Well Design – Spring 2012
Well Design
PE 413
Surface Equipments and Placement Techniques
Prepared by: Tan Nguyen
Well Design – Spring 2012
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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Well Design – Spring 2012
Surface Cementing Equiments
Jet Mixer
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Well Design – Spring 2012
Surface Cementing Equiments
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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Surface Cementing Equiments
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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Well Design – Spring 2012
Surface Cementing Equiments
Cement Head
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Well Design – Spring 2012
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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Well Design – Spring 2012
Downhole Cementing Equipments
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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Downhole Cementing Equipments
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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Well Design – Spring 2012
Downhole Cementing Equipments
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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Well Design – Spring 2012
Downhole Cementing Equipments
Wiper Plugs
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Well Design – Spring 2012
Downhole Cementing Equipments
Casing Centralizers
Casing centralizers are used to:
1.Improve displacement efficiency
2.Prevent differential pressure sticking
3.Keep casing out of key seats
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Well Design – Spring 2012
Downhole Cementing Equipments
Wipers and Scratchers
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.
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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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Well Design – Spring 2012
Primary Cementing Techniques
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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Well Design – Spring 2012
Primary Cementing Techniques
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 predetermined 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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Well Design – Spring 2012
Primary Cementing Techniques
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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Well Design – Spring 2012
Primary Cementing Techniques
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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Well Design – Spring 2012
Primary Cementing Techniques
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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Primary Cementing Techniques
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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Well Design – Spring 2012
Primary Cementing Techniques
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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Primary Cementing Techniques
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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Primary Cementing Techniques
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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Well Design – Spring 2012
Primary Cementing Techniques
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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Well Design – Spring 2012
Cementing Techniques
Single Stage Cementing Operation
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Well Design – Spring 2012
Cementing Techniques
Single Stage Cementing Operation
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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Well Design – Spring 2012
Cementing Techniques
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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Well Design – Spring 2012
Cementing Techniques
Multi-Stage Cementing Operation
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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Cementing Techniques
Multi-Stage Cementing Operation
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Well Design – Spring 2012
Cementing Techniques
Multi-Stage Cementing Operation
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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Cementing Techniques
Multi-Stage Cementing Operation
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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Cementing Techniques
Multi-Stage Cementing Operation
Stage collar installed in the
casing for multi-stage
cementing operation
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Cementing Techniques
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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Well Design – Spring 2012
Cementing Techniques
Liner Cementing
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Well Design – Spring 2012
Cementing Techniques
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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Well Design – Spring 2012
Cementing Techniques
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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Well Design – Spring 2012
Liner Cementing
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Well Design – Spring 2012
Cementing Techniques
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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Well Design – Spring 2012
Cementing Techniques
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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Well Design – Spring 2012
Cementing Techniques
Squeeze Cementing
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Well Design – Spring 2012
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
Prepared by: Tan Nguyen
Well Design – Spring 2012
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
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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Well Design – Spring 2012
Evaluation of Cement Jobs
Temperature Survey
(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.
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Evaluation of Cement Jobs
Radioactive Surveys
(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
radioactivity
identifying
where
the
decreases
to
the
background natural radioactivity of the
formation.
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Well Design – Spring 2012
Evaluation of Cement Jobs
Cement Bond Logs
(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).
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Well Design – Spring 2012
Evaluation of Cement Jobs
Cement Bond Logs
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