Challenges of Drilling in Saltwater Conditions

Drillers should adapt drilling fluids to saline conditions.

By Todd Tannehill

Mud rotary drilling with bentonite has long been one of the main drilling methods for water wells. This is
due to its exceptional swelling capacity, viscosity or thickness, and filtration control.

When hydrated with freshwater, sodium bentonite forms a strong gel structure that suspends cuttings, cools and lubricates the bit, supports the borehole, and forms a tight filter cake which reduces fluid loss. In short, it transports cuttings to the surface, unloads them in our mud pan, and heads back downhole to do it again.

Bentonite likes freshwater, but when drilling in saltwater conditions near the coast, in tidal zones, or through saline aquifers—the effectiveness of bentonite is greatly reduced. Understanding how salty conditions affect bentonite, and knowing the alternative fluids that are available, is essential for successful drilling and maximizing your drilling efficiency.

Why Bentonite Fails in Saltwater

The primary reason that bentonite struggles in saltwater is due to the electrochemical behavior of clay particles. Greg Plutko, research and development scientist at CETCO Drilling Products, says:

“This struggle is best to describe in terms of charge differential. It occurs when negative charge attracts positively charged cations or molecules with oppositely charged dipole moments (including water).

When salt levels are high in the bulk liquid, water is attracted to solubilized cations (sodium, calcium, etc.) and therefore not as strongly attracted to the charged bentonite surface or to the exchanged cations on the surface.

If the draw of water to the platelet surfaces is diminished by salt, fewer actual molecules of water intercalate between the platelets and provide osmotic pressure to push the platelets apart.”

More simply put, when salt concentrations are low, platelets want to push apart as they have the same negative surface charge. When salt content increases, the repulsive forces are diminished, and the platelets can get close enough to interact and flocculate.

Using Bentonite Successfully in Saltwater

Although bentonite is sensitive to salt, it can still be used effectively if you know the levels and properly treat it in advance. The salinity level (ppm of salt) and drilling objectives will help determine the fluid plan. (See sidebar showing saltwater concentration levels.)

Pre-Hydration in Freshwater

Bentonite should always be fully hydrated in freshwater before being exposed to the saltwater downhole. Once initial swelling occurs, the clay becomes more resistant to ionic interference.

Polymer Stabilization

Adding polymers, particularly xanthan, but also PAC (polyanionic cellulose) or CMC (carboxymethyl cellulose), can help encapsulate bentonite particles, protect against cation exchange, and improve filtration control. Use of these polymers also reduces the amount of bentonite needed, making the drilling fluid more cost effective.

Salt-Tolerant Bentonite or Modified Clay

Some bentonite products are chemically treated to improve performance in brackish or moderately saline conditions. These “salt gel” versions maintain viscosity better and resist flocculation. Ask your drilling fluid professional about these options.

When salinity is too high and the bentonite is not performing as expected, there are several alternative drilling fluids that can offer reliable performance. Again, drilling objectives will help determine which of these is right for your job.

Polymer-Only Systems (Non-Bentonite)

Natural polymers like xanthan gum (xanthan biopolymer) offer a highly salt-tolerant fluid that has excellent suspension properties and is ideal for lifting and evacuating cuttings. Xanthan works well in unconsolidated formations (i.e., sand, gravel) where suspension is critical. Because this is a natural polymer and can grow bacteria, they are not commonly available as NSF certified.

Another natural polymer is guar gum. This is a coarse granular, high-viscosity blend that is formulated for easy and quick dispersion in drilling applications. Coarser granules prevent lumps or encapsulation of the fluid. Guar gum is biodegradable, easy to mix in fresh or brackish water, creates high viscosities, and is quick dispersing.

Because it is natural and able to grow bacteria, many states have banned the use of guar gum in water well applications. While my company still offers this product, some manufacturers have discontinued it.

Drilling Foams

If you have the ability, switching to air drilling with foam is a viable option. In areas with extreme salinity or low water availability, air-foam can provide minimal fluid intrusion and good penetration rates.

While bentonite remains the go-to of water well drilling fluid systems, its performance in saltwater conditions is significantly reduced. Drillers operating in brackish conditions should adapt their drilling fluids to the conditions.

It is important to check your local water well regulations or guidelines to see what polymers are allowed before committing to an alternative fluid. By understanding bentonite’s chemistry and limitations, you will be able to choose the proper fluid to get the job done.

Todd Tannehill, based out of northeast Ohio, is the North American Director for the Drilling Products division of CETCO, a Minerals Technologies Company. Tannehill has been with CETCO since 2005 and is well known in the industry for providing strong technical support to contractors, distributors, and engineers. He is a past board member of the National Ground Water Association and can be reached at todd.tannehill@mineralstech.com.