Chemical Thickening-Time Test for Cement Blends
Authors: McElfresh Paul M. , Cobb Jo Ann 
Published: December 2007

Considerable time is lost during field operations waiting for cement-thickening times to be determined. Our method to minimize the amount of waiting time is a colonmetric procedure based on Basic Blue 9 dye for determining a relative measure of the thickening time for a cement blend. This test is applicable to any field blend regardless of the additives used, and can be run in a matter of minutes rather than hours.

Introduction

Consistometer thickening-time tests always have been used to establish whether a particular field blend will perform as expected downhole. These tests involve use of expensive equipment and one test sometimes requires an entire working day or more to complete. The consistometer thickening times are costly because of the equipment involved, the manpower costs associated in running the tests, and the time lost in the field. The chemical method presented here was developed to minimize the number of samples run on the consistometer and thus to reduce the time and cost factors involved.

The primary application of the chemical thickeningtime test is to verify the blended composition of field mixtures relative to a laboratory-prepared pilot mixture. Specifically, it is designed to monitor the uniformity of the blending process when blending large jobs. Each 100- to 200-sack portion of the total amount to be blended-a "drop"-is tested and compared to the pilot blend to ensure that the entire slurry will be uniform downhole. This test can be a substitute for rerunning consistometer thickening-time tests when time does not permit additional testing. The method also can be used to provide quality-control analyses for additives that normally require consistometer thickening-times. Data presented support the use of colorimetric determination to monitor thickening time, provided that a pilot blend that has had its thickening time measured on a consistometer is used as a standard. This approach also can be used to design slurries without repeated thickening-time determinations. This is accomplished by preparing two standards that bracket the desired thickening time.

Theory

The chemical thickening-time test is based on a colorimetric procedure that measures the amount of ionic dye in cement filtrate. Young proposes that cement particles have active sites onto which various additives are adsorbed. When cement is blended and hydrated, the additives are exposed thoroughly to these active sites and are adsorbed onto the cement particles. This adsorption determines the performance of the cement with regard to such properties as thickening time and fluid loss. The ionic nature of the dye added after the cement has been hydrated allows it to be adsorbed onto the remaining active sites of the cement. It is the number of these vacant active sites that determines the concentration of dye left in the cement filtrate, and, therefore, becomes an indirect measure of the total additive concentration.

Some of the additives that are used for extenders and for weighting materials are not adsorbed onto the cement, but these additives have active sites that are very similar to those on cement.

JPT

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