Introduction:

The key to understanding drilling fluids is to understand clay chemistry. And in order to understand clay chemistry, we should start with the basic notions; general definition of what is a clay, and different clay types.

The clay minerals are a part of a general group within the phyllosilicates that contain large percentages of water trapped between the silicate sheets. Most clays are chemically and structurally analogous to other phyllosilicates but contain varying amounts of water and allow more substitution of their cations.

From the standpoint of geology, clay (sediments less than 0.0039 mm in size) is a group of rock-forming, hydrous aluminum silicate minerals that are layered in morphology and can form by the alteration of silicate minerals. On the other hand, from the standpoint of drilling fluid technology, clay is a large family of complex minerals containing the elements magnesium, aluminum, silicon, and oxygen combined in a sheetlike structure.

Clay Types: We can divide clay minerals into 4 major groups:

• The Kaolinite group: This group has three members (kaolinite, dickite and nacrite)
• The Montmorillonite / Smectite group: This group is composed of several minerals including talc, vermiculite, sauconite, saponite, nontronite...
• The Illite ( Clay-Mica) group: The mineral illite is the only common mineral represented, however it   is a main component of shales.
• The Chlorite group: This group is not always considered a part of the clays and is sometimes left alone as a separate group within the phyllosilicates.

Various clays react to water at differing levels known as activity levels. The smectites are the most reactive with water, easily disassociating. The best known clay is sodium montmorillonite, better known as bentonite. Calcium montmorillonite is sometimes called subbentonite. And vermiculite is the least active of the Smectite group.
The next less-reactive clays are the illites, followed by the chlorites, and the kaolinites. Each of these clays is present in differing proportions in formations, a fact that can seriously complicate drilling fluid selection.

Molecular Structure:

Particles Sizes: The common size of a given particle is usually measured in microns (μm). Particles that are greater than 44 μm are considered sand-sized particles (regardless of their composition). Grains can be subcategorized as coarse (greater than 2 mm), intermediate (between 2 mm and 250 μm), medium (between 250 and 74 μm), and fine (between 74 and 44 μm). Particles sized between 44 and 2 μm are so-called silt-sized. And particles less than 2 μm are known as colloidal.

Clay particles are colloidal in size. While sand and silt-sized particles can be physically separated in a liquid, a colloidal-sized particle cannot. It must be removed using a chemical reaction, which typically enlarges the particle and makes it susceptible to physical separation.

Because of its remarkable surface area (which is about 6 m square for a sample of 1cm cube of clay, due to the platelet structure of clay molecules), clay has a high chemical reactivity.

*Ordinary kaolinite under an electron microscope. Source : Yongjae Lee, Yonsei University

Constituents of Clays: There are major and minor constituents. Major contituents are present in high proportions and are the building units of the crystalline structure of clays, they are Silicium, Aluminum, Oxygen and Hydrogen (Oxygen may be present alone or in combination with Hydrogen such as Hydroxyl (OH) or Water). Minor constituents are present in low proportions and there presence gives specific chemical or physical properties to the clay (reactivity, color, hardness...), the most predominant are Sodium, Calcium, Iron, Magnesium and Potassium.

Molecule Building: There are 2 basic building units from which all clay minerals are constructed:

     1. The Octahedral Layer : This consists of two sheets of closely packed hydroxyl ions in which aluminum, iron or magnesium ions are embedded. (See figure below [2])

     2. The Tetrahedral Unit: In each tetrahedral unit, a silicon atom is located in the centre of the tetrahedron, equidistant from the four oxygen atoms. (The OH groups may replace the oxygen atoms, if needed to electrically balance the structure.) (See figure below [2])

A model representation of the structure of sodium montmorillonite is shown in the Figure below. A central alumina octahedral sheet has silica tetrahedral sheets on either side. These sheetlike structures are stacked with water and the loosely held cations between them. Polar molecules such as water can enter between the unit layers and increase the interlayer spacing. This is the mechanism through which montmorillonite hydrates or swells.

                                                                                            *Structure of sodium montmorillonite [1]

Hydration: 

Hydration occurs as clay platelets absorb water and swell. The process involves cations exchange between the clay molecules and water. The figure below illustrates the various forms of clay behaviour

Dispersion (or disaggregation) causes clay platelets to break apart and disperse into the water because of loss of attractive forces as water forces the platelets farther apart.

Aggregation—a result of ionic or thermal conditions— alters the hydration of a layer around the clay platelets, removes the deflocculant from positive-edge charges, and allows platelets to assume a face-to-face structure.

Flocculation begins when mechanical shearing stops and platelets that previously dispersed come together because of the attractive force of surface charges on the platelets. 

Deflocculation, the opposite effect, occurs by addition of chemical deflocculant to flocculated mud; the positive edge charges are covered and attraction forces are greatly reduced.

                                                                                                           *Different Associations of Clay Particles [1]

Sodium montmorillonite (Bentonite) is known as a premium mud additive. It is considered higher quality swelling clay, while calcium-type montmorillonite is of lower quality and is treated during grinding by adding more additives for various commercial applications. Sodium montmorillonite is capable of swelling to approximately 10 times its original volume when mixed with fresh water. Calcium montmorillonite will swell only 2 to 4 times its original volume when mixed with water.

However, the ability of bentonite to swell when exposed to water is considerably affected when the salinity of the water is important. In the particular case of salt water, a fibrous, needle-like clay mineral called Attapulgite is used. Attapulgite (a mineral found near Attapulgus, Georgia, USA) is composed of magnesium-aluminum silicate and is incapable of controlling the filtration properties of the mud. The ability of attapulgite to build viscosity is thought to be a result of interaction between the attapulgite fibers rather than the hydration by the water molecules. A longer period of agitation is required to build viscosity with attapulgite than with smectite clays. However, with continued agitation, viscosity decreases are observed eventually because of the mechanical breakage of the long fibers. This can be offset through the periodic addition of a new attapulgite material to the system [1].

References:

[1] R. F. Mitchell, S. Z. Miska ''Fundamentals of Drilling Engineering'', 2011.

[2] ''Clay Chemistry'', KMC Oil tools.