Application of Enhanced Ultrasonic Measurements for Cement and Casing Evaluation

Abstract

The hydraulic isolation of the wellbore casing and cement is critical for the completion of production and injection wells. Zonal isolation prevents the production of fluids from non-completion intervals, contamination of ground water by fluids in the wellbore, and allows conformance control of injected fluids. Current acoustic evaluation techniques may be limited by the acoustic properties of the material behind casing and by the inability to see beyond the cemented region near the casing. A new ultrasonic imaging tool has been developed to address these limitations. The new imager tool combines the classical pulse-echo technique with a new ultrasonic technique that provides temporally compact echoes arising from propagation along the casing and also reflections at the cement-formation interface. Processing these signals yields unprecedented characterization of the cased-hole environment in terms of the nature and acoustic velocity of the material immediately behind casing, the position of the casing within the borehole, and the geometrical shape of the borehole.5 In order to provide answers to the casing/cement evaluation questions, a field study was performed to evaluate the results provided by both sonic and this new ultrasonic tool in the different cement materials, drilling fluids, and casing sizes. Field examples are presented to illustrate the actual response of the new ultrasonic tool to these various completion environments including wells cemented with conventional and lightweight cement. The results demonstrate enhanced cement evaluation for all cement types and a significant reduction in the uncertainty in making a squeeze or no-squeeze decision. The new cement evaluation tool implements both the traditional pulse-echo technique and the new flexural wave concept. The flexural mode enables deep imaging of the cement sheath up to the cement-formation interface. In addition, the measurement of the borehole geometrical shape makes it possible to evaluation double casing string conditions for potential damage. Introduction Sonic logging tools have been used since the 1960's to evaluate the placement of cement for hydraulic isolation of formations. There have been several advancements in the logging tools that improved the ability to evaluate the cement sheath since that time. During the same period of time there has been little change in the types of cement. In the past few years, however, there has been an emphasis on optimizing the cementing operation and reducing the overall cost of the completion. To the cementing operation, this meant developing lightweight and specialized cements that would allow setting casing strings deeper without worrying about lost returns. Other gains in efficiency were also achieved using lighter cements while drilling and completing weak formations. Changes in these cements, and their properties, have also brought about the need for re-evaluating the techniques and tools used for the evaluation of these cements with the sonic logging tools currently available. The Cement Bond Log (CBL) type tools, which include all tools that measure amplitude or attenuation, have a common theory of measurement, interpretation principles, strengths, and weaknesses. The principle of measurement of these tools is to measure the amplitude of a sonic signal, produced by a transmitter emitting a 20 kHz acoustic wave, after it has traveled through a section of the casing as an extensional mode. This amplitude is then converted into attenuation by either using a ratio of multiple transmitter and receiver amplitudes, or using chart book conversions. At this point the interpreter has to select a value for the attenuation of a 100% bonded interval. This can be done based on the CBL data collected in the well or it can come from the predicted cement properties. The value for the attenuation in a 100% bonded interval is the key to the interpretation of this type of log. Zonal isolation is estimated from an empirical data base. These tools also provide a qualitative indication of bond to the formation through the use of a Variable Density Log (VDL) waveform.

Targeting Zonal Isolation Challenges on the Strategic Szoreg UGS Project, Hungary

“…The new imager tool combines traditional pulseecho technique with a flexural wave that enables deep imaging of the cement sheath up to the cement/formation interface (Morris et al 2007). The interpretation of the imager log identified gas presence in a small section of the first well.…”

Research on Improvement Measures of Cementing Acoustic Amplitude Logging

“…Brown (1970) and others suggested that the minimum seal length of cement ring is related to cement cementing index and casing type, and the minimum effective interlayer seal length of cement ring increases with the increase of casing outer diameter [2]. The Drilling Manual developed by China National Petroleum Corporation (1990) stipulates that the evaluation index for casing with an outside diameter of 7in and a wall thickness of 0.408in is: relative sound amplitude less than or equal to 15% is evaluated as excellent cementing, relative sound amplitude between 15% and 30% is evaluated as qualified cementing, and relative sound amplitude greater than 30% is evaluated as poor cementing [3]. S. Talaba•ni (1993) established a fluid flow model and proposed effective measures to prevent gas fouling [4], which marked the generalized application of cementing theory research.…”

Microannulus and Cement Evaluation: Effectiveness of Cement Evaluation Using Sonic and Ultrasonic Technologies in Wells with Microannulus Between Casing and Cement Sheath (Russian)

“…Оба замера, а именно, акустический импеданс и затухание изгибной волны совмещаются для определения количества твердого вещества, жидкости и газа за обсадной колонной (Kuijk et al 2005;Morris et al 2007). Этот прибор имеет четыре излучателя/приемника установленных на вращающуюся муфту (Рис.…”