Abstract

Cement job evaluation has always been a major problem. Acoustic and ultrasonic logs are widely used to assess the quality of the cement job but even experts experience difficulties in locating cement on a bond log. Bond Index (BI) has been the only interpretation method to make quantitative evaluation of the cement job. Unfortunately, BI is far from being an exact figure and in many cases it will not be a valid method to use. Mores over, whenever BI is valid, it is very difficult at present to choose the right references for doing the interpretation. This paper demonstrates that the evaluation of cement jobs can be significantly improved when the design and actual cement job data are taken into consideration. Integration of this data, linked to characterizing the cement by its acoustic properties rather than its compressive strength, allows a calculation of all reference values for a better BI interpretation, and prevents misinterpretation in many cases. Field cases are also discussed which show the validity and the added benefits of the new integrated approach.

Introduction

Cement job evaluation is a critical step in the completion of a well. It is far from being obvious and too often not considered with the required care. Evaluation means to compare job results versus job objectives and expectations. In some cases, the evaluation is very straight-forward: cement to surface or good execution of the job. But in other cases it is not always that simple: -hydraulic isolation is required and to be verified by either destructive or non destructive techniques.

Destructive techniques such as communication test or dry test indeed give a definite answer, but imply some irreversible processes potentially detrimental to hydraulic isolation in the well. processes potentially detrimental to hydraulic isolation in the well. Non destructive techniques are generally preferred because they have no influence on hydraulic isolation. Temperature logs were first used but can only help in locating the top of cement. Introduction of Cement Bond Logs (CBL) in the early 60's, and ultrasonic cement evaluation techniques in the early 80's enabled a more quantitative analysis of the cement placement around the casing. In spite of the limitations of these measurements, decisions are taken based on CBL's or ultrasonic logs interpretation with no or little information on the cement job itself, and poor knowledge of parameters influencing the response of the logs. poor knowledge of parameters influencing the response of the logs. After a brief review of CBL principle, interpretation and limitations, this paper proposes an integrated approach for cement job evaluation. This new paper proposes an integrated approach for cement job evaluation. This new approach shows that considering well parameters, cement job real time monitoring and to some extent post-cement job history improves cement job evaluation and permits a better interpretation of cement bond logs.

CBL principle, interpretation and limitations

CBL principle The principle of the CBL was discovered over 30 years ago. When a sonic wireline tool is run inside a casing, a sonic transducer on the tool - the transmitter - sends an omnidirectional pulse which induces a vibration of the casing (see Figure 1). When bonded to a hard material, he vibration of the casing is attenuated.

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