The Significance of silica and other additives in geothermal well cementing

Abstract

Geothermal drilling is a capital intensive venture that needs extensive resources to explore and develop to production phase. The well drilling, for instance, involves geoscientific exploration, drilling and casing using cement slurry. The reliability of the power plant will depend on the lifespan of the production wells in the geothermal field. There are many factors that make a well be non-productive, such as poor well casing among others. In high temperature geothermal fields, which are rich with harmful geothermal fluids, there is a need to design and formulate cement slurry that can withstand these environments. Portland cement is stabilised by adding a silica component to achieve good compressive strength development and reduction of permeability in the cement. Pozzolonic Portland cement can be a viable option too. In Olkaria, Kenya, 20% silica flour by weight of cement is used to prevent strength degradation by the CO2 rich environment that causes gas channelling having an adverse impact on cement bond integrity, hence accelerating strength retrogression. Most wells are highly permeable and therefore one of the greatest challenges in cementing is due to circulation losses.

Mica flakes are currently used to prevent or minimize losses. New loss of circulation materials such as Cementing Lost Circulation Fibers (CLCF) could be a viable option. The new fluid loss control (FLC) agent ADVA Cast 530 gives improved rheological properties. Other possible alternatives to the challenges of lost circulation are to reduce the density of the slurry by using foam cement or to introduce new additives such as perlite, microspheres and other low density pozzolanic materials. To fully analyse the strength retrogression of well cements in Kenya, that are exposed to high temperatures over their lifetime, tests of cement slurries at the laboratory should ideally be simulated at close to actual well temperatures (bottom hole circulating temperature (BHCT) and bottom hole static temperature (BHST)) over a few months, since this would give a better indication of the strength retrogression. Current research in the geothermal sector seem to lack testing at high enough temperatures over a sufficiently long period.