Increased Certainty in the Determination of Zonal Isolation through the Integration of Annulus Geometry Imaging and Improved Solid-Fluid Discrimination

Abstract

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.

Swelling Elastomer Applications in Petroleum Drilling and Development

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Some downhole applications of swelling elastomers in oil and gas wells

“…Amongst all ultrasonic techniques employed, third interface echo (TIE) gives more promising results to ensure hydraulic seal and zonal isolation in the case of swelling elastomer. 18 Other authors [19][20][21] describe experimental, analytical, and numerical methods for performance evaluation of swell packers and other types of swelling elastomer seals deployed in downhole petroleum applications.…”

New Practices for Cement Integrity Evaluation in the Complex Environment of Xinjiang Oil Field

“…Typical cement properties (Al-Suwaidi et al 2009), together with those of mud, are listed in Table 1. Typical cement properties (Al-Suwaidi et al 2009), together with those of mud, are listed in Table 1.…”