A glossary of legacy well terms

Well status

Abandoned well: In common usage, a well that was plugged and taken out of service under the rules in force at the time, with the paperwork filed and the operator's obligations closed. Regulators use the word more loosely; EPA's definition takes in orphaned wells and long-idle wells as well. When someone says "abandoned," ask which meaning they intend, because the liability consequences differ.
Orphaned well: A well that is inactive and has no viable operator responsible for plugging it, in the Interstate Oil and Gas Compact Commission definition that most states and the federal program follow. The operator is gone, bankrupt, or can't be found, so the plugging cost falls to a state program or to federal funding. Orphaned wells are the narrow, countable subset of the legacy well population.
Idle well, inactive well: A well with an identifiable operator that hasn't produced, injected, or otherwise been used for a period the state defines. The period varies by state, from months to years, and so does what happens when it's reached: additional bonding, testing, fees, or a deadline to plug or return to production. Idle wells are the largest and least well defined category, and the one most likely to become orphaned when an operator fails.
Zombie well: A colloquial term rather than a regulatory category, which took hold in Texas, for a well that was plugged decades ago and has come back to life because the plug failed and fluid is moving up the wellbore again. A zombie well is abandoned on paper, isn't orphaned if a solvent successor operator exists, and isn't eligible for federal orphan well funding, which doesn't cover re-plugging.
Legacy well: The umbrella term this site uses for any well drilled under earlier standards that still exists in the ground: abandoned, orphaned, idle, or zombie, plugged or not. It carries no regulatory meaning and is chosen for that reason.
Undocumented well: A well that exists in the ground but appears in no regulatory database, usually because it predates permitting or its record was lost. Undocumented wells are found by field survey, old maps, magnetometry, and, increasingly, satellite deformation data, and each discovery widens the inventory.

Construction and failure

Casing: The steel pipe run into a wellbore and cemented in place to hold the hole open and isolate the formations it passes through from one another and from the surface. A well usually has several concentric strings of casing set to different depths. Corrosion of casing, particularly at threaded connections, is one of the two main ways a plugged well loses its seal over decades.
Cement sheath, annulus: The annulus is the space between a casing string and the rock, or between two casing strings. The cement pumped into it is meant to fill that space and bond to both surfaces so nothing can move up or down along the outside of the pipe. How much of the annulus the cement actually filled, and how well it bonded, is the question a cement bond log tries to answer.
Microannulus: A thin, continuous gap between cement and casing or between cement and rock, usually opened by cement shrinkage during curing or by thermal and pressure cycling afterward. It may transmit nothing for decades. It only needs to exist for a later pressure difference to use it.
Plugging and abandonment (P&A): The process of permanently closing a well: setting cement plugs at required intervals inside the wellbore to isolate formations and seal the hole, cutting and capping the casing below ground, and restoring the surface. Historically the rules specified where plugs went and how long they had to be, not how long they had to last.
Cement plug: A column of cement placed inside the casing or open hole at a specified depth during plugging. Its purpose is to stop vertical movement of fluid past that point. A plug that shrinks, cracks, or was never placed where the report says it was is the other main way a plugged well loses its seal.
Sustained casing pressure: Pressure that builds in a casing annulus at the surface, is bled off, and comes back. It means gas or fluid is entering the annulus from somewhere below, through a channel in the cement or a leak in the pipe, and it's one of the few surface signals that a wellbore's isolation has failed.
Wellbore integrity: Whether the casing, cement, and plugs of a well are still doing what they were put there to do: keeping fluids in the formations they came from. Integrity is a condition, not a certificate, and it changes over time.
Cement bond log (CBL): A wireline log run inside cased hole that measures how well the cement is bonded to the casing, and with some tools to the formation, by recording how sound travels through the pipe. It's a snapshot of the cement at the time the log was run, read through the casing, and it's only as good as the tool of its era could see.

Disposal and injection

Produced water: The water that comes up with oil and gas. It's salty, often far saltier than seawater, sometimes carries naturally occurring radioactive material, and there's little use for it, so most of it is injected back underground.
Saltwater disposal well (SWD): A well drilled or converted to inject produced water into a porous formation for permanent disposal, as distinct from injection meant to push oil toward producing wells. In the federal Underground Injection Control program these are Class II wells.
Class II well, Underground Injection Control (UIC): UIC is the federal program under the Safe Drinking Water Act that regulates injection wells to protect drinking water. Class II is its category for wells associated with oil and gas production, including disposal wells. Most oil and gas states administer the program themselves under delegated authority, which is why the rules vary from state to state.
Underground source of drinking water (USDW): An aquifer that supplies, or could supply, drinking water, defined in the federal rules by its salinity. A disposal permit is supposed to demonstrate that injected fluid will stay separated from any USDW.
Injection interval, receiving formation: The depth range and the rock unit the disposal well is permitted to inject into. The receiving formation's permeability, thickness, and compressibility decide how far the injected pressure spreads.
Confining layer: A layer of low-permeability rock above or below the injection interval that's meant to keep the injected fluid and its pressure in the receiving formation. Faults and old wellbores are the two things that cut through it.
Pore pressure: The pressure of the fluid in the pore space of a rock. Injection raises it; production lowers it. A change in pore pressure is what moves fluid through a compromised wellbore and what lowers the friction holding a fault in place. How pressure moves out from a disposal well covers the mechanism.
Area of review (AOR): The region around a proposed injection well that the applicant must examine for existing wellbores that could carry fluid or pressure out of the injection interval. State rules fix it as a radius. Pressure doesn't observe the radius, which is why the practice estimates the actual pressure footprint and compares the two.
Pore space: The void volume in subsurface rock that can hold fluid. Who owns it, the surface owner or the mineral owner, varies by state and matters when someone wants to put something into it, such as produced water or carbon dioxide, or when pressure from a neighbor's injection arrives in it.

Seismicity

Induced seismicity: Earthquakes caused by human activity, in this context by fluid injection raising pore pressure on a fault that was already close to slipping. The injection doesn't create the stress; it lowers the friction that was holding the fault in place. How an induced seismicity assessment is built covers the method.
Fault slip potential: An estimate, from a fault's orientation, the regional stress field, and the pore pressure, of how much additional pressure would bring the fault to failure. Published fault slip potential studies exist for the major producing basins and are one input to a seismicity assessment.
Earthquake catalog: The record of located earthquakes compiled by a seismic network, with a time, location, depth, and magnitude for each. State networks and the national network publish them. Every catalog has a magnitude below which it misses events and a location uncertainty that depends on how many stations recorded each one, and both limits matter when reading it.
Seismic response protocol: A regulator's published procedure for what happens to injection permits when earthquakes above given magnitudes occur within given distances of a disposal well: review, rate reduction, or suspension in stages. The thresholds are set by each state and change over time.
InSAR: Interferometric synthetic aperture radar, a satellite method that measures small changes in ground elevation over time. It can show ground rising over a disposal zone or sinking over a depleted one, and it's been used to find wellbore leakage and undocumented wells from the surface.

Money and liability

Financial assurance: The general term for money or guarantees an operator posts so that plugging and restoration get paid for if the operator doesn't do them. Bonds are the usual form; escrow, letters of credit, and insurance products are others.
Plugging bond: A surety instrument or cash deposit posted with the regulator, forfeited if the operator fails to plug. An individual bond covers one well or lease. The amount is set by rule and, historically, has stayed fixed for decades while plugging costs rose.
Blanket bond: A single bond that covers all of an operator's wells in a state, regardless of how many. Per-well coverage falls as the inventory grows, which is how coverage ratios of a few percent, or a fraction of one percent, come about.
Bonding gap: The difference between what financial assurance covers and what plugging and restoration would actually cost. It's usually a design feature of the framework rather than a recent failure, and it's the subject of Article 5.
Risk-based prioritization, triage: The scoring a regulator uses to decide which orphaned wells get inspected, monitored, or plugged first when money is limited: evidence of leakage, proximity to people and water, age, construction, and so on. It's a ranking at a moment in time. It doesn't say which wells will fail; it says where failure would be noticed first.
Successor liability: The question of whether the obligation to plug a well transfers with the well when it's sold, and whether it survives a bankruptcy. The answer differs by state and by the structure of the sale, and it's where a large share of orphaned wells come from.

Records

API number: The unique identifier assigned to a well in the United States: a state code, a county code, and a well number, with further digits for sidetracks and events. It's the key to every state database. Wells drilled before the system existed may have one assigned retroactively, or none.
Well file: The regulator's record of one well: permits, completion report, logs, production and injection history, operator transfers, inspections, and the plugging report. What it contains, and how much of it survives, is the subject of How to read a legacy well file.
Completion report: The form the operator files after drilling and completing a well, recording the casing strings and their depths, the cement volumes pumped, the perforated intervals, and the tests run. It records what was reported, which is not always what was done.
Plugging report: The form filed after plugging, recording where plugs were set, how much cement went into each, and the method. It's the document that turns a well into an "abandoned" well, and it's a report of work, not a verification of it.
Scout ticket, scout card: A one-page summary of a well's drilling and completion compiled at the time by commercial scouting services for the industry, often the only surviving record of early wells' depths and casing. They're useful, and they're secondhand.
Well log: A continuous record of a measurement against depth in the wellbore, made by a tool lowered on a wireline. Open-hole logs measure the rock before casing is run; cased-hole logs, including the cement bond log, measure through the pipe. Each tool has a generation, and each generation could see certain things and not others.

Definitions are given as the terms are used in the article series and on the practice page. Where a state's rule defines a term differently, the rule governs. This page isn't legal advice and doesn't describe any engagement.