Fluid filled cracks and seismic anisotropy: An experimental study

Abstract

Summary Seismic characterization of subsurface fractures has important applications in reservoir assessment. Recent studies have also shown that azimuthal anisotropy can not only help delineate orientation and intensity of fractures, but also indicate the fluid infill of these cracks. To explore the influence of different fluids on seismic response in a transversely isotropic background medium, a physical modeling study using vertically aligned and grooved plexiglass material embedded in isotropic resin was conducted. The theoretical motivation for this experimental study is the Hudson and Thomsen theories for pennyshaped cracks. The composite model was designed to simulate a single set of aligned vertical cracks (HTI symmetry) filled with fluids. In a series of ultrasonic and scaled seismic experiments, the model is initially filled with air (gas-saturated) and fluids are gradually injected into the composite material to simulate different rates of fluid saturation. Porosity in our assembled cracked media is 2.5% and crack density is estimated as 14%. Our results show changes in ultrasonic compressional and shear wave velocities ranging from 1% to 15% in different directions as a function of brine saturation. Most significant velocity change is observed in vertically propagating P-waves. We also detect changes in delay between fast and slow split shear waves with saturation. NMO velocity measured from scaled ultrasonic response shows not only changes in velocity values on saturation, but also a difference in the trend of NMO velocities as a function of source-receiver azimuth. Computed stiffness coefficient changes ranged from 3% to 30%. Anisotropic parameters also varied with different levels of saturation. Our experiments show that