I N AMER I S AMER I EUROPE I AFRICA I M EAST I ASIA-PAC I GLOBAL I

Well types

Drilled wells - are defined as those drilled by drilling rigs with intent to locate and exploit oil and gas resources. They are allocated to the year of spud, except for pre-spudded development wells, allocated to the year of return to the hole.  Sidetracks, drilled from existing well bores, become drilled wells if the original well has seen its rig stand down and the intent is to acquire new data and production.

Exploratory wells - are defined as the sum of exploration and appraisal wells.

Exploration wells - are defined as those drilled where a field is suspected (a prospect), to determine the presence of hydrocarbons and collect data to assess the merit of a prospect.

Appraisal wells - are defined as those drilled during appraisal of a prospect that has been drilled by a successful exploration well.

Development wells - are defined as all those drilled to produce a field once volumes have been proven by exploratory wells, including producers and injectors.

Surface-completed wells - are defined as those offshore wells on (usually fixed) platforms where the valve set (Christmas Tree) is above sea level (dry).

Subsea-completed wells - are defined as those offshore wells on (usually floating) platforms where the Christmas Tree is located on the sea floor (wet) and a riser runs to the production platform.

Drilled wells (onshore) - are defined as those that are not ‘Drilled Wells (Offshore)’. They include wells drilled from shallow water barges in lakes and swamps and wells drilled from onshore sites to tap offshore accumulations.

Drilled wells (offshore) - are defined as those that are drilled with rigs in marine environments, including land rigs, such as platform rigs, erected on platforms and artificial islands. The location of the wellhead defines the water depth. ‘Very shallow waters’ have a seabed location beneath marine water depths down to 100m, ‘medium shallow waters’ from >100 to 500m, ‘medium deep waters’ from >500 to 1000m, ‘very deep waters’ from >1000 to 2000m, and ‘ultra deep waters’ from >2000m.

Producing (active) wells - are defined as those that have been drilled into a reservoir and are active, used to produce oil and/or gas from that reservoir including wells that are used for injection of fluids.

Producing (active) wells (oil) - are all active wells that have been drilled into an oil or oil and gas reservoir and are used to produce oil (and any associated gas) from that reservoir. They include wells that are used for injection of fluids.

Producing (active) wells (gas) - are all active wells that have been drilled into a gas reservoir that has no associated liquids (including condensates stable at the surface). The wells are used to produce only gas and include wells that are used for injection of fluids.

More details

Drilled wells (very shallow waters) - are defined as having a seabed location beneath marine water depths ranging from greater than 0m down to 100m. If a mobile rig is used these wells are usually drilled with jackups.

Drilled wells (medium shallow waters) - are defined as having a seabed location beneath marine water depths ranging from greater than 100m down to 500m. If a mobile rig is used these wells are usually drilled with floaters (semisubs and drillships) although a few may be drilled with advanced jackups.

Drilled wells (medium deep waters) - are defined as having a seabed location beneath marine water depths ranging from greater than 500m down to 1000m. Floaters are used to drill nearly all wells in deep waters although in rare cases fixed rigs on platforms can be used.

Drilled wells (very deep waters) - are defined as having a seabed location beneath marine water depths ranging from greater than 1000 down to 2000m.

Drilled wells (ultra deep waters) - are defined as having a seabed location beneath marine water depths greater than 2000m.

Exploration well - is drilled where a field is suspected (called a prospect), to determine the presence of hydrocarbons and collect data to assess the merit of a prospect. Also known as new-field wildcats or new-pool wildcats.

Appraisal well - is drilled after an exploration well has been evaluated and a decision to go ahead with appraisal of a project has been made. Also known as delineation, offset, step-out or outpost wells. If a well has a dual or triple purpose it is defined as an appraisal.

Development well - is drilled to produce a field once volumes have been proven by exploratory wells; most are producers but some may be designated injectors used to pump fluids into the reservoir for pressure support and to conserve gas.

Producing wells (active) - comprise the number of wells in operation in a given year that have been drilled into an oil, gas, or oil and gas, reservoir AND are being used to produce oil and/or gas from that reservoir (including wells that are used for the injection of fluids). They do not include wells that have never been used, suspended and have been abandoned.  

Stripper wells (USA) - are defined as any well whose maximum daily oil production does not exceed 15 bbls, or gas production does not exceed 90 mcf during a 12-month period.

Marginal wells (USA) -are producing wells that is unable to operate at a lower price per unit of oil or gas because of low production rates and/or high production costs and/or high co-production of associated water and non-combustible gases.

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Modelling producing wells

Onshore model

In the absence of calibration data it is assumed that 10% of exploration wells drilled, 60% of appraisal wells drilled and 90% of development wells drilled each year are completed as producers or injectors to become part of the “active well stock”.

The attrition rate for active wells is between 2% and 5% each year. Thus every year 2% to 5% of the active well stock are abandoned whilst 10%/60%/90% of the E/A/D wells drilled are added to the existing years active well stock.

Where empirical numbers are available in the public domain calibrations are performed within reasonable limits of the model, including any rapid changes as a result of non-routine surface events (e.g. OPEC restriction of output).

Offshore model  

In the absence of calibration data, for surface-completed wells, it is assumed that all successful development wells drilled each year are completed as producers or injectors to become part of the “active well stock”.

The attrition rate for active wells is estimated at 2% each year. Thus every year 2% of the active well stock are abandoned whilst all of the successful development wells drilled are added to the preceding years active well stock.

For subsea-completed wells, it is assumed that all successful development wells drilled each year are completed as producers or injectors to become part of the “active well stock”.

Shallow and deep water subsea wells are expected to have an 8- and 10-year active life respectively before being abandoned. Thus every year all wells drilled 8 (or 10) years previously are abandoned whilst all of the development wells drilled are added to the preceding years active well stock.

Where empirical numbers are available in the public domain calibrations are performed within reasonable limits of the model, including rapid changes as a result of non-routine surface events (e.g. OPEC restriction of output).

Note that all empirical numbers in the public domain can be highly variable, with different definitions (often unstated)