The selection of the constituents, concentrations used and resultant properties are driven by the chemical interactions that occur. The following is a basic overview of the chemistry of four drilling fluid components: the make-up water, bentonite, polymers and surfactants.
Although the make-up water comprises the greatest percentage of a drilling fluid — as much as 97 percent by weight in the neat drilling fluid — the chemistry is often overlooked. This is unfortunate because accounting for the water chemistry is crucial to achieving an efficient and effective drilling fluid. The most significant issue to address in the water is the identity and concentration of anions (negative) and cations (positive). The ions, or charged atoms, present in the water source can impact hydration and property development of the additives, which will be discussed subsequently. For example, hardness at >100 ppm and/or anionic species (chlorides and sulfates are two examples) at >500 ppm can reduce viscosity development in polymers and bentonite, occupy reactive sites on surfactants and polymers and increase filtration volume. The bottom line is, know your water source and if conditions exist which can be detrimental to fluid performance, use a treatment such as soda ash to reduce hardness or design a fluid which can tolerate the water chemistry.
Now that the foundation of the fluid has been addressed, it is time to explore the additives. Bentonite is a naturally occurring mineral that is primarily comprised of montmorillonite, a water swellable phyllosilicate. Due to its morphology (platelets) and ability to absorb water, it provides viscosity, suspension and carrying capacity while also reducing penetration of water into porous formations or filtration control. Although bentonite can be highly effective at providing the properties listed above, not all results with bentonite are equal and that is where chemistry is crucial. When bentonite is mixed in water, stacks of platelets exfoliate and interact in solution. Additionally, there are also various ions associated with the bentonite. Beginning with the platelets (see Figure 1), there is a charge deficiency which is a result of the process by which the bentonite is formed in nature. This charge deficiency causes the faces of the platelets to be negative while the edges are positive. With these conditions, electrostatic interactions can occur between neighboring platelets and ions in solution which can give rise to gel strengths, suspension, and carrying capacity. As stated previously, a solution of bentonite is not simply comprised of the crystals themselves, but also of ions (commonly sodium and calcium) which are associated with the clay and driven by the depositional environment in which the clay formed.
These ions are critical to clay performance as they significantly alter the swelling capability of the clay. If the bentonite is primarily sodium in nature, the swelling capacity is typically 10 times greater than the calcium variety. Since producing effective properties with bentonite are highly dependent upon its interaction with water, the highest possible swelling capacity is critical for efficiency in its use.
In addition to bentonite, there are a variety of polymers that are commonly used for drilling fluids in HDD applications. The most commonly used polymers provide shale and clay stabilization, filtration control and suspension enhancement. For shale and clay stabilization, partially hydrolyzed polyacrylamides, or PHPAs, are the polymer of choice. These polymers can be anionic, cationic, or nonionic (no charge), but the anionic variety are the most common. By adhering to clay surfaces through primarily electrostatic interactions, these polymers can restrict water flow and stabilize the clay or shale. This inhibition can be critical in buying the time necessary to complete an installation in sloughing or swelling clay. For filtration control, polyanionic cellulose (PAC) is the most common tool of choice. These are usually slightly anionic, much lower than a PHPA, and are formed by the reaction of cellulose with monochloroacetic acid in the presence of sodium hydroxide. This reaction adds carboxylate groups to the cellulose backbone which are necessary for water solubility and filtration control. After modification, it then has the capability of adsorbing onto solids (Figure 2), with bentonite clay being an especially efficient substrate. This adsorption further reduces the permeability of the filter cake, which results in greater borehole stability. It is important to note that the solids are required and, that while filtration control is a result of PAC addition, it cannot provide it on its own.
Additional factors such as the specific chemical substitutions on the cellulose backbone and the degree of those substitutions also give rise to its useful attributes and result in the small amount required once a threshold of solids is present. In addition to shale and clay stabilizers and filtration control additives, suspension and carrying capacity is also enhanced through the polymers.
In HDD, this typically involves the addition of a polysaccharide, like xanthan gum. As with PAC, xanthan gum is slightly anionic and water soluble, but it differs from PAC in its highly branched structure. Due to this molecular architecture, it is able to form networks in the solution which are exhibited in viscosity, gel strength, and carrying capacity. Fortunately, these associations are weak and can easily be broken with shear, so the fluid will readily return to flow without excessive pressure being exhibited on the bore.
Now that the water, bentonite and polymers have been discussed, it is time to address surfactants. Surfactants or surface activating agents represent a broad category of chemistries that contain both a hydrophilic and hydrophobic portion. This simply means that there is a portion of the molecule that is water soluble and a portion that is oil soluble. When this architecture exists in a molecule, it can interact at the solid/liquid interface. This is especially useful in drilling fluids since a drilling fluid is a combination of a liquid, such as water in HDD applications, and solids, either additives or drilled solids from the formation. Surfactants can be anionic, cationic, or nonionic.
The anionic and nonionic are the most commonly used in HDD. There are two common methods by which these molecules provide a benefit while drilling. First, they can interact on the drilled solids and allow them to be dispersed and incorporated into the drilling fluid for removal. When bit balling has occurred, surfactants can be an effective chemistry for “cleaning” the bit so drilling efficiency can increase.
Second, they can interact at the interface of metal and drilling fluid, creating a barrier on bits and drill pipes which prevents the deposition of “sticky” cuttings.
Although the preceding is not an exhaustive representation of the chemistries in an HDD drilling fluid, it does cover a majority of the constituents. The unique chemistry of each additive gives rise to the distinct features, but ultimately, an effective drilling fluid requires a combination of the materials discussed. Not all are required for every scenario, but since no two bores are exactly the same, an engineered solution that is informed by the chemistry of each additive results in the highest probability of success.
Ryan Collins is Halliburton Industrial Products R&D manager.