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Cement that is pumped down into the annulus is used as a sealant to help protect:
1. Casing and wellbore from external pressure that could collapse the pipe or cause
a blowout
2. Oil- and gas-producing strata from extraneous fluids
3. Casing from possible corrosion and electrolysis caused by formation waters and
physical contact with various strata
4. Downhole production and drilling equipment
5. Pipe from the stresses of formation movement
IntroductionHistory and Overview
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Introduction
The cement composition and placement technique for each job must be chosen
so that the cement will achieve an adequate strength soon after being placed in
the desired location. This minimizes the waiting period after cementing.
However, the cement must remain pumpable along enough to allow placement
to the desired location. The main ingredient in almost all drilling cements is
Portland cement, artificial cement made by burning a blend of limestone and
clay. This is the same basic type of cement used in making concrete.
History and Overview
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Introduction
Cement is composed principally of a blend of anhydrous metallic oxides. The
addition of water to this material converts these compounds to their hydrated
form. After a period of time, the hydrates form an interlocking crystalline structure
which is responsible for the set cement's strength and impermeability.
Hydration of Cement
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The principal components of common Portland cement are
1. 50% tricalcium silicate (3CaO SiO 2) - C 3S
2. 25% dicalcium silicate (2CaO SiO 2) C 2S
3. 10% tricalcium aluminate (3CaO Al2O3) - C 3 A
4. 10% tetracalcium aluminoferrite (4CaO Al2O3 Fe 2O3) - C 4 AF
5. 5% other oxides
Composition of Portland Cement
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2(3CaO.SiO 2) + 6H 2O --> 3CaO.2SiO 2.3H 2O + 3Ca(OH) 2
2(2CaO.SiO 2) + 4H 2O --> (slow)3CaO.2SiO 2.3H 2O + Ca(OH) 2
4CaO.Al2O3.Fe2O3 + 10H2O + 2Ca(OH)2 --> (slow)6CaO.Al 2O3.Fe 2O3.12H 2O + Ca(OH) 2
3CaO.Al 2O 3 + 12H 2O + Ca(OH) 2 --> (fast)3CaO.Al 2O3.Ca(OH) 2.12H 2O
3CaO.Al 2O 3 + 10H 2O + CaSO 4.2H 2O --> 3CaO.Al 2O3.CaSO 4.12H 2O
Composition of Portland Cement
Oxide
Lime (CaO or C)
Silica (SiO 2 or S)
Alumina (Al 2O3 or A)
Ferric Oxide (Fe 2O 3 or F)
Magnesia (MgO)
Sulfur Trioxide (SO 3)
Ignition loss
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API uses the following equations for calculating the weight percent of the crystalline
compounds from the weight percent of the oxides present.
C3S = 4.07C 7.6S 6.72A 1.43F 2.85SO 3
C2S = 2.87S 0.754C 3S
C3 A = 2.65A 1.69F
C4 AF = 3.04F
These equations are valid as long as the weight ratio of Al 2O3 to Fe 2O3 present isgreater than 0.64
Composition of Portland Cement
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Example: Calculate the percentages of C 3S, C 2S, C 3 A, and C 4 AF from the
following oxide analysis of a standard Portland cement.
Example
Oxide Weight Percent
Lime (CaO or C)
Silica (SiO 2 or S)
Alumina (Al 2O3 or A)
Ferric Oxide (Fe 2O3 or F)
Magnesia (MgO)
Sulfur Trioxide (SO 3)
Ignition loss
65.6
22.2
5.8
2.8
1.9
1.8
0.7
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The A/F ratio is 5.8/2.8 = 2.07.
C3S = 4.07C 7.6S 6.72A 1.43F 2.85SO 3
C3S = 4.07(65.6) 7.6(22.2) 6.72(5.8) 1.43(2.8) 2.85(1.8) = 50.16%
C2S = 2.87S 0.754C 3S
C2S = 2.87(22.2) 0.754(50.16) = 25.89%
C3 A = 2.65A 1.69F
C3 A = 2.65(5.8) 1.69(2.8) = 10.64%
C4 AF = 3.04F
C4 AF = 3.04(2.8) = 8.51%
Example
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API Tests for Cementing
presents a recommended procedure for testing drilling cements. Cementspecifications almost always are stated in terms of these standard tests. The test
equipment needed to perform the API tests includes:
1. A mud balance for determining the slurry density,
2. A filter press for determining the filtration rate of the slurry,
3. A rotational viscometer for determining the rheological properties of the slurry,
4. A consistometer for determining the thickening rate characteristics of the slurry,
5. Specimen molds and strength testing machines for determining the tensile and
compressive strength of the cement
6. A cement permeameter for determining permeability of the set cement,
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The test consists essentially of filling the cup with a mud sample and determining
the rider position required for balance. Water is usually used for the calibration fluid.
The density of fresh water is 8.33 lbm/gal.
Mud Balance Slurry Density Test
API Tests for Cementing
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The rate at which a cement slurry loses
the water required for its fluidity through
a permeable barrier is called filtration
rate or fluid-loss rate .
The standard API filter press has an
area of 45 cm 2 and is operated at a
pressure of 100 psig (6.8 atm). The
filtrate volume collected in a 30-min time
period is reported as the standard water
loss.
Filter Press Fluid Loss Test
API Tests for Cementing
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The mud is sheared at a constant rate between an inner bob and an outer rotating
sleeve. Six standard speeds plus a variable speed setting are available with the
rotational viscometer.
Rotational Viscometer
API Tests for Cementing
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Flow curves of time-independent fluids
Newtonian fluids:
Power law fluids:
Bingham fluids:
Herschel-Bulkley(Yield power law fluids)
n K
p y
n y K
Rotational Viscometer
API Tests for Cementing
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For Bingham fluid:
N N
a
300
300600 P
p y 300
Rotational Viscometer
API Tests for Cementing
where: a(cp) - apparent viscosity,
N - dial reading in degrees,
N(RPM) - rotor speed,
p(cp) - plastic viscosity,
and y (lbf/100ft 2) - shear stress, and yield stress
(1/s) - shear rate,
and p(cp) - fluid viscosity and Bingham viscosity,
K (lbfxs n/100ft 2) - consistency index,
n - flow behavior index.
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The data below are obtained from a rotational viscometer. Determine type of fluid
and the rheological model of this fluid.
RPM Dial Reading
3 106 12
100 35200 48
300 60
600 75
Rotational Viscometer
API Tests for Cementing
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A device used to determine the thickening time of cement slurries under simulated
downhole pressure and temperature conditions. The thickening time is a
measurement of the time during which cement slurry remain in a fluid state and is
capable of being pumped. Thickening time is assessed under simulated downhole
conditions using a consistometer that plots the consistency of a slurry over time at
the anticipated temperature and pressure conditions. The end of the thickening
time is considered to be 50 or 70 Bc for most applications.
Cement Consistometer Thickening Time Test
API Tests for Cementing
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The thickening time of a slurry under realistic conditions must be established to
ensure adequate pumping time for slurry placement.
Excessive thickening time must be avoided to prevent:
1. Delays in resuming drilling operations
2. Settling and separation of slurry components
3. Formation of free-water pockets
4. Loss of hydrostatic head and gas cutting
Cement Consistometer Thickening Time Test
API Tests for Cementing
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The apparatus consists of a rotating cylindrical slurry container equipped with a
stationary paddle assembly, all enclosed in a pressure chamber capable of
withstanding temperatures and pressures encountered in well cementing
operations. The cylindrical slurry chamber is rotated at 150 rpm during the test. The
slurry consistency is defined in terms of the torque exerted on the paddle by the
cement slurry. The relation between torque and slurry consistency is given by
Cement Consistometer
API Tests for Cementing
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Consistometer for simulating down-hole conditions
Consistometer for simulatingatmosphere conditions
Cement Consistometer
API Tests for Cementing
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Typicalthickening time
test output
Cement Consistometer
API Tests for Cementing
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The torque required to hold the paddle assembly stationary in a cement
consistometer rotating at 150 rpm is 520 g-cm. Compute the slurry consistency.
Cement Consistometer
API Tests for Cementing
units yconsistencT
Bc 2202.20
2.78520
02.20
2.78
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Cement permeameter is an apparatus for measuring the permeability of a
core sample. The permeability of a set cement core to water is determined by
measuring the flow rate through the core at a given pressure differential across the
length of the core. The permeability then is computed using an appropriate form of
Darcys law:
Where K(mD) is the permeability, q(mL/s) is the flow rate, (cp) is the water
viscosity, L(cm) is the sample length, A(cm 2) is the sample cross-sectional area,
and DP(psi) is the differential pressure.
Cement Permeameter
API Tests for Cementing
P A Lq
K D
700,14
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Cement Permeameter
API Tests for Cementing
ll
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A class E cement core having a length of 2.54 cm and a diameter of 2.865 cm
allows a water flow rate of 0.0345 mL/s when placed under a pressure differential
of 20 psi. A second core containing 40% silica cured in a similar manner allows only
0.00345 mL/s of water to flow under a pressure differential of 200 psi. Compute the
permeability of the two cement samples.
Cement Permeameter
API Tests for Cementing
W ll D i S i 2012
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Cement Permeameter
API Tests for Cementing
P A Lq
K D
700,14
mD K 10
20865.24
54.20.10345.0700,14
21
mD K 1200865.2
4
54.20.100345.0700,14
22
W ll D i S i 2012
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The compressive strength of the set
cement is the compressional force
required to crush the cement divided
by the cross-sectional area of thesample.
Strength Testing Machine Compressive Strength Test
API Tests for Cementing
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