The key to understanding drilling fl uids is to understand clay chemistry. And to really understand clay chemistry, the understanding of particle sizes is critical.
3.5.1 Particle Sizes. The common size of a given particle is usually measured in microns (µm). This is 0.001 mm or 3.937 × 10–5 in. Particles that are greater than 44 µm are considered sand-sized particles (regardless of their material). These can be subcategorized as coarse (greater than 2 mm), intermediate (between 2 mm and 250 µm), medium (between 250 and 74 µm), and fi ne (between 74 and 44 µm). Particles sized between 44 and 2 µm are silt-sized. And particles less than 2 µm are called 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. The range of particle sizes are illustrated in Fig. 3.3.
The surface area of a clay particle is remarkable. For example, a 1 cm cube of clay has a surface area of 6 × 10–4 m2. Chop that same 1 cm cube into 1-µm cubes (the size of a clay particle) and the result would be 10,0003 (1012) particles. Each particle would have a surface area of 6 × 10–12 m2; 1012 of those particles would yield a surface area of 6 m2—this is an increase of 10,000 in the available surface area. And, because the chemical reactivity of clay is partially dependent on its surface area, the more dispersed a clay is, the more reactive it becomes.
The surface area of a clay particle typically has an ion associated with it. This cation links the platelet structure of the clay together. The strength of the cation, as well as other environmental conditions, dictates whether the platelets separate or not.
Hydration occurs as clay platelets absorb water and swell. Fig. 3.4 illustrates the various forms of clay behavior. 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 defl occulant from positive-edge charges, and allows platelets to assume a face-to-face structure. Flocculation begins when mechanical shearing stops and plate- lets that previously dispersed come together because of the attractive force of surface charges on the platelets. Defl occulation, the opposite effect, occurs by addition of chemical defl occulant to fl occulated mud; the positive- edge charges are covered and attraction forces are greatly reduced.
3.5.2 Clay Types. 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-fl uid technology, clay is a large family of complex minerals containing the elements magnesium, aluminum, silicon, and oxygen (magnesium, aluminum silicates) combined in a sheetlike structure (Darley and Gray 1991).
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 or gel. Calcium montmorillonite is sometimes called subbentonite. And vermiculite is the least active of the smectites. The next less-reactive clays are the illites, followed by the chlorites, and the kaolinites. Each of these clays is pres- ent in differing proportions in formations, a fact that can seriously complicate drilling-fl uid selection.
Wyoming bentonite is composed primarily of three-layer clays called montmorillonite (a mineral found near Montmorillon, France). The term now is reserved usually for hydrous aluminum silicates approximately repre- sented by the formula 4SiO2·A12O3·H2O + water, but with some of the aluminum cations Al3+ being replaced by magnesium cations Mg2+. This replacement of Al3+ by Mg2+ causes the montmorillonite structure to have an excess of electrons. This negative charge is satisfi ed by loosely held cations from the associated water. The name sodium montmorillonite refers to a clay mineral in which the loosely held cation is the Na+ ion.
Montmorillonite, a hydrophilic and dispersible clay mineral of the smectite group, is a mineral that tends to swell when exposed to water. This clay has the extraordinary capacity of exchanging cations, typically sodium
0.01 0.1 0.01 0.1 30 50 90 1 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 10 1 10 200 100 60 20 100 1000 10,000 Solids Percentage Bentonite 1 m 1 mm 1 cm Tobacco Centrifuge overflow Flour Silt Desilter underflow Desander underflow Fine Sand Coarse Sand Beach Sand Gravel Mesh Shaker Discard Mesh Mesh Mesh Milled
Settling rate of drilled solids in 68°F water, feet per minute Smoke
Drilled solids
Drilling Fluids 97
(Na+) and calcium (Ca+2). Sodium montmorillonite 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 two to four times its original volume when mixed with water. In mud parlance, bentonite is classifi ed as so- dium bentonite and calcium bentonite, depending on the exchangeable cation (Darley and Gray 1991).
Montmorillonite clay has a mica-type crystal structure made up of a crystal lattice of silica and aluminum, and the lattice is loosely bound with a cation such as sodium or calcium. In the presence of water, the crystal lattice absorbs water, allowing the crystal to swell. The covalent calcium ion holds the crystal lattice together tighter, allowing less swelling. A model representation of the structure of sodium montmorillonite is shown in Fig. 3.5 (Grim 1968). A central alumina octahedral sheet has silica tetrahedral sheets on either side. These sheetlike struc- tures are stacked with water and the loosely held cations between them. Polar molecules such as water can enter between the unit layers and inc rease the interlayer spacing. This is the mechanism through which montmorillonite hydrates or swells. A photomicrograph of montmorillonite particles in water is shown in Fig. 3.6 (Grim 1968). Note the platelike character of the particles.
In addition to the substitution of Mg2+ for A13+ in the montmorillonite lattice, many other substitutions are pos- sible. Thus, the name montmorillonite often is used as a group name including many specifi c mineral structures. However, in recent years, the name smectite has become widely accepted as the group name, and the term mont- morillonite has been reserved for the predominantly aluminous member of the group shown in Fig. 3.5. This re- cent naming convention has been adopted in this text.
The ability of smectite clays to swell when exposed to water is considerably affected when the salinity of the water is too great. In the particular case of salt water, a fi brous, needlelike 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 fi ltration properties of the mud. Attapulgite is approximately represented by the formula (Mg,AL)2Si4O10·4H2O , but with some pairs of the magnesium cations (2Mg2+) being replaced by a single trivalent cation. A photomicrograph of attapulgite in water is shown in Fig. 3.7a. The ability of attapulgite to build viscosity is thought to be a result of interaction between the attapulgite fi bers rather than the hydration of 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 fi bers. This can be offset through the periodic addition of a new attapulgite material to the system.
Aggregation (face-to-face) Flocculation (edge-to-face) (edge-to-edge) Deflocculation Dispersion
The clay mineral sepiolite, a magnesium silicate with a fi brous texture, has been proposed as a high-temperature substitute for attapulgite. A photomicrograph of sepiolite in water is shown in Fig. 3.7b. The idealized formula can be written Si12Mg8O32·nH2O. X-ray diffraction techniques and scanning-electron-microscope studies have estab- lished that the crystalline structure of this mineral is stable at temperatures up to 800°F. Slurries prepared from sepiolite exhibit favorable rheological properties over a wide range of temperatures.