Although utilized by an elite group of river crossing contractors just a few years ago, drilling fluids recycling systems have found favor with the mid-size rig contractors and are even being used with some mini-rigs today. Advantages such as cleaner locations, less water hauled in and less mud hauled off, less drilling fluid products required to complete the bore, and reduction in labor cost and drilling time are now being had by contractors throughout the industry. Just as special care is taken in selection of drilling rigs, drill stem, bits and reamers, the same care should be taken in selecting a system and training operators.
Principals of Operation
Drilling fluid from the bore is pumped from the spoils pit into a primary or scalping shaker (vibratory screening deck), where larger drill cuttings are removed. The fluid then drops into a compartment where it is recovered and pumped into hydrocyclones for removal of finer solids. From this point, some systems will discharge the hydroclone underflow slurry on the ground or over ultra fine screens for additional fluids recovery. The hydrocyclone overflow (clean liquid), is discharged into a clean compartment and is ready for reuse or conditioning. The processed volume that is not being sent back to the rig flows back into the first compartment for reprocessing.
Advantages and Disadvantages of Hydrocyclone Underflow Recovery
Although hydrocyclones are capable of removing very fine solids (four-inch cones make a cut point of around 15 microns; five-inch cones make a cut point of around 20 to 25 microns), the discharge is a slurry that can consist of up to 15 percent of total liquid processed. The advantage to discarding this slurry is in removing more ultra fine solids. The disadvantage is large amounts of waste to dispose of. Lots of drilling contractors will try to get around this by discharging slurry back into the spoils pit. The problem with this practice is as the solids content of the spoils pit increases, efficiency of the hydrocyclones decreases and more solids are returned to the rig's mud pump.
Discharging hydrocyclone underflow slurry over ultra-fine screens is an alternative to this problem. The solids discharge is much drier, and more fluid is recovered. This requires a high performance shaker with a large screen area. The screens should be at least a 200-mesh size. Shaker screen mesh size relates to the amount of openings per linear inch. The finer the screens, the more openings per linear inch. The downside to this type of system is a 200-mesh screen makes only a 74 micron cut (which is what the American Petroleum Institute classifies as sand).
What makes this system work is the solids are concentrated when discharged onto the screens by the hydrocyclones and a piggyback effect occurs when they convey across the screens so finer solids are grouped with larger ones. In removing the sand-size particles from the drilling fluid before they get ground down into smaller particles, the drilling fluid can remain relatively clean.
The performance of recycling systems should be monitored through regular testing. A sand content kit will determine the amount of solids 74 microns and larger. To check for solids below 74 microns, mud weight should be checked by a mud balance scale. Water weighs 8.33 pounds per gallon, and drilling fluids used in horizontal directional drilling will range from 8.5 to 8.9 pounds per gallon. If the weight gets to 9 pounds per gallon or higher, the mud is high in solids, and immediate action should be taken to avoid damaging the rig's mud pump and other components.
Selecting a System to Match Your HDD Rig
A good drilling fluids recycling system should include a great mixing system with a venture-style hopper and a charge pump to send positive pressure feed back to the rig's mud pump. Adequate tank volume is important, but processing capacity of the system is even more important. A recycling system should be capable of processing at least one and a half times the maximum capacity of the rig's mud pump. This is usually determined by the size and number of hydrocylones the unit has. Four-inch cones process about 50 gallons per minute per cone, and five-inch cones process around 80 gallons per minute per cone.
Some units come with additional features, such as spoils pumps and pressure washers. The system should require minimal effort for mobilization and demobilization. Along with being a piece of mud processing equipment, the system is also a work station and should have the required safety features such as adequate handrails and work area free of trip hazards. Workers are constantly climbing up and down the unit, so good stairways are a necessity.
