| Title | DURABILITY OF THE CEMENT IN GEOTHERMAL WELLS |
|---|---|
| Authors | N.B. Milestone, M.D.W. Sharp and A.T. Durant |
| Year | 2015 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Well cementing, durability, carbonation cement corrosion |
| Abstract | The cement surrounding the casing is an integral part of well design to exploit geothermal energy. It plays several roles including sealing in the high pressure steam and ensuring isolation of the various zones through which the well penetrates. Any degradation of cement may compromise the working life of the well where lifetimes in excess of 30 years are now being sought. New Zealand’s early wells were cemented with conventional construction cement after quenching to around 40°C and most of the wells associated with the Warakei development (the station came on stream in 1958) have lasted well with few problems. In later development at Ohaaki, where the wells were quenched to 60°C before cementing, significant corrosion problems due to acidic fluid interactions with the hardened cement were encountered. Our recent research is showing that the conventional engineering approach of developing the strongest binder through use of silica flour, has not taken into account any chemical reactions that: (i) occur during the initial formation of the hydrothermal cementing phases as the cement hardens, and (ii) subsequently occur between the hardened cement phases and the down hole geothermal fluids. These reactions can lead to issues with long term durability of the cement sheath. In recent well completions, the wells have been cooled by circulation of drilling fluid with cementing nominally undertaken around 90°C. The higher temperatures now encountered during cementing have changed the cement hydration mechanism so that some retarders will no longer function efficiently, and different hydration reactions can occur. The presence of CO2 affects the way in which cement hydrates and the phases that are formed. |