Reliability of Cement Bond Log Interpretations Compared to Physical Communication Tests Between Formations

Abstract

Abstract Two classes (sonic and ultrasonic) of cement bond log tools are run in tandem as part of ZADCO's standard cement evaluation program. The effectiveness of these tools and their evaluations are often challenged and are not regarded as a replacement for reservoir inter-zonal communication tests performed between producing reservoirs on every well. Consequently the value of continuing to run these tools was raised by management. In response, the reliability of these tools and their interpretations for determining the existence of poor behind casing cement quality and possibly hydraulically communicating layers was critically and systematically examined by a dedicated team of ZADCO and Schlumberger technical professionals. The criteria used to judge the usefulness of these logs was the present or not of communication behind casing as determined by the physical test. For the twenty-eight wells examined, twenty-five of the cement log interpretations matched the communication test results. One well which communicated had 10 feet of apparent good cement with the remainder poor. Its cement log suffered from eccentralization which negatively affected the cement bond log interpretation. Two wells with long intervals of excellent cement on logs suffered behind casing communication. The reasons for this remain unresolved but are a source of constant discussion. Possible explanations for the mismatch between cement evaluation log interpretation and the physical test results are explored. Introduction Evaluation of cement integrity and its ability to prevent fluid and gas migration in the annular space behind casing is a challenge. Historically, three measures are evaluated to check for isolation between reservoir units:pressure testing,physical communication tests between formations, andcement bond log interpretation. Pressure tests are restricted to localized areas of the casing such as the casing / liner shoe and squeeze perforations. Communication tests, regarded as the most definitive method of testing behind casing isolation jeopardize casing integrity, are costly, and directed at preventing reservoir to reservoir communication. Cement evaluation logs are time efficient, cover the majority of the casing and inexpensive compared to communication tests. Their interpretations however occasionally do not predict behind casing communication, creating a perception of unreliability. Determining the confidence which can be placed upon cement log interpretations is an objective of this task force. Behind casing communication of hydrocarbons via the cemented annular space means expensive remedial cement squeezes to cure the problem. The expense is considerably higher if the communication problem is discovered after the well is drilled and the rig has left location. Because of the importance and criticality of the zonal isolation between oil bearing formations for reservoir management, ZADCO's policy is to confirm isolation by physical communication tests regardless of the quality of cement interpreted from cement bond logs. The policy is applied selectively however. Only cement barriers between formations where production is planned are tested (Figure 1). Intervals between reservoirs not planned for production are not tested. Since the majority of cement squeeze decisions are based on the results of physical communication tests, the value of continuing to run cement bond logs was questioned. Conversely, applying the policy of communication tests was costly and time consuming and could not be practically applied to wells covering multiple reservoirs not intended for production. Consequently a task force comprised of representatives from drilling, well integrity, petroleum engineering, petrophysics and the primary service company was formed to review ZADCO's communication test policy and access the reliability of cement bond logs for evaluating behind casing cement quality and from this inferring the presence of or lack of isolation between formations.