According to
Wikipedia: “Shear wave splitting, also called seismic birefringence, is the
phenomenon that occurs when a polarized shear wave enters an anisotropic medium.
The incident shear wave splits into two polarized shear waves. Shear wave
splitting is typically used as a tool for testing the anisotropy of an area of
interest. These measurements reflect the degree of anisotropy and lead to a
better understanding of the area's crack density and orientation or crystal
alignment.” An anisotropic medium has properties that differ according to
direction, unlike an isotropic medium, which has uniform properties. Anisotropic
mediums may have different elastic, electrical, or hydraulic properties along
different axes. When a shear wave splits as it enters an anisotropic medium “it
splits into two shear waves. One of these shear waves will be faster than the
other and oriented parallel to the cracks or crystals in the medium. The second
wave will be slower than the first and sometimes orthogonal to both the first
shear wave and the cracks or crystals in the media. The time delays observed
between the slow and fast shear waves give information about the density of
cracks in the medium. The orientation of the fast shear wave records the
direction of the cracks in the medium.” Thus, split shear waves can yield
information about density and orientation of cracks or fractures in rock. Shear
wave splitting has been used to identify anisotropy in the upper mantle and to
predict earthquakes. They do this by detecting changes in the microcracks due
to changing stresses as the earth moves. Rocks in earthquake zones exhibit dilatancy,
volume changes in granular materials in response to shear deformation. Changes
in shear wave splitting measurements over time can be used to predict
earthquakes. In a similar manner the technique can be used to help predict
volcanic eruptions.
Schematic diagram of
two orthogonal polarized shear waves traveling through an anisotropic medium.
Source: Wikipedia.
Shear wave
splitting has also long been used to map fracture networks in hydrocarbon
reservoirs. Shear wave splitting measurements can give the degree of anisotropy
of a reservoir and generally the higher degree of anisotropy the larger the number
of open fractures and thus the highest fracture porosity. Studies have shown
that fluid injection can also affect split shear wave measurements, likely due
to changing the stress conditions in the rock. Logs depicting rock anisotropy
are used in characterizing unconventional reservoirs such as shale since
fracture characterization, orientation, and density can be key factors in reservoir
productivity. Rock anisotropy can also be used to analyze stresses on induced
fractures after wells have been hydraulically fractured. Rock anisotropy, a
proxy for fracture density and orientation, is why induced fractures have
favorable propagation directions. Induced fractures will favor a propagation direction
with lower fracture toughness.
Researchers in
China in 2022 proposed new methods of processing algorithms that are more
computationally efficient. The usual processing algorithm, grid search, they
say, involves many unnecessary computations in shear wave splitting (SWS)
analysis. The study considered gradient descent, Newton, and advanced-retreat
computational algorithms and found that the gradient descent method most
closely matched fracture porosity estimated by resistivity logs in a well and
also decreased the computational time by magnitudes over the grid search
method. Thus, they concluded that “SWS analysis combined with the
gradient-descent method can accurately and efficiently obtain SWS parameters
for fracture prediction.”
Source: Shear-Wave
Splitting Analysis Using Optimization Algorithms. Zhengtao He; Yuyong Yang;
Huailai Zhou. Lithosphere (2022) 2022 (1): 3318196. December 31, 2022.
Source: Shear-Wave
Splitting Analysis Using Optimization Algorithms. Zhengtao He; Yuyong Yang;
Huailai Zhou. Lithosphere (2022) 2022 (1): 3318196. December 31, 2022.
Carbonates,
which include limestones and dolostones, are the most prevalent reservoir rock
for oil and gas. Mineral-rich brines also occur in carbonates. These fluids are
contained in the open spaces in the rocks, which for carbonates, are primarily fabric
selective porosity but secondary porosity features like fractures and dissolution
features like vugs are also very important for important for many carbonate
reservoirs. Carbonates are also being used and hold future promise for storing
CO2 via sequestration wells. Better fracture characterization in carbonates
would help exploration efforts.
Researchers from Khalifa University in the UAE have developed a new seismic sensing technique based on shear wave splitting that can give more information about carbonate fracturing. The new method is cost-effective and gives quantitative objective results. One of the authors, Mohammed Ali, a professor of Earth Science at Khalifa University, notes that the next step is employing machine learning: “Recent successes in identifying key features with high sensitivity to shear-wave splitting have paved the way for employing machine learning algorithms. These algorithms will be instrumental in automating the analysis process and improving the accuracy of fracture characterization.” The goal of this new method of multicomponent shear wave velocity analysis is to obtain accurate fracture properties at seismic scale when integrated with data from well logs. They say the method is less sensitive to overburden anisotropy and random signal noise. They are also tweaking their source and receiver configurations for future improvements. They note that their conclusions are quite applicable to fractured reservoirs,“especially those located in Abu Dhabi, which are characterized by high heterogeneity and complex fracture network related to complex tectonic history.”
Map of Abu Dhabi Oil
Fields Showing Variance in Structural and Fracture Orientations Due to Complex
Tectonic History. Source: Investigation of fractured carbonate reservoirs by
applying shear-wave splitting concept. Alejandro Diaz-Acosta, Fateh Bouchaala,
Tadahiro Kishida, Mohamed S. Jouini, Mohammed Y. Ali. Advances in Geo-Energy
Research. Vol. 7, No. 2, p. 99-110, 2023.
References:
Shear-wave
splitting reveals oil reservoir secrets. Khalifa University Explorer. October 30,
2023. Shear-wave
splitting reveals oil reservoir secrets | KU Explorer
Investigation
of fractured carbonate reservoirs by applying shear-wave splitting concept. Alejandro
Diaz-Acosta, Fateh Bouchaala, Tadahiro Kishida, Mohamed S. Jouini, Mohammed Y.
Ali. Advances in Geo-Energy Research. Vol. 7, No. 2, p. 99-110, 2023. 642-2372-2-PB.pdf
Shear wave
splitting. Wikipedia. Shear wave splitting
- Wikipedia
Shear-Wave
Splitting Analysis Using Optimization Algorithms. Zhengtao He; Yuyong Yang;
Huailai Zhou. Lithosphere (2022) 2022 (1): 3318196. December 31, 2022. Shear-Wave
Splitting Analysis Using Optimization Algorithms | Lithosphere |
GeoScienceWorld
Fracture
toughness anisotropy in shale. Michael Chandler, Philip Meredith, N. Brantut,
B. Crawford. School of Geosciences, University of Edinburgh. Journal of
Geophysical Research: Solid Earth. March 31, 2016. Abstract. Fracture
toughness anisotropy in shale — University of Edinburgh Research Explorer
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