Acoustic evaluation of cementing quality using obliquely incident ultrasonic signals

Ultrasonic cement bond logging is a widely used method for evaluating cementing quality. Conventional ultrasonic cement bond logging uses vertical incidence and cannot accurately evaluate lightweight cement bonding. Oblique incidence is a new technology for evaluating cement quality with improved accuracy for lightweight cements. In this study, we simulated models of acoustic impedance of cement and cementing quality using ultrasonic oblique incidence, and we obtained the relation between cementing quality, acoustic impedance of cement, and the acoustic attenuation coeffi cient of the A0-mode and S0-mode Lamb waves. Then, we simulated models of different cement thickness and we obtained the relation between cement thickness and the time difference of the arrival between the A0 and A0′ modes.

An acoustic-wave theory for casing bond evaluation using viscoelastic slip boundary modeling

Slip boundary condition is commonly utilized to model elastic wave propagation through layered earth media. The same approach is used here to characterize acoustic wave propagation along a cased borehole with various cement bond conditions. By modeling the cement layer between casing and formation as a viscoelastic slip interface with complex coupling rigidity parameters, one can not only reduce the complexity in the classical elastic wave modeling of the problem, but also efficiently model various complicated wave phenomena that are difficult for the existing modeling. More specifically, the new theory can well describe the effect of the cement bond condition change and the location of the change(i.e., whether it is in the first interface between casing and cement, or the second interface between cement and formation) on the acoustic waves,demonstrating the good modeling capability and predicting power. Application of the theory to field data shows that the theory can correctly model the acoustic wave characteristics and interpret the cement bond condition, thus providing a useful fundament theory for casing bond evaluation using acoustic logging.

Evaluation of the cement bond quality through filtering the reflected ultrasonic waves

High cement bond quality is required to keep an oil well from hydraulic commu- nication between zones. In the cement bond evaluation, the ultrasonic echo method is widely used for its capability of channeling azimuth detection. Full waveforms reflected from the cased hole are simulated for different bonding conditions by the generalized transfer matrix method. Because of the high acoustic impedance of casing, the amplitudes of the reflected waves from the cement and the formation are small and cannot easily be used to evaluate the cementing condition. The wave that can propagate into the cement and the formation through the casing concentrates closely on the casing resonant frequency. To reduce the amplitude of the reflected wave from the casing inner surface and highlight the part of the reflected wave which carries the cementing quality information, the reflected full wave is filtered according to the casing resonant frequency. There are several wave packets in the filtered waveform. When the amplitude of the second wave packet is low, the cement bonds well with the casing, otherwise poorly. A low amplitude third wave packet is an indication of a good bond of the cement with the formation, otherwise poor. To reveal the sensitivity of the reflection wave amplitudes to the incident angle, reflected full waveform is modeled when an acoustic beam with finite width is incident on the casing. It is shown that the bond evaluation method based on filtered wave packets is valid for incident angle less than 5 degrees.

Assessing Leak Paths in the Cement Sheath of a Cased Borehole by Analysis of Monopole Wavefield Modes

Evaluation of possible leakage pathways of CO_(2) injected into geological formations for storage is essential for successful Carbon Capture and Storage(CCS).A channel in the borehole cement,which secures the borehole casing to the formation,may allow CO_(2) to escape.Risk assessment and remediation decisions about the pres-ence of such channels depend on channel parameters:radial position r from the center of the borehole;channel thickness d;azimuthal positionφof the channel;and az-imuthal extentθof the channel.Current state-of-the-art cement-bond logging technology,which uses only the first arrival at a centralized borehole receiver,can diagnose limited details about CO_(2) leak channels.To accurately characterize the possible leak paths in the cement,we use a 3-dimensional finite-difference method to investigate the use of the abundant data collected by a modernized monopole sonic tool that contains an array of azimuthally distributed receivers.We also investigate how to improve the tool design to acquire even more useful information.For cases where borehole fluid is either water or supercritical CO_(2),we investigate various receiver geometries,multi-modal analyses of multi-frequency data to discover the type of logging tool that provides the best information for CCS management.We find that an appropriate choice of wave modes,source frequencies,source polarities,and receiver locations and off-sets provides sensitivity to d,φ,θ.The amplitude of the first arrival from a monopole source is sensitive toθ.Amplitudes at receivers at different azimuths are sensitive toφ.The slow Stoneley mode(ST2)velocity is sensitive to d,but ST2 is not easy to pick whenθand d are small.Further improvement is necessary to provide comprehensive information about possible flow channels in casing cement.