Well Design – Spring 2012 Prepared by: Tan Nguyen Well Design PE 413 Surface Equipments and...

Well Design – Spring 2012

Prepared by: Tan Nguyen

Well Design

PE 413

Surface Equipments and Placement Techniques

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Surface Cementing Equiments

Jet Mixer

Dry cement must be mixed with the proper amount of water to ensure that slurry

and set-cement properties are as designed.

The jet mixer induces a partial vacuum at the venturi throat, drawing in the dry

cement. High stream turbulence then provides thorough mixing

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Jet Mixer

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Batch Mixer

Batch mixing and/or blending is achieved through use of propellers, paddle

mixers, pneumatic mixing, and rotation of the cement tank


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Pump Skid Truck

The typical slurry-pumping unit is truck-mounted, and contains diesel engines and

displacement tanks that are accurately graduated so that water or mud volumes

can be controlled to place the slurry downhole properly.


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Cement Head


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Downhole Cementing Equipments

Basic Equipments

A predetermined volume of slurry is pumped

into the casing between two wiper plugs.

The bottom plug ruptures when it seats

The top plug is displaced with mud or

completion fluid.

Flow stops and pressure builds when the top

plug lands.

Check valves in the float shoe to prevent

backflow of the heavier column of slurry

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Guide/Float Shoes and Collars

In most cases, except in certain shallow wells, a round-nosed shoe is run on

the bottom joint to guide the casing past borehole irregularities encountered

while the string is run.


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Guide/Float Shoes and Collars

Float equipment reduces derrick stress by

increased casing buoyancy. Float equipment

consists of casing shoes and collars which

contain check valves to prevent wellbore fluids

from entering. As the casing is lowered, the

hook load is reduced by the weight of fluid

displaced. The casing is filled from the surface

to prevent casing collapse.


Float collar

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Wiper Plugs

Wiper plugs are used to separate cement

from preceding or following fluids. The

bottom plug removes mud from the wall of

the casing, and prevents this mud from

accumulating beneath the top plug.


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Wiper Plugs


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Casing centralizers are used to:

1.Improve displacement efficiency

2.Prevent differential pressure sticking

3.Keep casing out of key seats

Casing Centralizers


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Wipers and scratchers are used

primarily to remove borehole mud cake.

They also aid in breaking up gelled

mud. Both rotating and reciprocating

styles are available.

Wipers and Scratchers


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Primary Cementing Techniques

Conductor

The conductor is usually the first and shortest casing string. Its purpose is to

protect shallow sands from being contaminated by drilling fluids, and help prevent

wash-outs which can easily occur near the surface because of loose,

unconsolidated formations. The depth is normally less than 300 ft. It can be used

for the attachment of a blowout preventer.

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Introduction

The objective of a primary cement job is to place the cement slurry in the annulus

behind the casing. In most cases this can be done in a single operation, by

pumping cement down the casing, through the casing shoe and up into the

annulus. However, in longer casing strings and in particular where the formations

are weak and may not be able to support the hydrostatic pressure generated by a

very long colom of cement slurry, the cement job may be carried out in two stages.

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Introduction

The first stage is completed in the manner described above, with the exception

that the cement slurry does not fill the entire annulus, but reaches only a pre-

determined height above the shoe. The second stage is carried out by including a

special tool in the casing string which can be opened, allowing cement to be

pumped from the casing and into the annulus. This tool is called a multi stage

cementing tool and is placed in the casing string at the point at which the bottom of

the second stage is required.

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Conductor

Recommended cements for use with conductor casing are:

• Accelerated neat

• Ready-mix concrete

• Thixotropic cement

• LCM additives

When cementing down casing, plugs may not be used; cement is simply placed

The cement must have a compressive strength high enough to support the

wellhead load; therefore, high-compressive-strength cements are best.

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Surface Casing

Surface casing is usually the second string of pipe set in the well.

Shallow surface casing is cemented in the same manner as conductor casing.

For deeper strings of surface casing, a lightweight lead cement is used, followed

by heavier-weight completion cement to strengthen the bottom of the surface

casing around the shoe. This creates a strong seal with the pipe and formation for

solid support of the casing.

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Surface Casing

Recommended cement types include:

• Accelerated cements

• LCM additives

• High-strength cements, which are often used on deep-well surface casing to

support future strings

The following is a brief summary of surface-casing cementing practices:

• Both bottom and top plugs should be used to prevent mud contamination.

• Centralizers should be used.

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Intermediate Casing

The intermediate casing is the first string of pipe set after the surface casing.

Intermediate casing strings extend from the surface to a formation able to hold the

mud weights expected at greater depth.

Unlike the conductor and surface casings, additives such as friction reducers,

fluid-loss additives, and retarders are required for intermediate slurries. Where the

annulus is small, friction reducers lower pump pressures and reduce the chance of

losing fluids in a lost-circulation zone.

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Intermediate Casing

The following is a brief summary of intermediate-casing cementing practices:

• Both bottom and top plugs should be used to minimize contamination of the

cement.

• Scratchers, centralizers, and flushes can be important in the successful

completion of an intermediate-casing cementing job.

• This casing string can be cemented in a single-stage primary cement job, but a

multistage job is often performed because such a tall annular column of cement

slurry would exert a hydrostatic pressure greater than the formation pressure.

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Production Casing

The production casing is the last full string of pipe set in the well, and extends to

the surface.

The production casing is normally run and cemented through a zone to be

produced, and then perforated to allow communication with the formation.

Sometimes it is set just above the zone, and an openhohle completion is

performed. The production casing is normally the last casing set in the well. It may

be subjected to maximum well pressures and temperatures, and must be designed

to withstand such conditions.

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Production Casing

Recommended types of cement are

• Filler cements with high-strength tail-in

• Low-water-ratio cements (for all potential pay zones)

• Densified cements (for high competency and pressure control)

• Fluid-loss control additives

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Cementing Techniques

Single Stage Cementing Operation

The single stage primary cementing operation is the most common type of

cementing operation that is conducted when drilling a well.

In the case of the single stage operation, the casing with all of the required

cementing accessories such as the float collar, centralisers etc. is run in the hole

until the shoe is just a few feet off the bottom of the hole and the casing head is

connected to the top of the casing. It is essential that the cement plugs are correctly

placed in the cement head. The casing is then circulated clean before the

cementing operation begins

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1. The first cement plug (wiper plug) is pumped down ahead of the cement to wipe

the inside of the casing clean.

2. The spacer is then pumped into the casing. The spacer is followed by the

cement slurry.

3. This is followed by the second plug (shutoff plug). When the wiper plug reaches

the float collar its rubber diaphragm is ruptured, allowing the cement slurry to

flow through the plug, around the shoe, and up into the annulus. At this stage

the spacer is providing a barrier to mixing of the cement and mud.

4. When the solid, shut-off plug reaches the float collar it lands on the wiper plug

and stops the displacement process..

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Multi-Stage Cementing Operation

When a long intermediate string of casing is to be cemented it is sometimes

necessary to split the cement sheath in the annulus into two, with one sheath

extending from the casing shoe to some point above potentially troublesome

formations at the bottom of the hole, and the second sheath covering shallower

troublesome formations. The placement of these cement sheaths is known as a

multi-stage cementing operation

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The reasons for using a multi-stage operation are to reduce:

• Long pumping times

• High pump pressures

• Excessive hydrostatic pressure on weak formations due to the relatively high

density of cement slurries.

• Cost due to the long distance between pay zones (reduce the high quality

volume of cement required for the production zones)

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First stage:

The procedure for the first stage of the operation is similar to the single stage

operation, except that a wiper plug is not used and only a liquid spacer is pumped

ahead of the cement slurry. The conventional shut-off plug is replaced by a plug

with flexible blades. This type of shut-off plug is used because it has to pass

through the stage cementing collar which will be discussed below.

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Second stage

The second stage of the operation involves the use of a special tool known as a

stage collar, which is made up into the casing string at a pre-determined position.

The ports in the stage collar are initially sealed off by the inner sleeve. This sleeve

is held in place by retaining pins. After the first stage is complete a special freefall

plug is released form surface which lands in the inner sleeve of the stage collar.

When a pressure of 1000 - 1500 psi is applied to the casing above the freefall plug,

the retaining pins on the inner sleeve are sheared and the sleeve moves down,

uncovering the ports in the outer mandrel. Circulation is established through the

stage collar before the second stage slurry is pumped.

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Stage collar installed in the casing for multi-stage cementing operation

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Liner Cementing

Liners are run on drillpipe and therefore the conventional cementing techniques

cannot be used for cementing a liner. Special equipment must be used for

cementing these liners.

As with a full string of casing the liner has a float collar and shoe installed. In

addition there is a landing collar, positioned about two joints above the float collar.

A wiper plug is held on the end of the tailpipe of the running string by shear pins.

The liner is run on drillpipe and the hanger is set at the correct point inside the

previous casing string. Before the cementing operation begins the liner setting tool

is backed off to ensure that it can be recovered at the end of the cement job.

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Liner Cementing

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Liner Cementing

The cementing procedure is as follows:

1.Pump spacer ahead of cement slurry

2.Pump slurry

3.Release pump down plug

4.Displace cement down the running string and out of the liner into the annulus

5.Continue pumping until the pump down plug lands on the wiper plug.

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Liner Cementing

6. Apply pressure to the pump down plug and shear out the pins on the wiper

plug. This releases the wiper plug

7. Both plugs move down the liner until they latch onto landing collar

8. Pump the plugs with 1000 psi pressure

9. Bleed off pressure and check for back flow

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Liner Cementing

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Squeeze Cementing

Squeeze cementing is the process by which hydraulic pressure is used to force

cement slurry through holes in the casing and into the annulus and/or the

formation. Squeeze cement jobs are often used to carry out remedial operations

during a workover on the well. The main applications of squeeze cementing are:

• To seal off gas or water producing zones, and thus maximise oil production from

the completion interval

• To repair casing failures by squeezing cement through leaking joints or

corrosion hole

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Squeeze Cementing

• To seal off lost circulation zones

• To carry out remedial work on a poor primary cement job (to fill up the annulus)

• To prevent vertical reservoir fluid migration into producing zones

• To prevent fluids escaping from abandoned zones.

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Squeeze Cementing

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Evaluation of Cement Jobs

A primary cement job can be considered a failure if the cement does not isolate

undesirable zones. This will occur if:

• The cement does not fill the annulus to the required height between the casing

and the borehole.

• The cement does not provide a good seal between the casing and borehole and

fluids leak through the cement sheath to surface.

• The cement does not provide a good seal at the casing shoe and a poor leak off

test is achieved

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A primary cement job can be considered a failure if the cement does not isolate

undesirable zones. This will occur if:

• The cement does not fill the annulus to the required height between the casing

and the borehole.

• The cement does not provide a good seal between the casing and borehole and

fluids leak through the cement sheath to surface.

• The cement does not provide a good seal at the casing shoe and a poor leak off

test is achieved

When any such failures occur some remedial work must be carried out. A number

of methods can be used to assess the effectiveness of the cement job. These

include:

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(a) Temperature surveys:

This involves running a

thermometer inside the casing

just after the cement job. The

thermometer responds to the

heat generated by the cement

hydration, and so can be used to

detect the top of the cement

column in the annulus.

Temperature Survey

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(b) Radioactive surveys

Radioactive tracers can be added to

the cement slurry before it is pumped

(Carnolite is commonly used). A

logging tool is then run when the

cement job is complete. This tool

detects the top of the cement in the

annulus, by identifying where the

radioactivity decreases to the

background natural radioactivity of the

formation.

Radioactive Surveys

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(a) Cement bond logs (CBL)

The cement bond logging tools have become the standard method of

evaluating cement jobs since they not only detect the top of cement, but also

indicate how good the cement bond is.

The CBL tool is basically a sonic tool which is run on wireline. The distance

between transmitter and receiver is about 3 ft.

Since the speed of sound is greater in casing than in the formation or mud the

first signals which are received at the receiver are those which travelled

through the casing. If the amplitude (E1) is large (strong signal) this indicates

that the pipe is free (poor bond).

Cement Bond Logs

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Cement Bond Logs

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