A New Portland Cement Derived Composite Binder for Use in Geothermal Wells

Abstract

The shortcomings of regular grades of Portland cements used in service temperature environments above 110°C are well known. The metastable Calcium Silicate Hydrate reaction products recrystallise as generally denser, lower volume forms such as α-C2SH, which leads to shrinkage and loss of compressive strength. In geothermal environments the associated cracking creates pathways for down hole fluid access. This process is known as strength retrogression. Modern type A cements also show additional volume loss with the conversion of ettringite into higher density forms.
Strength retrogression is traditionally controlled through the addition of 35-40 wt% silica to the cement which allows the preferential formation of tobermorite, xonotlite and gyrolite which give desirable well cementing qualities.
High levels of carbon dioxide are commonly found under geothermal conditions. The addition of excess silica to control strength retrogression results in a cement body very prone to rapid carbonation resulting in the deposition of lower volumes of aragonite, leading to shrinkage and reduced compressive strength. Associated cracking creates pathways for down hole fluids to accelerate this process further. To offset this the levels of silica are generally reduced but the result is a cement grout body that often rapidly degrades through both strength retrogression and carbonation processes.
This paper describes a new composite Portland derived cement designed to maximize durability and well life under geothermal conditions by taking advantage of the carbon dioxide concentrations to form desirable reaction products.