2. Aggregation
• Aggregation (Face-to-face) in drilling fluids involves clay particles coming
together to form larger aggregates or flocs. Hydration of clay separates the
clay particles. The sheets may then form aggregated systems that can
either be flocculated or deflocculated.
• The natural state of clay particles is aggregation when they are in close
proximity to each other. In this state, only a few particles remain
suspended, resulting in a low plastic viscosity of the mud. If, at any point,
the mud has been dispersed, aggregation can be restored by introducing
cations (such as Ca²⁺) to facilitate the bonding of the clay plates. Gypsum or
Lime are added to achieve this effect.
• Mechanism: Aggregation can occur when drilling fluids contain additives or
chemicals that encourage clay particles to clump together. Aggregation
improves the suspension properties of the drilling fluid.
• Effects: Aggregation enhances the viscosity and gel strength of drilling
fluids, which are important for carrying cuttings to the surface and
maintaining wellbore stability.
3. Dispersion
• Dispersion in drilling fluids involves the separation of clay particles to
ensure that they remain uniformly suspended throughout the fluid.
Dispersion is formed when there is a complete breakdown of aggregates.
Dispersion can either be flocculated or deflocculated.
• Mechanism: Dispersion takes place when the individual clay platelets are
separated by a certain mechanism, leading to an increase in the total
number of particles and causes an increase in plastic viscosity. In the
presence of freshwater, clays will naturally undergo dispersion, with the
process being further accelerated by the agitation of the mud. Bentonite
typically does not achieve complete dispersion in water. Drilling fluids often
include dispersants or deflocculants to maintain clay particles in a
dispersed state. These additives reduce the tendency of clay particles to
agglomerate.
• Effects: Effective dispersion prevents settling of clay particles in the drilling
fluid, ensuring that the fluid remains homogeneous and maintains its
desired properties.
4. Flocculation
• Flocculation (Face-to- Edge or Edge-to-Edge) in drilling fluids refers to the formation of clay aggregates or flocs due
to the action of flocculating agents
• Mechanism: 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. Decrease in the distance between the particles due to shrinkage by
temperature or other factors increases flocculation.
• Flocculating agents may be added to drilling fluids to promote the formation of larger, more stable clay aggregates.
This can be beneficial under certain conditions, such as controlling fluid loss or reducing the impact of reactive clays.
In a flocculated system, the net attractive forces is necessary to maintain a flocculated system. This can be achieved
through:
• a. Addition of Polymeric flocculants:
• i. in a clear water mud to remove cuttings in order to maintain the low mudweight
• ii. to stabilize hydrophilic formation.
• b. High concentration of electrolytes. Distance between the particles is minimized and the attractive force is high.
• c. Polyvalent Cations: a polyvalent cation can react with more than one clay platelets to form an ion bridge between
the clays to produce a flocculated structure. Example Ca, Mg and Al. Thus, if the clays in the mud are in sodium
form, then presence of Ca will cause a sudden change in the flow properties of the mud. By ensuring the drilling
mud are in the polyvalent form, the problem of flocculation can easily be avoided. Lime or Gypsum are the typical
source of Ca in drilling fluid.
• d. Low PH Conditions: Face to edge association is normally enhanced at low PH due to increased positive charges.
As a result, acid are added to flocculate mud.
• Effects: Flocculation can help improve the rheological properties of drilling fluids and reduce fluid loss during drilling
operations.
5. De-Flocculation
• Deflocculation in drilling fluids involves breaking apart clay aggregates or flocs into individual particles.
• Mechanism: Deflocculation takes place when the structure resembling a house of cards is dismantled,
and a substance is introduced into the mud to diminish the edge-to-face interactions among particles.
Chemicals known as "thinners" are incorporated into the mud for the purpose of accomplishing this
effect. Deflocculation can be maintain by ensuring maximum repulsive forces. This is achieved by:
• 1. Reducing salt (electrolyte) concentration to the minimum. This is to ensure maximum repulsive forces.
• 2. Maintaining maximum negative charge. There are two ways of achieving this as follows:
• a. High pH condition – number of negative silicic acid groups on the edge are increased by maintaining
pH above 8.0.
• b. Applying defloccculant or dispersants. Most of the dispersant are negatively charged polymers that
neutralize a positive charge on the edge to become adsorbed. The surface are at the edge is relatively
small, hence small amount of deflocculant can be very effective.
• Deflocculating agents or additives may be used to disperse clay aggregates and maintain the clay
particles in a more dispersed state. This is often done to control viscosity and prevent excessive gelling of
the drilling fluid.
• Overall repulsive force is required between the particles to maintain deflocculated system. In clay system
under alkaline conditions, there is a net negative charge.
• Effects: Deflocculation helps maintain the desired rheological properties of the drilling fluid, ensuring it
remains pumpable and easy to circulate downhole.
6. Charges on Clay
1. Based on structure on clay
2. Based on broken edges.
1. Charges on clay based on structure on clay (isomorphous substitution).
Silica tetrahedra and aluminium octahedra have a balanced electrostatically
neutral structures. 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
nature of this substitution depends on
i. tetrahedral or octahedral substitution
ii. Extent of substitution
iii. The nature of exchange cations: i.e. Na, K or Ca.
7. • If a negative charge on the clay lattice created by isomorphous substitution is neutralized 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.
8. 2. Broken Edge Charges (Broken edges)
• 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.
9.
10. Schematic representation of the flocculation-deflocculation mechanism and the aggregation-dispersion
mechanism (Courtesy of Shell Dev. Co.)
