CEMENT SLURRIES FOR GEOTHERMAL WELLS CEMENTING

Abstract

During a well cementing special place belongs to the cement slurry design.To ensure the best quality of cementing, a thorough understanding of well parameters is essential, as well as behaviour of cement slurry (especially at high temperatures) and application of proven cementing techniques.Many cement jobs fail because of bad job planning.Well cementing without regarding what should be accomplished, can lead to well problems (channels in the cement, unwanted water, gas or fluid production, pipe corrosion) and expensive well repairs.Cementing temperature conditions are important because bottomhole circulating temperatures affect slurry thickening time, arheology, set time and compressive strength development.Knowing the actual temperature which cement encounters durin placement allows the selection of proper cementing materiak for a specific application.Slurry design is affected by well depth, bottom hole circulating temperature and static temperature, type of drilling fluid, slurry density, pumping time, quality of mix water, fluid loss control, flow regime, settling and free water, quality of cement, dry or liquid additives, strength development, and quality of the lab cement testing and equipment.Most Portland cements and Class J cement have shown suitable performances in geothermal wells.Cement system designs for geothermal wells differ from those for conventional high temperature oil and gas wells in the exclusive use of silica flour instead of silica sand, and the avoidance of fly ash as an extender.In this paper, Portland cement behaviour at high temperatures is described.Cement slurry and set cement properties are also described.Published in literature, the composition of cement slurries which were tested in geothermal conditions and which obtained required compressive strength and water permeability are listed.As a case of our practice geothermal wells Velika Ciglena-1 and Velika Ciglena-la are described.