TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTwo 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.

The zonal isolation behind casing was critical during a major field development project in the Middle East. However, the assessment whether the cement bond will hold against hydraulic communication or not has not always been an easy task; slurry contamination during pumping, slurry invasion by formation hydrocarbons especially gas, channeling through the cement sheath, patchy cement, annulus shape and casing excentralization inside the outer wellbore are all factors that have traditionally made it difficult and sometimes even impossible to judge the zonal isolation behind casing between various zones of the reservoir. A new approach was adopted for the assessment of the cement bond behind casing in this project. The technique combines the conventional "pulse -- echo" ultrasonic measurement with that of a newly introduced flexural waveform. The analysis from this combination allows a better discrimination between solid, liquid and gas behind casing and it also provides, for the first time, a comprehensive assessment of the annulus shape and casing centralization inside open hole or inside the second casing string. This latter casing stand-off measurement has proven to be instrumental when deciding whether the cement sheath will hold against hydraulic communication or not. In this paper we will discuss the data that was acquired in four wells from this project. We aim at proving the clear benefit of this new approach for the assessment of zonal isolation behind casing. Introduction The traditional ultrasonic measurement estimates the acoustic impedance of the media behind the casing wall. The acoustic impedance of any material is the product of its density by the sound velocity through it (Z = density × acoustic velocity). Z is usually expressed in MRayl (106 kg.m-2.s-1). The readings are then binned in three "baskets" in order to determine what points denote gas, which ones indicate liquid and which ones indicate solid. Empirical cutoffs or thresholds are used to determine the extent of each basket. The default value of the threshold between liquid and solid is 2.6 Mrayls usually. The material at any point on the log where the acoustic impedance measures above this value is considered solid. hardened non-light weight cement acoustic impedance is usually distinctly higher than this value, whilst liquid and cement slurries have an acoustic impedance that is lower than 2.y Mrayls. There is no ambiguity in the classification of the material behind the casing in this case. However the acoustic impedance of hardened light weight cements can also be lower than this value (see Table 1). The contamination of the cement that might occur during the cement job can decrease its acoustic impedance below this threshold too. Consequently there is a range where acoustic impedances of slurries and heavy mud overlap with those of solid light weight and/or contaminated cement (Fig. 1). A "single axis" measurement of the acoustic impedance is not enough to decide on the nature of the material behind casing in this case. In the new technique, an additional measurement of the flexural attenuation has been added in order to solve this problem.

This paper describes optimal field development and appraisal in complex reservoirs and challenging environments in field ‘ABC’. Most of the wells are laterals with ICD (lower) completions across heterogeneous carbonate reservoirs. Highly corrosive environments i.e. up to 20% H2S present an added risk, particularly in the event of water encroachment. Optimal development needs a multi-disciplinary surveillance approach involving an integration of input form stakeholders, including geoscience and petroleum engineering, to ensure productivity optimization during the whole life of the field. Field ABC is an offshore field with extremely heterogeneous carbonate reservoirs and acid stimulation is usually done to improve production. The wells in the field are mostly horizontal, oil producers with ICD lower completions. The upper completion uses carbon steel L80 and for corrosion mitigation, inhibitors are injected through chemical injection valves. In this paper, a pilot well is reviewed where a methodical approach was used for evaluation. Baseline production logging and reservoir saturation monitoring were done in the lower completion and a corrosion log was acquired in both the upper and lower completions. Data acquired was integrated and observations show that the measurements correlate well with each other. This case study integrates and correlates downhole zonal contribution, phase holdups, pressure and temperature data from production logging with metal loss data from a high-resolution multi-finger caliper tool. Well trajectory shows a depression across the heel of the well which is incidentally between the EOT and the topmost ICD. Although there is no water production at surface, a static water sump is observed across this depression on the production logs. This static water is possibly completion fluid or unremoved fluid from the acid job. Minor localized corrosion is also observed across the same depression on the corrosion logs, also confirming presence of some water. The H2S production and the presence of water is an added risk to completion integrity as it creates a corrosive environment. Therefore, in such cases it will be necessary to monitor the production and corrosion at regular intervals of time. This case study shows that by applying a multi-disciplinary approach and integrating various measurements, well conditions can be viewed not just as pieces of a puzzle but as a complete picture to improve the understanding of the well behavior. Time-lapse monitoring of production and corrosion along with reservoir saturation is also necessary to prevent surprises and help in making informed decisions towards better field development.

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