Water based mud is one of type drilling fluid with most used in this world, normally the composition is water and bentonite. Water it self may be used as a drilling fluid. However, most drilling fluids require some degree of viscosity to suspend the Barrites and to carry drilled cuttings up the annulus of the well bore. The viscosity of water based mud is generated by the addition of clay or polymers. However the cheapest and most widely used additive for viscosity control is clay. The clay material in water based mud is responsible for two beneficial effects:
• An increase in viscosity which improves the lifting capacity of the mud to carry cuttings to the surface. (This is especially helpful in larger holes where annular velocity is low).
• Building a wall cake in permeable zones, thus preventing fluid loss. The clays are not the only solids in a drilling fluid. There are two types of solids which may be present in a water based mud:
• Active solid – these are solids which will react with water and can be controlled by chemical treatment. These may be commercial clays or hydratable clays from the formations being drilled.
• Inactive or inert solids – these are solids which do not readily react with water. These may be drill solids such as limestone or sand. Barite is also an inert solid.
In order to appreciate how clays play an important part in water based mud some understanding of clay chemistry is necessary.
The group of minerals classified as clays play a central role in many areas of drilling fluid technology. The clay group can be described chemically as aluminium silicates. Since the elements that constitute the clays account for over 80 % of the
mass of the earth (aluminium 8.1%, silicon 27.7% and oxygen 46.6%) it can readily be appreciated that virtually every stage in the drilling of a hole will bring contact with clay. Clays are often used to derive the viscous properties of the drilling fluid
and since clays will also be encountered during the drilling of the hole many of the chemicals used to ‘condition’ the mud are used to control these properties.
Clay minerals can be divided into two broad groups.
• Expandable (hydrophyllic) clays – these will readily absorb water (e.g. montmorillonite).
• Non-expandable (hydrophobic) clays – these will not readily absorb water (e.g. illite).
Clay minerals have a sandwich-like structure usually consisting of three layers. The alternate layers are of silica and alumina. A clay particle usually consists of several sandwiches stacked together like a pack of cards.
Expandable and Non-expandable clays in water
The most commonly clay used in drilling fluids is Wyoming Bentonite (sodium montmorillonite). Figure above shows a simplified diagram of its structure. In fresh water the clay layers absorb water on water based mud system, the chemical bonds holding them together are
weakened and the stack of layers disintegrates. This process is known as dispersion (i.e. less face-to-face association). Dispersion results in an increase in the number of particles in suspension, which in turn increases the number of suspended particles and causes the fluid to thicken or viscosify. During this process, positively charged cations separate from the clay surface leaving the flat surface of the particles negatively charged while the edges are positively charged. It is likely therefore that some plates will tend to form edge-to-face arrangements. This process is known as flocculation.
In a Bingham Plastic fluid, Plastic viscosity can be thought of as that part of the flow resistance caused by mechanical friction between the particles present in the mud and will therefore be dependant on solids content. Yield point is that component of resistance caused by electro-chemical attraction within the mud while it is flowing. There are 4 arrangements of clay particles which are commonly encountered.
Aggregation (Face-to-face) is the natural state for the clay particles. In this configuration there are a small number of particles in suspension and therefore the plastic viscosity of the mud is low. If the mud has, at some time been dispersed,
aggregation may be achieved by introducing cations (e.g. Ca2+) to bring the plates together. Lime or gypsum may be added to achieve this effect. Dispersion occurs when the individual clay platelets are dispersed by some mechanism. Dispersion increases the number of particles and causes an increase in plastic viscosity. Clays will naturally disperse in the presence of freshwater but this process will be enhanced by agitation of the mud. Bentonite does not usually
completely disperse in water. Flocculation is when a house of cards structure is formed because of the attraction between the positive charges on the face of the particles and the negative charges on the edge of the particles. Flocculation increases the viscosity and yield point of the mud. The severity of flocculation depends on the proximity of the charges acting on the linked particles. Anything that shrinks the absorbed water film around the particles (e.g. temperature) will decrease the distance between the charges on the particles and increase flocculation. De-flocculation occurs when the house of cards structure is broken down and something is introduced into the mud that reduces the edge-to-face effect. Chemicals
called “thinners’ are added to the mud to achieve this.
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