Well Design – Spring 2012
Well Design
PE 413
Prepared by: Tan Nguyen
Well Design – Spring 2012
Introduction
History and Overview
The basic principle of oil well cementing involves displacing cement slurry down
the casing to a predetermined point in the well. The slurry is formed by mixing
water with Portland cement, or with cement blended with additives. This
procedure controls gas/oil and water/oil ratios, and is used in various types of
liner jobs and remedial work. The casing must be cemented to exclude water
and other unwanted fluids. Cement slurry is forced into the annular space
between the casing and the wall of the hole, where the cement can set and form
a permanent barrier against water and other fluids.
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Well Design – Spring 2012
Introduction
History and Overview
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
Prepared by: Tan Nguyen
Well Design – Spring 2012
Introduction
History and Overview
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.
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Well Design – Spring 2012
Introduction
Hydration of Cement
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.
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Well Design – Spring 2012
Composition of Portland Cement
The principal components of common Portland cement are
1.
50% tricalcium silicate (3CaO·SiO2) - C3S
2.
25% dicalcium silicate (2CaO·SiO2) – C2S
3.
10% tricalcium aluminate (3CaO·Al2O3) - C3A
4.
10% tetracalcium aluminoferrite (4CaO·Al2O3·Fe2O3) - C4AF
5.
5% other oxides
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Well Design – Spring 2012
Composition of Portland Cement
2(3CaO.SiO2) + 6H2O
-->
3CaO.2SiO2.3H2O + 3Ca(OH)2
2(2CaO.SiO2) + 4H2O
-->
(slow)3CaO.2SiO2.3H2O + Ca(OH)2
4CaO.Al2O3.Fe2O3 + 10H2O + 2Ca(OH)2
-->
(slow)6CaO.Al2O3.Fe2O3.12H2O + Ca(OH)2
3CaO.Al2O3 + 12H2O + Ca(OH)2
-->
(fast)3CaO.Al2O3.Ca(OH)2.12H2O
3CaO.Al2O3 + 10H2O + CaSO4.2H2O
-->
3CaO.Al2O3.CaSO4.12H2O
Oxide
Lime (CaO or C)
Silica (SiO2 or S)
Alumina (Al2O3 or A)
Ferric Oxide (Fe2O3 or F)
Magnesia (MgO)
Sulfur Trioxide (SO3)
Ignition loss
Prepared by: Tan Nguyen
Well Design – Spring 2012
Composition of Portland Cement
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 – 2.85SO3
C2S = 2.87S – 0.754C3S
C3A = 2.65A – 1.69F
C4AF = 3.04F
These equations are valid as long as the weight ratio of Al2O3 to Fe2O3 present is
greater than 0.64
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Well Design – Spring 2012
Example
Example: Calculate the percentages of C3S, C2S, C3A, and C4AF from the
following oxide analysis of a standard Portland cement.
Oxide
Weight Percent
Lime (CaO or C)
65.6
Silica (SiO2 or S)
22.2
Alumina (Al2O3 or A)
5.8
Ferric Oxide (Fe2O3 or F)
2.8
Magnesia (MgO)
1.9
Sulfur Trioxide (SO3)
1.8
Ignition loss
0.7
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Well Design – Spring 2012
Example
The A/F ratio is 5.8/2.8 = 2.07.
C3S = 4.07C – 7.6S – 6.72A – 1.43F – 2.85SO3
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 – 0.754C3S
C2S = 2.87(22.2) – 0.754(50.16) = 25.89%
C3A = 2.65A – 1.69F
C3A = 2.65(5.8) – 1.69(2.8) = 10.64%
C4AF = 3.04F
C4AF = 3.04(2.8) = 8.51%
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
presents a recommended procedure for testing drilling cements. Cement
specifications 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,
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Mud Balance – Slurry Density Test
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.
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Well Design – Spring 2012
API Tests for Cementing
Filter Press – Fluid Loss Test
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 cm2 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.
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Well Design – Spring 2012
API Tests for Cementing
Rotational Viscometer
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.
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Well Design – Spring 2012
API Tests for Cementing
Rotational Viscometer
Flow curves of time-independent fluids
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Newtonian fluids:
  
Power law fluids:
  K n
Bingham fluids:
   y   p
Herschel-Bulkley
(Yield power law fluids)
   y  K n
Well Design – Spring 2012
API Tests for Cementing
Rotational Viscometer
For Bingham fluid:
a 
300 N
N
where: a(cp) - apparent viscosity,
N - dial reading in degrees,
N(RPM) - rotor speed,
 P   600   300
p(cp) - plastic viscosity,
 y   300   p
 and y (lbf/100ft2) - shear stress, and yield stress
(1/s) - shear rate,
 and p(cp) - fluid viscosity and Bingham viscosity,
K (lbfxsn/100ft2) - consistency index,
n - flow behavior index.
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Rotational Viscometer
The data below are obtained from a rotational viscometer. Determine type of fluid
and the rheological model of this fluid.
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RPM
Dial Reading
3
10
6
12
100
35
200
48
300
60
600
75
Well Design – Spring 2012
API Tests for Cementing
Cement Consistometer – Thickening Time Test
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.
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Cement Consistometer – Thickening Time Test
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
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Cement Consistometer
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
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Cement Consistometer
Bc 
T  78 .2
20 .02
T is the torque on the paddle in g-cm and Bc is the slurry consistency in API
consistency units designated by Bc. The thickening time of the slurry is defined as
the time required to reach a consistency of 100 Bc. This value is felt to be
representative of the upper limit of pumpability.
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Well Design – Spring 2012
API Tests for Cementing
Cement Consistometer
Consistometer for simulating downhole conditions
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Consistometer for simulating
atmosphere conditions
Well Design – Spring 2012
API Tests for Cementing
Cement Consistometer
Typical
thickening time
test output
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Well Design – Spring 2012
API Tests for Cementing
Cement Consistometer
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.
Bc 
T  78.2 520  78.2

 22 consistenc y units
20.02
20.02
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Well Design – Spring 2012
API Tests for Cementing
Cement Permeameter
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
Darcy’s law:
K  14,700
qL
ADP
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(cm2) is the sample cross-sectional area,
and DP(psi) is the differential pressure.
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Cement Permeameter
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Cement Permeameter
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.
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Cement Permeameter
K  14,700
qL
ADP
K1  14,700
0.03451.02.54  10 mD

2.8652 20
4
K 2  14,700
0.003451.02.54  1 mD

2.8652 200
4
Prepared by: Tan Nguyen
Well Design – Spring 2012
API Tests for Cementing
Strength Testing Machine – Compressive Strength Test
The compressive strength of the set
cement is the compressional force
required to crush the cement divided
by the cross-sectional area of the
sample.
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