Ever wonder about pressure when it comes to borehole stability in mud rotary? 

Whether you’re new to the industry or a longtime boots-on-the-ground professional, it should be a common thought before any borehole drilling. Knowing the right way could mean the difference between needing to rethink the whole process entirely.

There are many steps, from ensuring you have the correct PSI or pounds per square inch, or how drilling fluid density will increase. 

With that said. Here’s some guidance to that before you begin your next job:

Know your fluid properties.

Stability is dependent on the drilling fluid's properties to coat the borehole wall and its density to minimize fluid invasion to the formation. For fragile and highly porous formations, a bentonite-based fluid utilizing filtration-control additives such as PAC (poly anionic cellulose) to create a progressive filter cake or to form a stronger protective wall.

“In fragile formations, we use a 98 percent water/bentonite fluid to coat the borehole wall with bentonite platelets and  in our filtration control additives , and that creates your base,” said industry expert Brock Yordy.

Calculate your PSI. 

When understanding the PSI exerted by the fluid, here’s where the science including drilling math a bit of math comes in.  The standard calculation involves multiplying the mud weight by 0.052 which will gives the science and physics behind how much pressure is exerted on the borehole wall and the difference between stability and wall failure.

If I take an 8.45 mud and multiply it by roughly say 0.052, I get a .439 pounds per square inch that’s holding that formation back, said Yordy. “Especially if we have water flowing. It’s a good indicator that there’s some structure there. But I think the more important question here is ‘what do I need to maintain that borehole stability,’ and that really comes down to a low solids drilling fluid that is moving in the right direction, which is up. 

High solids are not your friend. 

Part of that weight and pressure analysis too is using mud that is too heavy (specifically anything between beyond 9.5lbs/gal). As high solids content increases, outward pressure on the formation also increases, which can break the fracture gradient, leading to loss of returns and borehole destabilization.

“That outward exertion of those undesirable solids, as we go from 3 percent solids to that 13, 14, 15 percent solids, [you have to ask] what is that force or exertion doing to the downhole stability or breaking the fracture gradient,” Yordy says. 

Recognize your up-hole velocity and rely on testing tools

Want to keep a hole stable, and facilitate proper removal of cuttings? In conventional mud rotary, maintain an up-hole velocity between 60 to 150 feet per minute, says Yordy. Also, when it comes to producing a well-rounded drilling program, lean on the tools designed to combine a drillers guts instincts with the science and math of drilling. Using a mud test kit to monitor fluid properties, including funnel viscosity, sand content, and utilizing a mud scale, determine your fluid density.

Using these tools ensures total control over the possible damaged or the right pressure exerted downhole. These are the essentials to doing the job right, minimizing the disruption of the drilling process, and maintaining borehole integrity.

Check out more videos from Brock Yordy via The Driller YouTube channel.