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 a clear compact borehole wall and needing to rethink the whole process entirely. 

There are many steps, from to ensuring you have the correct PSI or pounds per square inch, or how to avoid high solids that increases that pressure. With that said. Here’s some guidance to that before you begin your next job:

Know your fluid properties.

Stability is largely dependent on the drilling fluid's density and its ability to coat the borehole wall. For fragile formations, a bentonite-based fluid (typically 98% water) is used with filtration control additives like PAC (polyanionic cellulose) to form a protective wall.

“In fragile formations, we use a 98 percent water/bentonite fluid to coat the borehole wall with bentonite platelets 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 a bit of math comes in. Standard calculation involves multiplying the mud weight by 0.052 which will give a good indicator of how much pressure is on the hole and what it takes to keep it stable. 

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 10-10.5 lbs/gal), as high solids content increases outward pressure on the formation, 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 what we’re looking to break which is that fractured gradient,” Yordy says. 

Recognize your up-hole velo and rely on testing tools

Want to keep a hole stable, and facilitate proper removal of cuttings? Maintain an up-hole velocity between 60 to 150 feet per minute, says Yordy. Also, when it comes to producing a well-rounded job, lean on the tools designed to simplify a lot of the work. On major recommendation? 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 pressure exerted on the wall is not just a suggestion, it’s essential to doing the job right and maintaining the integrity of your drill.