A Scientific Approach to the Use of Thixotropic Cement
Authors: Clement Clark 
Published: December 2007

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Introduction
Thixotropic cements have been used in the oil field for many years. Severe lost-circulation and squeeze problems have been solved by these slurry systems. problems have been solved by these slurry systems. However, many times when thixotropic cements were necessary for difficult cementing situations. more conventional slurries were chosen instead. This preference for conventional slurries has been caused largely by apprehension of using thixotropic cements that exhibit unique physical properties and that have never been explained fully. Also, a definite set of guidelines on how to use thixotropic cements has not been available.

Definition
Thixotropic cement slurries have been described in the past as slurries that remain fluid while being sheared. but past as slurries that remain fluid while being sheared. but which immediately begin to gel when shear stops. A more thorough and accurate definition is that a good thixotropic slurry is a slurry that mixes easily, but which rapidly increases in viscosity both at room temperature and bottomhole temperature as the shear rate approaches zero. Furthermore, this increase in viscosity occurs repeatedly as the shear rate is alternately increased and decreased.

Laboratory Tests and Results
For comparison, the rheological properties of a commonly used 12.5-lb/gal lightweight slurry and a typical 12.5-lb/gal thixotropic cement were measured. Identical procedures at 120 deg. F were used for both slurries. procedures at 120 deg. F were used for both slurries. Figs. 1 and 2 compare the shear stress vs shear rate characteristics of the two slurries. The shear stress increases significantly for a thixotropic slurry as the shear rate increases (Fig. 1). Note that the shear-stress values maintain a single curve for about 20 minutes before increasing sharply. This leveling out is typical of thixotropic slurries and is important to remember when using these slurries in the field.
As expected, the rheological properties were significant for predicting the flow properties of both slurries and indicated the relative shear resistance of the slurries at different shear rates for various times. Correlated to downhole dynamics, these values indicate the shear stress necessary to initiate and to maintain slurry movement while the cement is in the pipe and in the formation.
The cement may be pumped in the pipe as long as the slurry is not stopped for more than 20 minutes. After more than 20 minutes the slurry would be immovable. In addition, the slurry could be stopped and pumped alternately several times and still be thixotropic, as gel tests proved.

A Simplified Mathematical Explanation
Thixotropic cement slurries act similarly to the fluids known as Bingham plastics. Bingham plastics have two properties that describe these fluids: (1) yield point, tau, properties that describe these fluids:yield point, tau, andplastic viscosity, mu.
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