Experimental Investigation of Second Interface Cement Bond Evaluation

Cement bond model wells （1：10 scaled-down） were made with a gradually degrading cement annulus for cement bond evaluation of the first interface （between the casing and the cement annulus） and the second interface （between the cement annulus and the formation）. Experimental simulation on cement bond logging was carried out with these model wells. The correlation of acoustic waveforms, casing wave energy and flee casing area before and after cement bonding of the second interface was established. The experimental results showed that the arrival of the casing waves had no relationship with the cement bonding of the second interface, but the amplitude of the casing head wave decreased obviously after the second interface was bonded. So, cement bonding of the second interface had little effect on the evaluation of the cement bond quality of the first interface by using casing head wave arrivals. Strong cement annulus waves with early arrivals were observed before the second interface was bonded, while obvious ＂formation waves＂ instead of cement annulus waves were observed after the second interface was bonded.

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.

Cementing Military Bonds

China and the United States incorporate military cooperation into their strategic dialogue The recently concluded fifth round of the China-U.S.Strategic Dialogue with its special focus on defense was an instrumental first step for the two countries to advance their mili- tary relations through formal discussions,