Paper: Cement and Casing Evaluation Using Sonic And Ultrasonic Techniques

The Cement Bond Log/Variable Denisty Log (CRL/VDL) service is used to determine the quality of cement bond to casing and the formation. Some of the recommended uses for the CBL/VDL are (1) Determine if vertical isolation exist between zones. (2) Check effectiveness of a squeeze cement job. (3) Check possible damage to cement by high pressure testing or injecting. (4) Locate cement top. (5) To study various cementing techniques. Figure 1 displays a standard Log presentation of the CBL/VDL. The CRL/VDL Log displays a CBL amplitude curve, VDL display and a transit time curve. In addition, the Gamma Ray, Casing Collar locator and Bond Index curves will appear. The CBL curve, which is located in track two is a continuous measurement of sound pulse amplitudes after they have traveled a specified length of casing. These amplitudes are at a minimum in well bonded casing and at a maximum in poorly bonded (free) casing. The VDL display in track three, complements the CBL amplitude curve by providing additional information about the quality of cement to formation bonding. The VnL displays the varying amplitudes of the arriving sound signals as variations of light intensities. The sonic transit time is recorded in track one and is used in conjunction with the CBL/VDL to verify their accuracy. The interpretation methods used with the CBL/VDL compare the response of all three displays to determine the quality of cement bond, since varying parameters may cause one curve to imply false information if analyzed alone.