ALBERT

Description: Carboniferous strata of the Wandel Sea Basin unconformably overlie the Laurentian Precambrian crystalline rocks of the Caledonian hinterland at its northernmost exposures in Holm Land (~80°N). Complex zircon from an intermediate gneiss gives an upper-intercept age of 1878 ± 71 Ma, a protolith age which fits with the regional 1.8–2.0 Ga calc-alkaline arc. A strongly deformed pegmatite was intruded at 435 ± 17 Ma, and it is a rare example of Caledonian magmatism in the northern sector of the orogen. Omphacite confirms the presence of eclogite (sensu stricto) lenses in the basement complex, thus documenting the northern extent of the North-East Greenland eclogite province formed during the Caledonian collision with Baltica. Holm Land lies in the eastern block of the eclogite province, where an ultrahigh-pressure (UHP) metamorphic event took place at 365–350 Ma. Zircon from a Holm Land eclogite lacks a Eu anomaly, has a flat heavy rare earth element pattern, and gives a sensitive high-resolution ion microprobe U-Pb age of 423 ± 7 Ma, and it is thus interpreted as the time of high-pressure (HP) metamorphism. This age overlaps with the established age of widespread HP metamorphism for the eclogite province (i.e., 415–395 Ma), rather than the younger UHP metamorphism. Westward thrusting of the North-East Greenland eclogite province onto the Laurentian margin after 395 Ma and subsequent exhumation of this uppermost thrust sheet provided a substrate for Carboniferous deposition. Detrital zircon age spectra from arkosic sandstones of the late Viséan (ca. 330–340 Ma) Sortebakker and early Moscovian (ca. 310–315 Ma) Kap Jungersen Formations record the progressive unroofing of the North-East Greenland Caledonides. All seven samples have a major peak at 1.8–2.0 Ga, and five also have a 1.75 Ga peak, matching the Paleoproterozoic arc and later anorogenic granitoids that comprise the crystalline basement. Paleozoic grains are sparse in the Sortebakker sandstones, but they constitute a pronounced 400 Ma peak in the younger Kap Jungersen Formation. The composition of the detritus—including garnet clasts, the high amount of discordant zircon (40%), and the large numbers of grains with metamorphic rims that cluster around 410 Ma—reflects a local provenance sourced in the North-East Greenland eclogite province, with some input from the overlying thrust sheets. Other Devonian and Carboniferous basins within and peripheral to the Caledonides also show distinct signatures, demonstrating that there is not a simple, representative detrital zircon signature for the Caledonian orogen.

Description: The Fjerritslev Formation in the Norwegian–Danish Basin forms the main seal to Upper Triassic–Lower Jurassic sandstone reservoirs. In order to estimate the sealing potential and rock properties, samples from the deep wells Vedsted-1 in Jylland, and Stenlille-2 and Stenlille-5 on Sjælland, were studied and compared to samples from Skjold Flank-1in the Central North Sea. Mineralogical analyses based on X-ray diffractometry (XRD) show that onshore shales from the Norwegian–Danish Basin are siltier than offshore shales from the Central Graben. Illite and kaolinite dominate the clay fraction. Porosity measurements obtained using helium porosimetry–mercury immersion (HPMI), mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) techniques on the shale samples show that MICP porosity is 6–10% lower than HPMI or NMR porosity. Compressibility, from uniaxial loading, and elastic wave velocities were measured simultaneously on saturated samples under drained conditions at room temperature. Uniaxial loading tests indicate that shale is significantly stiffer in situ than is normally assumed in geotechnical modelling. Permeability can be predicted from elastic moduli, and from combined MICP and NMR data. The permeability predicted from Brunauer–Emmett–Teller (BET)-specific surface-area measurements using Kozeny’s formulation for these shales, being rich in silt and kaolinite, falls in the same order of magnitude as permeability measured from constant rate of strain (CRS) experiments but is two–three orders of magnitude higher than the permeability predicted from the 1998 model of Yang & Aplin, which is based on clay fraction and average pore radius. When interpreting CRS data, Biot’s coefficient has a significant and systematic influence on the resulting permeability of deeply buried shale.

Description: Potential changes in the recent tectonic stress field as a result of increasing pore pressure due to injection of CO2 may induce mechanical failure of the storage formation and its caprock, reactivation of existing faults or ground uplift if disadvantageous geological conditions are encountered. Coupled hydro-mechanical simulations can be applied to analyse and predict to which extent stress field and mechanical material property changes resulting from pore pressure elevation pose a serious impact on the mechanical stability of faults, reservoir and caprock. Within the frame of the on-going EU-funded SiteChar project, an assessment of CO2 storage performance and associated geomechanical impacts was investigated at a prospective deep saline onshore CO2 storage formation, the Gassum formation at Vedsted, situated in Northern Denmark. Simulations consider the constant injection of 3.15 Mt CO2/year over a time span of 40 years into the marine and fluvial sandstone aquifer of Upper Triassic to Lower Jurassic age, located at a depth between 1,700 m and 2,000 m. For this purpose, coupling of two reservoir and two geomechanical simulators was undertaken by different partners. The widely used commercial Eclipse software and the scientific code TOUGH2-MP were used for the multi-phase flow simulations, whereas the geomechanical modelling was carried out with the commercial codes FLAC3D (coupled to TOUGH2-MP) and VISAGE TM (coupled to ECLIPSE). Pore pressure distribution at selected time steps of the flow simulations conducted was used as input to the geomechanical codes to compute displacement and stress changes. A 3D reservoir-scale model with a lateral extent of 12 km x 16 km was initially applied for the flow simulations and extended to 50 km x 50 km for the geomechanical simulations to minimise the influence of boundary effects. The extended 3D geological model of the Vedsted site also comprises ten different stratigraphic units from the Lower Triassic up to the ground surface to include the overburden. Five northwest-southeast trending major faults implemented into both models were treated as either hydraulically open or closed for cross-flow. Simulation results indicate that changes in the recent stress field after 40 years of CO2 injection are mainly confined to the vicinity of the injection well due to the fact that the greatest pressure build-up occurs in the near-well area of the injector. The magnitudes of stress changes near the faults closest to the injector were insignificant and as such there is as negligible risk for their reactivation. Although shear strength is slightly reduced close to the injection well, the mechanical stability of the reservoir and its caprock is not decreased below critical values at any time during the injection period of 40 years and after. The simulation results further suggest that CO2 injection over a time span of 40 years may induce a vertical displacement of 9.4 cm at the top of the storage formation and a ground uplift of up to 10 cm. The results of the present study show that coupled hydro-geomechanical modelling can be well applied to improve our understanding of the coupled processes during CO2 storage and evaluate associated potential impacts such as ground deformation, fault reactivation and caprock failure.

Description: Pore pressure variation resulting from geological CO2 storage may compromise reservoir, caprock and fault integrity. Therefore, we investigate the mechanical impact of industrial-scale CO2 storage at a prospective Danish site by coupled 3D hydro-mechanical simulations carried out by two independent modelling groups. Even though the two chosen modelling strategies are not identical, simulation results demonstrate that storage integrity is maintained at any time. Vertical displacements are mainly determined by hydraulic fault conductivity influencing spatial pore pressure elevation. The introduced fault zone implementation in the hydro- mechanical model allows for localization of potential leakage pathways for formation fluids along the fault plane.

Description: We assessed the synergetic benefits of simultaneous formation fluid extraction during CO2 injection for reservoir pressure management by coupled hydro-mechanical simulations at the prospective Vedsted storage site located in northern Denmark. Effectiveness of reservoir pressure management was investigated by simulation of CO2 storage without any fluid extraction as well as with 66% and 100% equivalent volume formation fluid extraction from four wells positioned for geothermal heat recovery. Simulation results demonstrate that a total pressure reduction of up to about 1.1 MPa can be achieved at the injection well. Furthermore, the areal pressure perturbation in the storage reservoir can be significantly decreased compared to the simulation scenario without any formation fluid extraction. Following a stress regime analysis, two stress regimes were considered in the coupled hydro-mechanical simulations indicating that the maximum ground surface uplift is about 0.24 m in the absence of any reservoir pressure management. However, a ground uplift mitigation of up to 37.3% (from 0.24 m to 0.15 m) can be achieved at the injection well by 100% equivalent volume formation fluid extraction. Well-based adaptation of fluid extraction rates can support achieving zero displacements at the proposed formation fluid extraction wells located close to urban infrastructure. Since shear and tensile failure do not occur under both stress regimes for all investigated scenarios, it is concluded that a safe operation of CO2 injection with simultaneous formation fluid extraction for geothermal heat recovery can be implemented at the Vedsted site.