The Evaluation of Specialized Cements

Abstract

In recent years specialized cements have been developed for use in the oilfield. These cements have ranged from very lightweight to very heavy cements. They have included additives such as glass or ceramic beads to alter their properties. With an increased use of these cements, concerns have risen about the cement evaluation. Quite often the question is asked, "Can we evaluate these cements?" rather than, "How do we evaluate these cements?" This paper will discuss some logging techniques that can be used to evaluate these new cements. The desire to cement longer casing intervals without exceeding the formation fracture gradient has led to the use of very light weight cements. These cements have low acoustic impedance values that are not much higher than the acoustic impedance values of drilling fluids. The compressive strength of these cements can also vary significantly. Acoustic impedance and compressive strength are the properties that cement evaluation tools use to differentiate between the fluids and the solids in the casing formation annulus. To identify these differences it is critical that the proper logging techniques and input parameters are selected for the sonic and ultrasonic tools. A statistical analysis of acoustic impedance measurements from the ultrasonic tools has also proven to be valuable for the evaluation of these special cements. Examples from Alaska, and other areas where these types of cements are being used to solve difficult completion problems, will be shown to demonstrate the use of these cement evaluation techniques. Introduction Since the 1960s sonic logging tools have been used to evaluate the placement of cement for hydraulic isolation of formations in both production wells and injection wells. Since this time, there have been several advancements in the logging tools that improved the ability to evaluate the cement sheath. During the same period of time the cements being used changed little. During 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), while still in existence in the same form it was originally developed, has also evolved into several other cement logging tools using the same physics of measurements with improved measurement outputs. The first tool of this type uses two transmitters and three receivers to make a direct measurement of attenuation that compensates for many of the problems observed in the basic CBL measurement of amplitude. This measurement was then put into another tool that could measure the attenuation in six sectors around the circumference of the casing. Since the use of the Cement Bond Log first began, there have been several different versions of the standard CBL tool to measure the amplitude in different sectors around the casing.

Stress Changes in Cemented Borehole Annulus

“…All materials are considered as elastic. This software allows to simultaneously analyze multiple models, so that we can considerchanging drilling conditions for different geometries of wells and determine the optimalsolution that provides satisfactory stability of connections for casing-cement-stone formations [5]. Table 2 shows the model used for the analysis of stress and deformation of cement stone [6].…”

Cement Evaluation Under Extreme Conditions

“…1,2,3,4 This is accomplished by using a transducer to project a short pulse of acoustic energy with a bandwidth of 200 kHz to 700 kHz toward the casing. The transducer then becomes a receiver and measures the returning echo from the casing.…”