A review of casing cementing practice for geothermal wells in Menengai field, Kenya

Abstract

The nature of highly fractured and weak rock formations can make cementing of geothermal wells challenging. A primary cementing job is usually designed to completely fill the annulus up to the surface. This is usually not achieved in the Menengai geothermal area due to loss of cement slurry into the formation. As a result, it becomes necessary to carry out cement backfill jobs until the annulus is completely filled. During backfilling, there is a risk of trapping water in the casing-to-casing annulus. Such water could enter the annulus from the surface through the side valves, or it could be free water segregating from the cement slurry placed in the annulus. Investigations in Iceland using downhole video cameras have revealed cases of casing collapse attributable to pressure from the expansion of trapped water as the well heats up during production. It is therefore imperative to try to minimise the risk of trapped water in the casing-to-casing annulus as much as possible during cementing. An assessment of the possibility of having trapped water between the anchor and production casing annuli of the existing wells in Menengai, Kenya, is made. Ways to minimise this risk without compromising the integrity of the cement sheath are explored, through a review of backfill volumes and design of an appropriate slurry for backfill jobs using currently available materials.