Aqueous drilling through clay has always been a potentially difficult situation and, over the years, sundry solutions have been presented to combat this sticky enemy. These have ranged from simple, store-bought dish soap to high-tech and highly expensive block-co-polymer technology. In this article, I will cover a few of the more commonly used anti-clay additive classes and give an overview of why each type works to improve clay drilling.
Let’s begin with the basics of why clay and shales are so difficult to drill. Clay types can cover a wide range and some are more reactive than others. While phyllosilicates like montmorillonite are certainly the worst of the lot, most of us are dealing with kaolin, illites and other less reactive clay types. The term clay technically refers to a particle size rather than a distinct crystalline makeup. Shale or clay-stone and mudstone are composed primarily of clay and silt that have been compressed over time to drive out any water, thus forming a hard, cohesive rock. However, when pulverized finely enough and allowed to hydrate with water, these supposed rocks can become quite sticky and even swell on us, just like the moist, garden-variety clay near the surface.
What Does It Mean to be ‘Sticky’?
Being so small and typically sheet-like in morphology, clays have massive surface area in comparison to their volume. This entire surface is hydrophilic, or able to adsorb water molecules. When a surface wants to adsorb water molecules but does not have enough water to fully wet the surface, then the material is sticky. It wants more water. Typically, dry soil will not be sticky and surfaces that have their full saturation of water will not be sticky. It is only when soil has some water, but not enough water, that it becomes sticky.
The Main Four Types
As far as aqueous drilling is concerned, we can roughly divide most of the commonly used anti-clay additives into four main groups based on how they behave. These are:
- Wetting agents
- Inhibitors, coagulants and flocculators
When stuck in the gumbo, the first anti-clay additive that most drillers reach for are wetting agents, or surfactants. Surfactants are chemicals that can change the surface tension of water, allowing it to more effectively wet hydrophobic or water-disliking surfaces. Picture a water droplet sitting on a non-stick pan. It has the shape of a partially deflated basketball, but is still generally rounded with a nearly 90-percent contact angle. In simple terms, this happens because water molecules love to be ordered and want to form as tightly packed an arrangement of molecules as possible with the lowest surface area. Non-polar surfaces would interrupt this ordering, and are thus pushed away. When we add a bit of surfactant, this interferes with the hydrogen bonding that gives the water droplet its tight shape. The water molecules are now able to take on a slightly less rigid conformation, allowing the drop to spread out and wet more of any hydrophobic surface.
Now, think of the shale and clay particles as being like a deck of cards that are very tightly shuffled together. A drop of water wants to maintain its spherical shape and squeezing it in between the cards is difficult. By adding the surfactant, we can deform the sphere of water into a more flattened shape, which allows it to slip in between the cards and wet them. The surfaces of the clay particles that were originally starved for water now find themselves with enough to satisfy them and we have decreased the stickiness of the clay.
The upside of using surfactants is that they are highly effective at reducing bit-balling. The downside is that all of the newly wet-out clay surfaces can become suspended in your drilling fluid where they increase solids content and reduce suspension, and can slowly but dramatically increase pump pressures.
In the aqueous drilling industry, most wetting agents tend to be non-ionic or zwitterionic (having both positive and negative charges). These surfactants are typically non-foaming or low-foaming, although any surfactant will act to decrease surface tension and help wet-out surfaces.
Encapsulating the drill cuttings is the second most common method of dealing with sticky clay. This is typically achieved by adding a slug of some high molecular-weight anionic polymer like PHPA. Partially hydrolyzed polyacrylamide or PHPA is certainly the most common class of polymer used for this purpose. It is cheap and can reach astoundingly high molecular weights (roughly 30 million). The PHPA polymer is anionic, which means that it has a negative charge that helps it hydrate and disperse quickly in water.
Highly viscous polymer slurries will hold onto their water quite tightly and render the water molecules immobile. Additionally, viscous liquids move into and through narrow cracks and fissures more slowly than thinner liquids like water. Because of this, water bound to a PHPA chain is not really available to wet-out the surfaces of clay platelets. By immobilizing water from contacting clay particles, we effectively delay their ever becoming sticky. Note that I say delay, because the one disadvantage of using polymer encapsulation is that, eventually, the clay will get wet and become sticky. Indeed, PHPA slurries may actually increase the ultimate swelling volume of the clay. This takes time, though. With luck, your drilled shaft will be completed before the clay wets-out and begins to swell. Typically, higher molecular weight and/or more neutrally charged polymers make more effective encapsulators, though they take longer to hydrate themselves.
Inhibitors, Coagulants and Flocculators
Clay inhibition and clay flocculation are two sides of the same coin. When we prevent the hydration of native clays, we are inhibiting, and when we remove hydrated clay solids out of suspension, we are flocculating. At first blush, these may seem like completely different effects, but the additives that inhibit clay will also tend to flocculate clay. The similarity has to do with the reversal or negating of surface charges and limiting the absolute number of discrete particles in suspension.
Most particles in suspension have a charge associated with their surface — typically anionic. As is the case with magnets, two anionic-charged poles will repel one another and surfaces with opposite charges will attract one another. The mechanism by which water is drawn toward a soil surface is via osmotic potential. The more densely packed the charges, the more water is drawn toward the charged surface. When water is adsorbed, it helps push particles apart. This is clay hydration. We can interrupt this repulsion in two major ways. First, we can insert an oppositely charged particle in between two like-charged particles to help bind them. This is opposite-charge flocculation. Secondly, we can effectively minimize or mask the surface charge using a non-ionic polymer or a salt. When a charged surface is rendered effectively chargeless it has a tendency to attract other, similarly low-charge surfaces. Technically referred to as coagulation, the outcome is essentially the same as flocculation, since particles are brought together.
Often, when using flocculants, we see the low-shear viscosity start to climb as inter-particle interactions are built. This has the temporary benefit of potentially increasing sand and solids suspension. There is an ideal dosage window, however, and it can be quite narrow. Also, be warned that there are two potential dangers that can crop up when using anti-clay flocculants. The first is that the viscosity builds until we have achieved peanut-butter-mud requiring ridiculous pump pressures to move. The other is that all of our drill solids and also bentonite can become completely flocculated out onto the bottom of the hole. Flocculation works on both native clays and the bentonite used in your drilling fluid, so care needs to be exercised when using this type of anti-clay strategy. This also points to another use of flocculants, which is as a dewatering aid. Since native clays are the same density as bentonite mud, flocculants are really the only way to remove colloidal solids from your drilling fluid once they are present.
When we use a flocculation aid before soil encounters water, we can prevent the clay particles from effectively hydrating. Their requirement for water is decreased. This is termed inhibition, and most flocculants make for very effective inhibitors. One of the most effective clay inhibition ingredients is potassium chloride salt, or KCl, but a fantastic anti-clay drilling fluid can be built utilizing cationic or even non-ionic polymers such as guar gum along with a multivalent salt such as calcium chloride.
Thinners, or deflocculants, are typically very low molecular weight, highly anionic polymers. They work by adsorbing onto the surfaces of clay, increasing like charge leading to dispersion and also preventing the clay particles from interacting. By dispersing clay particles, we see a decrease in the mud viscosity, but it needs to be pointed out that there are two major downsides to using a thinner to combat clay. Firstly, the solids suspension ability of your drilling fluid will be adversely affected and secondly, adding a dispersant will increase the absolute number of distinct particles in suspension. Thinners are fantastic as a slug treatment to get through a sticky spot, but if a dispersed fluid is left to build solids, pump pressure can escalate quickly.