Seismic wave propagation in fractured anisotropic carbonate rock: Experiments and theory

Carbonate rocks play a major economic role in the oil and gas industry because over 50% of the world\u27s oil reserves are contained in carbonate reservoirs and the potential for additional gas reserve in these reservoirs is enormous. However, understanding and quantifying the physical properties and processes that occur in carbonate reservoir is complicated because heterogeneity in physical properties occurs on all length scales in carbonate reservoirs, i.e., from the micro-scale to the reservoir scale. Thin section analysis showed that Austin Chalk contained a weakly directed fabric and was composed mainly (99%) calcite. X-ray tomographic imaging of several samples showed that the samples contained layers, layer thickness varied among the samples and within a sample, and also showed that the density varied along and among the layers in a sample (1700 kg/m3 to 2300 kg/m3). Thin sections showed that the sources of the density variation within the sample arise from the amount of cement in each layer, the number of pellets and the porosity. These small-scale features played a significant role in the observed seismic anisotropy of Austin Chalk. An important finding from seismic imaging was that the axis of seismic symmetry of this finely layered sample was not perpendicular to the layers by was oriented roughly 15 degrees from the normal to the layering. Seismic monitoring of the fluid-front during saturation of Austin Chalk indicates that the fine bedding also affected the hydraulic properties of the sample, i.e. a preferential path for fluid flow. When water was invaded into the samples, the seismic data indicated that fluid would preferentially flow either up-dip or down-dip (parallel to the layering) rather then across the layers. When a sample contained a fracture, there were preferential flow paths along the fracture plane as well in the matrix of the rock. In addition, the fracture plane was geochemically altered during fluid invasions resulting in a reduction in average fracture aperture from, 1.2 mm to 0.8 mm. An important finding from this research is the complications that arise in interpreting fracture specific stiffness when a fracture occurs in a layered medium. The displacement discontinuity theory describes a fracture as a low-pass filter, i.e. a fracture preferentially attenuates the high frequency components of a signal relative to the intact portions of the rock. However, from the experiments, a fracture in a layered medium (i.e. Austin Chalk samples) caused either an increase or decrease in the dominant frequency of the signals relative to that for the intact sample. From simulations, the presence of a fracture introduces additional multiples that ultimately affect spectral content of the signal and affect the accurate interpretation of fracture specific stiffness. The anisotropy of Austin Chalk and the effect of fluid substitution on seismic anisotropy in a transversely anisotropic medium were studied. Calculations showed that the Thomsen parameters for Austin Chalk are less than 1 indicating that this type of carbonate rock is weakly anisotropic. Both low frequency limit (Backus Averaging & Gassmann\u27s Equation) and high frequency limit (Wylie Time Averaging) approaches were taken for studying the observed effects of fluid substitution. The deviations between the experimental data and the two theoretical approaches arise because of the complexity of carbonate rock, i.e., the layering, density variation, pore structure, etc. The wavelength of both shear waves was roughly 2 mm, which is close to the thickness of the layers in the sample (0.5 mm – 1mm). This places these carbonate rock systems neither in the low or high frequency limit for a frequency of 1 MHz (λ/d ∼ 4 – 2) but in the transition zone between ray theory and effective medium theory. (Abstract shortened by UMI.