Design considerations for using Distributed Acoustic Sensing for cross-well seismics: A case study for CO2 storage

Downhole monitoring with fibre-optic Distributed Acoustic Sensing (DAS) systems offers unprecedented spatial resolution. At the same time, costs are reduced since repeated wireline surveys can be replaced by the permanent installation of comparatively cheap fibre cables. However, the single component nature of fibre data requires novel approaches when designing a monitoring project such as cross-well seismics. At the example of the shallow CO₂ injection test site in Svelvik, Norway, we model the evolution of velocity changes during CO₂ injection based on rock physics theory. Different cross-well seismic design scenarios are then considered to evaluate the best design and the limits of this method to detect containment breach. We present a series of evaluation tools to compare the effect of different well spacings for cross-well seismic tomography. In addition to travel-times, we also consider characteristic amplitude changes along the fibre unique to DAS strain measurements, which might add a constraint to the inversion. We also compare the effect of using helical fibres instead of classical straight fibres. We thus present a toolbox to evaluate and compare different monitoring design options for fibre optic downhole installations for cross-well monitoring.     

Interpretation of tracer tests performed in fractured rock of the Lange Bramke basin, Germany

 Two multitracer tests performed in one of the major cross-fault zones of the Lange Bramke basin (Harz Mountains, Germany) confirm the dominant role of the fault zone in groundwater flow and solute transport. Tracers having different coefficients of molecular diffusion (deuterium, bromide, uranine, and eosine) yielded breakthrough curves that can only be explained by a model that couples the advective–dispersive transport in the fractures with the molecular diffusion exchange in the matrix. For the scale of the tests (maximum distance of 225 m), an approximation was used in which the influence of adjacent fractures is neglected. That model yielded nearly the same rock and transport parameters for each tracer, which means that the single-fracture approximation is acceptable and that matrix diffusion plays an important role. The hydraulic conductivity of the fault zone obtained from the tracer tests is about 1.5×10^–2 m/s, whereas the regional hydraulic conductivity of the fractured rock mass is about 3×10^–7 m/s, as estimated from the tritium age and the matrix porosity of about 2%. These values show that the hydraulic conductivity along the fault is several orders of magnitude larger than that of the remaining fractured part of the aquifer, which confirms the dominant role of the fault zones as collectors of water and conductors of fast flow. Received, April 1997 Revised, January 1998, August 1998 Accepted, August 1998     

Mobility and H2O loss from fluid inclusions in natural quartz crystals

Petrographic observations on quartz crystals from the Mole Granite (Australia) and other localities shed new light on the mechanisms of post-entrapment modification of fluid inclusions. These modifications include migration away from pseudosecondary trails, changes in fluid salinity and density, shape distortion and the formation of “sweat-haloes” around strongly deformed inclusions. Increases in fluid salinity, which usually are associated with inclusion migration, indicate water-losses of up to 50%. However, LA-ICP-MS-analysis of unmobilized and mobilized inclusions of the same trail reveals basically unchanged ratios of major – and trace element cations, with the exception of Li, which seems to be incorporated into the crystal lattice during migration. Despite the fact that all these modifications are closely related to deformation processes, they occur not only in mechanically deformed quartz, but also in free-standing crystals. In the latter samples, stress has been generated internally as a result of brazil-twinned growth and compositional zonation. These observations and their interpretation leads to a list of practical criteria that should help in differentiating between reliable and suspect fluid inclusions in other samples. Received: 17 November 1998 / Accepted: 16 April 1999     

A Density Correction for Lagrangian Particle Dispersion Models

Current Lagrangian particle dispersion models, used to simulate the dispersion of passive tracers in the turbulent planetary boundary layer (PBL), assume that the density is constant within the PBL. In deep PBLs, where the density at the boundary-layer top may be lower by more than 20% than at the surface, this assumption leads to errors in the tracer concentrations on the order of 10%. In the presence of a vertical wind shear, this also leads to inaccurate calculations of the horizontal tracer transport. To remove this deficiency, a Langevin equation is presented that contains a density correction term. The effect of the density correction is studied using data from a large-scale tracer experiment. It is found that for this experiment, the main effect of the density correction is an increase in the surface tracer concentrations, whereas the horizontal tracer transport patterns remain largely unaffected.     

The Transport Time pdf for a Simple Turbulent Flow

In a turbulent fluid, the time a particle needs to travel from a point source to the observation point, can be considered as a random variable. It is shown that the probability density function (pdf) for this random variable is determined by the Lagrangian particle position pdf. The characteristics of the transport time pdf are discussed for the simple case of a turbulent fluid moving with a constant mean velocity.     

Relative sea level changes during the Holocene in the Sisimiut area,south‐western Greenland

Holocene relative sea level (RSL) changes have been investigated by analysing and dating isolation sequences from five lakes near Sisimiut in south‐western Greenland. The transitions between marine and lacustrine sediments were determined from elemental analyses and analyses of macroscopic plant and animal remains. Radiocarbon dating was used to provide minimum ages for the transitions and to construct a RSL curve. Dating of a shell of the marine bivalve Macoma balthica indicates that deglaciation of the lowlands occurred in the early Holocene, at around 10 900 cal a BP. The RSL curve shows initial rapid regression from the marine limit at around 140 m, implying strong glacio‐isostatic rebound. We suggest that the margin of the Greenland Ice Sheet was located at the shelf break during the Last Glacial Maximum. Frequent remains of the ostracode Sarcypridopsis aculeata, which is a thermophilous brackish water species that is unknown from the extant fauna of Greenland, in one of the basins around 8500 cal a BP may mark the beginning of the Holocene thermal maximum in the region. Copyright © 2011 John Wiley & Sons, Ltd.     

Carbon and oxygen diffusion in calcite: Effects of Mn content and P H2O

The diffusion rates of carbon and oxygen in two calcite crystals of different Mn contents have been studied between 500° and 800° C in a CO₂-H₂O atmosphere (P _{CO 2}=1?5 bars, P _H2_O=0.02?24 bars) labeled with ¹³C and ¹⁸O. Isotope concentration gradients within annealed specimens were measured using a secondary ion microprobe by depth profiling parallel and perpendicular to the c axis. Despite the anisotropic structure of calcite, the diffusion of carbon and oxygen are both very nearly isotropic. Least-squares fitting of the carbon data to an Arrhenius relation gives an activation energy of 87±2 kcal/mole, with D ₀ terms dependent only slightly upon direction: 1 $$D_{\text{0}} {\text{(}}\parallel c{\text{) = }}\left( {9\frac{{ + 12}}{{ - 5}}} \right){\text{x10}}^{\text{2}} cm^2 /s$$ , 2 $$D_{\text{0}} {\text{(}} \bot c{\text{) = }}\left( {5\frac{{ + 6}}{{ - 3}}} \right){\text{x10}}^{\text{2}} cm^2 /s$$ . These results are in close agreement with previous determinations. Results for oxygen diffusion, however, give D values much larger than those previously reported for dry conditions; at 650° to 800° C the D values are two orders of magnitude larger. The diffusion of oxygen, unlike carbon, is strongly dependent on water pressure, as well as Mn content, and does not fit an Arrhenius relation over the entire temperature range. On the basis of these observations and considerations of the defect chemistry of calcite, it is proposed that carbon migrates as a Frenkel pair. The diffusion of oxygen, however, appears to be more complicated and may depend upon several simultaneous mechanisms.     

Mesozoischer Vulkanismus in Nordspanien: Rifting im Keuper und Kreide-Vulkanismus auf Transform-Störungen?

Zusammenfassung Während des Keupers und des frühen Jura wurden in Nordspanien petrographisch und chemisch recht einheitliche Vulkanite, die sog. Keuper-Ophite, gefördert. Ihre Haupt- und Spurenelement-Chemie weist sie als Tholeiite aus. Diskriminierungs-Diagramme ordnen sie Ozeanboden-Basalten zu, ihre Gehalte an Leichten Seltenen Erden sind sehr gering. Dieser Befund, der zunächst den geologischen Verhältnissen zu widersprechen schien, ließe sich interpretieren als der fehlgeschlagene Versuch einer Ozean-Bildung. Während des Keupers entwickelte sich in Nordspanien ein Rift-Graben. Eventuell stand dieser Rift-Graben von einem Tripelpunkt ausgehend mit dem später einsetzenden Spreading im westlichen Mittelmeer in Verbindung.Während der Kreide (unteres Cenoman bis Coniac) stiegen Alkali-Basalte auf, die in Gängen erstarrten oder als Pillow-Laven ausflössen. In Diskriminations-Diagrammen fallen sie in die Felder der Intraplatten-Basalte. Der Vulkanismus ist eng verknüft mit der Öffnung der Biskaya. In der östlichen Biskaya kam es zu einer Aufspaltung der Spreading-Achse. Ein nördliches Becken (in der heutigen Biskaya gelegen) und ein südliches Becken (der Basko-Kantabrische Trog) wurden von einer Mikroplatte, dem sog. Biskaya-Hoch, getrennt. Zwischen den beiden Divergenz-Zonen vermittelten vermutlich ozeanische Bruchzonen bzw. undichte Transform-Störungen, die stumpfwinklig zu den Beckenachsen verliefen, und auf denen die Kreide-Vulkanite gefördert wurden.

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During Late Triassic and Early Jurassic the so-called Keuper-Ophite volcanics were extruded in North Spain. These volcanics are petrographically and chemically rather uniform with a main and trace element composition which is typical for tholeiites. Their position in discriminationdiagrams shows that they are related to oceanfloor-basalts. The tholeiites show very low contents of light rare earths. This result, which contradicts geological evidence, could be interpreted as an aborted attempt of ocean-forming. A rift-graben developed in North Spain during Late Triassic. Possibly this rift-graben extended into a triple junction, which was connected with the spreading-zone in the western Mediterranean that became active at a later stage.During the Late Cretaceous (lower Cenomanian to Coniacian) alkali basalts formed dykes or extruded forming pillow-lavas. In discrimination-diagrams these basalts plot in the fields of intra-plate basalts. The volcanism is closely related to the opening of the Bay of Biscay. In the eastern Bay the spreading-axes split. A northern basin (situated in the present Bay of Biscay) and a southern basin (the Basco-Cantabrian Trough) were separated by a microplate, the so-called Biscay-High. Both zones of divergence were probably connected by obtusely striking oceanic fracturezones or leaky transform faults along which the Cretaceous volcanics were extruded.

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Résumé Dans le nord de l'Espagne, des roches volcaniques chimiquement et pétrographiquement uniformes ont été extrudées au cours du Keuper et de l'Eo-jurassique; elles sont connues sous le nom de »Keuper-ophites«. Le chimissme de leurs éléments majeurs et en traces les rattachent aux tholéiites. Leurposition dans les diagrammes de discrimination les rattache aux basaltes de la crocute océanique. Leur teneur en terres rares est très faible. Ces données, qui semblement en contradiction avec la situation géologique, peuvent s'interpréter comme témoignant d'une ouverture océanique avortée. Un rift s'est développé dans le nord de l'Espagne au Triassique supérieur. Il est possible que ce rift, formant une des branches d'une tripöe jonction, ait été en relation avec l'expansion qui a eu lieu plus tard en Méditérrannée occidentale.Au cours du Crétacé (du Cénomannien inférieur au Coniacien), des basaltes alcalins se sont mis en place sous forme de filons et de laves en coussins. Sur les diagrammes de discrimination, ils se situent dans les champs des basaltes intra-plaques. Le volcanisme est étroitement asocié à l'ouverture du golfe de Gascogne. Dans la partie E du golfe, l'axe d'ouverture s'est dichotomisé, donnant naissance à deux bassin méridional (la fosse basquo-cantabrique), séparés par une micro-plaque. Ces deux branches étaient probalement reiées par des zones de fractures océaniques ou par des failles transformates, qui faisaient un angle obtus avec les axes des bassins et le long desquelles ont pu monter les basalter crétacés.

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A versatile flow channel for laboratory experiments with running water macroinvertebrates

Experiments with running water organisms should be conducted under running water conditions. A flow channel which has proved useful for long-term rearing of running water macroinvertebrates is described. Through its arena-shaped design and water jet drive, damage to drifting animals is practically excluded.