Experimental study on the effects of fractures on elastic wave propagation in synthetic layered rocks

Abstract

Fractures greatly increase the difficulty of oil and gas exploration and development in reservoirs consisting of interlayered carbonates and shales and increase the uncertainty of highly efficient development. The presence of fractures or layered media is also widely known to affect the elastic properties of rocks. The combined effects of fractures and layered media are still unknown. We have investigated the effects of fracture structure on wave propagation in interlayered carbonate and shale rocks using physical models based on wave theory and the similarity principle. We have designed and built two sets of layered physical models with randomly embedded predesigned vertically aligned fractures according to the control variate principle. We have measured the P- and S-wave velocities and attenuation and analyzed the effects of fracture porosity and aspect ratio (AR) on velocity, attenuation, and power spectral dimension of the P- and S-waves. The experimental results indicated that under conditions of low porosity ([Formula: see text]), Han’s empirical velocity-porosity relations and Wang’s attenuation-porosity relation combined with Wyllie’s time-average model are a good prediction for layered physical models with randomly embedded fractures. When the porosity is constant, the effect of different ARs on elastic wave properties can be described by a power law function. We have calculated the power spectrum fractal dimension [Formula: see text] of the transmitted signal in the frequency domain, which can supplement the S-wave splitting method for estimating the degree of anisotropy. The simple power law relation between the power spectrum fractal dimension of the P-waveform and fracture density suggests the possible use of P-waves for discriminating fracture density. The high precision and low error of this processing method give new ideas for rock anisotropy evaluation and fracture density prediction when only P-wave data are available.

Spatiotemporal Variations in Crustal Seismic Anisotropy Surrounding Induced Earthquakes Near Fox Creek, Alberta

“…The approximately N‐S‐aligned normal faults exhibit Gaussian distributions with wide strike orientations ranging from N50°W to N40°E (Figure a). Second, stress‐aligned fluid‐filled fractures in the shale formation (which are often associated with HF stimulation) can produce a fast splitting direction parallel to fracture orientation (Anderson et al, ; Li et al, ; Nolte et al, ). These two types of anisotropy may be distinguishable from their depths: The latter scenario with fractures aligned approximately with the NE‐SW maximum horizontal stress (Figure a) may result in anisotropy in the vicinity of the HF (typically 3‐ to 4‐km depth for Duvernay around Fox Creek), whereas fault‐induced anisotropy could potentially extend from the basement (considering the fault is deep‐seated) to the subsurface and affect the entire ray path.…”

Micromechanical modeling of ultrasonic velocity for pore-structure and porosity characterization considering anisotropy in carbonate samples

“…These were analyzed using a two-layer physical model with a vertical fracture system. The results indicated that the wave velocities usually increase when the fracture aspect ratio is also increased (Li et al, 2016).…”

Using a Synthetic Borehole Model to Determine the Pore Aspect Ratio Dependence of Velocities from Acoustic Well-Logging Data

“…Currently, techniques such as acoustic emission experiments, acoustic logging, and seismic monitoring, which are based on elastic wave theory, have become important methods for detecting underground structures, oil and gas minerals, and geothermal resources. However, the changes in pore structure can significantly impact the elastic frame moduli of naturally occurring rocks, resulting in changes in their elastic wave velocities [4][5][6][7][8][9].…”

“…The pore aspect ratio (AR; the ratio of the minor axis to the major axis) dependence of velocities of crustal rocks, affecting the energy exchange between the formation and pore fluids, has been widely reported [5,[10][11][12][13]. The power relationship between elastic wave velocities and pore AR has been proposed in the theoretical model (e.g., [14,15]) and rock physics experiments at the core scale (e.g., [4,8]). However, the frequency dependence of velocities can lead to errors and difficulties in the dynamic elastic parameters of rocks when attempting to apply the laboratory results in the ultrasonic frequency band (0.1 MHz-10 MHz) to the acoustic well-logging interpretation in the mediumfrequency range (20 Hz-200 kHz) [16][17][18][19] and seismic exploration (~100 Hz) [20].…”