Well Types

A well is a hole drilled to find or delimit a petroleum deposit and/or produce petroleum or water for injection purposes, inject gas, water or another medium, or map or monitor well parameters.  There are many types of wells.  Well construction requirements are determined by geologic, hydrologic and environmental factors.  A particular well type may be best suited or most economic in the efforts to drain a specific configuration of hydrocarbons.  Various drilling strategies can be adopted to place wells in specific patterns with the aim of optimizing production from a field.

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Common Well Types

• Abandoned Well  -  A well permanently plugged in the drilling phase for technical reasons.
• Appraisal Well  -  An oil and gas company will typically go on to drill an appraisal well once a discovery has been made.  The wells, which have a higher chance of success and are more expensive than exploration wells, are used to determine the size of an oil gas field (both physically and in terms of its reserves) and its expected production rate.  They are also used by a company when looking at the most efficient way of developing a discovery or whether it makes economic sense to drill at all.
• Brine Well  -  A well used for injecting fresh water into geologic formation comprised mainly of salt.  The injected freshwater dissolves the salt and is pumped back to the surface as a saturated sodium chloride brine solution used as a feedstock in petrochemical refineries and in oil and gas well drilling and workover operations.
• Closed-in Well  -  A well with a valve closed to halt production.  Wells are often closed in for a period of time to allow stabilization prior to beginning a drawdown-buildup test sequence.
• Developmental Well  -  A well drilled in a proven area of an oil or gas reservoir with the goal of maximising economic gloss production and recovery of reservoirs known reserves.  They are typically drilled to the depth of a stratigraphic horizon that is known to be productive. The probability of success increases as more development wells are drilled.  Development wells differ to exploratory wells as they are wider in diameter and drill deeper.
• Deviated Well  -   A well that is drilled at an angle from the vertical, but not to the extent of a horizontal well.  Deviated wells are typically used when the oil or gas reservoir is located at an angle to the surface or when there are other geological barriers that prevent vertical drilling.  The angle of deviation can vary depending on the specific circumstances, but typically deviated wells have a deviation angle of less than 60 degrees from vertical.
• Directional Well  -  A wellbore that requires the use of special tools or techniques to ensure that the wellbore path hits a particular subsurface target, typically located away from (as opposed to directly under) the surface location of the well.
• Discovery Well  -  An exploratory well that encounters a previously untapped oil or gas deposit.
• Disposal Well  -  A well, often a depleted oil or gas well, into which waste fluids can be injected for safe disposal.  Disposal wells typically are subject to regulatory requirements to avoid the contamination of freshwater aquifers.
• Dry Hole Well  -  A well that is drilled but does not produce any commercially viable quantities of oil or gas.  This can happen due to a variety of reasons, such as drilling in the wrong location or encountering a reservoir that has been depleted.
• Exploration Well  -  Drilling carried out to determine whether hydrocarbons are present in a particular area or structure.  Sometimes known as a ‘wildcat well’, particularly in areas where little drilling has taken place previously.
• Fishbone Wells  -  A series of multilateral well segments that trunk off a main horizontal well.  The appearance closely resembles the ribs of a fish skeleton trunking off the main backbone.
• Flowing Well  -  A well in which the formation pressure is sufficient to produce oil at a commercial rate without requiring a pump.  Most reservoirs are initially at pressures high enough to allow a well to flow naturally.
• Gas Well  -  A well that primarily produces natural gas.
• Horizontal well  -  Wells where the reservoir section is drilled at a high angle, typically with a trajectory to keep the well within a specific reservoir interval or hydrocarbon zone.  In a strict sense, these wells are rarely perfectly horizontal, but they tend to be near horizontal mostly, generally at an angle greater than 80 degrees from vertical.
• Image Well  -  A virtual well used to mathematically create the effect of a flow barrier.  The pressure transient behavior both at the well and in the reservoir is identical for the following two cases: 1) a well near a barrier represented by a plane normal to the bedding, or 2) a well producing or injecting at the same rate as the tested well.  In the second case, the effect is of a barrier bisecting the space between the two wells.
• Injection Well  -  A well in which fluids are injected rather than produced, the primary objective typically being to maintain reservoir pressure.  Two main types of injection are common: gas and water.  Separated gas from production wells or possibly imported gas may be reinjected into the upper gas section of the reservoir. Water-injection wells are common offshore, where filtered and treated seawater is injected into a lower water-bearing section of the reservoir.
• Intelligent Well  -  A well equipped with monitoring equipment and completion components that can be adjusted to optimize production, either automatically or with some operator intervention.
• Junked Well  -  A well which during the drilling phase, has been permanently plugged for technical reasons.
• Marginal Well  -  A well that, for reasons of depletion or natural low productivity, is nearing the limits of viable production and profitability.
• Multibranch Well  -  A well drilled to produce and/or inject from several well paths simultaneously.
• Naturally Flowing Well  -  A well in which the formation pressure is sufficient to produce oil at a commercial rate without requiring a pump. Most reservoirs are initially at pressures high enough to allow a well to flow naturally.
• Observation Well  -  A development well or test development well used to measure specific well parameters.
• Oil Well  -  A producing well with oil as its primary commercial product.  Oil wells almost always produce some gas and frequently produce water.  Most oil wells eventually produce mostly gas or water.
• Off-pattern Well  -  A production or injection well that has a lateral or diagonal displacement with respect to the other wells in an injection pattern.  The existence of an off-pattern well affects oil recovery and water/oil ratio.
• Offset Well  -  An existing wellbore close to a proposed well that provides information for planning the proposed well.  In planning development wells, there are usually numerous offsets, so a great deal is known about the subsurface geology and pressure regimes.  In contrast, rank wildcats have no close offsets, and planning is based on interpretations of seismicdata, distant offsets and prior experience.  High-quality offset data are coveted by competent well planners to optimize well designs.  When lacking offset data, the well planner must be more conservative in designing wells and include more contingencies.
• Offshore Well  -  Wells that are drilled in the ocean or other bodies of water.  These wells can be located in shallow waters close to shore or in deep waters far from shore.  Offshore wells require specialized equipment and facilities, such as offshore drilling rigs and offshore platforms, to access and process the oil or gas.  These facilities can be expensive to build and maintain, and the drilling process can be more challenging due to the ocean environment, such as storms and waves.
• Orphan Well  -  A well or associated site that does not have a legally responsible and/or financially viable party to deal with its decommissioning and reclamation.
• Plugged Well  -  To fill a well’s bore hole with cement or other impervious material to prevent the flow of water, gas or oil from one strata to another when a well is abandoned.
• Producing Well  -  A well producing fluids (gas, oil or water).
• Production Well  -  Collective term for wells used to recover petroleum, including injection wells, observation wells and possible combinations of these.
• Prorated Well  -  A well in which the maximum production rate is fixed by law.  These laws were developed by producing states primarily to control the market and avoid periodic price collapses.
• Pumping Well  -  A well produced by use of some kind of downhole pump.  Pumps are required when the formation pressure is not sufficient to allow flowing production of fluids at the desired or necessary rate.  The performance of well tests on pumping wells is always complicated by the presence of the pump, which often must be removed to take downhole pressure measurements.  Downhole pressure measurements in pumping wells are usually made by measuring the rise in liquid level in the well.  This is often accomplished by sonic devices, like well sounders, that measure the response time of sound waves bounced off the downhole liquid surface.  Most oil wells are eventually put on pumps as pressure declines during production.  The exceptions are in strong waterdrive reservoirs or in settings where pressure maintenance by gas or water injection is sufficient to maintain a high reservoir pressure.
• Recovery Well  -  A well used for production or injection.
• Relief Well  -  Used to stem oil or gas flow from a separate, damaged well.  Companies will often base these wells at some distance from the damaged well before using directional drilling to intersect the damaged well at a great depth.  Specialized liquid like heavy drilling mud or cement is then pumped into the damaged well to stem any unwanted flow.
• Salt Water Disposal Well  -  A well, sometimes a formerly producing well, that is used to inject produced salt water into a formation or formations for disposal.
• Service Well  -  Wells that are drilled for purposes other than oil or gas production, such as to monitor reservoir conditions or to access the wellbore for maintenance or repairs.
• Sidetrack Well  - Used when a well has already been drilled or part-drilled, but there is a need to drill out of one side of a well to a different target.  There are many reasons for doing this.  For example, a section of the original well may have been rendered unusable by an irretrievable object such as a fish or collapsed wellbore.  Another reason could be that all the hydrocarbons present at the original wellbore have now been depleted, making it necessary for a business to deviate from its well's casing and head towards a new target.
• Slimhole Well  -  An inexact term describing a borehole significantly smaller than a standard approach, commonly a wellbore less than 6 inches in diameter.  The slimhole concept has its roots in the observed correlation between well costs and volume of rock extracted.  If one can extract less rock, then well costs should fall.  One form of slimhole work involves using more or less conventional equipment and procedures, but simply reducing the hole and casing sizes for each hole interval.
• Stabilized Gas Well  -  A gas well producing at a constant rate in which wellhead pressure changes no more than a small amount as a function of time.  The actual amount of change permitted in a given time period to allow a well to be designated as stabilized may be fixed by law.  Alternatively, the target stabilization for rigorous flow-after-flow testing in gas wells is pseudosteady-state flow, and this may be recognized as the pressure change versus time predicted from formation properties and drainage area size.
• Steam Injection Well  -  An improved recovery technique in which steam is injected into a reservoir to reduce the viscosity of the crude oil.
• Step-out Well  -  Wells that are drilled outside of the known producing area of an oil or gas reservoir.  These wells are drilled to determine if the reservoir extends beyond the known boundaries and to discover new producing areas.
• Stripper Well  -  An oil well in the final stages of production.
• Subsea Well  -  A well in which the wellhead, christmas tree and production-control equipment is located on the seabed.
• Suspended Well -  A well that is not currently producing oil or gas but has been safely secured and may produce in the future.
• Well  -  A hole drilled to find or delimit a petroleum deposit and/or produce petroleum or water for injection purposes, inject gas, water or another medium, or map or monitor well parameters.  A well may consist of one or more well paths and may have one or more terminal points.
• Wildcat Well  -  Exploration well drilled to establish (prove) whether petroleum exists in a potential petroleum deposit.