Cement Bond Evaluation Using Early Refracted Arrivals

Abstract

The cement bond evaluation tool is a device used to examine the integrity of cement bonding to the casing. A conceptual tool operating between 80-200 kHz is considered here, with a transmitter and two receivers, oriented parallel to the axis of the borehole and next to the casing. The compressional head wave in the casing, excited by the transmitter, will be the first arrival to be measured by the two receivers in most situations. With both receivers on the same side of the transmitter, the attenuation of this wave in traveling between the two receivers is dependent on the properties of the medium immediately outside the casing. The radially layered borehole was modeled as a layered plane medium for large operating frequency. A spectral integral approach (complete wave synthesis) was used to compute the response at the receiver locations, which then provided attenuations. Different parameters, such as transducer separation (1-12 in), annulus thickness (0-6 in), annulus impedance (free pipe to good cement), casing thickness (0.25-0.45 in), standoff distance (0.5-1 in) and source frequency (80-200 kHz) were varied in the evaluation of the operation of the tool. The parameter studies based on the theoretical computations revealed that free pipe could be distinguished from the presence of cement in a variety of situations. Additionally, lower bounds on receiver separations are given for reliable operation of the tool.