Clay Mineralogy And The Colloid Chemistry Of Drilling Fluids

Anyone concerned with drilling fluids technology should have a good basic knowledge of clay mineralogy, as clay provides the colloidal base of nearly all aqueous muds, and is also used in oil-based drilling fluids. Drill cuttings from argillaceous formations become incorporated in the drilling fluid, and profoundly change its properties. The stability of the borehole depends to a large extent on interactions between the drilling fluid and exposed shale formations. Interactions between the mud filtrate and the clays present in producing horizons may restrict productivity of the well if the wrong type of mud is used. All of these point out the need for a knowledge of clay mineralogy.

The drilling fluid technologist should have a basic knowledge of colloid chemistry as well as clay mineralogy, because clays form colloidal suspensions in water, and also because a number of organic colloids are used in drilling muds.

Both clay mineralogy and colloid chemistry are extensive subjects, but in this chapter it will only be possible to summarize briefly those aspects which affect drilling fluid technology.

Characteristics of Colloidal Systems

Colloids are not, as is sometimes supposed, a specific kind of matter. They are particles whose size falls roughly between that of the smallest particles that can be seen with an optical microscope and that of true molecules, but they may be of any substance.

Actually, it is more correct to speak of colloidal systems, since the interactions between two phases of matter is an essential part of colloidal behavior. Colloidal systems may consist of solids dispersed in liquids (e.g.. clay suspensions), liquid droplets dispersed in liquids (e.g., emulsions), or solids dispersed in gases (e.g., smoke). In this chapter, we shall only be concerned with solids dispersed in water.

One characteristic of aqueous colloidal systems is that the particles are so small that they are kept in suspension indefinitely by bombardment of water molecules, a phenomenon known as the Brownian movement. The erratic movements of the particles can be seen by light reflected off them when they are viewed against a dark background in the ultramicroscope.

Another characteristic of colloidal systems is that the particles are so small that properties like viscosity and sedimentation velocity are controlled by surface phenomena. Surface phenomena occur because molecules in the surface layer are not in electrostatic balance; i.e., they have similar molecules on one side and dissimilar molecules on the other, whereas molecules in the interior of a phase have similar molecules on all sides. Therefore, the surface carries an electrostatic charge, the size and sign of which depends on the coordination of the atoms on both sides of the interface. Some substances, notably clay minerals, carry an unusually high surface potential because of certain deficiencies in their atomic structure, which will be explained later.

The greater the degree of subdivision of a solid, the greater will be its surface area per unit weight, and therefore the greater will be the influence of the surface phenomena. For example, a cube w ith sides one mm long would have a total surface area of 6 mm2. If it were subdivided into cubes with one micron sides (1 micron = 1 x 10 3 mm) there would be 10" cubes, each with a surface area of 6 x 10 6 mm2, and the total surface area would be 6 x 10' mm:. Subdivided again into milli-micron cubes, the total surface area would he 6 x K)6 mm2, or 6 square meters.

The ratio of surface area per unit weight of particles is called the specific surface. Thus if a 1 cm3 cube were divided into micron sized cubes, the specific surface would be 6 x 106 12.1 = 2.2 x: 106 mmz/g = 2.2 nr/g, assuming the specific gravity of the cube to be 2.7.

F igure 4-1 shows specific surface versus cube size. To put the values in perspective, the size of various particles, expressed in equivalent spherical radii (esr), are shown at the top. The esr of a particle is the radius of a sphere that would have the same sedimentation rate as the particle. The esr may be determined by applying Stokes' Law (see Chapter 3) to the measured sedimentation rate.

The division between colloids and silt, shown in Figure 4-1, is arbitrary and indefinite, because colloidal activity depends (a) on specific surface, which varies with particle shape, and (b) on surface potential, which varies with atomic structure.

minerals, and (b) organic colloids, such as starch, the carboxycelluloses, and the polyacrylamide derivatives. These substances have macro-molecules, or jre long-chain polymers, whose size gives them colloidal properties. We will consider the clay minerals first.

Clay Mineralogy

The upper limit of the particle size of clays is defined by geologists as 2 microns, so that virtually all clay particles fall within the colloidal size range. As they occur in nature, clays consist of a heterogeneous mixture of finely divided minerals, such as quartz, feldspars, calcite, pyrites, etc., but the most colloidally active components are one or more species of clay minerals.

Ordinary chemical analysis plays only a minor part in identifying and classifying clay minerals. Clay minerals are of a crystalline nature, and the atomic structure of their crystals is the prime factor that determines their properties. Identification and classification is carried out mainly by analysis of X-ray diffraction patterns, adsorption spectra, and differential thermal analysis. These methods have been summarized in the abundant literature on the subject.1 - ; 1

Most clays have a mica-type structure. Their flakes are composed of tiny crystal platelets, normally stacked together face-to-face. A single platelet is called a unit layer, and consists of:

1. An octahedral sheet, made up of either aluminium or magnesium atoms in octahedral co-ordination wih oxygen atoms as shown in Figure 4-2. If the metal atoms are aluminium, the structure is the same as the mineral gihhsite, Al2 (OH)6. In this case, only two out of three possible sites in the structure can be filled with the metal atom, so the sheet is termed dioetahedral. If, on the other hand, the metal atoms are magnesium, the structure is that of brucite, Mg3 (OH)6. In this case all three sites are filled with the metal atom, and the structure is termed triartahedral.

Figure 4-2. Octahedral sheet. Structure shown is that of brucite. Magnesium atoms = % Hydroxyls = @

(After Grimm.2)

Figure 4-2. Octahedral sheet. Structure shown is that of brucite. Magnesium atoms = % Hydroxyls = @

(After Grimm.2)

2 One or two sheets of silica tetrahedra, each silicon atom being coordinated with four oxygen atoms, as shown in Figure 4-3. The base of the tetrahedra form a hexagonal network of oxygen atoms of indefinite area! extent.

The sheets are tied together by sharing common oxygen atoms. When there arc two tetrahedral sheets, the octahedral sheet is sandwiched between (hern, as shown in Fig. 4-3. The tetrahedra face inwards and share the oxygen atom at their apexes with the octahedral sheet, which displaces 2 out of 3 of the hydroxyls originally present. This structure is known as the Hoffmann structure,5 the dimensions of which are shown in Figure 4-4.

Note that the oxygen network is exposed on both basal surfaces. When there is only one tetrahedral sheet, it is bonded to the octahedral sheet in the same manner, so that, in this case, the oxygen network is exposed on one basal surface, and hydroxyls are exposed on the other, as shown in Figure 4 5

Figure 4-3. Bonding between one octahedral sheet and two tetrahedral sheets through shared oxygen atoms.

Aluminum or magnesium

Oxygen = O

(After Grimm.2)

Figure 4-3. Bonding between one octahedral sheet and two tetrahedral sheets through shared oxygen atoms.

Aluminum or magnesium

Oxygen = O

(After Grimm.2)

0 0

Post a comment