One of the most commonly used products in the drilling industry is bentonite; also known as ‘gel’. As it is one of the most common products, it is also not unusual to see it being used in ways that do not utilize all the benefits that this basic product can provide.
The function of bentonite in a drilling fluid system is to provide viscosity as well as reducing fluid loss in the formation by forming filter cake on the borehole walls. In order to optimize these functions, it must be given the correct conditions to do so.
Bentonite yields best in fresh water with a pH between 8.5 to 9.5. It is common that normal tap water is used and this often has a neutral pH of around 7. While bentonite will still yield at this neutral pH, it is best to raise the pH using a little soda ash to improve the performance. Groundwater is also commonly used to provide water for the drilling operation and this can often be saline or have low pH. The result of using water of this quality will be that the bentonite will not mix with the water and there will be separation in the mixing tank. There is no fix to using saline water as bentonite will not hydrate if the salinity is above around 7000 ppm however pH can be treated with soda ash.
Hard water (very mineralised water with calcium or magnesium) can also cause bentonite not to mix and soda ash can help precipitate some of the calcium from the water.
A recent Tiger Fluids trip to the Philippines found that a customer was using groundwater with a pH of around 6 (acidic) this was the root cause for the bentonite not mixing properly. With treatment of soda ash, this was quickly remedied. The pictures below show a Tiger engineer demonstrating the effects of low pH and bentonite hydration and the difference the use of soda ash can make.
Another of the main issues is the time given for bentonite to hydrate. Often, it is mixed into a tank and immediately pumped down the hole. We would recommend giving it at least 20 mins to hydrate or longer if possible. In the oilfield, bentonite is often given 24 hours to allow it to yield to its full potential.