Charges on clay surfaces
Charges on clay surfaces arise from two mechanisms. One is related to the structure of the clay and is a characteristic of the particular mineral. The other arises from the broken edges.
a. Isomorphous Substitution
The idealised combinations of silica tetrahedra and aluminium octahedra sheets give structures in which the charges are balanced and are electrostatically neutral. However, if a metal ion within the layers is replaced by an ion of lower charge valency, a negative charge is created. For example, in the tetrahedral layer, some of the silica may be replaced by iron, or in the octahedral layer some of the aluminium may be replaced by magnesium. This creates a negative potential at the surface of the crystal structure.
The pattern of isomorphous substitution is random and varies in the different minerals according to the following :
(a) Tetrahedral or octahedral substitution
(b) Extent of substitution
(c) The nature of the exchanged cations, i. e. Na, K or Ca.
The negative charge on the clay lattice created by isomorphous substitution is neutralised by the adsorption of a cation. In the presence of water the adsorbed cations can exchange with other types of cations in the water. This gives rise to the important property of the clays known as cation exchange capacity, because the ions of one type may be exchanged with ions of the same or different type. This property is often used to characterise clays, shales and drilling fluid and is determined by measurement of the adsorption of a cationic dye, methylene blue. The result is quoted as the milli-equivalents of dye adsorbed per 100g of dry clay. The replaceability of cations depends on a number of factors such as:
• Effect of concentration
• Population of exchange sites
• Nature of anion
• Nature of cation
• Nature of clay mineral.
This large number of variables creates a complex system to analyse. It has been shown that different ions have different attractive forces for the exchange sites. The relative replacing power of cations is generally Li+ < Na+ < K+ < Mg ++ < Ca++ < H+. Thus at equal concentrations, calcium will displace more sodium than sodium will displace calcium. If the concentration of the replacing cation is increased, then the exchanging power of that cation is also increased. For example, high concentrations of potassium can replace calcium. Also, in some minerals such as mica, potassium is particularly strongly adsorbed and not easily replaced, except by hydrogen.
b. Broken Edge Charges
When a clay sheet is broken, the exposed surface will create unbalanced groups of charges on the surface. Some of the newly exposed groups have the structure of silica, a weak acid, and some have the structure of alumina or magnesia, a weak base. Therefore, the charge on the edge will vary according to the pH of the solution. One of the reasons for the pH values of drilling fluid to be kept on the alkaline side is to ensure that the clay particles are only negatively charged so that electrostatic interactions are kept at a minimum. Chemical treatment of drilling fluids is often aimed at a reaction with the groups on the broken edges. Since the edge surface is created by grinding or breaking down the clays, chemical treatment costs can be minimised by ensuring that the formation clays are removed as cuttings, rather than broken down at the bit into finer sized particles.
Clays play a significant role in drilling fluids. They may be added intentionally to control viscous flow properties and fluid loss or they may build up in an uncontrolled fashion in the drilling fluid whilst drilling through a clay formation. In both cases control of the resulting flow properties must be maintained. These properties may be modified intentionally by chemical treatment or as a consequence of drilling through water soluble formations, such as cement, anhydrite, salt or magnesium.
3.1 Particle Associations
The associations between clay particles are important as they affect viscosity, yield point and fluid loss. The terms describing the associations are as follows :
Deflocculated System
A system of suspended particles is described as de-flocculated or dispersed, when there is an overall repulsive force between the particles. This is normally achieved by creating the conditions in which the particles carry the same charge. In clay system under alkaline conditions, this is normally a net negative charge.
Flocculated Systems
A system may be described as flocculated when there are net attractive forces for the particles and they can associate with each other, to form a loose structure.
Aggregated Systems
Clays consist of a basic sheet structure, and the crystals consist of assemblages of the sheets, one upon the other. During clay swelling the sheets can be separated. The sheets may then form aggregated systems. These aggregates may be flocculated or deflocculated.
Dispersed Systems
A system in which the breakdown of the aggregrates is complete is called a dispersed system. The dispersed particles may be either flocculated or deflocculated.
The forces acting on clay particles may be either repulsive or attractive. The particles approach each other due to Brownian motion. The particle associations that they assume will depend on the summation of these forces.
a. Repulsive Forces
Electrical Double Layer Repulsion
The clay particles have been described as small crystals that have negatively charged surfaces. A compensating charge is provided by the ions in solution that are electrostatically attracted to the surface. At the same time there is a tendency for the ions to diffuse away from the surface, towards the bulk of the solution. The action of the two competitive tendencies results in a high concentration of ions near the surface, with a gradual falloff further from the surface. The volume around the clay surface is called the Diffuse or Gouy Layer. The thickness of the layer is reduced by the addition of salt or electrolyte.
When two particles approach each other there is an interference that leads to changes in the distribution of ion sin the double layer of both particles. A change infers that energy must be put into the system and once this is not the case there must be a repulsive force between the particles, that will become larger as the particles come
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closer together. However, since the electric double layer can be compressed by electrolytes, then as the electrolyte concentration is increased so the particles can approach closer to each other before the repulsive forces are significant.
b. Attractive Forces
Van der Waals Forces
Van der Waals forces arise through the attraction of the spontaneous dipoles being set up due to distortion of the cloud of electrons around each atom (Van der Waals dipoles). For two atoms, the attractive force decays very rapidly with distance (1/d7) but for two spherical particles, the force is inversely proportional to only the third power of the distance (1/d3). Thus, for a large assemblages of atoms, such as in a clay platelet, this force can be significant as it is additive. The attractive force is essentially independent of the electrolyte concentration.
To maintain a system in a deflocculated state the repulsive forces must be maximised. This can be achieved by two mechanisms.
Low Salt Concentrations
In order to maximise the electrostatic repulsion, the electrolyte concentration has to be as low as possible.
Maximum Negative Charge
The conditions have to be chosen so that the negative charges on the clay particles are at a maximum. This can be done in two ways:
(1) High pH conditions
A pH of above 8.0 will increase the number of negative silicic acid groups on the clay edges. Thus, maintenance of alkaline pH conditions with caustic soda will stabilise the clay system.
(2) Addition of deflocculants or dispersants
There is a wider range of chemicals known as dispersants or thinners, that have a wide range of chemical structure. However, they can all be described as negatively charged polymers which can neutralise a positive charge on the edge to become adsorbed. Then, the other negative groups increase the negative charge density on the clay platelet. Since the deflocculants are reacting with the positive sites on the edges, and the edge surface area is relatively a small proportion of the total, the chemicals can be effective at low dose rates. Also note that the materials tend to be acidic. Thus, caustic soda additions should also be made with the thinner. The other fine particulate solids, such as sand, calcium carbonate or barites, will react in essentially the same way.
In many drilling fluid systems the clays are deflocculated and the change to a flocculated condition can drastically alter the fluid properties. There are a number of mechanisms by which the interparticle attractive forces can be increased and repulsive forces decreased:
High Salt Concentrations
Higher salt levels allow the particles to approach each other close enough for the shorter range attractive forces to predominate. The upper limit of salinity, for bentonite to yield satisfactorily, is about 2% sodium chloride. In drilling practice this reaction occurs when a fresh-water clay-based fluid is used to drill into a salt section when a fresh-water system has salt added to it in preparation to drill evaporite sequences.
Polyvalent Cations
A soluble cation containing more than one positive charge can react with more than one exchange site on the surfaces of more than one clay platelet, to form an ion bridge between the clays to produce a flocculated structure. Calcium is the most common ion, although aluminium, magnesium and zirconium ar other examples. Calcium is often encountered in the form of gypsum (calcium sulphate) and cement. If the clays in the drilling fluid are in the sodium form, then the contact with calcium will drastically alter the properties. Some mud systems overcome this problem by ensuring that the clays are already in the calcium form before the contaminant is encountered. Thus, lime or gypsum are added in excess to ensure a source of calcium is available. The aluminium and zirconium ions have been suggested as treatments for production sands to flocculate the clay minerals and thus prevent their mobilisation to block the pores of the production zone. The flocculation is followed by aggregation of the clays.
Addition of Polymeric Flocculants
These polymers extend the concept of an “ion bridge” or the polyvalent cations, to a polymer bridge between clay platelets. The main feature of the flocculants is a very high molecular weight, so that the molecule spans the distance between particles. The molecules must also absorb onto the particles, so the presence of anionic or cationic groups often makes the molecules more effective. There are two cases where the polymeric flocculants are used. One is in clear-water drilling where the drilled solids are removed by the flocculant in order to keep the density low. The other is where the polymer is added to stabilise a hydrateable formation.
Low pH Conditions
Since the edge charges are pH dependent, a low pH will generate more positive sites and encourage face to edge association. Values of pH below 7, and no caustic soda treatment, would probably induce this reaction. Acid may be added to flocculate drilled solids in a sump clean-up operation.