2 - Clay Chemistry_PTM_Handout

8/22/2019 2 - Clay Chemistry_PTM_Handout

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Global Research & Technology Centre/ GRTC

Training Department

CLAYCLAY

CHEMISTRYCHEMISTRY

CLAYCLAY

CHEMISTRYCHEMISTRY

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Introduction

Clays play a major role in drilling fluid tech.

Every stage of drilling a hole brings in contact withthe Clays.

Chemical & Mechanical properties of the rock depend on the type & quantity of clay minerals.

Understanding of clay chemistry is essential inselection of drilling fluid system & bore holestability.


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Clays - Origin

Clays originate from the Rocks due to weatheringprocess.

Sedimentary rocks are the most abundant rock typeon the Earths Surface and Crust.

Shale is one most abundant rock type and clayminerals, its chief constituents.

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Genesis and Composition

Chemically Clays are Aluminosilicates.

Clay minerals are a part of a general group withinthe phyllosilicates (layered silicates).

Most clays are chemically and structurally analogousto each other but contain varying amounts of waterand allow varying levels of substitution in theircations

Shales are classified by age, water content, clay

content, and type o hardness of the shale.


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Composition of Clays

Major Constituents

Silica, Aluminum and Oxygen

(The above 3 elements constitute >80% of earthsmass)

Minor Constituents

Iron, Magnesium, Sodium and Potassium.

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Composition of ClaysPhysical Properties (structural details)

Size- Fine to Very Fine (0.1-5)

Surface area- Large to Very Large (12-300M2/g)

Chemically Reactive Surface.


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Building BlocksThere are two basic building units from which all the

different clay minerals are constructed :

Tetrahedral Layer

In each tetrahedral unit a silicon atom is located in thecentre of the tetrahedron, equidistant from the fouroxygen atoms.

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Building BlocksThe Tetrahedral Unit

The OH groups replace the oxygen atoms to electricallybalance the structure.

(a) (b)

Oxygen Atom Silicon Atom


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Building Blocks

The Octahedral Layer

In each octahedral unit an aluminium (or magnesium)atom is located in the centre of the octahedron,

equidistant from the six oxygen atoms.

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Building BlocksThe Octahedral Layer

This consists of two sheets of closely packed hydroxoyl ionsin which aluminum, iron or magnesium ions areembedded.


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Aluminums Silicons Hydroxyls Oxygens

Silica

(tetrahedral)

layer

Octahedral

layer

Building Blocks

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Groups of Clay There are > 400 reported clay mineral names due to

different combination of the basic building blocks and 26different clay mineral groups.

Clay minerals are divided into 7 major groups for drillingfluid purpose:

1.Kaolinite, 2.Illite, 3.Chlorite, 4.Mica,

5.Montmorillonite, 6. Attapulgite


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Groups of Clay

Each clay mineral type exhibits different characteristics andwas deposited in a different environment!

Montmorillonite/Smectite clays are expandable, thusabsorb water

Kaolinite, Illite, Chlorite are not expandable, thus do notabsorb water.

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Why are Clays Important

Clays in the Drilling Fluid :

Bentonite(gel, smectite, montmorillonite) for viscosityand fluid loss control in some WBM

Organophilic bentonitefor viscosity and fluid losscontrol in NAF system

Bentoniteis a key component of MMH systems

Attapulgitefor viscosity in salt

Sepiolitefor viscosity in very high temperature WBM

Drilled solids help with fluid loss control but can giveunwanted viscosity


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Why are Clays Important

Clays in Rocks :

In shales / mud rocks / clays causing possibledrilling problems

In reservoirs giving possible formation

damage

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Clay Structures

KAOLINITE: TO or 1:1

MONTMORILLONITEAND MICA (INCLUDE ILLITE) :

TOT or 2:1

CHLORITE:

+ + +

ATTAPULGITE/SEPIOLITE:

TOT :0: TOT or 2:1:1

TOT or 2:1

KEY: SILICATE SHEET (T)SILICATE SHEET (T) ALUMINA SHEET (O)ALUMINA SHEET (O)


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Comparison of Structures

Property Kaolin Mica Mont Attap Chlorite

Layer type 1 : 1 2 : 1 2 : 1 2 : 1 2 : 1 : 1

CrystalStructure Sheet Sheet Sheet Sheet Sheet

Particle Hexagonal Extensive Flakes Needles PlatesShape Plate Plates

Particle 0.5 - 5 0.5 - Large 0.1 - 2 0.1 - 1 0.1 - 5Size () Sheets

Surface AreaBET-N2-m

2/g 15 - 20 50 - 110 30 - 80 200 140BET-H2O-m

2/g - - 200 - 800 - -

CEC-meq/100g 3 - 15 10 - 40 80 - 150 15 - 25 10 - 40

Viscosityin Water Low Low High High Low

Effects ofSalts Flocculates Flocculates Flocculates Flocculates Flocculates

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Ca++Ca++

12.1 Ao

Ca++Ca++

17 A0

Hydration

Limited separation between clay plateletsdue to divalent charge of calcium.

Divalent charge cations will hold the clay

platelets closer together.


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Na+Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+9.8 A

o

+

water

Expansion from 17 A

to infinite separation

Hydration

Infinite separation between clay platelets,

due to monovalent charge of sodium.

Monovalent charge cations will not create a

bonding power between platelets.

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Hydration of Sodium and Calcium

Montmorillonites


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Charges on Clay Particles

Clay charges are important as they determine properties such as :

Ion Exchange

Swelling Behavior

Viscosity of Muds

Charges can arise from :

Broken edges on clay particles (Induced charges)

Substitution of Ions in the clay structure (Permanent

charges)

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Broken Edge Charges

Clay sheets can be broken due to mechanical action.

When a clay sheet is broken, the exposed edges will have

unbalanced charges which can either be +ve or -ve.

In an acidic environment the charges will tend to be +ve. In an

alkaline environment charges tend to be -ve.

One reason for keeping an alkaline pH in the drilling mud is to

keep all the clay charges -ve.

The -ve charges will repel each other thus reducing the

tendency for flocculation.


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Charges Due to Ion Substitution

Tetrahedral Layer : Some Si4+ can be replaced by Al3+ or Fe3+

Octahedral Layer : Some Al3+ can be replaced by Mg2+ or Fe2+

All Si Charges

All Al Balanced :

All Si Net Charge = Zero}

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Charges Due to Ion Substitution

These substitutions produce sheets with net negative charge

satisfied by adsorption of cations.

Unlike edge charges, these are permanent and not affected

by pH changes

Isomorphous substitution is the main reason why clays have

ion exchange properties and is the reason why

montmorillonite swells in water

Not All Si Charges

Not All Al Not Balanced :

Not All Si Net Charge}


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Ion Exchange Properties of Clays

The negative charge generated by isomorphous substitution is

balanced by cations held near the clay surface.

Common charge - balancing cations are Na, K, Ca, Mg; these

cations are readily exchangeable in montmorillonite

Cation exchange capacity of clay can be measured by methylene

blue test (MBT) or chemical analysis of displaced cations

++

+ +

eg. KCl solution +

Na+ Na+

Na+ Na+

K+ K+

K+ K+

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Factors Affecting Substitution of Exchangeable Cations:

Nature of Clay Mineral

Montmorillonite : Easy

Mica / Illite : Difficult

Chlorite : Impossible

Nature of original and substituted cations

Concentration of exchange solution

Cation Exchange


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Assuming all the cation concentrations are the same, theorder of increasing replacing power of cations is generally :

Li+ < Na+ < K+ < Mg2+ < Ca2+ < H+

At equal concentrations potassium will displace more

sodium than sodium will displace potassium.

Increasing the concentration of any given cation will increase

the probability that it will displace another cation.

It is possible for high concentrations of potassium to

displace calcium

Cation Exchange

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Hydration of clays is due to adsorption and absorption of

water.

Adsorption is the attachment of water molecules to the

external surface of clay particles, causing interlayer swelling.

It is either physical or chemical adsorption.

Absorption is the entry of water into the structure of the clay

particles, either by osmosis or by capillary action. It is only

physical and weak forces.

Hydration of Clays


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Hydration properties of the exchange cations have animportant influence on clay properties.

Hydration of cations depends on their charge and size.

High charge & small diameter cations are usually most

highly hydrated

Low charge & large diameter cations are usually least

hydrated

Hydration of Cations

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Clay HydrationThe important diameter is the hydrated ionic diameter.

Atom Dehydrated Ion Hydrated Ion

Diameter A Diameter A

Na - Sodium 1.90 11.2

K - Potassium 2.66 7.6

Cs - Cesium 3.34 7.6

Mg - Magnesium 1.30 21.6

Ca - Calcium 1.90 19.0

Hydrated Ionic Diameter

CATION

HH

-

H

H

-

H

-

H


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Clay Swelling

The most common swelling clay mineral is montmorillonite.

Montmorillonite (bentonite) is used in some drilling fluids to

give viscosity and fluid loss control.

Montmorillonite is found in many reactive shales.

Montmorillonite is found in some sandstones (including

reservoir sands).

The amount of water taken up by a montmorillonite (& hence

the degree of swelling) depends on :

Layer charge of the clay / Ion exchangeNature of the exchangeable cation

Nature of the external solution

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Clay SwellingCations Exchange Capacity / Layer Charge

Kaolinite Montmorillonite Mica (Illite)

Layer Charge Low Intermediate High

CEC Low (3-15) Intermediate (80-150)Low (10-40)

Swelling in None High None

Water


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Clay Swelling : Nature of ExchangeableCation

Swelling promoted by highly hydrated, low charge exchangeablecations

eg. Li+ , Na+

Swelling reduced by high charge, less hydrated cations

eg. Al3+

K+ reduces swelling because poorly hydrated even though low

charge.

Ca2+, Mg2+ reduces swelling because high charge, though highly

hydrated.

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Clay Dispersion / DeflocculationThere are four basic colloidal states of clay particles in a fluid :

Deflocculated. There is an overall repulsive force between the

particles. This is done by ensuring all the particles have the

same charge. (The particles may be aggregates)

Flocculated. There are net attractive forces for the particles

and they can associate with each other to form a loose

structure.


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Clay Dispersion / Deflocculation

Aggregated. The clay sheets are still attached to each otherand hydration has not occurred, or the hydration process has

been reversed.

Dispersed. This is where the aggregates have all been broken

down. The dispersed clays may be flocculated or

deflocculated.

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Clay Dispersion

Mechanical energy causes DISPERSION of aggregates

Mechanical energy can also break individual mineral grains

Leads to increased surface area of solids

MECHANICALMECHANICAL

ENERGYENERGY

MECHANICALMECHANICAL

ENERGYENERGY

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Clay DeflocculationChemical energy is used to deflocculate clays

The state of deflocculation is determined by surface charges

and electrical double layers surrounding particles in

suspension

FLOCCULATEDFLOCCULATED DEFLOCCULATEDDEFLOCCULATED

chemical energy


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+ - - - +

+ - - - +

1. Change pH- - - - -

- - - - -

- - - - -

- - - - -

- - - - -

add alkali (OH-)

add acid (H+)

< ~ pH 6.5FLOCCULATED

>> ~~ pH 8pH 8DEFLOCCULATEDDEFLOCCULATED

2. Add chemical deflocculants

+ - - - +

+ - - - +- - -

- - -

-- -- --

-- --

-- --

-- --

--

--

----

add deflocculant

+

+

+

++

+

--

-

-

-

-

-

-

-

+

++

+

--

--

-

-

-

Clay Deflocculation

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Effect of Clay Dispersion/Deflocculation

on Suspension ViscosityTo increase viscosity

Increase level of solids

Add high molecular weight viscosifying polymer

Flocculate with calcium or other polyvalent cation

Flocculate with salts

Flocculate with low pH conditions


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Effect of Clay Dispersion/Deflocculationon Suspension Viscosity

To decrease viscosity

Dilute with water

Deflocculate with low molecular weight polymers

Remove calcium by chemical treatment

Deflocculate with higher pH conditions

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-+

---

+ ++

+

--

-

-

-

-

- -

+

+

+

+

+ ++

+ +

++

-

-

--

-

-

-

- -

+

+

+

+ +

+

+

++-

-

--

++

+

+

+

++

+

-

---

-

-

-

-

-

+

++

+

Flocculated clay

Absorption of low M.W. polymercreates overall negative charge

resulting in deflocculation

High M.W. polymeracting as bridgebetween particles

to form layeraggregate

- --- - - ---

Deflocculant Flocculant

Clay States


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Na+

Na+

Na+

Na+

Na+

Na

+

Na

+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Ca++

Ca++

Ca++

Ca++

Ca++

Ca++

Ca++

+ Ca++

Dispersed sodium

montmorilonite

Flocculated systemcaused by calcium

bridges between particles

Aggregated calcium montmorilonite

Clay States

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Table of Viscosities in Different

Solutions

0

5

10

15

20

50,000 100,000 150,000 200,000

1500 3000 4500 6000

SALT

CALCIUM

PPMPPM

VISCOSITY (cP)VISCOSITY (cP)

A

B

A Dry bentonite in salt solution

B Dry bentonite in calcium solution


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Clays in Drilling Fluids

Clays are added to some water based muds to give :

Viscosity : Bentonite, Sepiolite / Attapulgite

Fluid loss control : Bentonite

Organophilic bentonite added to oil based muds to give viscosity

and fluid loss control.

Clays entrained in mud as drilled solids. These give viscosity andfluid loss control.

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Grades of BentoniteWyoming bentonite

Pure sodium montmorillonite. This is the best grade of

bentonite

API Bentonite

Is montmorillonite that meets API standards on viscosity and

filtration control. It may be (and usually is) treated with

polymers/extenders (Sodium Polyacrylate) to attain the API

grade.

OCMA Bentonite

Calcium montmorillonite

Documents

2 - Clay Chemistry_PTM_Handout