My last article, ‘Solid Control Equipment Types and Usage,’ focused on the different types of solid control equipment, and during the research and interviews for that article, a common drilling rule of thumb was both stated as a fact and challenged by the industry experts that I interviewed. That belief is that you must have at least 1.5 times the hole volume in our solids control system that we can expect for the sand content and density in our system to increase throughout the drilling process. 

So, I began thinking while I was preparing to evaluate the first group of potential driller apprentices entering our industry in 2024, I wondered if I could find a way to test this theory. In 2023, my apprenticeship program purchased a mud technologies RSS-990 that had been generously discounted by the manufacturer. In the past, we have used some large HDD mud recyclers that were a little large for the depth and diameter of the hole we typically drill during training. 

This new unit did not arrive until December, so it has had limited use over the winter. This would be the first real opportunity for us to utilize this piece of equipment. Therefore, I began to theorize how I could set this piece of equipment up in different ways to simulate either someone running an undersized piece of equipment or running a piece of equipment that is the correct size or slightly larger than the 1 1/2 times circulating volume recommendation. I decided by varying the size of the pit or pan that the pickup pump sits in, I could effectively change the volume of our solids control unit while keeping other variables constant with the same screens, shaker, and cones. My hope was that I could get a definitive answer on the hole volume requirement by doing this.

The system:

  • Versa drill V-100 Water Well rig with three-by-four centrifugal pump 

  • Mud technologies RSS-990 solid control unit with 500-gallon “dirty” mud tank and 500-gallon “clean” mud tank 300+ cleaning capacity

  • Electric linear shaker unit, 16.8 square feet of area

  • 40 mesh screens, 8.4 square feet of area

  • 165 mesh screens, 8.4 square feet of area

  • Four 5” cones 40gpm. with 20-micron cut points 

  • 400 gpm 7.5 horse electric pick up “trash” pump

The test was to drill 8-inch holes to 400 feet, one with a small splash pan as a sump for our pickup pump and one with a larger pit for the pickup pump, effectively making the same hole with two different-sized systems. Drill times and mud parameters were tested and documented every five feet of drilling by the future apprentices working as drillers' assistants. Geology consists of clay soils for the top 60 feet, broken rock from 60 feet to 67 feet, and limestone and shale from 67 feet to 400 feet. 

The first step is to calculate the volume of the hole we intend to drill. This is one of the first calculations that Apprentices must know, so we gave them the formula and let them calculate our total hole volume on the whiteboard prior to going out and beginning work. 

Our hole is 8 inches in diameter, so the first step is to figure out how many gallons per foot in an 8-inch hole. We do this by dividing the diameter of the hole squared by 24.52. 

  • 8x8= 64  
  • 64/24.52= 2.61 gallons per foot
  • 2.61 x 400 feet =1044.05 gallons of total hole volume

That is 5.6 cubic yards of material, which highlights the need for effective solid control; when you put it that way, asking someone to shovel a mud pan of that much material in six hours while also helping around the drill site seems a lot to ask... (remind me to have a talk with the drillers I worked with early in my career. LOL) 

The next step was figuring out the actual circulating volume of our solids control system, as the system would never operate completely filled to the top. That came out to be 880 gallons of effective circulation with just our solid control unit. To get our 1 1/2 times hole volume circulating volume, we would need to add 686 gallons of fluid in our pickup pump sump. To do this we dug an earthen pit 5 feet deep by 3 feet wide by 6 feet long with a volume of 673 gallons and a 18” wide by 1 foot deep by 6 foot long “trough” with a volume of 67.3 gallons for a total sump mud capacity of 740 just 54 gallons beyond our target total system. With our pickup pump running at over 300 gallons a minute, the only real challenge was to avoid the pickup pump overfilling the mechanical solids control system when making rod changes or other downtime. This was accomplished by cycling the pickup pump off when making rod changes well as continuing to run the de-sanding/ desilting cones to ensure enough clean mud was available for the rig. 

The first step was mixing mud to a 32 to 35-second viscosity using a medium-yield bentonite gel; a med yield instead of a high-yield bentonite was used to increase the suspension and filter cake characteristics of the drilling mud as no fluid additives were going to be used for this test. 

Mixing 20 pounds of mud per 100 gallons of bentonite produced an initial viscosity of 32 seconds per quart and at theoretical 8.48 pounds per gallon mud weight. Once tested, the actual mud weight was 8.43 pounds per gallon. Some of the bentonite added to the system did not mix properly and remained in suspension. That makes our total solids 1.1% starting the hole. (8.34 divided by the weight of mud per gallon gives you the percent fluid. The remaining percentage is solids entrained in the drill mud.) 

As drilling progressed, viscosity and density, as well as sand content, were tested. The max viscosity reached was 37 seconds per quart at 105 feet, and the max density and sand content reached the bottom of the hole at 2.5 percent sand and 8.9 pounds per gallon density. After circulating for about 20 minutes with a solid control system running, the mud weight decreased to 8.7 lbs. per gallon, and the sand content reduced to 1.75%. showing the effectiveness of the system at altering mud properties.

The next hole completed did not use appear but a drill through splash pan with 150 gallons of capacity. The pickup pump was positioned in the splash pan, and the charge pump was plumed so that it could discharge directly into the splash pan when making rod changes and so that the system could continue to clean. The total circulating volume was 1030 gallons or 99%of our total hole volume. Mud was again mixed with 20 lbs. of medium-density mud per 100 gallons of bentonite and produced an initial viscosity of 32 seconds per quart and a mud weight of 8.45 pounds per gallon. The maximum viscosity reached during drilling was 135 feet at 40 seconds per quart, and the maximum density reached was 9 1/2 lbs. per gallon at 400 with 3% sand. 

Both holes were completed in roughly the same amount of time, giving credence to the adage that 1 1/2 times the hole volume for our circulating system with mechanical solids control is the minimum we should be using during Water Well drilling operations. However, like everything in drilling, the right answer is that it depends if we're doing geothermal drilling and not attempting to develop water wells. We may be able to get by with smaller systems and still maintain the desired properties needed for the type of operation. 

With this in mind, my students and I visited an active geothermal job site where they were using a much smaller solids control system than the recommended 1 1/2 times total circulating volume. The system they were using had roughly half the total hole volume available for circulation, and make-up water had to continually be added in order to maintain adequate fluid volumes. My students tested fluid properties while they were drilling, and although the weight and sand content did reach relatively high values, the end result was the loops were installed and grouted in a timely fashion.

So, in closing, the answer to the question of whether a solid control unit needs to be 1.5 times the total hole volume to be an effective tool is that it depends. If you have the ability and the equipment is 1.5 times the hole volume, it will make drilling easier, faster, and cleaner and reduce equipment wear and the possibility of fracking out. Still, some mechanical solids control is better than no mechanical solid control, so use what you have. It will save some work for your crew, help clean up the site, and assist your company’s image.

Until next time, keep your mud clean and turn to the right.