WHY STUDY CLAYS
Clay is a mineral, (either rock or soil), that becomes “plastic” when wet. This property is the result of the small particle size and the plate like nature of the particles. Most clays become sticky and slippery when wetted but only a few clays actually swell to the same degree as Wyoming Bentonite (Gel).
Almost all drilling fluids contain some clays. In the mud business clays are an essential ingredient in almost all drilling fluids yet they are also one of the most common contaminants. Excessive amounts of clay cause problems with rheology and density. This situation is very prolific in California.
Clays are found in most drilling locations in the oil and gas fields in California. Shales resplendent with clays make up the majority of the formations drilled. Understanding the nature of clays is an important part of controlling drilling mud characteristics and anticipating problems.
The addition of clay is the primary method of increasing viscosity in many drilling fluids. The graph in Figure 1, below, illustrates how efficient Wyoming Bentonite (GEO Gel) is at raising viscosity.
Shale and clay in their many forms are also the primary source of undesirable changes to viscosity/rheology. You can see from the graph (Figure 1) that at concentrations above 30 ppb each additional unit of clay causes a much larger change than the previous unit. The addition of polymers, thinners, and dispersants can extend the point at which viscosity increases dramatically, but there comes a point in any water based mud system where every additional ppb of clay causes a significant increase in Yield Point and Gel Strengths.
In addition, filtration control in drilling fluids is usually based on the use of Bentonite with the addition of other products to enhance fluid loss properties. By itself, Bentonite provides substantial fluid loss control in a mud system. The American Petroleum Institute (API) requires that API Bentonite mixed at 22.5 ppb and aged for 24 hours have a filtrate loss less than or equal to 15.0 ml. The thin, pliable clay platelets are ideally suited to form a semi-permeable barrier to water. However, Bentonite is easily disturbed by contamination of the water with calcium, chloride, or hydroxyl ions. The addition of polymer filtration control products reduces the effects of these contaminants and provides broader particle size distribution in the filter cake which aids in filtration control.
Geologists use the term “clay” to describe an alumina silicate mineral with a particle size of four microns 1 (μm) or less. In general, clays are classified as mineral particles 2 μm or smaller. Particles larger than 4 μm are considered to be silt, larger than 74 μm are considered sand. The internal dimensions of the actual clay particles in Bentonite are measured in Angstroms2, on the order of 10-10 meters.
All clays contain 5%-50% non-clay materials, usually larger than 2 μm. Shale, a metamorphic claystone, is harder and more laminated than clay. There is a continuous grading from shale to clay. These fine grained materials, sometimes referred to as “argillaceous” can have the size of clays but lack the laminated structure.
WHERE DO CLAYS COME FROM
Clay is also a classification for rocks. It is characterized by its ability to become plastic or flexible when wet. The technical definition of clays involves the molecular structure. A material that has loosely bonded layers which are only molecules thick is generally classified as a clay. Some clays absorb a great deal of water and can produce higher viscosity, others merely become flexible. All clays tend to break down to very small particles when immersed in water, especially under high shear conditions.
Drilling muds generally use Wyoming Bentonite if it is available because it meets the specifications set by the American Petroleum Institute (API). These “Wyoming” Bentonite deposits spread across several states and are formed from massive volcanic ash falls. During the Cretaceous period, 135 to 65 million years ago, there were nearly continuous eruptions of ash from a string of volcanoes in the area that is now Idaho. East of these volcanoes covering what is now Montana, Wyoming and North and South Dakota there was a large inland sea. The prevailing winds blew the ash into the sea where it was under the unique conditions needed to developed into nearly pure Sodium Montmorillonite.
Other types of clay are formed by the weathering of granite into decomposed granite which is easily broken into smaller and smaller particles. Granite contains primarily Quartz, Mica and Feldspar. The quartz particles remain unchanged and only get smaller, producing most of the sand we find. However, Feldspar and Mica degrade into very small particles that are subject to molecular changes when exposed to various chemicals found in the water that washes through them. With extended exposure to this water and chemicals they will eventually form clays such as kaolin.
Classification of the clay minerals 3
A. Two- layer type (sheet structures composed of units of one layer of silica tetrahedrons and one layer of alumina octahedrons)
Kaolinite, nacrite, etc.
B. Three- layer types (sheet structures composed of two layers of silica tetrahedrons and one central dioctahedral or trioctahedral layer)
1. Expanding lattice
Montmorillonite, sauconite, etc.
Nontronite, saponite, hectorite
2. Nonexpanding lattice Illite group
C. Regular mixed-layer types (ordered stacking of alternate layers of different types) Chlorite group
D. Chain-structure types (hornblende- like chains of silica tetrahedrons linked together by octahedral groups of oxygens and hydroxyls containing Al and Mg atoms)
3 From “Clay Mineralogy, Second Edition,” p. 32.
Clays can also be formed from the weathering products of sedimentary rocks, igneous rocks or even through hydrothermal reactions
THE FAMILY OF CLAY MINERALS
Amorphous clays are in a group called Allophanes. The makeup of these clays is silica tetrahedrons and aluminum octahedrons intermixed with crystalline material. The Allophane clays have little or no discernable structure. They seem to be a precursor to the more structured clays.
Cation Exchange Capacity (CEC) measures the exchangeable cations. Anions also can be exchanged but they are not as well understood. The size of various cations determines how readily they are exchanged and how tightly it is held once replaced, Figure 11. The larger the non-hydrated size of the cation the more easily it drives out the existing cation. In addition, the smaller the hydrated size of the ion is the more strongly it holds onto its position in the clay matrix. The tetrahedral sheets form a series of holes which perfectly match the Potassium ion size such that once it has replaced Sodium or Calcium it is very hard to displace it. This helps explain the importance of K+ in the long term inhibition of clays.
Kaolinite, Figure 3, has a two layered crystalline structure bonded by hydrogen. The clay is non-swelling due to its tight bonds and resistance to substitution within the lattice structure. The CEC ranges from 3-15 milliequivalents (meq)/100 gram. See Figure 10 for CEC of other clays.
The Smectites are layered crystalline clays with an expanding lattice which makes them highly susceptible to swelling. The group includes Aliettite, Beidellite, Hectorite, Montmorillonite, Nontronite, Saponite, Sauconite, Stevensite, Swinefordite, Volkonskoite, Yakhontovite, and Zincsilite. All these clays are very fine grained, no large crystals are known to exist. They have a small layer charge allowing exchange of interlayer cations. The weak bonding between layers allows water to form a spacing between the molecules of 10Å to 18Å.
Bentonite is a generic name given to a rock type, not a mineral classification like Illite or Kaolinite. It is usually a distinctive bed formed by the deposition and alteration of volcanic ash. Typically, Bentonite is composed of Montmorillonite but may also contain other minerals including other types of clay.
Montmorillonite has oxygen bonding which makes for weak attraction and substantial cleavage between layers. It is a 2:1 clay, meaning that it has two tetrahedral sheets sandwiching a central octahedral sheet (see Figure 4). The average diameter of a single platelet is approximately 1 öm. The particle thickness is extremely small, about one nanometer (1/1000 μm).
Chemically Montmorillonite is hydrated Sodium Calcium Aluminum Magnesium Silicate Hydroxide (Na,Ca)x(Al,Mg)2(Si4O10)(OH)2·nH2O. Its many uses include performing as a soil additive to hold water in drought prone soils, in the construction of earthen dams and levees, to prevent the leakage of fluids from ponds, as a component of foundry sand, and as a desiccant to remove moisture from air and gases. It is the major constituent in non-explosive agents for splitting rock in natural stone quarries in order to limit the amount of waste and for the demolition of concrete structures where the use of explosive charges is unacceptable. It also has been used in cosmetics and has reputed therapeutic effects having been used for medicinal purposes by the ancient Egyptians.
Water and other polar molecules can enter between the layers causing expansion. It is extremely active due to the exchangeable cations between layers. Volume changes of 1400 to 2000 percent are reported from laboratory tests in which samples of dry Sodium Montmorillonite were immersed in water, and changes of 45 to 145 percent are reported for Calcium Montmorillonite. The CEC for Montmorillonite ranges from 80-150 meq/100 grams.
Nonexpanding lattice clays have a similar structure to the expanding lattice but due to the variations in the atoms included in the matrix the behavior is greatly altered. The Illite group include micas. The crystalline structure is identical to Montmorillonite except some silicon is always replaced by Aluminum. Kaolinite can be converted to Illite by the addition of Potassium.
The activity of Illite is due to the missing silicon in the matrix which results in a negative charge between layers. Potassium ions are attracted to the negative charge. The size fit is almost perfect for K+ ions and once they are in place it is very hard to remove them. If illitic clay material is subjected to acidic water the Potassium ion may be replaced by Magnesium. This would cause some limited swelling in the otherwise non-swelling clay. The illitic minerals would then be referred to as Degraded Illites. The CEC ranges from 10 meq/100 grams to about 40 meq/100 grams, still a fairly low level of activity. Illites can be a source of increased viscosity but to a much lesser extent than Montmorillonite. It can cause formation damage by swelling and thus the potassium is important in completion fluids for zones with Illite in the sand matrix.
Mixed layer clays show stacking of alternate layers of different types of clays. The vertical stacking can be regular (ordered), segregated regular, or random. Chlorite is essentially non-swelling in nature. There is no precise construction of the lattice but it always has the same general framework. The basic structure of chlorites consists of negatively charged mica-like (2:1) layers regularly alternating with positively charged brucite-like (octahedral) sheets. The CEC can range from 10-40 meq/100 gram.
Chain structure clays include Attapulgite, Figure 5, Sepiolite, Figure 6, and Palygoskite. They are all non-swelling and require substantial shear to generate viscosity. The chains are composed of only tetrahedrons, forming a material similar to the silica sheets of Smectite but only 11.5 Å thick. The range of CEC is only 3-15 meq/100 gram.
THE STRUCTURE OF MONTMORILLONITE
The structure of clay minerals ranges from very organized to totally disorganized. Because the particles are so small and their arrangement being slightly irregular it is very difficult to arrive at a clear picture of their structure. Two different molecular structures are presented at the end of this paper (Figures 9 and Figure 10). The basic components shown in Figure 4 are octahedral groups (blue) and tetrahedral groups (red). These are arranged in sheets that are repeated in two directions many times over.
The fundamental structure has two tetrahedral sheets sandwiching an octahedral sheet (see Figure 7). Between the octahedral sheet and one of the tetrahedral sheets there is very strong bonding. This remains in tact. Between the Octahedral sheet and the other tetrahedral sheet the bonding is very weak, using a layer of cations (Calcium, Sodium or Potassium) as a link to hold the faces together. Potassium holds the particles close together as seen in the chart, Calcium does a fair job of holding the particles together while Sodium does a poor job. Water is adsorbed (drawn into the gap between the layers) in varying amounts, depending on the cation. This determines what we observe as swelling. With Sodium, the water absorbed can be so great that there is no longer any attraction between the layers.
The bonding of the particles within a clay body is the result of the following forces:
- The attraction of the mass of one particle for another (the force of gravity).
- Overlapping molecular forces.
- Cation attraction pulling two platelets together.
- Water molecules, 1-3 molecules thick, attracting the 2 platelets to one another.
Clays with the same chemical constituents may have very different properties and clays with identical properties may have very different chemical constituents. It all depends on how the atoms are arranged. That’s why a scanning electron microscope is needed to determine the types of clays in a sample.
The positive charge sites correspond to the OH sites on the next two pages. Cations, Na+, Ca2+ or K+ are found in this interlayer region and can be exchanged for each other. The proclivity to exchange varies with the size of the hydrated ion (Figure 9), the concentration of ions in the electrolyte (the water), and the type of clay. The negative sites correspond to Oxygen atoms at the edges of the platelet. As you can see on the next two pages those Oxygen atoms that are in the middle of the structure have bonds to 6 other atoms while the ones on the edge have only 3 bonds. This deficiency, leads to a net negative charge at the edges.
About.com, About, Inc., A part of The New York Times Company, http://composite.about.com/library/glossary/v/bldef-v5956.htm, 2006.
Gershteins, Sergey & Anna, http://www.convert-me.com/en/convert/length, Copyright 1996-2005.
Grim, Ralph E., “Clay Mineralogy, Second Edition”, McGraw-Hill Book Company, New York,1968.
Theng, B. K. T., “Formation and Properties of Clay Polymer Complexes”, Elsevier Scientific Publishing
Company, New York, 1979.
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