me4105 nus offshore oil and gas technology lecture 7

7/27/2019 ME4105 NUS Offshore Oil and Gas Technology Lecture 7

1/32

Offshore Oil & Gas Technology


2/32

WELL COMPLETION


3/32

Well Completion Once the well is drilled to the desired depth, the

formation must be tested and evaluated to determine

whether the well will be completed for production, orplugged and abandoned.

If the decision is made to go ahead to complete the well,the production casingis run by the drilling crew, usingthe drilling rig for the purpose.

A well is always cased(set pipe) to complete the well.Casingis relatively thin-walled, steel pipe. Numerous

joints of the same size casing are screwed together toform a long length of casing, called a casing string. Thecasing has an outer diameter of at least 2 less then the

wellbore diameter. The casing string is run into the welland cemented to the sides of the well in a cement job.

Casing stabilizes the well and prevents the sides fromcaving into the well. It protects fresh water reservoir fromthe oil, gas and salt water brought up the well duringproduction. Casing also prevents the production frombeing diluted by waters from other formations in the well.


4/32

Many wells requires four concentric strings of large pipe:

Conductor pipe Surface casing,

Intermediate casing(s)

Production casing (often call the oil stringor long stringin the

oil patch)

The production casing is the last and deepest string of casing run in

the well. Size of the production casing, which provides a conduit from

the surface to the producing formation, is determined principally by

subsurface considerations, such as:

Subsurface artificial lift equipment required

Multiple-zone completions requiring several different strings of

rubing isolated from each other by packers.

Types of completion method to be used: open hole, perforated

casing, screened open hole, or screened perforated casing.

Prospects of deepening the well at a later date.

Well Completion


5/32

Conductor, surface, intermediate & production casing cemented in the well

Source: Gerding, Fundamentals of Petroleum, 3rdEd.


6/32

Relationship of

strings of casing

Source: Berger, Anderson, Modern Petroleum - A Basic Primer of the industry, 3 rdEd.


7/32

Another type of string/pipe that is not common in wells over 10,000 ft is

called a liner. Liners are sometimes set in a hole as a protective string,serving the same function as an intermediate string. A liner is usually

run only from the bottom of the previous string to the bottom of the open

hole. Liners are suspended from a previous string with a hanger. They

may suspendin the well either with our without cementing.

The final string of pipe usually run in a producing well is thetubing. Tubing is nearly freely suspended in the well from

the tubing head. In a flowing well, its small diameter

production more efficient results than casing. Also, tubing is

considerably easier to remove than casing when it

becomes plugged or damaged. Tubing, when used in

conjunction with a packer, keeps well fluids away from the

casing because the packer seals the space between the

tubing and the casing. Well fluids corrode the casing and

thus cause costly repair job later.

Well Completion


8/32

The packer consists of a pipe-like device through which

well fluids can flow, a rubber sealing element that forms

a fluid-tight seal, and gripping elements (called slips)

that hold the packer in the tubing-casing annulus just

above the producing zone. Since the packer seals off the

space between the tubing and the casing, formationfluids flowing into the well are forced into and up the

tubing.

Another device frequently installed in the tubing string

near the surface is a subsurface safety valve. Thevalve remains open as long as fluid flow is normal. When

the valve senses something amiss with the surface

equipment of the well, it closes, preventing the flow of

fluids.

Well Completion


9/32

Completedwell

Source: Dawe, Modern Petroleum Technology, Vol 1, Upstream


10/32

Well ready to produce

Source: Berger, Anderson, Modern Petroleum - A Basic Primer of the industry, 3 rdEd.


11/32

Tubing and packer

in completed well



12/32

Cementing Primary cementingpertains to the initial cementing jobs

performed in conjunction with setting the various casingstrings. It also afford additional support for the casing, andhelp retard corrosion.

Secondary cementingis done after primary cementing,most often because of some functional failure of the

primary cement job. It may be used to squeeze cementthrough perforations, to plug a dry hole, or it may be usedto plug an open zone. Secondary cementing normallyinvolves the injection under pressure of a slurry into theporous formation. Secondary cementing is used for: Supplement of repair a primary job

Repair defective casing or improperly place perforations

Abandon a permanent nonproductive or depleted zone

Reduce the danger of lost circulation in open hole while drilling deeper

Isolate a zone before perforating

Fracture the formation


13/32

Casing cementing job



14/32

Completion Methods Open-hole completion An open-hole, or barefoot,

completion has no production casing or liner set oppositethe production formation. Instead, reservoir fluids flow

unrestricted into the open wellbore. This type of

completion, which is rarely used and is generally

restricted to limestone reservoirs, is useful where only

one production zone and low-pressure formations exist.

In an open-hole completion, casing is set just above the

pay zone, and drilling proceeds into the production zone

as far as necessary to complete the well. Open-hole

completed wells are usually stimulated by Hydraulic oracid fracturing.


15/32

Open-hole completion



16/32

Perforated completion The perforated completion, by

the most popular method of completing a well, required agood cementing job and proper perforating method.Perforation is the process of piercing the casing wall andthe cement to provide openings through which formationfluids may enter the wellbore. Perforating is accomplished

by lowering a perforating gun down the production casingof the tubing until it is opposite the zone to be produced.The gun is fired to shoot bulletsor, more often, to set offspecial explosive charges known as shaped charges. Ashaped charge is designed so that an intense, directionalexplosion is formed. Because the explosion is actually a

jet of high-energy gases or particles, it is calledjetperforating. If a production liner is used rather thanproduction casing, the liner is perforated to complete the

well.

Completion Methods

f f


17/32

A perforating gun creates holes or perforation in the casing cement

and producing formation and allows fluids to enter the well.



18/32

Another type of completion involves a wire-wrapped

screen, which is a short length of pipe that has openingsin its sides and is wrapped with a specially shaped wire.

One method involves attaching the screen to the bottom

of the tubing string and lowering it into a well that has

been perforated. Usually wire-wrapped screens are run

in conjunction with a gravel pack, i.e. gravel is placed inthe hole outside the screen. Well fluids flow through the

gravel pack, through the wire-wrapped screen, and into

the tubing. Wire-wrapped screen completion are most

often utilised in cases where the producing zone orzones are likely to produce sand as well as oil and gas.

The screen and gravel pack minimise the entry of sand

into the wellbore, thus preventing problems caused by

sand.

Completion Methods

Wire wrapped screen completion run with a gravel pack


19/32

Wire-wrapped screen completion run with a gravel pack

inside the perforated casing



20/32

Tubeless completion Although most wells are

completed with a tubing, a small-diameter well that usessmall-diameter casing may be completed without the

tubing. The small casing is cemented and perforated

opposite the producing zones. Tubeless completions are

used mostly in small gas reservoirs that produce few, ifany, liquid and are low in pressure.

Multiple completions Multiple completions are be

used when one wellbore penetrates two or more

producing zones. Usually, a separate tubing string inwith packers is run inside the production casing for each

producing zone. For example, in a triple completion,

three tubing strings and three packers can be utilized in

a single production string.

Completion Methods

T bi l l ti ith i t bi t i


21/32

Tubingless completion with no inner tubing string



22/32

Multiple completion



23/32

Wellhead The wellheadis the equipment used to confine and

control the flow of fluids from the well. It forms a seal to

prevent well fluids from blowing out or leaking at the

surface. The conditions expected to be encountered in

the individual well determine the type of wellhead that is

needed. Sometimes, all that is required is a simpleassembly to support the weight of the tubing in the well.

In other cases, the control formation pressure is

necessary, and a high-pressure wellhead is required.

Pressures greater than 20,000 psi have been found insome fields.

Basically the wellhead is made up of combination of

parts called casinghead(s), tubing head, and

Christmas tree.


24/32

Wellhead


A Ch i t t ith l t t l th fl f fl id f th ll


25/32

A Christmas tree with valves to control the flow of fluids from the well



26/32

Casinghead The casinghead is a heavy steel fitting at

the surface to which the casing is attached. Duringdrilling and workover operations, the casinghead is used

as an anchor for the pressure control equipment that may

be necessary. If several strings of casing are used in the

well, more than one casinghead may be used on awellhead assemble.

Tubing head Similar in design and use to the

casinghead, the tubing head supports the tubing string,

seals off pressure between the casing and the inside oftubing, and provides connections at the surface with

which the flowing liquid or gas can be controlled. The

tubing head is supported by the casinghead if a

casinghead is used on the well.

Wellhead


27/32

Christmas Tree The assembly of control valves,

pressure gauges and chokes located at the top of a well

is known as the Christmas tree, so named because of its

treelike shape, with a large number of fitting branching

out above the wellhead. The valves are opened and

closed to control the flow of oil and gas from the wellafter it has been completed. The master valve can shut

off the flow entirely. Also in the Christmas tree is a

choke, or restriction in the line, that control the amount

of fluids flowing from the well. The pressure gaugesreveal casing and tubing pressure. By knowing these

pressures under various operating condition better well

control is possible.

Wellhead


28/32

Swabbing Swabbingis an operation that temporarily lowers the

fluid level in the well so that it can begin to produce. It is

sometimes necessary if the well does not flow afterperforating.

The purpose of swabbing is to swab, or lift, enough fluids

out of the tubing so that the hydrostatic pressure is

reduced to a value below that of the formation pressure.If it is successful, formation fluid will start to flow

immediately. Swabbing is done with a swab cup, which

picks up fluid in the tubing and raises it to the surface by

means of continuous pull on a cable attached to ahoisting winch.

If the well does not begin to produce, it may need to be

stimulated, or a pump may have to be installed as a

permanent lifting device to bring the oil to the surface.


29/32

Offshore Completions A well completed offshore is much like one on land,

except that a different technique is used to control thefluids as they grow from the top of the well. The wellhead

valves and fittings may be placed above the surface of

the water (surface completion), or they may be

installed on the seafloor (subsea completion).

A well drilled from a platform is usually completed with a

Christmas tree on the deck of the platform, while a

subsea completion, the wellhead is located on the

seafloor.

A wetsubsea installation is one installed on thewellhead, exposed to the surrounding water.

A drysubsea installation is one housed in a protective

dome.


30/32

Subsea completion



31/32

Well Testing

A variety of well test are conducted to determine

production rates for oil and gas wells. Each test reveals

certain information about a particular well and the

reservoir in which it is completed. Accuracy is, of course,

very important, and these test data form the case history

of a well. Potential test It is a measurement of the largest

amount of oil and gas that a well will produce in a 24-

hour period under certain fixed conditions. This test is

made on each newly completed well and at other timesduring the wells producing life. Potential test information

is generally required by the state regulatory group, which

uses it to establish the producing allowable for the well.

W ll T ti


32/32

Bottomhole pressure test It measures the reservoirpressure of the well at a point opposite the producingformation. The test is usually conducted after the well hasbeen shut for 24 to 48 hours. When this test is conducted atscheduled intervals, valuable information about the declineor depletion of the zone in which the well is producing will begathered.

Productivity test It is conducted to determine the effectsof different flow rates on the pressure within the producingzone of the well. This reveals certain physical characteristicsof the reservoir, and the maximum potential rate of flow canbe calculated without risking the damage that might occur if

the well were produced at its maximum possible flow rate.The closed-in bottomhole pressure of the well is firstmeasured. Next the flowing bottomhole pressure at severalstabilised flowrates were measured. This type of testing isdone on both oil and gas wells, and is the most widely

accepted method of determining the capacity of gas wells

Well Testing

me4105 nus offshore oil and gas technology lecture 7

Documents