Wayne Nash answers two questions from drillers this month - both relating to problems associated with bottom hole assemblies.

Got a couple questions from drillers this month - both relating to problems associated with bottom hole assemblies.%

Drilling in Limestone for installation of nested monitoring wells. Courtesy of Paddock Drilling Ltd.
Wayne: We drill mostly sand and gravel formations with layers of fairly hard limestone. When we drill down to the limestone, the bit "jumps" over to the side of the hole to drill the limestone, and then jumps back in the sand below. This leaves the hole i n the limestone offset to the rest of the hole and makes it hard, if not impossible to run casing. Any ideas?
Bob in Florida

Bob: This is called "bit walk" and is fairly common in the water well industry. Two things aggravate the problem - small diameter drill pipe in relation to bit size (i.e. 2 3?-inch pipe and a 7 -inch bit) and pulldown.

All bits tend to walk to the le ft somewhat during normal drilling. If the bedding plane of the rock is not perfectly level (is it ever?), the bit will tend to deviate one of two ways: If the rock-face is just a few degrees off horizontal, the bit will tend to walk up-gradient. If the b edding plane is more nearly vertical, the bit will tend to walk down the rock-face. Either way, when the bit finally penetrates the rock layer and goes back into soft formation, the drill pipe will tend to go back to vertical, leaving an offset in the h ole.

The cure to this is twofold. First, you should run a straight blade stabilizer immediately above the bit - two feet will do it, but the longer the better. This will keep the bit centered in

the hole as it starts to 'track' in the top of the ro c k. Secondly, don't run any pulldown in the top of the rock until you are sure the bit has 'tracked' squarely into the rock face.

If you need more weight to make hole, consider a drill collar above the stabilizer. This will help keep the string in ten si on, and the hole straight. The stabilizer should be nearly full gauge (say, 1?inch under) and be built up when it gets to 1?inch undergauge. If you still are having problems, you may be able to 'mule-shoe' the bottom of the casing. That is, cut the bottom o f the casing at a 45-degree angle. Then when you hit a ledge, you may be able to rotate the casing off the ledge and into the hole. The only problem with this: it is significantly harder to get a good seal when you land the casing, and you may have to pressure cement to get a good shoe-seal. h¿¿

Wayne: We drill mostly clay formations, and have a hole balling problem. The clay balls up in the hole and is very hard to circulate out. We usually have to 'trip' the hole an extra time to make sure we can get the casing in. Sometimes even this doesn't d o it, and we have to push to get the casing in. What would you recommend for this problem?
Steve in Texas y

Use pH test strips to determine the pH of the make-up water.
Steve: Good to hear from a fellow Texan. We had that problem a bunch on the Gulf Coast and the learning curve seemed never-ending. What seemed to work for me was running a spiral blade stabilizer one joint above the bit. If a clay ball formed right behind the bit, the stabilizerwould re-cut it, allowing it to be circulated out. The other advantage to a string stabilizer was, when we were tripping out of the hole to set the casing, if the stabilizer pulled into a clay ball, the string would turn as it was pulled, indicating a tight hole. Since the bit is still one joint below it, just put the kelly back on and circulate the ball out.

The mud companies have done a lot of research over the years on bit balling and such. What they found:

* Often the cla y particles will have such a high electrical charge, that they try to reassemble themselves as soon as they leave the bit. Several things will help this. First, run as low a viscosity as you can and still maintain the hole. This keeps the water phase of t he mud high in proportion to the drilled solids. Second, check the pH of your make-upwater. You don't need a bunch of fancy instruments to do this; pH test strips are available in beauty supply houses (ask your wife; she'll know where to get them). Keep t he pH above 7 with soda ash. When we drill balling clay, I like to see the pH at least 9, maybe 10. Use a polyphosphate thinner. This helps strip the electrical charge off the clay particles, making them much easier to circulate out. I won'tsay that these solutions have solved all my problems, but I haven't had to cut the casing out and re-drill in years. I have 'pushed' a few strings though. Hope this helps.

If you have any questions or comments you can reach me at 912-265-1839, or at rockbit@compuse rve.com. I try to answer all questions and if I have missed something, I'd like to know, and will be glad to pass it on to everyone else. Who knows, you could win fabulous prizes such as worn out drill bits or empty dope buckets! Just last month, I gave a driller a brand-new (1 hole) string of drill pipe. All he had to do was go get it! Of course it was 700 feet in the ground and stuck, but you can't have everything.