Abstract

The objective of this paper is to describe and quantify the influence of cement slurry composition on the mechanical properties of the hardened cement and on the main output of the cement bond log: attenuation of the first arrival amplitude.

Laboratory experiments were performed in ambient conditions of pressure and temperature, with a laboratory type cement bond tool coupled to an oscilloscope and a data acquisition unit. The tool was immersed in oil filled, 4.5 inch, 11.3 lbm/ft casing. The annulus between the casing and a 9.5 inch PVC pipe backed by air was filled with the cement slurry.

The following mechanical properties were determined as a function of time: compressive strength, shear bond strength, total chemical contraction, velocity of compressional waves and resonance frequency.

More than 20 different cement slurry formulations were tested, with densities ranging from 10 to 19 lbm/gal, and containing as widely different additives as bentonite, soluble silicate, silica microspheres, hematite, salts, latex, dispersant and fluid loss agent.

Amongst many results and correlations, several conclusions can be drawn. The relationship between compressive strength and velocity of compressional waves, verified for most slurries, is not valid for salt containing slurries, when for all the systems tested, a single relationship exists between Young's modulus and Poisson's ratio. Particulate extenders like silica microspheres provide, for a given slurry density, higher acoustic impedance and CBL attenuation rate than chemical extenders like soluble silicates. Finally, latex in the cement does not influence the CBL.

Introduction

Cement job evaluation is mainly based on the interpretation of acoustic logs, like the Cement Bond Log (CBL). In the early 60's, some theoretical work [1] stated the CBL attenuation rate was related, amongst many parameters, to 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 well-known as " CBL interpretation chart". This single chart could not be used to evaluate every cement job and has been later modified to take foamed cements into account [2].

Later developments in logging [3] give access to the direct measurement of the acoustic impedance of the material located immediately behind the casing, with an angular distribution which enables to locate mud channel when cement and mud acoustic impedances are different.

It seems now obvious that the knowledge of acoustic properties of the cement will improve the evaluation of cement jobs through a better interpretation of acoustic logs. However, the use of ultrasonic methods to characterize oil well cements is fairly new [4]: most of the recommended methods used in the oil field, like compressive strength determination, are destructive, thus little acoustic data exist which could help for cement job evaluation.

Ultrasonic methods, which have been widely used for more than 40 years in the concrete industry [5], offer one major advantage over traditional methods: they are non destructive and can be used "in situ". Furthermore, ultrasonic properties of a material are directly related to its elastic properties.

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