If a driller uses a dug pit, it must be big enough to allow ample settling time. According to the Johnson Book, this process requires a pit at least twice the volume of the hole, which might take a mighty big pit on some wells. It also requires that the mud change direction several times between the hole and the suction. If you drill a 7-7⁄8-inch hole 400 feet deep, it means the pit should be at least 2,000 gallons. This is a lot of pit, and it takes a lot of water just to start drilling. Also, the amount of gel it takes to build the initial mud viscosity is large and expensive.
A lot of drillers solve this problem by using a small steel pit they can carry on the rig and rig up quickly. It allows the hole to be started with less water and mud, but requires constant shoveling and the addition of mud and water to maintain the mud. Not to mention that it’s getting harder and harder to find someone to shovel cuttings. Steel pits work pretty well for settling out the large cuttings, but the natural action of shoveling the cuttings tend to stir up the sand and fines, and sending them back to the suction before they get a chance to re-settle. I’ve seen many drillers using steel pits circulating mud that was 5-percent to 10-percent sand, which causes all kinds of problems in the finished borehole, and wear on the rig.
Look at it this way: If you are circulating mud that has a 5-percent sand content at 200 gallons per minute, you are circulating 450 gallons of sand per hour of drilling time. This is a tremendous amount of abrasive material working on the pump, swivel, drill pipe and bit. It also raises the specific gravity, or weight, of the mud, causing increased water loss to the formation, and a thick and fluffy wall cake, which leads to difficulty running casing, and potentially severe development problems.
Most mud engineers like to see a sand content of less than 1 percent. Achieving this ratio is difficult to do with dug pits, and impossible to do with small steel pits without help. One way to do it is to run at least one desander cone to help reduce the finer cuttings before they have a chance to recirculate. This system works fairly well on small-diameter, shallow holes, but as diameter and depth increase, a single-cone desander set-up won’t be able to keep up. It allows recirculation of the cuttings and regrinding by the bit until the cuttings are too small for even a desander to remove. At this point, mud weight goes up, water loss goes up and lost circulation is a danger.
The only way to maintain the mud at the weight, viscosity and sand content for most effective drilling is to treat the mud mechanically. This entails removing the cuttings before they are recirculated. In water well drilling, it usually is done with a scalping shaker and a desander system large enough to handle more than 100 percent of the circulated volume of mud. These systems typically clean the mud to less than 1 percent sand content before recirculating the mud back to the rig.
One of the problems I’ve seen is in fast-hole drilling in a sticky or balling clay. The cuttings sometimes come out of the hole the size of softballs or larger, and the pick-up pumps can’t handle pumping them to the shaker. This problem is commonly seen on the gulf coast and other areas. The cure is to run a gumbo chain before the pick-up pump. A gumbo chain removes cuttings larger than 1 inch before returning to the mud system, thus making pumping and scalping much more efficient. This system was developed in the oilfield and has just come to the water well industry, and it greatly improves the usefulness and efficiency of a mud system.
To sum up, whatever type of mud system you use, you should shoot for a sand content under 1 percent, and do what it takes to get there. The mud companies make a nifty little sand content kit that is inexpensive and easy to use, and that should be a regular part of a driller’s tool kit along with a marsh funnel and a set of mud scales.