A Preliminary Study of Factors Influencing Cement Bond Logs

Abstract

Abstract To improve the interpretation of cement bond logs, a series of measurements have been made on casing surrounded by a sheath of various cementing mixtures. The following properties were measured on each test:the acoustic transmission of the sheathed casing;the resistance to hydraulic communication of the sheath-casing interface; andthe support coefficient or mechanical bonding strength of cement to casing, with accompanying compressive-strength values for the cement. In addition, test were made to determine the degree of intimacy of the pipe-sheath interface necessary to achieve reduced acoustic transmission. These tests indicate that cement bond logs are a measure of the degree of contact or acoustical coupling of the casing and cement sheath, and are not always related to the adhesion of the cement. Increased acoustic transmission is more pronounced when the sheath is of low-density material. Log interpretation requires knowledge of the instrument sensitivity, the physical properties of the cementing material used, the hole size, the type of formation, the temperature and the pressure conditions during cement curing. Introduction The "bond log", as the name indicates, is intended to provide information regarding the degree of bonding to the pipe by the cement. It has also been used to detect the presence or absence of a channel in the cement sheath surrounding the casing. Since the introduction of "bond" logging, many questions have arisen as to proper field application and interpretation of field results. To further a better understanding of bond logging, a series of laboratory controlled tests were made. These studies were conducted without formation in the proximity of the cement sheath; they included not only the acoustic amplitude measurement of the transmitted pipe signal, or the degree of acoustic transmission, but also some pertinent physical characteristics of various cementing compositions relating to resistance to communication between the pipe and cement sheath. Additional tests are contemplated. Scope of Tests The effects of various oilwell cementing mixtures on acoustic attenuation were studied, as well as related physical properties. These tests were subject to the limitation that they fail to completely simulate well conditions, in that only the casing and cement sheath were present. Table 1 lists these various cementing compositions and shows the identification number assigned to each material in the graphic illustrations. To date, tests have been performed only on these compositions; however, additional work is being prepared for other cementing mixtures now being used by the oil industry. Description of Logging Apparatus Many conventional bond logging tools employ a transmitter which emits a sound pulse at controlled intervals and a receiver, acoustically isolated from the transmitter, which receives the sound pulse whether transmitted through the wellbore fluid, pipe, cement sheath or formation. A gating circuit is commonly used to allow measurement of that portion of the received signal which is considered pertinent to cement bonding. The amplitude of the received signal in the gated portion is measured and retransmitted in a form suitable for recording. This measure is proportional to the acoustic transmission. The tool used for the tests described herein employs a magnetostrictive transmitter coupled to the wellbore fluid. A crystal receiver isolated by a 4-ft Teflon section was also coupled to the wellbore fluid. JPT P. 369^