1.1.    Christmas Tree

A Christmas Tree is an assembly of valves, spools, and fittings used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well, and other types of wells. It is named for its resemblance to a decorated tree at Christmas.

1.2.    Well Head

It is the surface termination of a wellbore that incorporates facilities for installing casing hangers during the good construction phase. The wellhead also incorporates a means of hanging the production tubing and installing the Christmas tree and surface flow-control facilities in preparation for the production phase of the well. Wellhead provides mechanical support to casing structures.

1.3.    Valves on Christmas Tree and Well Head

1.3.1.    Swab Valve

The topmost valve on a Christmas tree provides vertical access to the wellbore during online flow for well intervention operations.

  • Slick Line used for BHP survey and stationing of gauges.
  • Wire Line inserted for providing well real-time data.
  • Coil Tubing inserted for pumping acid to clear tubing perforations when blocked.

1.3.2.    Wing Valve

A wing valve is located on the side of a Christmas tree and it is used to control or isolate production from the well into surface facilities. Depending on each design of a Christmas tree, it can be equipped with one or two wing valves.  In the case of two production wing valves, one serves as the main production valve and another one as a backup.

1.3.3.    Master Valve

A master valve is located above the tubing hanger and its function is to allow the well to flow or shut well in. Typically, there are two master valves. One is called a lower master valve and another is an upper master valve. Two valves are often used because they provide redundancy. If one master valve cannot function properly, another valve can perform the function. The lower master valve is manually operated normally, while the upper master valve is often hydraulically actuated.

1.3.4.    Subsurface Safety Valve

A subsurface safety valve is essentially a shutdown valve installed at the upper wellbore for emergency shutdown to protect the production tubing and wellhead in case of overpressure. The purpose of a subsurface safety valve (SSSV) is to avoid the ultimate disaster which can result in the release of reservoir fluids to the surroundings.

A subsurface safety valve is typically a unidirectional flapper valve, directed in such a way that the flappers open downwards when pressure is applied from an upward direction. The flapper can only open in the downward direction. This hydraulic pressure is responsible for keeping the flappers of SSSV open and loss of hydraulic pressure results in the closing of the valve. Thus, the wellbore can be isolated in case of system failure or damage to the surface facilities.

1.3.5.    Annular Valve

To check annulus gauge pressures. 03 annular valves installed. The first one provides surface casing and intermediate casing pressure, the second one provides intermediate casing and production casing pressures and the third one provides production casing and well tubing gauge pressures.

1.3.6.    Choke   

A choke is a device that is used to control fluid flow rate or create downstream pressure, also known as backpressure. Chokes are available in several configurations for both fixed and adjustable modes of operation.

1.3.6.1.     Adjustable Choke

A valve, located on the Christmas tree is used to control the production of fluid from a well. Opening or closing the variable valve influences the rate and pressure at which production fluids progress through the pipeline or process facilities.

The adjustable choke is commonly linked to an automated control system to enable the production parameters of individual wells to be closely controlled. enable the fluid flow and pressure parameters to be changed to suit process or production requirements.

1.3.6.2.    Fixed Choke

A device is used to control the flow of fluids by directing flow through a restriction or hole of a fixed size. The fluid characteristics and the pressure differential across the choke determine the flow rate through a fixed choke. They are more resistant to erosion under prolonged operation or the production of abrasive fluids.

The first flow is adjusted by varying adjustable choke size, when desired flow at required parameters is obtained, flow is transferred to the fixed choke line and carried away further.

1.3.7.    Casing

The casing is the major structural component of a well. The casing is needed to:

  • Maintain borehole stability
  • Prevent contamination of water sands
  • Isolate water from producing formations
  • Control well pressures during drilling, production, and workover operations

The casing provides locations for the installation of:

  • Blowout preventers
  • Wellhead equipment
  • Production packers
  • Production tubing

The cost of the casing is a major part of the overall well cost, so the selection of casing size, grade, connectors, and setting depth is a primary engineering and economic consideration.

There are four basic types of casing strings:

1.3.7.1.      Conductor casing

Conductor casing is the first string set below the structural casing (i.e., drive pipe or marine conductor run to protect loose near-surface formations and to enable circulation of drilling fluid). The conductor isolates unconsolidated formations and water sands and protects against shallow gas. This is usually the string onto which the casing head is installed. A diverter or a blowout prevention (BOP) stack may be installed onto this string. When cemented, this string is typically cemented to the surface or to the mud line in offshore wells.

1.3.7.2.    Surface casing

Surface casing is set to provide blowout protection, isolate water sands, and prevent lost circulation. It also often provides adequate shoe strength to drill into high-pressure transition zones. In deviated wells, the surface casing may cover the build section to prevent key seating of the formation during deeper drilling. This string is typically cemented to the surface or to the mud line in offshore wells.

1.3.7.3.      Intermediate casing

Intermediate casing is set to isolate:

  • Unstable hole sections
  • Lost-circulation zones
  • Low-pressure zones
  • Production zones

It is often set in the transition zone from normal to abnormal pressure. The casing cement top must isolate any hydrocarbon zones. Some wells require multiple intermediate strings. Some intermediate strings may also be production strings if a liner is run beneath them.

1.3.7.4.      Production casing

Production casing is used to isolate production zones and contain formation pressures in the event of a tubing leak. It may also be exposed to:

  • Injection pressures from fracture jobs
  • Down casing, gas lift
  • The injection of inhibitor oil
  • A good primary cement job is very critical for this string.
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