Ultra-lightweight cements have been used throughout the Industry for many years as a specialty cement for application in areas of weak formations. Typical light weight slurries use hollow ceramic microspheres or nitrogen and other low specific gravity materials to maintain light slurry densities while providing good compressive strength development. The one problem with these systems has been the evaluation of the job via sonic tools. This paper will introduce a new technique to allow an accurate Interpretation of a sonic cement evaluation and will be demonstrated from laboratory model studies.
Light cement evaluation studies show a statistical variance technique to be the preferred method of evaluation in its class. Standard methods of cement evaluation rely on the contrast in acoustic properties of materials behind pipe. The Cement Bond Log (CBL)acoustic amplitude is used to derive compressive strength. The ultrasonic tool uses acoustic Impedance to derive compressive strength. However, in the process of lightening the cement slurry, the acoustic properties of the cement can approach the acoustic properties of a fluid. Modeled values were compared to measured values of various slurries from 16.4 lb/gal to 9.0 lb/gal, confirming the trend from high acoustic Impedance contrast of a cement to a drilling fluid in the case of a 16.4 lb/gal slurry to a negative contrast in the case of a 9.0 lb/gal slurry.
Situations can arrive where drilling mud will be displaced by a cement slurry of the same density. Standard cement evaluation techniques would not be able to identify the cement In this case because of a lack of acoustic impedance contrast. Another technique is available for low-density cement evaluation that identifies the cement from the fluid behind pipe by quantifying the other statistical variance of the acoustic impedance measurement. It will be demonstrated that there is a difference in the impedance measurement statistically between a solid crystalline cement and fluid, even though the absolute values are the same.
Additional work will demonstrate the difference between designed lightweight cement with nitrogen and cement that has been subjected to gas migration.
Low-density cements have been used to cement casings in wells for a number of years. The reasons that lightweight cements are used are as follows:
Low density usually means lower cost.
Low density is used for low fracture gradient.
Special low-density slurry properties are needed for lost circulation or gas migration.
Three basic types of lightweight cement are used to cement a well, water-extended cement, high-strength ceramic bead cement, and nitrified cements. The disadvantage of using a water-extended cement at ultra low densities of less than 12.5 lb is that the compressive strength development is slow and of low magnitude. The hollow ceramic bead cements and the nitrified cements have a much higher strength than the water-extended slurries at comparable densities and are able to obtain reasonable compressive strength in the range of 9.0 to 12.5 lb/gal density. Figure 1 shows the relationship between strength and density of all three kinds of cement compositions.