14. Case Studies of Multicomponent Seismic Data for Fracture Characterization: Austin Chalk Examples

Shear wave studies of multicomponent seismic data were done along the Austin Chalk trend in Texas. Six surface seismic lines of four-component shear wave data from Pearsall and Giddings fields and three zero-offset vertical seismic profiles (VSPs) from three sites with different production rates were studied to demonstrate the applications of shear wave splitting for fracture reservoir delineation. The three seismic lines (l-3) from Pearsall field formed a classic experiment for studying shear wave splitting. They have different (line) azimuth with respect to the regional fracture strike (parallel, perpendicular, and at -4O”, respectively), are in areas with different hydrocarbon production, and have different split shear wave behavior. The anisotropy along line 1 is small, which correlates with the absence of nearby commercial production. There is an increasing trend in anisotropy from line 2 to 3, which correlates with line 2 being close to and line 3 being within the producing Austin Chalk acreage. The trend of anisotropy variation along line 3 also correlates with the distribution of producing oil wells along that line. In particular, production from the three horizontal wells (Wl-W3) drilled nearby correlates with the variation in shear wave polarization, time delay, and amplitude. Line 4 from Giddings field had a horizontal well drilled on it, and mud logs were obtained for identifying fracture zones. The fracture swarms identified by the mud logs are coincident with the dim spots identified from the section of the slow split shear wave. Lines 5 and 6, also from Giddings field, demonstrate classic S, dim spot and S, versus S2 mistie behavior, respectively. The three VSPs, from three wells with different production rates, show different shear wave responses. VSP 1, from a nonproductive well, shows minimal shear wave time delay and no amplitude anomalies. VSP 2, from a water-producing well, shows some amount of splitting but no anomalies in shear wave attributes. VSP 3, from an oil-producing well, shows clear shear wave splitting and anomalies in shear wave amplitudes. Amplitude corrections are necessary for preserving and recovering shear wave anisotropy information associated with the target, and the linear transform technique (LTT) simplifies the processing sequence for examining anisotropy in shear wave reflection data. Stacked polarization logs can be used for identifying local variations in polarization (often associated with local variations in crack geometry) and for better imaging of subsurface structure, as in line 3.

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