Preparation of setting slurries

Abstract

When preparing a settable slurry such as a cement slurry for cementing an oil well, cement or other solid powder is mixed with water and a set retarder. In order to be able to run a convenient and rapid check on the concentration of retarder after dilution at the site of use, a tracer material is mixed with the retarder in known amount during manufacture. The tracer is chosen to enable its concentration to be determined analytically after dilution at the site of use, thereby providing a way to determine the concentration of set retarder after such dilution. The tracer may be a redox-active material and its concentration may be determined by voltammetry. Tracer may likewise be mixed with additives other than set retarder.

1 . A process of preparing a settable slurry by mixing a solid powder, water and one or more additives, characterised by mixing a tracer material with an additive before mixing that additive with the solid powder and the water and analysing a mixture containing at least the additive and at least part of the water to determine the concentration of tracer therein. 
     
     
         2 . A process according to  claim 1  wherein the step of analyzing a mixture is performed on the slurry. 
     
     
         3 . A process according to  claim 1  wherein the step of analyzing a mixture is performed on a mixture of the additive and water before mixing with the solid powder. 
     
     
         4 . A process according to  claim 1  wherein the solid powder is cement, so that the slurry is a cement slurry. 
     
     
         5 . A process according to  claim 4  wherein said additive with which the tracer is mixed is a cement set retarder. 
     
     
         6 . A process according to  claim 5  which includes a step of pumping the cement slurry into a space between a borehole and a casing within the borehole. 
     
     
         7 . A process according to  claim 1  wherein the tracer is mixed with the additive at a manufacturing or storage facility and the additive with admixed tracer is subsequently transported to another site for preparing the slurry by mixing with the solid powder and water. 
     
     
         8 . A process according to  claim 1  wherein analysis to determine the concentration of tracer is carried out by electrochemistry. 
     
     
         9 . A process according to  claim 8  carried out with an array of laminar electrodes formed on an insulating substrate. 
     
     
         10 . A process according to  claim 8  wherein the tracer undergoes electrochemical oxidation and/or reduction and analysis is carried out using voltammetry to determine the concentration of tracer. 
     
     
         11 . A process according to  claim 10  where analysis is carried out using square wave voltammetry. 
     
     
         12 . A process according to  claim 8  wherein
 the step of analyzing a mixture is performed on a mixture containing a mediator compound which undergoes electrochemical oxidation and reduction 
 the tracer undergoes chemical reaction with the mediator compound, and 
 analysis is carried out using voltammetry to observe the effect of tracer on electrochemical reaction of the mediator compound.