Completion (oil and gas wells)
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In petroleum production, completion is the process of making a well ready for production (or injection). This principally involves preparing the bottom of the hole to the required specifications, running in the production tubing and its associated jewellery and perforating and stimulating as required. Sometimes, the process of running in and cementing the casing is also included.
[edit] Lower completion
This refers to the portion of the well across the production or injection zone. The well designer has many tools and options available to design the lower completion according to the conditions of the reservoir. Typically, the lower completion is set across the productive zone using a liner hanger system, which anchors the lower completion to the production casing string. The broad categories of lower completion are listed below.
[edit] Barefoot completion
This type is the most basic, but can be a good choice for hard rock, multi-laterals and underbalance drilling. It involves leaving the productive reservoir section without any tubulars. This effectively removes control of flow of fluids from the formation; it is not suitable for weaker formations which might require sand control, nor for formations requiring selective isolation of oil, gas and water intervals. However, advances in interventions such as coiled tubing and tractors means that barefoot wells can be successfully production logged, zonal isolation of the toe-end can be achieved (e.g. cemented off), and sidetracks can readily be drilled from within the barefoot section.
[edit] Open hole completion
This designation refers to a range of completions where the tubulars across the production zone are not cemented in place. Openhole completions have seen significant uptake in recent years, and there are many configurations, often developed to address specific reservoir challenges. There have been many recent developments which has boosted the success of openhole completions, and they also tend to be popular in horizontal wells, where cemented liners are more expensive and technically more difficult. The common options for openhole completions are;
1) pre-holed liner (also often called pre-drilled liner). The liner is prepared with multiple small drilled holes, then set across the production zone to provide wellbore stability and an intervention conduit. Pre-holed liner is often combined with openhole packers, such as swelling elastomers, mechanical packers or external casing packers, to provide zonal segregation and isolation. It is now quite common to see a combination of pre-holed liner, solid liner and swelling elastomer packers to provide an initial isolation of unwanted water or gas zones. Multiple sliding sleeves can also be used in conjunction with openhole packers to provide considerable flexibility in zonal flow control for the life of the wellbore.
This type of completion is also being adopted in some water injection wells, although these require a much greater performance envelope for openhole packers, due to the considerable pressure and temperature changes that occur in water injectors.
Openhole completions (in comparison with cemented liners) require better understanding of formation damage, wellbore clean-up and fluid loss control. A key difference is that perforating penetrates through the first 6-18inches (15-45cm) of formation around the wellbore, whilst openhole completions require the reservoir fluids to flow through all of the filtrate-invaded zone around the wellbore and lift-off of the mud filter cake.
Many openhole completions will incorporate fluid loss valves at the top of the liner to provide well control whilst the upper completion is run.
There are an increasing number of ideas coming into the market place to extend the options for openhole completions; for example, electronics can be used to provide a self-opening or self-closing liner valve. This might be used in an openhole completion to improve clean-up, by bringing the well onto production from the toe-end for 100 days, then self-opening the heel-end. Inflow control devices and intelligent completions are also installed as openhole completions.
Pre-holed liner may provide some basic control of solids production, where the wellbore is thought to fail in aggregated chunks of rubble, but it is not typically regarded as a sand control completion.
2) Slotted liner can be selected as an alternate to pre-holed liner, sometimes as a personal preference or from established practice on a field. It can also be selected to provide a low cost control of sand/solids production. The slotted liner is machined with multiple longitudinal slots, for example 2mm x 50mm, spread across the length and circumference of each joint. Recent advances in laser cutting means that slotting can now be done much cheaper to much smaller slot widths and in some situation slotted liner is now used for the same functionality as sand control screens.
3) Openhole sand control is selected where the liner is required to mechanically hold-back the movement of formation sand. There are many variants of openhole sand control, the three popular choices being stand-alone screens, openhole gravel packs (also known as external gravel packs, where a sized sand 'gravel' is placed as an annulus around the sand control screen) and expandable screens. Screen designs are mainly wire-wrap or premium; wire-wrap screens use spiral-welded corrosion-resistant wire wrapped around a drilled basepipe to provide a consistent small spiral gap (such as 0.012inch, termed 12 gauge). Premium screens use a woven metal cloth wrapped around a basepipe. Expandable screens are run to depth before being mechanically swaged to a larger diameter. Ideally, expandable screens will be swaged until they contact the wellbore wall.
[edit] Cased hole completion
This involves running casing or a liner down through the production zone, and cementing it in place. Connection between the well bore and the formation is made by perforating. Because perforation intervals can be precisely positioned, this type of completion affords good control of fluid flow, although it relies on the quality of the cement to prevent fluid flow behind the liner. As such it is the most common form of completion...
[edit] Completion components
The upper completion refers to all components from the bottom of the production tubing upwards. Proper design of this "completion string" is essential to ensure the well can flow properly given the reservoir conditions and to permit any operations as are deemed necessary for enhancing production and safety.
[edit] Christmas tree
This is the main assembly of valves that controls flow from the well to the process plant (or the other way round for injectors) and allows access for chemical squeezes and well interventions.
[edit] Tubing hanger
This is the component, which sits on top of the wellhead and serves as the main support for the production tubing.
[edit] Production tubing
Production tubing is the main conduit for transporting hydrocarbons from the reservoir to surface (or injection material the other way). It runs from the tubing hanger at the top of the wellhead down to a point generally just above the top of the production zone.
[edit] Downhole safety valve
This component is intended as a last resort method of protecting the surface from the uncontrolled release of hydrocarbons. It is a one way valve, which allows fluids to flow down, but not up, when closed. It is placed as far below the surface as is deemed safe from any possible surface disturbance including cratering caused by the wipeout of the platform.
[edit] Annular safety valve
On wells with gas lift capability, many operators consider it prudent to install a valve, which will isolate the 'A' annulus for the same reasons a DHSV may be needed to isolate the production tubing in order to prevent the inventory of natural gas downhole from becoming a hazard as it became on Piper Alpha.
[edit] Side pocket mandrel
This is a welded/machined product which contains a 'side-pocket' alongside the main tubular conduit. The side pocket, typically 1" or 1½" diameter is designed to contain gas lift valve, which allows hydrocarbon gas from the 'A' annulus to be injected into the flow stream.
[edit] Electrical submersible pump
This device is used for artificial lift to help provide energy to drive hydrocarbons to surface if reservoir pressure is insufficient.
[edit] Landing nipple
This is a receptacle to receive wireline tools. It is also a useful marker for depths in the well, which can be difficult to accurately determine.
[edit] Sliding sleeve
The sliding sleeve is hydraulically or mechanically actuated to allow communication between the tubing and the 'A' annulus. They are often used in multiple reservoir wells to regulate flow to and from the zones.
[edit] Production packer
The packer isolates the annulus between the tubing and the inner casing and the foot of the well. This is to stop reservoir fluids from flowing up the full length of the casing and damaging it. It is generally placed close to the foot of the tubing, shortly above the production zone.
[edit] Downhole gauges
This is an electronic or fibre optic sensor to provide continuous monitoring of downhole pressure and temperature. Gauges use a 1/4" control line clamped onto the outside of the tubing string to provide an electrical or fibre optic communication to surface.
[edit] Perforated joint
This is a length of tubing with holes punched into it. If used, it will normally be positioned below the packer and will offer an alternative entry path for reservoir fluids into the tubing in case the shoe becomes blocked, for example, by a stuck perforation gun.
[edit] Formation isolation valve
This component, placed towards the foot of the completion string, is used to provide two way isolation from the formation for completion operations without the need for kill weight fluids. Their use is sporadic as they do not enjoy the best reputation for reliability when it comes to opening them at the end of the completion process.
[edit] Centraliser
In highly deviated wells, this components may be included towards the foot of the completion. It consists of a large collar, which keeps the completion string centralised within the hole.
[edit] Wireline entry guide
This component is often installed at the end of the tubing (the shoe). It is intended to make pulling out wireline tools easier by offering a guiding surface for the toolstring to re-enter the tubing without getting caught on the side of the shoe.
[edit] Perforating and stimulating
In cased hole completions (the majority of wells), once the completion string is in place, the final stage is to make a connection between the wellbore and the formation. This is done by running perforation guns to blast holes in the casing or liner to make a connection. Modern perforations are made using shaped explosive charges, similar to the armor-penetrating charge used on antitank rockets (bazookas).
Sometimes once the well is fully completed, further stimulation is necessary to achieve the planned productivity. There are a number of stimulation techniques.
[edit] Acidising
This involves the injection of chemicals to eat away at any skin damage, "cleaning up" the formation, thereby improving the flow of reservoir fluids. Acid can also be used to clean the wellbore of some scales that form from mineral laden produced water.
[edit] Fracturing
This means creating and extending a fracture from the perforation tunnels deeper into the formation increasing the surface area for formation fluids to flow into the well as well as extending past any possible damage near the wellbore. This may be done by either injecting fluids at high pressure (hydraulic fracturing), injecting fluids laced with round granular material (proppant fracturing) or using explosives to generate a high pressure and high speed gas flow (TNT or PETN up to 1,900,000 Psi) and (propellent stimulation up to 12,000 Psi).
[edit] Acidising and fracturing (combined method)
This involves use of explosives and injectition of chemicals to increase acid-rock contact.
[edit] Nitrogen circulation
Sometimes, productivity may be hampered due to the residue of completion fluids, heavy brines, in the wellbore. This is particularly a problem in gas wells. In these cases, coiled tubing may be using to pump nitrogen into the bottom of the hole to circulate out the brine.
