363 research outputs found
Development of anisotropic contiguity in deforming partially molten aggregates:2. Implications for the lithosphereâasthenosphere boundary
Multifluid squirt flow and hysteresis effects on the bulk modulus - water saturation relationship
Sensitivity of time lapse seismic data to the compliance of hydraulic fractures
We study the sensitivity of seismic waves to changes in the fracture normal and tangential compliances by analyzing the fracture sensitivity wave equation, which is derived by differentiating the elastic wave equation with respect to the fracture compliance. The sources for the sensitivity wavefield are the sensitivity moments, which are functions of fracture compliance, background elastic properties and the stress acting on the fracture surface. Based on the analysis of the fracture sensitivity wave equation, we give the condition for the weak scattering approximation to be valid for fracture scattering. Under the weak scattering approximation, we find that the percentage change of fracture compliance in hydraulic fracturing is equal to the percentage change of the recorded time-lapse seismic data. This could provide a means for monitoring the opening/closing of fractures in hydraulic fracturing through time-lapse seismic surveys.Eni-MIT Energy Initiative Founding Member Progra
Detectionof gas hydrates infaults using azimuthal seismic velocity analysis,Vestnesa Ridge, W-Svalbard Margin
Accepted for publication in Journal of Geophysical Research. Solid Earth. Copyright 2020 American Geophysical Union. Further reproduction or electronic distribution is not permitted.Joint analysis of electrical resistivity and seismic velocity data is primarily used to detect the presence of gas hydrateâfilled faults and fractures. In this study, we present a novel approach to infer the occurrence of structurallyâcontrolled gas hydrate accumulations using azimuthal seismic velocity analysis. We perform this analysis using oceanâbottom seismic (OBS) data at two sites on Vestnesa Ridge, WâSvalbard Margin. Previous geophysical studies inferred the presence of gas hydrates at shallow depths (up to ~190â195 m below the seafloor) in marine sediments of Vestnesa Ridge. We analyze azimuthal Pâwave seismic velocities in relation with steeplyâdipping near surface faults to study structural controls on gas hydrate distribution. This unique analysis documents directional changes in seismic velocities along and across faults. Pâwave velocities are elevated and reduced by ~0.06â0.08 km/s in azimuths where the raypath plane lies along the fault plane in the gas hydrate stability zone (GHSZ) and below the base of the GHSZ, respectively. The resulting velocities can be explained with the presence of gas hydrateâ and free gasâfilled faults above and below the base of the GHSZ, respectively. Moreover, the occurrence of elevated and reduced (>0.05 km/s) seismic velocities in groups of azimuths bounded by faults, suggests compartmentalization of gas hydrates and free gas by fault planes. Results from gas hydrate saturation modelling suggest that these observed changes in seismic velocities with azimuth can be due to gas hydrate saturated faults of thickness greater than 20 cm and considerably smaller than 300 cm
Volumetric bounds on subsurface fluid substitution using 4D seismic time shifts with an application at Sleipner, North Sea
Quasi-periodic Fibonacci and periodic one-dimensional hypersonic phononic crystals of porous silicon:Experiment and simulation
Fluid Flow and Defect Formation in the Three-Dimensional Dendritic Structure of Nickel-Based Single Crystals
Seismic attenuation in fractured porous media: insights from a hybrid numerical and analytical model
Seismic attenuation in fluid-saturated porous rocks can occur by geometric spreading, wave scattering or the internal dissipation of energy, most likely due to the squirt-flow mechanism. In principle, the pattern of seismic attenuation recorded on an array of sensors contains information about the medium, in terms of material heterogeneity and anisotropy, as well as material properties such as porosity, crack density, and pore-fluid composition and mobility. In practice, this inverse problem is challenging. Here we provide some insights into the effects of internal dissipation by analysing synthetic data produced by a hybrid numerical and analytical model for seismic wave propagation in a fractured medium embedded within a layered geological structure. The model is made up of one anisotropic and three isotropic horizontal layers. The anisotropic layer consists of a porous, fluid-saturated material containing vertically aligned inclusions representing a set of fractures. This combination allows squirt-flow to occur between the pores in the matrix and the model fractures. Our results show that the fluid mobility and the associated relaxation time of the fluid-pressure gradient control the frequency range over which attenuation occurs. This induced attenuation increases with incidence angle and azimuth away from the fracture strike-direction. Azimuthal variations in the induced attenuation are elliptical allowing the fracture orientations to be obtained from the axes of the ellipse. These observations hold out the potential of using seismic attenuation as an additional diagnostic in the characterisation of rock formations for a variety of applications including hydrocarbon exploration and production, subsurface storage of CO2, and geothermal energy extraction
Sensing, measuring and modelling the mechanical properties of sandstone
We present a hybrid framework for simulating the strength and dilation characteristics of sandstone. Where possible, the grain-scale properties of sandstone are evaluated experimentally in detail. Also, using photo-stress analysis, we sense the deviator stress (/strain) distribution at the microscale and its components along the orthogonal directions on the surface of a V-notch sandstone sample under mechanical loading. Based on this measurement and applying a grain-scale model, the optical anisotropy index K0 is inferred at the grain scale. This correlated well with the grain contact stiffness ratio K evaluated using ultrasound sensors independently. Thereafter, in addition to other experimentally characterised structural and grain-scale properties of sandstone, K is fed as an input into the discrete element modelling of fracture strength and dilation of the sandstone samples. Physical bulk scale experiments are also conducted to evaluate the load-displacement relation, dilation and bulk fracture strength characteristics of sandstone samples under compression and shear. A good level of agreement is obtained between the results of the simulations and experiments. The current generic framework could be applied to understand the internal and bulk mechanical properties of such complex opaque and heterogeneous materials more realistically in future
- âŠ