How to read a well schematic

A well schematic (also called a wellbore diagram) is the one drawing that describes how a well is actually constructed: which pipes are in the ground, how deep they go, what is cemented, and how the well produces. Handed a schematic, an experienced engineer can reconstruct most of the drilling history of the well. Here is how to read one, top to bottom.

Start at the datum

Every depth on a schematic is measured from a stated reference — the datum. On land wells this is usually the kelly bushing (KB/RKB) on the rig floor; ground level (GL) and mean sea level (MSL) are the other common references. A professional schematic always states its datum in the title block, because a depth without a datum is ambiguous by the height of the rig floor — often 20–30 ft.

The casing strings, outside-in

Wells are built like telescopes: each string of casing is run inside the previous one, so diagrams read outside-in and shallow-to-deep. A typical land well has:

• Conductor — the first, largest pipe (20" in the example), often driven rather than cemented. It keeps loose surface ground out of the hole.
• Surface casing — protects groundwater and carries the BOP loads. Regulators almost always require cement back to surface, which is why its cement column reaches the top of the drawing.
• Intermediate casing — isolates troublesome zones (lost circulation, unstable shales, abnormal pressure) between surface casing and the reservoir.
• Production casing or liner — the string across the reservoir that contains producing pressures for the life of the well.

The small triangle at the bottom of each string is the casing shoe. Next to each string you will normally find its size (outside diameter), weight (lb/ft), grade, and setting depth — for example 9 5/8", 53.5#, N-80 at 7,930 ft MD.

Cement: what is isolated, and what is not

The shaded column in the annulus (the space between casing and hole) is cement. It runs from the shoe up to the top of cement (TOC). Where there is cement, zones are hydraulically isolated; where the annulus is open, they are not. In the example, the intermediate string is cemented from its shoe at 7,930 ft up to a TOC of 5,500 ft — everything above that is open annulus. Reading TOCs is one of the fastest ways to assess well integrity from a drawing.

Inside the casing: the completion

• Tubing — the narrow string (2 7/8" here) that actually carries production. Casing is the structure; tubing is the flow path, and it can be replaced without redrilling anything.
• Packer — the seal between tubing and production casing, drawn as a crossed box. It forces flow up the tubing and keeps produced fluids off the casing wall.
• Perforations — the tick marks across the casing at reservoir depth, where the casing and cement have been shot to let the formation flow in.
• Open hole — any drilled section below the last shoe with no pipe across it.

A reading checklist

When you pick up an unfamiliar schematic, run this order:

• Datum and depth type (MD vs TVD) from the title block.
• Trajectory — vertical, deviated or horizontal, and the kickoff point.
• Each casing string: size, weight/grade, shoe depth, hole size.
• Each TOC — what is isolated and what is open annulus.
• Completion: tubing depth, packer depth, perforated intervals.
• Anything unusual: sidetracks, liners and tie-backs, slotted sections, fish left in hole.

Once that order becomes habit, a schematic stops being a picture and becomes the well's résumé.