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WELL COMPLETION

Oil production can be described as the activity that takes place on the completion of well
drilling, when a large quantity of recoverable oil has been found, to the point where it is sent
to the oil processing facilities to be refined or treated for shipment to the customer.

Production activities consist of well completion, when a well is made ready to produce oil in
large quantities, or well workover when an operating well is reworked to replace old tubing
and other components to improve the well's oil flow.
  
First a hole is drilled in the ground to a depth below where oil or gas is trapped.

In the drilling process the hole is lined or cased with large diameter pipe that is screwed
together in sections. This pipe is called production casing.

Cement is pumped into the space between the outside of the casing and the inside of the
drilled hole.

The cement secures the casing to the rock and seals the face of the rock.

Liquids or gases cannot flow from the rock into the hole or into nearby rocks.

So far we only have a steel lined hole in the ground full of drilling mud.

We have to finish or complete the well.


Drilled Hole Ready for Completion

PERFORATING

Holes or channels have to be made to let the oil or gas flow from the rock into the well.

This is done with a perforating 'gun'.

The gun has special explosives called 'shape charges

The shape charges direct a super hot stream of gas at the casing.

This gas stream melts (vaporises) the casing, the cement and some of the rock to leave a
hole or channel.

The fluids in the reservoir rock can now flow into the well.

The perforating can be done before any equipment is run into the cased hole or after all the
equipment is already set in the hole.

Then the perforating gun is fired the perforating is done very quickly in microseconds.


Perforating the Well

COMPLETIONS

There are two main types of completion (see figure 1‑3):

·      Single completion which produces from a single zone or formation.

·      Dual completion (the most common type of completion), which produces from two     different zones or formations up to the surface through separate pipes.

Single and Dual Zone Completions

COMPLETION COMPONENTS

A lot of individual components go to make up well completion. The components used and
where they are placed in the well piping depends on many different things. There are so
many different types of well completion that it would be impossible to include them all here.

When the completion has been done, a careful record must be made of what has been put in the well.

1. Production Tubing

This is smaller diameter pipe which is lowered inside the casing to a depth just above the
perforations.

The tubing is in 30 foot (approximately 10 metre) lengths and goes all the way to the
surface.

The oil is produced through the tubing.

Production Packer and Sliding Side Door

2. Production Packer

This is a device that anchors (is fixed) inside the casing just above the perforations.

The production packer also seals against the inside of the production casing and the outside
of the production tubing.

Oil and gas produced from the perforations flows up the production casing until it meets the
packer. The flow then has to go up the production tubing.

3. Sliding Sleeves or Sliding Side Door

These devices allow circulation of fluids between the tubing and the annulus.

The annulus is the space between the outside of the tubing and the inside‑of the casing.

4. Expansion or Travel Joint

This device is screwed into the production tubing string to allow for expansion and
contraction of the tubing.

No‑Go Nipples in a Dual Completion

 5. Wireline Landing Nipples

These are short pieces of pipe screwed into the production tubing string that have special
internal profiles (shapes).

Flow control and plugging devices can be set in these nipples.

Landing nipples can be 'Selective' or 'No‑Go'.

With selective nipples the operator chooses which nipple he wishes to use.

With No‑Go nipples the device being run stops at the No‑Go nipple. It cannot pass through the nipple. This is the type preferred by oil companies.


No‑Go Nipple in a Single Completion

6. Flow Couplings

These are thick walled pieces of pipe screwed each side of a landing nipple.

They resist erosion caused by turbulent flow each side of a restriction in the tubing string. 


Single Completion with Flow Couplings and Pup Joints

7. Blast Joints 

Blast Joints are heavy wall tubes which are run as an integral part of the tubing string.

Used to act as a sacrifice element and minimize the erosive blasting action opposite
perforation in dual completions​.

8. Pup Joints

These are thick walled, hard faced, pieces of pipe screwed into the tubing string.

They are placed opposite upper zone perforations to prevent flow damage to the outside of
the 'longstring' tubing. Longstring tubing is the name given to the longer tubing string in a
dual completion well.

These are short sections of tubing of different lengths.

They are used to 'space out' (to get tubing strings to the correct length) between packers
and the well head.

Perforated pup joints are short sections of tubing full of holes which hang below the packers. They permit flow through the holes when measuring devices are placed in the landing nipple below the perforated pup joint.


Dual String Retrievable Packer Installation
Large Bore Single String, Completion With Tapered No‑Go Nipples
Tubing‑Conveyed Perforating Completion
Completion Equipment and Well Data Guide Example

LEGEND


Dual Tubing Completion Guide

COMPLETION FLUIDS

Drilling mud is left in the hole when drilling is finished.,

This must be removed when the well is completed.

The drilling mud is replaced by a special fluid called completion fluid.

Completion fluid is normally water based and contains special chemicals to stop corrosion
of the production tubing and casing.

Completion fluid also exerts a hydrostatic pressure which helps to keep the packer seated
and also provides a means of 'killing' the well. (The hydrostatic pressure of the completion
fluid is higher than the formation pressure).

Completion fluid is pumped down the tubing to displace the drilling mud up the annulus.

This is done before the packers are set or before the tubing is stabbed (inserted) into the
packer.

Once the drilling mud has been replaced with completion fluid, the tubing is spaced out and
connected to the well head.

WORKOVER OPERATIONS

workover is the re‑completion of an existing or old well.

it is done to correct downhole problems that have caused a loss of oil production.

downhole problems that make workover necessary could be:

• Mechanical failures.

• Reservoir (producing formation) problems.

 

TECHNICAL FAILURES AND LEAKS

Failures include, (but are not limited to):

Primary cement failure.

Casing, tubing and packer leaks.

Wellbore communication in multiple completion. (Oil flow from one producing formation
into another one through the wellbore).

Leaks can be caused by:

·                  Pressure differences.

·                  Corrosion of completion equipment.

·                  Erosion of completion equipment.

·                  Elastomer seal failure.

Behind Casing Flow Resulting From Cement Sheath Failure

RESERVOIR (PRODUCING FORMATION) PROBLEMS

1. Decreasing Reservoir Pressure and Solution Gas

The initial reservoir pressure depletes as the well produces oil. The pressure drop from the
reservoir to the well bore also increases. When the pressure goes below the bubble point
pressure solution gas comes out of the oil. This gas will flow easier than the oil to the well
bore.

To explain 'solution gas' and 'bubble point' think of a bottle of lemonade or cola. When you
buy the bottle and look at it all you see is the lemonade or cola as a liquid. When you take
the top off the bottle gas bubbles form in the liquid and rise to the surface of the liquid. This
gas was in solution in the liquid so it is called solution gas. When the top was taken off the
bottle the pressure inside the bottle was reduced to below the bubble point. That is, the
point at which the solution gas comes out of solution. 

2. Coning


Coning
If the rate at which oil is taken out of the reservoir (the production rate) is too high, the water below the oil will be drawn up into a cone and will enter the production tubing through the perforations. Coning can be prevented by reducing the production rate.

3. Increasing Gas Cap

Increasing Gas Cap
As the oil is removed from the reservoir the gas above the oil (the gas cap) expands to fill
the space left. In time the gas/oil interface will be at the area of the perforations and gas will
be produced with the oil.

This problem can be corrected by sealing off the original perforations with cement and making new perforations lower in the well.

4. Advancing Oil/Water Contact

Advancing Oil/Water Contact
As the oil is removed from the reservoir the water below the oil increases to fill the space
left. In time the oil/water interface will be at the area of the perforations and water will be
produced with the oil.

This problem can be corrected by sealing off the original perforations with cement and making new perforations higher in the well.

5. Permeability Irregularities

Permeability Irregularities
Permeability means how easy or difficult it is for liquid to flow through the formation rock of
the reservoir.

The well has four perforations in different areas of permeability. In the area of high
permeability the oil has been able to flow easily so the oil has come out of this area first.
This has allowed the water to reach the perforations in that area.

Even though there is plenty of oil in the area of the other three perforations water is coming
in through the perforations in the high permeability area and is being produced with the oil.

To stop this the perforations in the area of high ‑permeability are sealed with cement.

6. Reservoir Fluid Flow During Workover

Reservoir Fluid Flow During Workover
The well may have been completed into two separate producing zones which are at different
pressures. During a workover of this type of well, when everything except the casing is
removed from the well, the pressure in the two separate zones will equalise with reservoir
fluid flowing from the high pressure zone to the low pressure zone.

The flow of fluid from one zone to the other can be prevented but it is a specialised job carried out by a specialist company.

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