This paper describes and quantifies the influence of cement slurry composition on the mechanical properties of the hardened cement and on the cement bond log (CBL). Laboratory experiments were performed on more than 20 different cement slurry formulations with densities ranging from 1200 to 2280 kg/m3 [10 to 19 lbm/gal]. Results show that
CBL attenuation rate is directly related to the acoustic impedance of the cement;
particulate extenders like silica microspheres provide, for a given slurry density, higher acoustic impedance and CBL attenuation rate than chemical extenders like soluble silicates;
latex in the cement does not influence the CBL;
the relationship between compressive strength and velocity of compressional waves, verified for most slurries, is not valid for salt-containing slurries; and
a single relationship exists between Young's modulus and Poisson's ratio.
Cement job evaluation is based mainly on the interpretation of acoustic logs, like the CBL. In the early 1960's, theoretical work showed that the CBL attenuation rate was related to, among many parameters, the cement density and velocity of compressional and shear waves through the cement. Experimental work was performed at the same time, leading to the construction of a nomograph known as the CBL interpretation chart. This single chart could not be used to evaluate every cement job and has since been modified to take foamed cements into account. Later developments in logging gave access to the direct measurement of the acoustic impedance of the material located immediately behind the casing, with an angular distribution that enables mud channels to be located when cement and mud acoustic impedances are different.
It now seems obvious that the knowledge of acoustic properties of the cement will improve the evaluation of cement jobs through better interpretation of acoustic logs. However, the use of ultrasonic methods to characterize oilwell cements is fairly new; most of the recommended methods used in the oil field, like compressive-strength determination, are destructive, so few acoustic data exist to aid cement job evaluation.
Ultrasonic methods, which have been widely used for more than 40 years in the concrete industry, offer one major advantage over traditional methods: they are nondestructive and can be used in situ. Furthermore, the ultrasonic properties of a material are directly related to the elastic properties. This is not the case in cube-crushing tests, which do not directly measure any fundamental property. In particular, the propagation of a compressional wave through a material and its natural resonance frequencies are dependent on the elastic modulus of the material. During cement hardening, both mechanical strength and elastic modulus increase. This can also be seen by measurements of pulse velocity or natural resonance frequency.
Spinner and Tefft proposed methods to determine mechanical resonance frequencies of cylinders and bars and derived equations to compute elastic moduli from natural resonance frequencies, density, and geometric characteristics. A standard test method was subsequently defined that covers the measurement of fundamental transverse, longitudinal, and torsional resonance frequencies for calculating dynamic moduli and Poisson's ratio. Equations show that even when Poisson's ratio is unknown, a good estimate of Young's modulus can still be made from measurements of natural resonance frequency. This is not possible with the pulse-velocity measurement because the relationship between Young's modulus and sound velocity is influenced much more by the Poisson's ratio.
Studies have been carried out to relate pulse-velocity measurements to the strength of the concrete. Elvery et al. found that the relationship between ultrasonic pulse velocity and cube strength is practically independent of the water/cement ratio and temperature between 1 and 60 degrees C [34 and 140 degrees F] but is influenced by the aggregate content and the type of cement. The method has also been used successfully to monitor the influence of various additives on the hydration of the cement. Sturrup et al. concluded that pulse-velocity/strength relationships could be established for concrete at early ages, but that pulse velocity is rather insensitive to even major increases in strength at later ages and should not be used for matured concretes. Furthermore, pulse velocity can also be affected by cracks, voids, or other discontinuities in the concrete.
In the course of the work described here, extensive testing with both destructive and nondestructive techniques has been conducted on a large variety of cement formulations. The purpose of this paper is to show how cement slurry composition affects various physical and acoustic properties of set cement and the CBL. It shows also how the data generated and cross correlations made between different types of measurements can improve cement job evaluation.
CBL Measurements. The experimental setup, which was used previously to study the influence of borehole geometric parameters on the CBL output, can be divided into three sections.
The mixing area is where the volume of slurry required for the test is prepared. It is located 3m [10 ft] above the test cell so that the slurry can flow by gravity. The slurry is kept under agitation with a high-shear paddle until its rheology becomes constant and is then pumped by gravity into the test cell.
The test cell is a 1.2-m [4-ft] section of sandblasted casing (114-mm [4.5-in.] OD, 102-mm [4-in.] ID) positioned vertically in a 241-mm [9.5-in.] -diameter PVC pipe backed with air but providing enough cement thickness to have a CBL peak measurement free of reflections. The annular space is cemented, while the inside of the casing is full of low-viscosity oil where the sonic sonde is located.
The sonic sonde consists of a cylindrical body in Teflon holding four ceramic transducers arranged along the axis with a 0.152-m [0.5-ft] spacing.
These transducers, used on standard field sonic sondes, have a ringing frequency of about 17 kHz [17,000 cycles/sec]. The sonic sonde, which is kept centered inside the casing, is fired 5 times/sec, and the sonic signal received is displayed on a digital storage oscilloscope. The amplitude of the first peak, A1, of the sonic wave is monitored vs. time, and the attenuation rate of the signal can be derived from the following relationship:
where = attenuation rate, dB/m [dB/ft], Ls = transmitter/receiver spacing, m [ft],