Well Design – Spring 2012 Prepared by: Tan Nguyen Well Design PE 413

Well Design – Spring 2012

Prepared by: Tan Nguyen

Well Design

PE 413

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Introduction

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.

History and Overview

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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·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

Composition of Portland Cement

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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

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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 – 2.85SO3

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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Example: Calculate the percentages of C3S, C2S, C3A, and C4AF from the

following oxide analysis of a standard Portland cement.

Example

Oxide Weight Percent

Lime (CaO or C)

Silica (SiO2 or S)

Alumina (Al2O3 or A)

Ferric Oxide (Fe2O3 or F)

Magnesia (MgO)

Sulfur Trioxide (SO3)

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 – 2.85SO3

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%

Example

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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,

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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


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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 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.

Filter Press – Fluid Loss Test


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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


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Flow curves of time-independent fluids

Newtonian fluids:

Power law fluids:

Bingham fluids:

Herschel-Bulkley (Yield power law fluids)

nK

py

ny K



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For Bingham fluid:

NN

a

300

300600 P

py 300



where: a(cp) - apparent viscosity,

N - dial reading in degrees,

N(RPM) - rotor speed,

p(cp) - plastic viscosity,

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.

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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 10

6 12

100 35

200 48

300 60

600 75



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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


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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


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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


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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.



02.20

2.78T

Bc

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Consistometer for simulating down-hole conditions

Consistometer for simulating atmosphere conditions



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Typical thickening time

test output



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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.



unitsyconsistencT

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

Darcy’s 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(cm2) is the sample cross-sectional area,

and P(psi) is the differential pressure.

Cement Permeameter


PA

LqK

700,14

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Cement Permeameter


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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


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Cement Permeameter


PA

LqK

700,14

mDK 1020865.2

4

54.20.10345.0700,14

21

mDK 1200865.2

4

54.20.100345.0700,14

22

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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 the

sample.

Strength Testing Machine – Compressive Strength Test


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Documents

Well Design – Spring 2012 Prepared by: Tan Nguyen Well Design PE 413