Optimising Silica Additions in a Durable Geothermal Cementing Formulation to Improve Rheological Properties

Abstract

Following extensive research, we have recommended the addition of Microsilica 600 (MS600) at 20% addition by weight of binder to a Portland based oil well cement as the best option for providing carbonation resistance in geothermal well cementing. MS600 is a fine amorphous silica mined locally that reacts quickly with Ca(OH)2 formed from the hydration of cement and inhibits crystallisation of any hydrothermally formed calcium silicate hydrates. This reaction with Ca(OH)2 also prevents the phenomenon known as strength retrogression, caused by the crystallisation and growth of a phase known as ?-C2SH, from occurring. Its use also stops the rapid carbonation of some cement formulations arising from the crystallisation of tobermorite (C5S6H5) which forms when silica flour is used.
One of the drawbacks of using MS600 is its very fine particle size, which creates a high water demand which results in a cement slurry that is thick and more difficult to pump and place than regular cementing formulations when silica flour is used. Chemical admixture additions can be used to control and improve rheology but an increased use of these chemicals can be costly.
We have investigated a range of cement-silica blends containing MS600 to try to improve the rheological properties of the blend. By using silica containing materials in conjunction with MS600, lower water demand formulations and reduced levels of chemical admixtures can be obtained, while still maintaining the carbonation resistant properties of the amorphous binder associated with the use of MS600.