An eigenfunction representation of deep waveguides with application to unconventional reservoirs

Guided waves that propagate in deep low-velocity zones can be described using the displacement-stress eigenfunction theory. For a layered subsurface, these eigenfunctions provide a framework to calculate guided-wave properties at a fraction of the time required for fully numerical approaches for wave-equation modeling, such as the finite-difference approach. Using a 1D velocity model representing the low-velocity Eagle Ford Shale, an unconventional hydrocarbon reservoir, we verify the accuracy of the displacement eigenfunctions by comparing with finite-difference modeling. We use the amplitude portion of the Green’s function for source-receiver eigenfunction pairs as a proxy for expected guided-wave amplitude. These response functions are used to investigate the impact of the velocity contrast, reservoir thickness, and receiver depth on guided-wave amplitudes for discrete frequencies. We find that receivers located within the low-velocity zone record larger guided-wave amplitudes. This property may be used to infer the location of the recording array in relation to the low-velocity reservoir. We also study guided-wave energy distribution between the different layers of the Eagle Ford model and find that most of the high-frequency energy is confined to the low-velocity reservoir. We corroborate this measurement with field microseismic data recorded by distributed acoustic sensing fiber installed outside of the Eagle Ford. The data contain high-frequency body-wave energy, but the guided waves are confined to low frequencies because the recording array is outside the waveguide. We also study the energy distribution between the fundamental and first guided-wave modes as a function of the frequency and source depth and find a nodal point in the first mode for source depths originating in the middle of the low-velocity zone, which we validate with the same field data. The varying modal energy distribution can provide useful constraints for microseismic event depth estimation.

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Validating the origin of microseismic events in target reservoir using guided waves recorded by DAS

The Leading Edge ◽

10.1190/tle39110776.1 ◽

2020 ◽

Vol 39 (11) ◽

pp. 776-784

Author(s):

Owen Huff ◽

Ariel Lellouch ◽

Bin Luo ◽

Ge Jin ◽

Biondo Biondi

Keyword(s):

Wave Energy ◽

Guided Waves ◽

Velocity Structure ◽

Guided Wave ◽

Accurate Estimation ◽

Low Velocity ◽

Eagle Ford ◽

Low Velocity Layer ◽

Microseismic Events ◽

Velocity Layer

We develop a new algorithm that uses guided-wave energy in distributed acoustic sensing (DAS) records to identify microseismic events originating within or very close to a shale reservoir. Guided waves are dispersive waves that propagate in a low-velocity layer bounded by two high-velocity layers. This is a geologic structure that is seen for some shale reservoirs, most notably the Eagle Ford. Only microseismic events originating within or close to the low-velocity layer will excite significant guided-wave energy, which can be observed in DAS records. We confirm the relationship between guided-wave energy and event depth relative to the reservoir by using synthetic modeling. Given the known velocity structure, we can predict the dispersion curves for guided waves and use them to separate body and guided waves. We demonstrate a method to quantify the amplitude of guided waves in field DAS data recorded directly above the Eagle Ford Shale. Using this technique, we can separate events that originate within or close to the Eagle Ford from events that do not, thus circumventing the large depth uncertainty in a microseismic catalog derived from surface geophones. Our analysis shows that events classified as originating within or close to the Eagle Ford are horizontally closer to the stimulating well than non-Eagle Ford events. This is interpreted as representing different hydraulic fracture geometries in the Eagle Ford compared to its bounding formations, the Buda Limestone and Austin Chalk. The application of our method yields a new catalog that highlights the events relevant to stimulation and production in the target reservoir. It also provides a strong depth constraint that can improve relocation attempts using surface data, enabling a more accurate estimation of stimulated rock volume geometry.

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Fault Zone Guided Wave generation on the locked, late interseismic Alpine Fault, New Zealand

10.26686/wgtn.13818791.v1 ◽

2021 ◽

Author(s):

JD Eccles ◽

AK Gulley ◽

PE Malin ◽

CM Boese ◽

John Townend ◽

...

Keyword(s):

Fault Zone ◽

Plate Boundary ◽

Guided Wave ◽

S Wave ◽

Low Velocity ◽

Catchment Scale ◽

Near Surface ◽

Low Velocity Zone ◽

Alpine Fault ◽

Velocity Zone

© 2015. American Geophysical Union. All Rights Reserved. Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7-35Hz) have been recorded on shallow borehole seismometers installed within 20m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW-generating microseismic events are located beyond the catchment-scale partitioning of the fault indicating lateral connectivity of the low-velocity zone immediately below the near-surface segmentation. Initial modeling of the low-velocity zone indicates a waveguide width of 60-200m with a 10-40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.

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Topology of ray surfaces in low-velocity zones

Bulletin of the Seismological Society of America ◽

10.1785/bssa0690020369 ◽

1979 ◽

Vol 69 (2) ◽

pp. 369-378

Author(s):

George A. McMechan

Keyword(s):

Guided Waves ◽

Turning Points ◽

Three Dimensional ◽

Focal Depth ◽

Low Velocity ◽

Low Velocity Zone ◽

Ray Trajectories ◽

Definition Of ◽

Single Set ◽

Velocity Zone

abstract Plotting of three-dimensional ray surfaces in p-Δ-z space provides a means of determining p-Δ curves for any focal depth. A region of increasing velocity with depth is represented in p-Δ-z space by a trough, and a region of decreasing velocity, by a crest. Two sets of ray trajectories, the arrivals refracted outside a low-velocity zone, and the guided waves inside the zone, can be merged into a single set along the ray that splits into two at the top of the low-velocity zone. This ray is common to both sets. This construction provides continuity of the locus of ray turning points through the low-velocity zone and thus allows definition of p-Δ curves inside as well as outside the low-velocity zone.

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Discontinuity of the Mozumi–Sukenobu fault low-velocity zone, central Japan, inferred from 3-D finite-difference simulation of fault zone waves excited by explosive sources

Tectonophysics ◽

10.1016/j.tecto.2003.09.008 ◽

2004 ◽

Vol 378 (3-4) ◽

pp. 209-222 ◽

Cited By ~ 10

Author(s):

Yutaka Mamada ◽

Yasuto Kuwahara ◽

Hisao Ito ◽

Hiroshi Takenaka

Keyword(s):

Finite Difference ◽

Fault Zone ◽

Low Velocity ◽

Central Japan ◽

Low Velocity Zone ◽

Velocity Zone ◽

Fault Zone Waves

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Seismic characteristics of a Precambrian pluton and its adjacent rocks

Geophysics ◽

10.1190/1.1441487 ◽

1983 ◽

Vol 48 (5) ◽

pp. 569-581 ◽

Cited By ~ 13

Author(s):

Z. Hajnal ◽

M. R. Stauffer ◽

M. S. King ◽

P. F. Wallis ◽

H. F. Wang ◽

...

Keyword(s):

High Frequency ◽

Low Frequency ◽

Seismic Energy ◽

Seismic Profile ◽

Open Fractures ◽

Low Velocity ◽

Near Surface ◽

Low Velocity Zone ◽

Plutonic Rocks ◽

Velocity Zone

Surface, borehole, and laboratory acoustic measurements all confirm the existence of a near‐surface low‐velocity zone in metavolcanic, metasedimentary, and plutonic rocks of the Flin Flon region of Canada. This zone is caused by a high frequency of open fractures and extends from the surface to depths of between 5 and 44 m, although occasional open fractures extend to at least 60 m. There is a linear decrease in sonic velocity with increasing frequency of large fractures; the details, however, vary for different sites, depending upon several geologic features including rock type and nonfracture porosity. Laboratory sonic data indicate very low microcrack densities in the volcanic and plutonic rocks. Synthetic seismograms derived from sonic log information from the center of the granitic pluton have been compared with a nearby multifold seismic profile. This shows that the near‐surface low‐velocity zone attenuates most of the high‐frequency seismic energy. However, the remaining low‐frequency portion of the seismic spectrum can be used to map some features of the pluton.

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