How to read a legacy well file

What a well file is

Every oil and gas well drilled under a permit has a file with the state regulator, keyed by its API number. In most states the file is online now, scanned from paper, and it holds whatever the operator filed and whatever the agency generated over the life of the well: the drilling permit, the completion report, any logs that were submitted, production or injection reports, operator transfers, inspection notes, complaints, and, if the well was plugged, the plugging report.

The file is the record of what was reported. That's the sentence to keep in mind for everything that follows. A well file is a stack of forms, filed by different people at different times under the rules of their day, rather than a description of the well as it exists in the ground, and the well in the ground has been changing ever since.

The permit and the location

The drilling permit or application establishes that the well was authorized, when, by whom, and where it was supposed to go. It gives the surface location, usually as a footage call from lease or section lines, the proposed depth, and the target formation.

What it establishes is intent. The well may have been drilled somewhere slightly different, deeper or shallower than proposed, or not at all. Older permits carry locations that were surveyed to the standards of the time, and a location that's off by a few hundred feet is common in files from before satellite positioning. When a permit location and a plugging report location disagree, or neither matches what's found in the field, that's the first gap to record.

The completion report

The completion report is the most important construction document in the file. Filed after drilling, it lists the casing strings run and the depth each was set to, the volume of cement pumped behind each string, the perforated or open intervals, and the tests performed. From it you can draw the wellbore: which formations are behind steel and cement, which are behind steel alone, and where the well is open to the rock.

Three cautions apply. Cement volume pumped isn't cement in place; the report says how many sacks went down the hole, not where they ended up, and the difference between the two is where channels and gaps come from. A cement top reported as calculated is an estimate from the volume, while a cement top reported as measured came from a log, and the report doesn't always say which. And a completion report describes the well on the day it was filed. Workovers, recompletions, and casing repairs in later years may or may not have generated their own paperwork.

The logs

A well log is a measurement recorded against depth by a tool lowered into the hole on a wireline. Open-hole logs, run before casing is set, measure the rock: resistivity, natural radioactivity, porosity, and the diameter of the hole. They establish what formations the well penetrated and at what depth, which is how a legacy well gets tied to the receiving formation of a disposal permit half a mile away. A caliper log, which records hole diameter, also tells you where the hole was washed out and where cement would have had to fill more space than planned.

Cased-hole logs, run after casing is set, are the ones that speak to isolation. The cement bond log records how sound travels through the pipe and reads the result as bond quality along the string. It's a snapshot of the cement on the day it was run, through steel, with the resolution its generation of tool allowed. A good bond log from the year the well was completed says the cement was sound then; it says nothing about fifty years of shrinkage, corrosion, and pressure cycling since. A poor bond log says less than it seems to, because the tool can be fooled by a microannulus that's mechanically fine and by fast formations behind the pipe. What the tool could and couldn't see in its era is a question for someone who ran them.

Many legacy files hold no logs at all. Logs were the operator's property, filing requirements came late, and paper logs from before the 1970s were often never submitted or didn't survive scanning. An absent log is a fact about the file, not about the well.

Production, injection, and transfers

Monthly production reports, where they exist, establish when the well was active, what it produced, and roughly how much reservoir pressure it drew down over its life. For an injection well the equivalent is the injection report: volumes and, in later years, pressures. These are the records that let you say whether a well sat in a depleted formation for decades or in one that's been repressurized.

Operator transfer records establish who held the well and when. They matter for liability, and they matter for the record itself, because each transfer is a point where paperwork could stop following the well. A file that shows a producing well transferred three times in a decade to progressively smaller operators, then goes silent, is telling you how it became an orphan.

The plugging report

The plugging report is the form filed after a well is closed: the plugs set, at what depths, with how much cement, by what method, and who witnessed it, if anyone. It's the document that changes a well's status to plugged and abandoned and ends active regulatory oversight.

It's also a report of work rather than a verification of it. Plug depths were often measured by pipe tally rather than logged. Cement volumes are what was pumped. Whether the plugs were set across the intervals the rules required, whether they bonded, and whether they're still there are separate questions the report doesn't answer. For a well plugged in the middle of the last century, the plugging standard of the day may have called for far less than the rules require now, and the report will show compliance with the old standard, which is what it should show. Compliance and isolation are different findings.

Scout tickets and other secondhand records

For early wells, the regulator's file may be thin or empty, and the best surviving record is often a scout ticket: a one-page summary of the well's drilling and completion compiled at the time by a commercial scouting service and sold to the industry. Scout tickets give depths, casing, formation tops, and initial production, and they're frequently the only source for wells drilled before the state required completion reports.

They're secondhand. A scout recorded what he was told or could observe, and the information was gathered for competitors, not regulators. Old county plat maps, lease files, and the operator's own records, where a successor still holds them, fill the same role with the same caveat.

Where the record runs out

The record runs out in predictable places. Wells drilled before permitting have no file at all and may have no API number; they exist in old maps, scout tickets, and the ground. Wells from the early permitting decades have files that were built on paper, moved, microfilmed, and scanned, with pages lost at each step. Wells transferred among small operators lose their paperwork at transfer. And plugged wells stop generating records the day the plugging report is accepted, so a well plugged decades ago has a file that ends there, however much has happened to it since.

A gap in the file is a finding. It belongs in the analysis stated as a gap, with what would fill it, rather than papered over with an assumption. "No cement bond log; cement top calculated from volume; no record of the well after the operator transfer" is a complete and useful sentence.

Reading the file as a set

The documents check one another. The completion report's casing depths should agree with the plugging report's plug depths and with the depths on the logs. The formation tops on an open-hole log should match the target on the permit and the tops on the scout ticket. The last production report should sit close to the date the well was reported idle or plugged. Where they agree, the file is probably describing one well. Where they don't, something happened that the file doesn't explain, and that's where to look.

This is what the practice's wellbore integrity screening does across a population of wells: reconstruct each wellbore from its file and the secondhand records, rank the population by the likelihood that a wellbore is an open conduit, and say plainly which wells have records good enough to say either way. The output is a ranked list rather than a verdict on any well: a record of what each file does and doesn't establish, and a shorter list of wells that need a field program before anyone relies on them.

This page describes records and a method. It isn't engineering certification, a regulatory determination, or legal advice, and it doesn't describe any engagement. Terms are defined in the glossary.