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What is TOC, and how is it shown on schematics?

Top of cement is where the cement column in the annulus ends — and therefore where zonal isolation ends. Here's what sets it, how it's verified, and how to read it off a well schematic at a glance.

By WellSchema Engineering · July 9, 2026

When casing is cemented, slurry is pumped down the inside of the pipe, out the shoe, and up the annulus — the ring-shaped space between the casing and the hole. Pumping stops before the annulus is necessarily full, so the cement column ends somewhere: that depth is the top of cement (TOC). It is one of the most consequential numbers on the entire schematic, because everything below the TOC is hydraulically isolated and supported — and everything above it is not.

Schematic of a vertical well showing four concentric casing strings — conductor, surface, intermediate and production — with cemented annuli, production tubing, and a depth ruler marked in true vertical depth. — Cement is the hatched band in the annulus, and the top of each band is that string's TOC. Here the conductor, surface and intermediate strings are cemented back to surface, while the production casing is cemented only from about the 8,000 ft intermediate shoe down to its own shoe at 9,500 ft — its annulus above that point was never cemented.

Why TOC matters

Zonal isolation. Cement is the barrier that stops formation fluids migrating up the annulus between zones — the exact failure mode behind sustained casing pressure and, in the worst case, groundwater contamination.
Regulatory compliance. Most regulators mandate minimum cement coverage — surface casing cemented to surface is a near-universal rule, and many jurisdictions require cement a stated distance above the shallowest hydrocarbon or water zone.
Casing support and protection. Cement carries part of the load, restrains buckling, and shields the pipe from corrosive formation fluids across the covered interval.
Later operations. Where you can perforate, where you can safely cut casing, what a plug-and-abandonment program must add — all read directly off the TOCs.

Where the number comes from

The planned TOC comes from a volume calculation: slurry volume versus annular capacity from the caliper log, plus an excess factor for washouts. The actual TOC gets verified after the job:

Cement bond log (CBL/VDL) — the definitive measurement, run inside the casing after the cement sets.
Temperature survey — curing cement releases heat; a temperature anomaly a few hours after the job marks the top.
Job records — lift pressure and returns during the job give an immediate, rougher estimate.

A schematic should reflect the verified TOC where one exists, not the design number. The polite fiction of a planned TOC has ruined more than one workover program.

Reading TOC on the schematic

Conventionally, cement is drawn as a shaded or hatched column in the annulus, from the casing shoe up to the TOC, usually with the depth labeled ("TOC 5,000 ft"). Three patterns cover most wells:

Cement to surface — the column reaches the top of the drawing (surface casing, and any string where full coverage was required).
Partial column — the column stops mid-annulus at the TOC; the annulus above it is open (often intentional on intermediate and production strings).
Uncemented sections — some pipe is deliberately not cemented at all: a slotted production liner in an open-hole completion has no cement column, and drawing one on it is a genuine error. The same goes for driven conductor pipe.

Two quick integrity questions you can answer from TOCs alone: is every permeable zone behind cemented pipe or deliberately open? And does any casing shoe rely on a cement column that stops suspiciously close above it? If the drawing is accurate, those answers take seconds — which is precisely why the drawing has to be accurate.