Shear Wave Travel Time Determination Using an Unconventional Approach

Abstract

Abstract In the past, numerous techniques have been used in attempting the downhole determination of formation shear travel time. These include visual inspection of full wave train recordings (e.g., variable density, signature, microseismogram), monitoring the amplitude characteristics of the acoustic wave train in selected gate(s), digital analysis of the acoustic wave train in selected gate(s), experimentally defining a relationship between compressional and shear wave transmit times, digital analysis of the acoustic wave train at various transmitter-receiver (T-R) spacings, etc. These techniques have had only limited success, particularly in shaly, unconsolidated, undercompacted and/or gas bearing particularly in shaly, unconsolidated, undercompacted and/or gas bearing formations. A new analytical model has been developed and tested which overcomes some of these constraints. By utilizing the standard compressional travel time in conjunction with supplementary non-acoustic logging data, the shear travel time can be estimated with reasonable accuracy in clastic (sand/shale) sequences. Introduction In acoustic logging, the total acoustic wave train consists of several wave types travelling in a fluid-filled borehole. These components include non-dispersive body waves and dispersive guided waves. The two body waves of main interest to acoustic (sonic) logging are compressional (P-waves) and shear (S-waves). P-waves are propagated away from the source parallel to the direction of particle displacement. The P-wave propagates as a compressional wave in the borehole fluid, is critically refracted into the formation in a compressional fashion and then returns through the borehole fluid to the instrument as a compressional wave. Since solids, fluids and gases tend to oppose compression, P-waves can be propagated through all of these media. S-waves propagate perpendicular to the direction of particle displacement. These waves propagate as compressional waves in the borehole fluid, are critically refracted into the formation as shear waves (provided the shear velocity in the formation exceeds that of the borehole fluid) and return as compressional waves into the borehole fluid. Since fluids and gases exhibit no rigidity (i.e., opposition to shearing), S-waves cannot be propagated through them. propagated through them. Dispersive guided waves caused by the wave guide effect of the wellbore are the pseudo-Raleigh wave and the Stoneley (tube) wave. The Stonley wave, which exists in all formations, exhibits travel times exceeding those of formation shear and fluid travel times. As such, Stonley waves are of no consequence to the present discussion. The pseudo- Raleigh wave exhibits a phase velocity between formation shear and the fluid velocities, i.e., the pseudo-Raleigh arrival is always later than the shear. Therefore, at long transmitter-receiver spacings, the pseudo- Raleigh wave should have a negligible effect. Finally, at later arrival times occurs the fluid (mud) wave which propagates from the transmitter through the borehole fluid to the receiver. Log-derived information on compressional and/or shear waves assists in: drilling operations - overpressure detection and evaluation; fracture gradient determination; mud weight requirements; casing seat selection, etc.