6.
Cementing
PEG1200
Drilling and Well Completion for Technicians
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Objectives
1. Distinguish between primary and remedial cementing operations.
2. Describe
a) Cement functions
b) Equipment
c) Slurry properties
d) Cement evaluation
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Well cementing
Well cementing consists of two principal operations:
1. Primary cementing: the process of placing a cement sheath in the
annulus between the casing and the formation. Cementing
completes the isolation step that was started with casing
2. Remedial cementing: occurs after primary cementing, when
engineers inject cements into strategic well locations for various
purposes, including well repair (squeeze cementing) and well
abandonment.
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Cementing Problems
Cementing is critical step of well construction.
Poor cement jobs can lead to:
1. well control situations
2. low productivity
3. gas channeling in annulus
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Primary Cementing
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Functions of Primary Cementing
Primary cementing is a critical procedure in
the well construction process. Cement:
1. provides a hydraulic seal that
establishes zonal isolation, preventing Cement
fluid communication between
producing zones in the borehole and Low
Pressure
blocking the escape of fluids to the Loss Zone
surface.
2. anchors and supports the casing string High
and protects the steel casing against Pressure
Permeable
corrosion by formation fluids. Zone
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Primary Cementing Objectives
The objectives of the primary cementing operation are to:
1. remove drilling fluid from the casing interior and borehole
2. place a cement slurry in the annulus
3. fill the casing interior with a displacement fluid such as drilling fluid,
brine or water.
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Primary Cementing
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Cement Head
Cementing Plug Container
Is mounted on top of casing
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to be cemented.
Subsea Cement Head
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© 2011 BP America Inc.
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Primary cementing operations - Casing Shoes
• After drilling through an interval to a desired depth, a drilling crew
removes the drillpipe, leaving the borehole filled with drilling fluid.
• The crew then lowers a casing string to the bottom of the borehole.
• The bottom end of the casing string is protected by a guide
shoe or float shoe.
• The guide shoe helps guide the casing past ledges and doglegs
(sudden hole direction changes) during pipe running.
• The float equipment helps prevent backflow of cement after full
displacement.
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Use a float shoe and a float collar
for redundancy.
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Primary cementing operations - Centralizers
• Centralizers are placed along
critical casing sections to:
1. help prevent the casing from
sticking while it is lowered into
the well
2. keep the casing in the center of
the borehole to help ensure
placement of a uniform cement
sheath in the annulus between
the casing and the borehole
wall.
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Centralizers
Bow Spring Centralizers
Solid Centralizers
Rigid Centralizers 15
Casing Movement – Reciprocation
Reciprocation helps in removing mudcake.
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Casing Movement – Rotation
Beneficial in removing loose mudcake and
providing for a good cement bond.
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Scratchers and Wipers
Scratchers
• Mechanical wall cleaning devices attached to
casing.
• Help remove filter cake and gelled mud from
the well as during casing movement.
• Provides a better bonding for the cement.
Wipers
• Have looped cables to help clean the wellbore
– with large flow areas between the loops to
permit the filter cake to pass through.
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Cement Mixing and Pumping System
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Primary cementing operations - Spacers
• Cement slurries and drilling fluids are
usually chemically incompatible. Therefore,
engineers employ chemical and physical
means to maintain fluid separation.
• Chemical washes and spacer fluids may be
pumped after the drilling fluid and before
the cement slurry. These fluids have the
added benefit of cleaning the casing and
formation surfaces, which helps achieve
good cement bonding.
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Primary cementing operations - Wiper Plugs
• Wiper plugs are elastomeric devices that
provide a physical barrier between fluids
pumped inside the casing.
• A bottom plug separates the cement slurry
from the drilling fluid, and a top plug
separates the cement slurry from the
displacement fluid.
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Primary cementing operations - Wiper Plugs
• The bottom plug has a membrane that
ruptures when it lands at the bottom of the
casing string, creating a pathway through
which the cement slurry may flow into the
annulus.
• The top plug does not have a membrane;
therefore, when it lands on top of the
bottom plug, hydraulic communication is
severed between the casing interior and the
annulus.
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Primary cementing operations - Wiper Plugs
• After the cementing operation, engineers
wait for the cement to cure, set and
develop strength - known as waiting on
cement (WOC).
• After the WOC period, usually less than 24
hours, additional drilling, perforating or
other operations may commence.
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Primary Cementing Operation Summary
1. Bottom rubber wiper plug is released to minimize cement
contamination from drilling fluid. Spacer fluid (or mud preflush)
may be pumped.
2. Desired volume of slurry is pumped
3. Top wiper plug is released
4. Drilling fluid displaces the top plug down the casing
5. When bottom plug reaches the float collar, its diaphragm ruptures.
6. The whole cement slurry has been fully displaced when the top
plug bumps the bottom plug.
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Courtesy of Weatherford
Typical Primary
• Reciprocating wiper
Cementing
Equipment • Centralizer
• Top plug
• Bottom plug
• Float collar
• Float shoe
Cement
• Nearly all well cementing operations use portland cement, which
consists mainly of anhydrous calcium silicate and calcium aluminate
compounds that hydrate when added to water.
• The hydration products, principally calcium silicate hydrates, provide
the strength and low permeability required to achieve zonal
isolation.
• Cement manufacturers produce special versions of portland cement
for use in wells
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API Classification for Oil Well Cements
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Cement additives
• >100 cement additives are available to adjust cement performance,
allowing engineers to customize a cement formulation for a particular
well environment.
• The principal objective is to formulate a cement that is pumpable for
a time sufficient for placement in the annulus, develops strength
within a few hours after placement and remains durable throughout
the well's lifetime.
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Cement Additives
Additives
Density Setting Lost Friction
Viscosity
Control Time circulation Reducers
Light Heavy
Accelerators Retarders
weight weight
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Cement additives
• Accelerators: reduce the cement setting time and increase the rate of
compressive strength development.
Setting
Time • Retarders delay the setting time and extend the time during which a
cement slurry is pumpable.
• Extenders lower the cement slurry density, reduce the amount of
Density cement per unit volume of set product, or both.
• Weighting agents increase the density of the cement.
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Cement additives
• Fluid loss control agents control leakage of water from the cement
slurry into porous formations, thereby preserving the designed
cement slurry properties.
• Lost circulation control agents limit flow of the entire cement slurry
out of the wellbore into weak, cracked or vugular formations and help
ensure that the cement slurry is able to fill the entire annular space.
• Dispersants reduce the viscosity of the cement slurry, which allows a
lower pumping pressure during placement.
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Cement Slurry
• The final slurry design has a certain slurry density, usually between 11
and 18 ppg, with typical class G or H being about ppg.
• More water can be added, but the water will separate as it sets,
resulting in channels or weak cement
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Cement Evaluation
Methods for evaluating the quality of cement around the casing may
detect the presence of channels that might exist, indicating poor
cement fill-up efficiency and risk of poor or no zonal isolation. These
include:
1. Pressure testing
2. Various well logging methods: where the logging crew lowers
measuring devices into the well:
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Pressure testing
• Pressure testing is the most common hydraulic testing method.
• The driller first performs a casing pressure test to verify the
mechanical integrity of the tubular string and then drill out the casing
shoe.
• Next, the driller performs a pressure integrity test by increasing the
internal casing pressure until it exceeds the pressure that will be
applied during the next drilling phase. If no leakage is detected, the
cement seal is deemed successful.
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Cement evaluation
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Temperature logs
• Temperature logs log run within 12 hours
of cementing help locate the top of the
cement column in the annulus.
• Cement hydration is an exothermic
process that raises the temperature of the
surrounding environment.
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Cement Bond Logs
• The cement bond log
presents the reflected
amplitude of an acoustic
signal transmitted by a
logging tool inside the
casing.
Casing collars
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Ultrasonic Logs
• Ultrasonic logging tools transmit
a short ultrasonic pulse, causing
the casing to resonate.
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Remedial cementing: Squeeze cementing
• When logging operations indicate that the cement job is defective,
either in the form of poor cement bonding or communication
between zones, a remedial cementing technique known as squeeze
cementing may be performed to establish zonal isolation.
• A cementing crew perforates the casing at the defective interval and
forces, or squeezes, cement slurry through the perforations and into
the annulus to fill the voids.
• In addition, squeeze cementing may be an effective technique for
repairing casing leaks caused by a corroded or split casing.
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Cementing stats
•Primary cementing cost about 5% of well cost.
•About 15% of primary cement jobs require squeeze cementing
immediately.
•Total cost of cementing when squeeze cementing is required is about
17% of well cost.
•Typical number of squeeze cementing required to fix a problem in a
primary cement job = 3.
•One out of every three workovers are done to repair bad primary
cement jobs.
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Remedial cementing: Plug cementing
• When a well has reached the end of its productive life, operators
usually abandon the well by performing plug cementing.
• Engineers fill the casing interior with cement at various depths,
thereby preventing interzonal communication and fluid migration into
underground freshwater sources.
• The ultimate objective is to restore the natural integrity of the
formations that were disrupted by drilling.
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prevents interzonal
communication and
fluid migration
Well
abandonment
Plugging a
plugs
depleted zone
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