Publications

Crack detection is an important property in carbonate or unconventional reservoirs for permeability and hydrocarbon reserve estimation. Shear wave splitting can be used in crack detection. The method is based on the difference in the arrival time between the vertical shear wave and the horizontal shear wave. A shear wave propagating through the fractured rock will be split into two waves with orthogonal polarizations. The wave with the higher velocity is polarized parallel to the fracture plane and the wave with the lower velocity is polarized perpendicular to the fracture plane. The ratio between the slow shear wave velocity (Vs_slow) and the fast shear wave velocity (Vs_fast) is called the splitting parameter. The splitting parameter is therefore proportional to the crack density, hence can be used as a crack indicator. We used the splitting parameter for crack estimation in a carbonate reservoir. Using a sonic log as input, Gassmann (1951) fluid substitution equation, fluid replacement modeling, and Thomsen (1986) anisotropy parameters were used for shear wave velocity prediction. Vs_fast and Vs_slow were estimated from the stiffness tensor matrix for a Horizontal Transverse Isotropy (HTI) medium using a model of a single set of aligned fractures. The splitting parameter was estimated from the ratio between Vs_fast and Vs_slow. The average crack density value was 0.0675, with an average splitting parameter of 8.46%. The high presence of crack density resides at a depth between 7900–8855 feet with a maximum value of 0.338. Keywords: Shear wave splitting, carbonate reservoir, crack density.