A Probable Example of Acoustic Anisotropy in Sedimentary Sandstone, Middle Ground Shoal Field
Authors: Jorden J.R. 
Published: December 2007

Purpose and Scope Purpose and Scope The acoustic anisotropy of some sedimentary formations has been well-documented in prior laboratory experiments. In particular, Podio el al. present a good review of previous work in measuring present a good review of previous work in measuring the elastic properties of rock using acoustic data and present original data showing, for Green River present original data showing, for Green River shales, the compressional wave velocity as a function of the direction of wave propagation. Schroeder and Gray, using critical angle wave propagation techniques, show that compressional wave velocity varies as a function of the direction of wave propagation for both slate and Green River shale propagation for both slate and Green River shale samples.To our knowledge, acoustic anisotropy has not been considered previously as an important variable affecting the acoustic transit time response of porous sandstones. The purpose of this paper is to present data that support a hypothesis that anisotropy effects indeed may influence the acoustic response in sedimentary sandstones. Consequently, such effects perhaps should be considered in estimating porosity perhaps should be considered in estimating porosity in sandstones when the angle between borehole and bedding plane is significantly different from 90 %. That is to say, conventional interpretation of acoustic logs perhaps can be misleading in high-angle boreholes.Available data are from two wells in the Middle Ground Shoal field in Alaska. Acoustic log response shows definite anomalies between correlative sands in these wells; these anomalies are attributed to anisotropy effects. It is recognized that factors other than anisotropy (e.g., clay content, in-situ stress) could account for the observed anomalies; available data are inadequate to eliminate completely these factors as the causative factors. Hence, this work must remain only a hypothesis until additional data are available for further study.

Investigation Methods
Fig. 1 shows a dip section of the Middle Ground Shoal field in the vicinity of the two wells under study, C-11-26 and C-31-26. As indicated in Fig. 1 and Table 1, the angle between borehole and structural dip is relatively "standard" (i.e., about 90 degrees) in all of the sandstone members in Well C-31-26. On the other hand, this angle deviates moderately from "standard" in the HC-HD through HR-HZ sandstones and deviates significantly in the GN-GO and GR-GS sandstones in Well C-1 1-26.Curve C in Figs. 2 and 3 illustrates the calibration of acoustic log response and density log response, respectively, to core porosity under stressed conditions. Porosities under stressed conditions were available on 12 samples; Table 2 presents some pertinent data on these samples. The porosity at pertinent data on these samples. The porosity at 5,000-psi (34 474-kPa) differential pressure was taken as in-situ porosity. It is known that the stress field at Middle Ground Shoal (Fig. 1) is significantly different than that of a tectonically relaxed, subsiding basin, which has been the model for most prior porosity/stress investigations. Additionally, the porosity/stress investigations. Additionally, the laboratory procedures used on these 12 samples may not have permitted the measurement of an "equilibrium" porosity at the various stress states (i.e., pore volume changes with time are very small, say 0.001 cm /24 hours). Nonetheless, the typical porosity/stress relation illustrated in Fig. 4 suggests a porosity/stress relation illustrated in Fig. 4 suggests a possible "knee" at about 5,000 psi (34 474 kPa) and possible "knee" at about 5,000 psi (34 474 kPa) and indicates that the porosities measured at high stresses may not deviate substantially from "equilibrium" values.Curve D in Figs. 2 and 3 indicates the calibration of acoustic log response and density log response, respectively, to in-situ porosity, as determined by other Shell Oil Co. staff (J.D. McLehaney and C.G. McMaster). They applied an average reduction factor (derived from the 12 samples discussed) to more than 200 porosity measurements taken at zero differential pressure. These corrected porosities then were related pressure. These corrected porosities then were related to the log responses to obtain the relationship of Curve D in Figs. 2 and 3. Reasonable agreement between the two methods of calibrating porosity to log response is noted.For comparison, Curves A and B in Fig. 2 illustrate the acoustic-transit-time/porosity relationship developed by Pickett.
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