Interactions Of The Cement With The Formation In Geothermal Wells

Abstract

Cementing the steel casing to the formation is one of the most important operations in completing a well for obtaining steam from geothermal fields. The cement is pumped in to fill the annulus between the steel casing and the formation on the side of the borehole and performs several vital roles. Portland cements are normally used so the binder is a calcium silicate hydrate. The reactions that occur hydrothermally with temperature within the hardened cement are reasonably well known but little is known about the interactions that occur between the cement and the formation. The minerals found in the formation, rhyolite and andesite, when used in conventional concrete can react with cement alkalis to give expansive products which cause cracking and low concrete durability. Our work has shown that Ca2+ and OH- ions migrate from the cement through to the porous rock forming an interstitial transition zone (ITZ) between cement and rock where new compounds are formed. This migration is modified by additions of silica and interactions with dissolved carbon dioxide. If a layer of drilling mud adheres to the formation, then alternative compounds are formed and a poor cement bond occurs.