The completions of ultra-deepwater wells in the Gulf of Mexico are requiring higher mud weights and heavier casing strings in order to economically maximize production rates and to control high-pressure reservoirs. Many operators are completing multiple wells using synthetic oil-based mud weights in excess of 15 lbm/gal, casing with a wall thickness of more than 0.625 in. and special cement materials. Standard cement bond logging tools, however, may not be well suited to evaluate the extreme casing conditions or analyze the casing/cement interface because of the acoustic properties of these materials.
In order to obtain quality answers to the casing/cement evaluation questions, a study was performed to evaluate the results provided by sonic and ultrasonic tools in the different casing materials. The results were obtained from a combination of special small-scale experiments and field applications. This work provides the technical background for extending the ultrasonic measurements beyond normal industry conditions. The measurement resolution and limitations in these extreme conditions are also discussed. It was found that the latest generation of ultrasonic cement evaluation tools could provide high-quality measurements of the casing dimensions and the cement acoustic impedance properties when utilizing the proposed procedures. Field examples are presented to illustrate the actual application and the response of the ultrasonic tools to various extreme cement evaluation conditions.
Cement bond logging tools have been used successfully for many years to evaluate casing and cement conditions in both production and injection wells. These tools utilize either sonic or ultrasonic measurement techniques designed for the conventional steel casing and cement environment.
The Cement Bond Log (CBL) type tools, which include all tools that measure amplitude or attenuation, have 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. This amplitude is then converted into attenuation by either using a ratio of multiple transmitter and receiver amplitudes, or using chart book conversions. The percent of the circumference of the casing bonded is then computed by calculating a Bond Index.
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 cement properties provided by the cementing company. The value for the attenuation in a 100% bonded interval is the key to the interpretation of this type of log and the cement properties are critical to the interpretation. It is important to make accurate estimates of the properties if they are not known from the cementing company. These tools also provide a qualitative indication of bond to the formation through the use of a variable density log (VDL) waveform.