World According to Wayne: Differential Sticking Tips for Oil Well Drilling
Another winter has pretty much passed in the Williston basin, and I am glad of it. We didn’t have much snow this winter, which made for better road conditions, but the temperature and wind chills set records. On the 15th of January, the wind chill was -51. I couldn’t get my quarters warm enough to keep from seeing my breath, so I slept in the truck. The wind chill on the first of March was only -40 or, as the natives put it, a sign of spring. A local town has passed an ordinance that all buried piping must be 7 feet below ground! Not too much trenching going on for a while.
My fishing work has slowed down a little this year due to a variety of factors. While my company is well known for open hole fishing expertise, we have not pushed the cased hole/production side of the business too much in this area. The drillers active in this area are getting better at what they do and make fewer mistakes. For instance, when the boom took off a few years ago up here, most of the rigs and crews were from Texas and Oklahoma. They knew darned good and well that lots of drill collars were necessary for a good hole. After planting and losing most of their drill collars, they decided that heavy weight drill pipe would provide a much safer weight-on-bit. We fish drill collars less now than we used to.
Differential sticking is all about formation pressure, columnist Wayne Nash says. Source: iStock
One of the problems with drill collars is the diameter. They are large in relation to the hole size, and the chances of wall-sticking, or differential sticking, is greater. Differential sticking is a phenomenon where the hydrostatic pressure inside the borehole is greater than the natural formation pressure. If the pipe penetrates the wall cake and becomes subject to the formation pressure it will stick tighter’n Dick’s hatband. Another problem that adds to differential sticking is the fact that there are several formations above the Bakken and Three Forks. These formations have been produced for many years and are, in places, pretty much depleted. The formation pressures are much lower than the mud weights we need to control the deeper formations. This leads to ideal places for differential sticking.
Usually, when I arrive at the rig, the company man has finally come to the conclusion that he’s stuck, and can’t just pull it loose. Circulation is usually good and the mud engineer is trying to sell some “magic potion” that will free the pipe. Most of the time, it will only free up money from the oil companies’ operating funds. I have found that my first job is to explain to the company man how, exactly, differential sticking works, and what we need to do to free it. Brute force won’t do it. Period. The best method is to equalize the pressure in the borehole and the formation. Since we usually have good circulation, conditioning the mud to a lighter weight is step one. As the hydrostatic pressure in the borehole gets lighter, working the pipe and attempts at rotation will sometimes free it.
Sometimes the mud is as light as it can be made with conventional additives like water. About 9 pounds per gallon is about as light as you can go with any water based mud and additives. When the hydrostatic pressure requires less than 8 pounds per gallon to equalize the formation pressure, the hole is often displaced with either diesel or a light distillate. This can lower the mud weight and hydrostatic pressure to around 7.5 ppg. After soaking for a few hours, the pipe might come free and conventional mud is then pumped to stabilize the hole and recover the diesel. Obviously, this is not a technique to be used in a water well.
If a lighter mud weight and hydrostatic pressure is necessary, often the driller will lighten the mud by aerating it. This is commonly done with nitrogen, an inert gas. It is added to the mud as it is circulated in a proportion calculated to reduce the hydrostatic pressure in the borehole. This has worked with fair success in the past, but there are a couple distinct problems with this method. First, it is a dynamic process. Pumping and injecting must be continuous. If the pumps are shut down, the nitrogen floats to the surface and the downhole effect is gone. Second, injecting a gas through the bit and causing rising and expanding gas bubbles in the annulus tends to tear up the wall-cake and the stability of the hole. This may lead to more severe problems.
A newer, safer method has been developed to lower the formation pressure, temporarily, to whatever hydrostatic pressure it takes to free the pipe.
First, the mud is fully conditioned to the lightest practical weight and the hole circulated clean. Then, open the Kelly line bleed off valve and close the annular preventer. Displace nitrogen into the annulus, pushing the mud down and returning through the bit to the pits. By carefully monitoring the amount of mud returned to the pits, the nitrogen/mud interface can be calculated. In no case should you displace nitrogen below the casing shoe, which may damage the formation. When the column of mud is deep enough to lighten the hydrostatic head on the formation the desired amount, open the choke and quickly exhaust the nitrogen. This will rapidly lower the formation pressure at the stuck point, freeing the pipe. As soon as the pipe is freed, it should be rotated and reciprocated as much as possible to prevent re-sticking. The Kelly line bleed off valve can be closed and good mud, which is already at the bit, can be circulated and thus condition the hole with very little gas or formation fluids entering the borehole.
Although this is a fairly new technique, it works very well, is quick, and does not require expensive downhole tools and trips.
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.