Lateral Velocity Variations Near Boreholes

Abstract

Difficulties occur in obtaining accurate two‐receiver velocity logs in formations sensitive either to damage by exposure to drilling mud or to mechanical stress relief. Some shales are so altered by the drilling operation that their elastic properties are modified. Vertical velocity measured immediately adjacent the boreface is lower than if it were measured at a greater radial distance from the bore. These damaged shales require relatively deep penetration by the acoustic signal; consequently, the transmitter‐to‐first‐receiver spacing in a two‐receiver velocity logging system should be long enough to refract the sound waves through virgin formation. Experiments in one predominantly shaly section show a difference of almost 10 percent between times measured using transmitter‐to‐first‐receiver spacing of 4.3 ft compared to 8.8 ft. A limited amount of field data suggest that sodium montmorillonite is the clay type most sensitive to hydration and swelling. Studies of areal prevalence of the shale damage problem are incomplete.

Shear Wave Velocities Determined From Long- and Short-Spaced Borehole Acoustic Devices

“…The 14-to 16-ft (4.3-to 4.9-m) P-wave ITT averages about I JLs/ft (3.3 JLs/m) less than the 3to 5-ft (0.9-to 1.5-m) P-wave ITT, while S-wave ITT is normally 1 to 3 JLs/ft (3.3 to 9.8 JLs/m) less than the 3-to 5-ft (0.9-to 1.5-m) curves. These magnitudes also can be observed on the curve comparisons Similar discrepancies have been observed 19,20 in sand/shale sequences, where lower ITT on longer spacing tools is attributed to the ability of this tool to "see" beyond the shallow shale hydration effects caused by the drilling process. The offset between short-and long-spaced curves is somewhat surprising in the test well because it occurs in dense carbonates and sandstones as well as in fairly high-velocity siltstones and shales.…”

Seismic Refraction Prospecting

“…Tw(• receiver CVL's with long spacing between transmitter and first receiver should be used in measuring the velocity to avoid the effects of shale damage on velocity logs (Hicks, 1959). Tw(• receiver CVL's with long spacing between transmitter and first receiver should be used in measuring the velocity to avoid the effects of shale damage on velocity logs (Hicks, 1959).…”