Well engineering and construction ch 7, 8, 9, 10, 16 17

Well Engineering & Construction

24 Kilometers

Hussain Rabia Index

Well Engineering &Construction i

Well Engineering &

Construction Table of Contents

Chapter 1 : Pore Pressure 1

Chapter 2 : Formation Integrity Tests 49

Chapter 3 : Kick Tolerance 71

Chapter 4 : Casing Properties 101

Chapter 5 : Casing Design Principles 145

Chapter 6 : Cementing 203

Chapter 7 : Drilling Fluids 267

Chapter 8 : Practical Rig Hydraulics 305

Chapter 9 : Drill Bits 339

Chapter 10 : Drillstring Design 383

Chapter 11 : Directional Drilling 443

Chapter 12 : Wellbore Stability 531

Chapter 13 : Hole Problems 575

Chapter 14 : Horizontal & Multilateral Wells 631

Chapter 15 : High Pressure & High Temperature Wells 681

Chapter 16 : Rig Components 717

Chapter 17 : Well Costing 749

Well Engineering &Construction 267 . . . . .

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DRILLING FLUIDS 7

Contents

1 Drilling Fluid Selection: data Requirements

2 Drilling Fluid Functions

3 Drilling Fluid Additives

4 Drilling Fluid Types

5 Drilling Mud Properties

6 Drilling Fluid Problems

7 Solids Control Equipment

8 Learning Milestones

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I NTRODUCTION

Drilling mud is one of the most important elements of any drilling operation. The mud has a

number of functions which must all be optimised to ensure safety and minimum hole

problems. Failure of the mud to meet its design functions can prove extremely costly in

terms of materials and time, and can also jeopardise the successful completion of the well

and may even result in major problems such as stuck pipe, kicks or blowouts.

There are basically two types of drilling mud: water-based and oil-based, depending on

whether the continuous phase is water or oil. Then there are a multitude of additives which

are added to either change the mud density or change its chemical properties.

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The following information should be collected and used when selecting drilling fluid or

fluids for a particular well. It should be noted that it is common to utilise two or three

different fluid types on a single well.

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• Pore pressure /fracture gradient plots to establish the minimum / maximum mud

weights to be used on the whole well, see Chapters One and Two for details.

• Offset well data (drilling completion reports, mud recaps, mud logs etc.) from

similar wells in the area to help establish successful mud systems, problematic

formations, potential hazards, estimated drilling time etc.

• Geological plot of the prognosed lithology.

• Casing design programme and casing seat depths. The casing scheme

effectively divides the well into separate sections; each hole section may have

similar formation types, similar pore pressure regimes or similar reactivity to

mud.

• Basic mud properties required for each open hole section before it is cased off.

• Restrictions that might be enforced in the area i.e. government legislation in the

area, environmental concerns etc.

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The drilling mud must perform the following basic functions:

1. To control sub-surface pressures by providing hydrostatic pressure greater than the

formation pressure. This property depends on the mud weight which, in turn,

depends on the type of solids added to the fluid making up the mud and the density

of the continuous phase.

2. To remove the drilled cuttings from the hole. The removal of cuttings depends on

the viscous properties called "Yield Point" which influences the carrying capacity of

the flowing mud and "gels" which help to keep the cuttings in suspension when the

mud is static. The flow rate of mud is also critical in cleaning the hole.

3. To cool and lubricate the drill bit and drillpipe.

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4. To prevent the walls of the hole from caving. This function is provided by the

formation of a stable mud cake on the walls of the wellbore, somewhat like

plastering the walls of a room to keep them from flaking.

5. To release the drilled cuttings at the surface.

6. To prevent or minimise damage to the formations penetrated by having minimum

fluid loss into the formation.

7. To assist in the gathering of the maximum information from the formations being

drilled.

8. To suspend the cuttings and weighing material when circulation is stopped

(gelation). This property is provided by gels and low shear viscosity properties.

9. To minimise the swelling stresses caused by the reaction of the mud with the shale

formations. This reaction can cause hole erosion or cavings resulting in an unstable

wellbore (See Chapter 13 ). Minimisation of wellbore instability is provided by the

"inhibition" character of the drilling mud.

The chemical additives required to achieve the above functions will be explained in the

following section.

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There are many drilling fluid additives which are used to develop the key properties of the

mud.

The variety of fluid additives reflect the complexity of mud systems currently in use. The

complexity is also increasing daily as more difficult and challenging drilling conditions are

encountered.

We shall limit ourselves to the most common types of additives used in water-based and oil￾based muds. These are:

• Weighting Materials

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Weighting Materials

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• Viscosifiers

• Filtration Control Materials

• Rheology Control Materials

• Alkalinity and pH Control Materials

• Lost Circulation Control Materials

• Lubricating Materials

• Shale Stabilizing Materials

3.1 WEIGHTING MATERIALS

Weighting materials or densifers are solids material which when suspended or dissolved in

water will increase the mud weight. Most weighting materials are insoluble and require

viscosifers to enable them to be suspended in a fluid. Clay is the most common viscosifier.

Mud weights higher than water (8.3 ppg) are required to control formation pressures and to

help combat the effects of sloughing or heaving shales that may be encountered in stressed

areas.

Table 7.1 gives a list of the most commonly used weighting materials. The specific gravity

of the material controls how much solids material (fractional volume) is required to produce

a certain mud weight. For example, to produce a mud weight of 19 ppg (2.28 gm/cc), the

solids content from using only barite (sg = 4.2) is 39.5% compared with haematite (sg = 5.2)

with solids content of 30%.

Table 7.1 : Materials used as densifiers, After Reference 1

Material Principal

Component

Specific Gravity %Acid

Soluble

Galena PbS 7.4-7.7 0

Haematite Fe2O3 4.9-5.3 50+

Magnetite Fe3O4 5.0-5.2 0

Illmenite FeO.TiO2 4.5-5.1 20

Barite BaSO4 4.2-4.6 0

Siderite FeCO3 3.7-3.9 95+

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3.1.1 DESCRIPTION OF MOST COMMONLY USED WEIGHTING MATERIALS

1. Barite

Barite (or barytes) is barium sulphate, BaSO4 and it isthe most commonly used weighting

material in the drilling industry. Barium sulphate has a specific gravity in the range of 4.20 -

4.60. The specific gravity of Most commercial barite contain impurities including quartz,

chert, calcite, anhydrite, and various silicates which slower its specific gravity. It is normally

supplied to a specification where the specific gravity is about 4.2.

Barite is preferred to other weighting materials because of its low cost and high purity.

Barite is normally used when mud weights in excess of 10 ppg are required. Barite can be

used to achieve densities up to 22.0 ppg in both water- based and oil -based muds. However,

at very high muds weights (22.0 ppg), the rheological properties of the fluid become

extremely difficult to control due to the increased solids content.

2. Iron Minerals

Iron ores have specific gravities in excess of 5. They are more erosive than other weighting

materials and may contain toxic materials. The mineral iron comes from several iron ores

sources including: haematite/magnetite, illmenite and siderite.

The most commonly used iron minerals are:

Iron Oxides: principally haematite, Fe2O3. Haematite can be used to attain densities up to

22.0 ppg in both water- based and oil -based drilling fluids. Iron oxides have several

disadvantages including: magnetic behaviour which influences directional tool and magnetic

logs, toxciticity and difficulty in controlling mud properties.

Celestite SrSO4 3.7-3.9 0

Dolomite CaCO3.MgCO3 2.8-2.9 99

Calcium Carbonate CaCO3 2.6-2.8 99

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Iron Carbonate: Siderite is a naturally occurring ferrous carbonate mineral (FeCO3). It has

a specific gravity ranging from 3.70 - 3.90. Both water- based and oil- based muds can be

successfully weighted with siderite to 19.0 ppg.

Illmenite: The mineral illmenite, ferrous titanium oxide (FeTiO3), has a specific gravity of

4.60. It is inert but abrasive. Ilmenite can be used to attain densities up to 23.0 ppg in both

water-based and oil- based drilling fluids. Illmenite is the main source of titanium.

3. Calcium Carbonates

Calcium carbonate (CaCO3) is one of the most widely weighting agents especially in non￾damaging drilling fluids. Its main advantage comes from its ability to react and dissolve in

hydrochloric acid. Hence any filter cake formed on productive zones can be easily removed

thereby enhancing production. It has a specific gravity of 2.60 - 2.80 which limits the

maximum density of the mud to about 12.0 ppg

Calcium carbonate is readily available as ground limestone, marble or oyster shells.

Dolomite is a calcium - magnesium carbonate with a specific gravity of 2.80 - 2.90. The

maximum mud density achieved is 13.3 ppg.

4. Lead Sulphides

Galena (PbS) has a specific gravity of 7.40 - 7.70 and can produce mud weights of up to 32

ppg. Galena is expensive and toxic and is used mainly on very high pressure wells.

5. Soluble Salts

Soluble salts are used to formulate solids free fluids and are used mainly as workover and

completion fluid. Depending on the type of salt used, fluid densities ranging from 9.0 - 21.5

ppg (sg =1.08 - 2.58) can be prepared. Table 7.2 gives the maximum densities that can be

attained for single salt systems.

Table 7.2 Maximum Densities Of Single Salt Brines, After Baroid 1

Material g/cm3 lb/gal

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Potassium Chloride (KCl) 1.16 9.7

Sodium Chloride (NaCl) 1.20 10.0

Sodium Formate (NaHCO2) 1.33 11.1

Calcium Chloride (CaCl2) 1.42 11.8

Potassium Formate (KHCO2) 1.60 13.3

Calcium Bromide (CaBr2) 1.85 15.4

Caesium Formate 2.36 19.7

Zinc Bromide (ZnBr2) 2.46 20.5

3.2 VISCOSIFIERS

The ability of drilling mud to suspend drill cuttings and weighting materials depends entirely

on its viscosity. Without viscosity, all the weighting material and drill cuttings would settle to

the bottom of the hole as soon as circulation is stopped. One can think of viscosity as a

structure built within the water or oil phase which suspends solid material. In practice, there

are many solids which can be used to increase the viscosity of water or oil. The effects of

increased viscosity can be felt by the increased resistance to fluid flow; in drilling this would

manifest itself by increased pressure losses in the circulating system.

A list of some of the materials used to provide viscosity to drilling fluids is given in Table

7.3. We will begin our discussion of viscosifers with clay minerals.

Table 7.3 Materials used as viscosifiers, After Reference 1

Material Principal Component

Bentonite Sodium/Calcium Aluminosilicate

CMC Sodium Carboxy-methyle cellulose

PAC Poly anionic Cellulose

Xanthan Gum Extracellullar Microbial Polysaccharide

HEC Hyroxy-ethyl Cellulose

Guar Gum Hydrophilic Polysaccharide Gum

Resins Hydrocarbon co-polymers

Silicates Mixed Metal Silicates

Synthetic Polymers High molecular weight Polyacrylamides/polyacrylates

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Viscosifiers

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3.2.1 CLAYS

Clays are defined as natural, earthy, fine-grained materials that develop plasticity when wet.

They are formed from the chemical weathering of igneous and metamorphic rocks. The

major source of commercial clays is volcanic ash; the glassy component of which readily

weathers very readily, usually to bentonite.

A clay particle has a characteristic atomic structure in which the atoms form layers, see

Figure 7.1. There are three layers which give the clays their special properties:

• tetrahedral layers: These are made up of a flat honeycomb sheet of tetrahedra

containing a central silicon atom surrounded by four oxygens. The tetrahedra

are linked to form a sheet by sharing three of their oxygen atoms with adjacent

tetrahedra.

• Octahedral layers: These are sheets composed of linked octahedras, each made

up of an aluminium or magnesium atom surrounded by six oxygens. Again, the

links are made up by sharing oxygen atoms between two or three neighboring

octahedras.

• Exchangeable layers: These are layers of atoms or molecules bound loosely into

the structure, which can be exchanged with other atoms or molecules. These

exchangeable atoms or molecules are very important as they give the clays their

unique physical and chemical properties.

The nature of the above layers and the way they are stacked together define the type of clay

mineral. For this reason, they are several types of clays available. The most widely used clay

is bentonite.

Bentonite

This is the most widely used additive in the oil industry. The name, bentonite, is a

commercial name used to market a clay product found in the Ford Benton shale in Rock

Creek, Wyoming, USA.

Bentonite is defined as consisting of fine-grained clays that contain not less than 85%

Montmorillonite which belongs to the class of clay minerals known as smectites. Bentonite

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is classified as sodium bentonite or calcium bentonite, depending on the dominant

exchangeable cation. In fresh water, sodium bentonite is more reactive than calcium

bentonite and hence, in terms of performance, bentonite is classed as "high yield" (Sodium

Bentonite) or "low yield" (Calcium Bentonite).

Swelling

Tetrahedral silica

Tetrahedral silica

Octahedral alumina

Exchangeable cations nH2O (adsorbed water)

Interlayer

distance

Next unit:

Tetrahedral silica

Figure 7.1 Atomic structure of smectite clays, after reference 2

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Viscosifiers

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Bentonite is used to build viscosity in water which is required to suspend weighting

materials and drillcuttings. When clay is dispersed in water, viscosity is developed when the

clay plates adsorb water layers on to their structure. Each or several stacked water layers are

shared by two clay plates; these repeating structures of clay plates and their attached water

layers result in a viscous structure. The dispersion process will only take place in fresh water.

If the clay is used in salt muds it has to be prehydrated in fresh water.

Attapulgite

Attapulgite belongs to a quite different family of the clay minerals. In this family, the

tetrahedra in the tetrahedral sheets of atoms do not all point in the same way, but some

tetrahedra in the sheets are inverted. Instead of crystallising as platy crystals, attapulgite

forms needle-like crystals.

Attapulgite-based muds have excellent viscosity and yield strength and retain these

properties when mixed with salt water. However, they have the disadvantage of suffering

high water loss thereby giving poor sealing properties across porous and permeable

formations.

Organophillic Clays 1

Organophillic clays are made from normal clays (bentonite or attapulgite) and organic

cations. The organic cations replace the sodium or calcium cations originally present on the

clay plates. Organophillic clays can be dispersed in oil to form a viscous structure similar to

that built by bentonite in water.

3.2.2 POLYMERS

Polymers are used for filtration control, viscosity modification, flocculation and shale

stabilisation. When added to mud, polymers cause little change in the solid content of the

mud.

Polymers are chemicals consisting of chains made up of many repeated small units called

monomers.Polymers are formed from monomers by a process called polymerization. The

repeating units (monomers) that make up the polymer may be the same, or two or more

monomers may be combined to form copolymers. Structurally, the polymer may be linear or

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branched and these structures, either linear, branched, or both, may be cross-linked, i.e. tied

together by covalent bonds 1

, see Figure 7.2.

Types of Reactive Groups

The chemical reactivity of polymers is

mainly dependent on the type of

groups that are attached to the

molecule and the number of groups.

The groups that can be attached to the

polymer can be divided into three

groups:

1. Nonionic or neutral

2. Anionic or negatively

charged

3. Cationic or positively

charged

Examples of Polymers

Starches

Starch is a natural polymer used in drilling muds primarily to reduce filtrate loss and to

provide viscosity. Starch is the principal component of the seeds of cereal grains (such as

corn, wheat and rice) and of tubers (such as potato and tapioca). Starches1 are subject to

fermentation by many micro organisms (yeasts, molds, bacteria) and unless a mud

containing starch is saturated with salt or the pH is about 12, a biocide should be added.

Starch disperses in water to form a swollen particle that physically blocks the pore spaces.

This action is independent of the salt level in the mud. The addition level of starch is

relatively high in the region of 3-6 lb/bb.

Figure 7.2 Structure of Polymers

Crosslinked Linear Branched

Graft

Block

Random

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Viscosifiers

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Chemical modification of starch can significantly increase its stability to temperature and

mechanical shear and the correct type of starch must be selected to match the prevailing

bottom hole temperatures.

Guar Gum1

Guar gum is a natural polymer produced from the seeds of guar gum plants. Guar gum is an

nonionic polysaccharide polymer with a molecular weight of about 220,000. Guar gum can

also be attacked by micro organisms unless protected by high pH, high salinity, or a biocide.

Guar gum flocculates drilled cuttings when added in low concentrations while drilling with

water.

Xanthan Gum

Xanthan gum (Microbial Polysaccharides) is a water-soluble biopolymer produced by the

action of bacteria on carbohydrates The bacteria are killed after the fermentation process and

the gum extracted by precipitation with isopropyl alcohol. After the alcohol is recovered, the

gum is dried and milled. The polymer has a molecular weight of around 5,000,000.

Xanthan gum can build viscosity in fresh, sea and salt water without the assistance of other

additives. Uniquely the molecule forms a rigid rod like structure in solution. This gives very

high viscosities or gels at low shear rates. Consequently, xanthan polymer gives excellent

suspension properties that cannot be matched by other polymers at equivalent

concentrations.

Xanthan gum polymer muds are resistant to contamination by anhydrite, gypsum and salt.

This polymer1

has particular application in clay free, potassium based fluids where it will

increase the carrying capacity of mud without increasing its viscosity. The polymer also has

application in completion fluids where suspension of weighting materials is required

Carboxymethylcellulose (CMC)

Sodium carboxymethylcellulose (usually abbreviated as CMC) is an anionic polymer

produced by the treatment of cellulose with caustic soda and then monochloro acetate. The

molecular weight ranges between 50,000 and 400,000.

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Being anionic, CMC easily adsorbs on clay surfaces. Filtration is sharply reduced by low

concentrations of CMC in clay based drilling fluids. Higher molecular weight CMC is most

effective in filtration control. CMC is used for viscosification and filtration reduction in

heavily weighted muds and wherever little viscosification of the fluid phase of the mud is

desirable.

Polyanionic Cellulose 1

Polyanionic cellulose (PAC), is a semi-synthetic polymer which has been modified to

increase its tolerance to salt (up to saturation) and calcium.

Hydroxyethyl Cellulose

Hydroxyethyl cellulose (HEC) is made by a similar process to CMC but with ethylene oxide

after the caustic soda. Its main advantage lies in its ability to hydrate in all types waters.

The polymer 1

does not contain any ionic groups and therefore it is ideally suited as a

viscosifier for clear fluid completion and other brine- based fluids. The polymer exhibits

highly developed thioxotropy or shear thinning characteristics, but does not exhibit any yield

stress or gellation properties.

3.3 FILTRATION CONTROL MATERIALS

Filtration control materials are compounds which reduce the amount of fluid that will be lost

from the drilling fluid into a subsurface formation caused by the differential pressure

between the hydrostatic pressure of the fluid and the formation pressure. Bentonite,

polymers, starches and thinners or deflocculants all function as filtration control agents.

Bentonite imparts viscosity and suspension as well as filtration control. The flat, "plate like"

structure of bentonite packs tightly together under pressure and forms a firm compressible

filter cake, preventing fluid from entering the formation

Polymers such as Polyanionic cellulose (PAC) and Sodium Carboxymethylcellulose (CMC)

reduce filtrate mainly when the hydrated polymer chains absorb onto the clay solids and plug

the pore spaces of the filter cake p preventing fluid seeping through the filter cake and