Why We Cement a Well

A drilled hole passes through many rock layers. Some layers hold water, some hold oil or gas, some are weak and some carry high pressure. When the hole reaches a planned depth, the crew runs a steel pipe called casing into it. The casing is smaller than the hole, so a ring-shaped gap is left between the casing and the rock. This gap is the annulus.

Primary cementing is the operation that fills this annulus with cement slurry, a mixture of cement powder, water and additives. The slurry is pumped down the inside of the casing, out through the bottom, and up into the annulus. There it sets into a hard, low-permeability solid called the cement sheath.

Primary cementing is done once for each casing string, at the time it is run. Any later cement work to repair or change the well is called remedial cementing.

Job 1 — Zonal isolation

The most important job of the cement sheath is zonal isolation. This means stopping fluids from moving along the annulus between one rock layer and another, or up to the surface.

Without isolation, gas from a deep zone could travel up the annulus and reach the wellhead. Salt water could flow into an oil zone. Oil could leak into a shallow freshwater aquifer that a town uses for drinking water. Each of these is a failure of the well, and some are very hard to repair.

Good isolation needs a continuous sheath. The cement must fill the annulus all the way around the casing, bond to the casing and to the rock, and contain no channels of mud or gas.

Why the job matters so much

A cement job is done once, in a few hours, in a place nobody can see. The result must last for the whole life of the well, which may be thirty years or more, and then through abandonment. A poor job can cost far more to fix than the original job cost. Remedial cementing is not always successful, and some defects cannot be repaired at all.

For this reason, every person on the job — from the bulk operator to the drilling supervisor — needs to understand what the cement is for. A crew member who knows that the aim is a complete seal will notice the things that put the seal at risk: poor mud conditioning, a density outside the range, or a plug that did not land.