Reservoir Geophysics

Seismic imaging in the broad sense is to derive any information for the Earth's interior using seismic waves. However, in the exploration seismology, we use 'seismic imaging' to specificallymean finding structural boundaries of the layers, hence the name: structrual seismic imaging. What's inside the layer is also critical for us to understand the subsurface system, the reservoir fabric, and the geomechanical stress. The "Reservoir Geophysics" in my group centers on creating novel algorithms to characterize the subsurface reservoirs. We also created novel ways to model and image the subsurface challenges.

Seismic inversion of subsurface fracture networks

Why we study fractures? Fractures are fundamental objects in Earth sciences. It would be difficult to imagine what Earth sciences would look like if there were no fractures. The spatial scale of the fracture can be either very large or very small:

• Plate tectonics are about motion of fractured plates.

• Earthquakes are due to relative motion on a fault which is a fracture zone.

• Volcanos are fluid motion through fractures.

• unconventional oil/gas development depends on knowledge of the subsurface fractures

  • geothermal energy untilization

  • CO2 sequesteration

  • storage of nuclear wastes

  • ... etc.

all depend on fractures. Our group have created several novel methods to "find" and "characterize" the subsurface fracture networks and fracture clusters with verification. The specific topics our reserach has been centered on are:

• • algorithm to detect and characterize fractures

  • using surface seismic data to deliver spatially dependent fracture parameters: orientation, density, and compliance

  • mechanical properties of the fracture (compliance) and fluid flow

  • interaction of fractures with seismic waves

  • multiscale fractures

Our work has gone beyond just modeling and theoretical research. We constantly use real data and drilling operations to check our results.

Double-beam fracture characterization

Fracture clustering effect on effective seismic anisotropy

Motivation. In seismology, fractured reservoirs are often treated as anisotropic media where people use either shear-wave splitting or reflected wave amplitude-variation-with-azimuth (AVAz) to find the anisotropy parameters then to infer fracture properties. However, this equivalent anisotropy is based on an implicit "single-scale" assumption about the fractures which says that fractures are periodic in space. Nature does not creat periodic fractures and many outcrop studies showed power-law distribution of fractures. Our research is to ask a basic question: what's the effect on the equivalent anisotropy due to 'random" fractures? Can we still use the traditional AVAz to infer fracture properties? To learn more, please check out our recent paper: (Fang, X., Y. Zheng, and M.C. Fehler (2017). ”Fracture clustering effect on amplitude variation with offset and azimuth analyses.” Geophysics, 82(1), N13-N25. )

Seismic wave propagation and scattering in karstic media and karstic topography

We developed hypersingular boundary element method to model elastic seismic wave scattering/propagation in media containing 'holes' or 'voids'.

[ref Yingcai Zheng, Adel H. Malallah, Michael C. Fehler, and Hao Hu (2016). ”2D full-waveform modeling of seismic waves in layered karstic media.” GEOPHYSICS, 81(2), T25-T34.

doi: 10.1190/geo2015-0307.1]. See the following numerical examples. This method has been benchmarked with the SEM. The method may be useful in learning how to image karsts, sinkholes, and clandestine tunnels using seismic/acoustic waves.

Modeling Scholte wave propagation along the rugged seafloor

Like the 'ground rolls', there are also 'ground-roll'-like waves along the seafloor, called Scholte waves. The Scholte wave dispersion is very useful in investigating shallow S-wave velocity near the seafloor if we have ocean-bottom cable data. To model these waves, finite-difference methods (i.e., staggered grids) may generate numerical artefacts. Care must be taken to ensure proper fluid-solid boundary conditions. Here, we use boundary element to do the numerical modeling. See the following examples: