The Basics of Drilling Mud
I’ve had a lot of calls and emails/messages about mud lately, for some reason. The drilling kind, not the kind we get to drive in. Maybe things go in cycles. Anyway, I thought it might be a good time to go over some mud basics.
First, drilling mud, properly circulated, removes the cuttings as the bit drills. They circulate to the surface and most of them settle in the pits, or are expelled by the shale shaker and other solids control equipment. I say “properly circulated” because, in the water well business, it is common to drill very large holes with very small pumps. I once drilled a 48-inch hole with a 5½-by-8 pump. Talk about slow circulating times! The problem is a combination of low annular velocity and slip velocity. Slip velocity is the speed that cuttings fall in fluid. Obviously, the annular velocity must be higher than the slip velocity to clean the hole.
One solution to this is to increase the viscosity of the drilling fluid to help “float” the cuttings. This lifts cuttings, but it has its drawbacks. Very high vis mud doesn’t drill as fast, and it tends to retain more small particles during recirculation. This causes trouble with wall cake, and erosion of pumps and plumbing. A better way is to pump a high vis “pill” before each connection to help clean the hole and lift the cuttings. The volume doesn’t have to be large, so it doesn’t add to the viscosity of the overall system much. Plus, a little fresh mud helps the overall system and the well. The pill will come back to the pits, drop the cuttings and blend in with the mud as it is recirculated. I use a small tank separate from my main mud pits. My helper mixes up some fresh mud as I drill the connection down and we are ready. Even if you don’t use it on every connection, it is good to use at the end of the day to clean your hole so that you don’t end up redrilling a bunch the next morning.
The next thing drilling mud does is counter the natural formation pressure and keep the hole open. Air drillers don’t have to worry about this — granite ain’t going anywhere. But, with unconsolidated formations, especially sand and gravel, with nothing in the hole, the formation will flow into the hole and drilling stops. The mud must exert enough hydrostatic pressure to hold back the formation pressure. This is usually not too high in water wells, and additional weight material, like barite, is not necessary. Usually it goes the other way. Even a column of clear water is often enough to break down the formation and cause lost circulation. This is why, for most water well situations, the mud should be kept as light as possible.
Properly designed pits and a good desander go a long way toward a successful hole. Another related aspect of good, bentonite-based mud is the ability to build a good wall cake. The wall cake is a thin membrane of mud that coats the wall of the borehole, stabilizes the formation and provides lubricity to the hole. Cuttings move upward faster, and there is less drag and wear on the drill string. One thing to watch is the amount of micro-sized solids in the mud. Without a desander, small cuttings contaminate the mud and build a thick, fluffy wallcake that allows fluid loss into the formation, decreases the diameter of the hole, and increases friction with the bit and drillstring. A thick, fluffy wallcake is also much harder to develop out when completing a well. Not always good for a harmonious outcome.
The bottom line is, mud is much more than just some brown liquid that can be pumped. It has properties that need attention to drill a good hole. The first ingredient is water. It should be clean and potable. I don’t like to drill with city water because it is chlorinated. Chlorine kills the polymers in the mud, forcing you to use more gel. I drill with untreated well water.
Next is pH. A pH higher than neutral causes the mud to yield much better, so you use less of it, saving money and keeping the solids content of your mud as low as possible. A higher pH also causes the mud to react less with swelling clays, which can be a big problem. Soda ash is cheap, safe and available, and works well. It doesn’t take much. In the oilfield, we use caustic soda. It works well, but it is a dangerous compound that requires special handling, so it is usually not worth it on water wells, except in special circumstances.
Next is polymer. There are several on the market and they do very well at increasing viscosity, but used by themselves do not build wallcake. Also, the bentonite must be mixed and completely hydrated before adding polymer. If you start with a straight polymer mud and add bentonite, it will just fall to the bottom of the pits and be useless. Mix the gel first.
When I have the hole drilled, and before setting casing, it is usually a good idea to condition the hole and adjust the mud to the next operation. You no longer have to carry cuttings, and hopefully you have built a good wallcake while drilling, so a thorough bottoms-up circulation and mud conditioning are in order. I usually thin up as much as I can to run casing and run a polymer sweep through the hole. This gets the last of the cuttings out and makes a nice, slick hole to run casing in. When the casing is on bottom, and before development, I usually circulate a couple gallons of Clorox. This kills the polymer, making development faster.
Hope this helps. If you need any additional info, you can call or email me anytime. Keep turning to the right!
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.