Free Pipe Effects On Guided Wave Propagation

Abstract

The effects of free pipe on the pseudo-Rayleigh and Stoneley waves are studied by introducing a fluid annulus between the casing and cement layers of a cased borehole model. The existence of a fluid annulus region results in a second Stoneley mode being generated which propagates primarily in the annulus. The propagation and attenuation of this second mode are controlled by the fluid annulus, cement, and formation properties. The primary or central Stoneley mode becomes decoupled from the effects of formation when the free pipe situation exists, and its propagation and attenuation are controlled by the central fluid and casing properties only. The amplitude of the second Stoneley mode increases as the annulus thickness increases, but in most cases is an order of magnitude less than the primary mode amplitude. The pseudo-Rayleigh wave becomes more dispersive as the annulus region increases in thickness.