north-south trending crestal graben filled with Paleogene sediments. This phase of salt remobilization is contemporaneous with the reintroduction of intraplate stress triggered by the Alpine and Pyrenean orogenies in the late Eocene. In the eastern Bay of Kiel and in the Bay of Mecklenburg, Late Eocene and younger sediments are largely absent due to Neogene uplift and erosion. Deepening of rim-synclines and synchronous infill of Paleogene strata give evidence for commencing salt pillow growth. Crestal faults pierce the Paleocene and Eocene strata, indicating salt movement at least during the later Eocene. This phase of salt movement occurred contemporaneously with salt remobilization in the Glückstadt Graben, initiation of the European Cenozoic Rift System and increased activity in the Alpine realm in the Late Eocene to Oligocene. We conclude that the rise of salt pillows since the Eocene significantly exceeds the growth during late Cretaceous to Paleocene inversion phase at the northeastern North German Basin.</p>

Cenozoic salt remobilization at the Baltic Sea sector of the northeastern North German Basin margin

<p>In the scope of the “StrucFlow” project, we study salt tectonics at the salt-floored northeastern North German Basin margin, which is part of the Central European Basin System. Salt pillows are located in the Bays of Kiel and Mecklenburg, in the SW Baltic Sea, east of the Glückstadt Graben and west of the Tornquist-Teisseyre Zone. Salt pillow growth initiated in the Late Triassic and rejuvenated in Late Cretaceous to Tertiary times. We combine offshore and nearby onshore wells, shallow seismic surveys and high-resolution seismic sections from the BalTec data to derive a detailed seismo-stratigraphic correlation of Cenozoic units. This allows a more precise analysis of Cenozoic salt movement in the transition zone between the Glückstadt Graben and Tornquist Zone. We present key profiles and time-isochore maps revealing new insights into salt pillow evolution at the northeastern North German Basin margin and discuss active phases of salt movement in the context of the regional tectonic framework.</p><p>We associate the Late Cretaceous phase of salt pillow growth with far-field effects of the Africa-Iberia-Europe convergence and the consequent Pyrenean orogeny. The resulting change from extensional to compressional intraplate stress caused graben inversion and thrust faulting in northern Europe. However, Early Cenozoic successions reveal no indications for ongoing salt movement and suggest a phase of salt tectonic quiescence. Within the Eocene, salt was remobilized at the Baltic Sea sector of the North German Basin, leading to renewed salt pillow growth and erosion above pillow crests. We propose that this phase of salt remobilization is controlled by the coeval initiation of the European Cenozoic Rift System, between the rising Alps in the south and the opening North Atlantic Ocean in the northwest. Faulting within Quaternary deposits above a salt wall in the Bay of Kiel could indicate continuous salt movement and was possibly amplified by glacial isostatic adjustment.</p>