Hydraulics Fantasy Land
A couple of years ago, we took a trip with our then four-year-old and two-year-old children to Walt Disney World's Magic Kingdom near Orlando, Florida. One of the shows we saw was a three dimensional movie, complete with the red-on-one-side and green-on-the-other-side glasses. During the showing of the movie, a most amazing thing happened. The scene was one of a beach...waves gently crashing on the sand and so forth. Then the camera turned to the sky on a small flock of seagulls in flight. After a few seconds, the seagulls turned and flew toward us. Spontaneously, every child in the theater reached their hands up to touch the birds! I suspect most adults felt the urge to do the same. The illusion was masterful. Someone (hopefully) got an 'attaboy' for it. The children were all convinced that the birds were real.
Later that year, while planning a well for the Gulf Coast of Texas, a geologist warned me not to 'run the pumps too fast'. He was convinced, just as the children were of the reality of the seagulls, that aggressive hydraulics programs 'washed out' the hole. He imagined that the flow through the jets and the annular return flow mechanically eroded the hole, leading to difficulties in logging, cementing, completing, and producing. He was also convinced that this effect was a universal phenomenon, regardless of geography, geology, or depth.
I politely disagreed. He resisted. I disagreed. He resisted. I tried to explain my reasoning. He resisted, but not as strongly. I agreed to run a test in the well. He agreed, "Just this one time."
I've spent many moons looking at all angles of well bore hydraulics. I will be the first to agree that some erosion can theoretically take place in certain fully unconsolidated 'beach sand' formations or soft gumbo. I have formed the opinion that rarely (if ever) is this the case in real life for 'normal' formations. Please bear with me while I explain why 'the birds are not real'.
The 'hole washout' problem usually surfaces when caliper logs indicate excessive hole enlargement, usually in shales. It is not uncommon for the caliper log arms to 'max out' and generate several feet of '16" hole' that was drilled with a 8 1/2" bit. Your geologist will say that all sorts of evils will befall this well, including poor logs, hidden pays, poor cement jobs, and reduced productivity. (Don't even mention that the objective sands, which are the subject of all of the complaints, are 'gage'!) Furthermore, the geologist will decree that the hole-crazy drillers washed out the hole with the bit nozzles. They've made the accusation so many times, I'll bet you can even find drillers and drilling engineers who believe it. Perhaps you do too. Please read on.
Consider first the jet velocity. While high, it quickly dissipates after the mud exits the bit nozzles. Indeed, bit companies understand this quick dissipation effect when they try to get the nozzles as close to the bottom of the hole or the bit teeth as possible. They've even used extended and mini-extended nozzles to put the 'exit point' closer to the rock. One researcher at a major bit company, (whose founder was an eccentric recluse multi-billionaire), found that the velocity of the jet as it actually impinged the rock face needed to be around 250 feet per second to do any good in certain shales. He found that it didn't even clean well below that point, much less erode the rock.
I personally once supervised a test in a facility not unlike APR's drilling machine in Tulsa. I was testing an extended nozzle bit. After the rock sample had been drilled I instructed the lab personnel to leave the bit on the bottom of the hole, leave the pumps on full, but with no rotation. The picture is that of extended nozzles only about 1 1/2" above the rock face (soft shale), blasting the rock with three high pressure jets of mud in the exact same three locations for thirty minutes or so. When the test was over, you could indeed tell where the nozzles had been, but the eroded depressions were only about 1/2" deep! Enough said.
Some folks will say that annular velocities can erode the rock. Don't reach for that bird either. The annular velocities are slooowwww, on the order of 100-500 feet per MINUTE or less. To put that in perspective, that's slower than most rivers. It's slower than walking. It's slower than babies can crawl. It's slower than...well, you get the picture. Further, the hydraulic and flow regime gurus will tell you that near the actual rock wall of the annulus the fluid is not moving at all! Ask yourself, is this rock that you just spent so much time and energy to drill with an expensive hardened steel (or even diamond) bit really going to simply erode away with a fluid velocity like rainwater gently running off of a driveway? Hardly! Cementers will tell you that you can't even erode the filter cake with fluid velocity, much less the rock itself!
If mechanical erosion were really a problem, why is it that oil-based and inhibitive water-based mud systems will drill a 'gun barrel' hole with exactly the same hydraulics as 'eroded' or 'washed out' the hole when a conventional mud was used? If mechanical erosion due to high fluid velocity was the problem, it would exist with the oil muds as well. It doesn't. Hence, the hole washout is not caused by high velocities.
Last, if high velocities were causing the problem, at the very worst it would be a 'self-limiting' problem. To be more specific, as the hole 'eroded', it would become larger, the cross-sectional area of the annulus would increase, and hence, the fluid velocity (whether from jets or in the return annulus) would decrease. It's not rocket science, just mathematics. A given flow rate through a larger hole means a lower velocity. To illustrate, assume we are drilling an 8 1/2" hole, have 6" drill collars, and are pumping at 450 gallons per minute. The annular velocity while the hole was gage would be 304 feet per minute by the collars. With the hole only 9", the velocity will already be down to 245 feet per minute. At 10" hole size, the velocity is down to 172 feet per minute or nearly half of what the gage hole velocity was. To reach this 172 feet per minute in a gage hole by reducing the pump rate, you would have to throttle the pumps back from 450 gpm to only 254 gpm! (And if you tried this approach, what would your velocity in the hole-by-drill pipe annulus do?)
So the next time your friends bring up hole washouts, you can unequivocally blame unconsolidated formations, reactive water sensitive shales, time, or even the phase of the moon. But please don't buy the Mickey Mouse assertion that excessive hydraulics causes it!
That fantasy is just plain Goofy!
Very truly yours,
Mark S. Ramsey, P.E.