Drilling fluid maintenance cost, clean up and disposal cost, as well as the overall cost of boring, can be reduced dramatically when proper solids-control techniques are utilized. These facts were recognized in the oil industry in the late 1800s, when open earthen pits were used to separate the cuttings from the borehole. This was accomplished by a series of weirs and settling pits that allowed the solids to settle out naturally by using gravity. The clean mud then flowed into a suction pit to be pumped back down the hole. This was the first solids-control technique ever used.
The next innovation in solids-control came when the shale shakers were introduced in the early 1930s in the oil industry. The shale shakers were derived from technology used in the mining industry. Today, the shale shaker remains the primary piece of solids-control equipment utilized in the industry. Another machine borrowed from the mining industry in the 1930s was the cone classifier, or hydrocyclone. The basic principle of this device involves the centrifugal forces brought about by the high velocity of the drilling fluid spinning in the cone forcing the larger and heavier solids to settle outward toward the cyclone wall and downward toward the underflow solids discharge. Together with the shale shaker, hydrocyclones have become an integral part of today’s solids-control system.
The state-of-the-art solids-control system includes improved versions of this original equipment, first introduced into the oil industry so many years ago. Although much more efficient and robust, the core technology has change little over the past few decades.
The future path of solids-control systems will continue to increase the overall removal efficiency of undesirable solids from the drilling fluid. This will include continued improvements in shale shakers and screen life. Research investigating alternate technology such as using vacuum techniques and different motions may prove more effective in the future. The continuing trend of more stringent environmental regulations around the world will require more and more solids-control systems to be implemented to minimize haul off of drilling waste, not to mention the cost savings on equipment such as mud pumps and mud motors.
Drilling fluid and equipment maintenance costs can decrease greatly when proper solids-control practices are utilized. From a fluid control standpoint, it would be desirable in most cases to remove all drilled solids. Although this is possible, it would be cost-prohibitive. The goal of a solids-control system is to achieve the balance between mechanical solids separation and dilution that will result in drill solids being maintained at an acceptable level, with minimum cost.
Particle Size and Effects
The solid phase of any drilling fluid is either commercial solids or drilled solids. Most commercial solids, such as bentonite, have a relative particle size of less than 1 micron (0.000039 in.). Drilled solids are those particles that enter the mud system in the form of cuttings from the bit or back reamer, or from borehole debris. These solids vary in size from less than 1 micron to larger, depending on the carrying capabilities of the drilling fluid. Note that spindle speed and the amount of push or pull force required to drill are important factors on the particle size of the cuttings. One of the most important objectives in solids control is to remove as many of the large particles as is practical the first time that these solids are pumped out of the borehole. This requires properly designed and installed solids removal mechanical treating equipment, sized to process a minimum of 100 percent and up to 125 percent of the mud circulation rate. Solids that are not removed during the first circulation through the surface equipment are subjected to mechanical degradation by the drill bit, reamers and mud pumps during each circulation cycle – until they are too fine for removal by mechanical means. In order to evaluate the removal capabilities of the various pieces of mechanical treating equipment, it is necessary to consider the source of the solids and classify them according to the following sizes:- 440 microns or larger – large drilled solids (cuttings)
- 74 microns to 440 microns – sand
- 2 microns to 74 microns – silt
- 0.5 microns to 2 microns – clay
- 0.5 microns and smaller – colloids
Five key benefits of low solids in drilling mud:
- Increased drilling penetration.
- Increased bit or back reamer life.
- Reduced mud cost.
- Reduced triplex mud pump, mud motor and surface equipment maintenance costs.
- Reduced clean up and hauling/disposal costs.
Methods of controlling solids:
- mechanical treatment
- chemical treatment
- dilution of mud with water
- discard mud and mix new mud
Mechanical Treatment
This is the method of mechanically removing solids using shale shakers or hydrocyclone devices, such as desanders and desilters. Each piece of equipment generally is limited to the following ranges of particle removal:- Standard shale shaker – 440 microns or larger (also referred to as the scalping shaker).
- Fine screen shaker – 74 microns or larger.
- Desanders – 100 microns and larger.
- Desilters – 15 micron and larger.
Mechanical Separation Basics
Mechanical separation equipment employs mass differences, size differences or a combination of both to selectively reject undesirable solids and retain desirable drilling fluid. The desanders and desilters utilize centrifugal force and mass difference between the solids density and liquid density for solids removal. The shale shakers employ a vibrating screen of various micron-sized differences. A standard rig shaker or fine screen shaker is vital to the solids control and should process 100 percent of the mud returning from the starting pit before allowing this mud to be processed by any of the downstream equipment you may utilize in your solids control system. Located directly downstream from the shale shaker will be one of the hydrocyclone devices – a desander or desilter.These pieces of equipment should be sized to process at least 125 percent of the rig circulation rate, while discarding undesirable cuttings and solids down to the 50-micron size range. The desander removes the majority of the solids down to the 100-micron size range and prevents the desilter from being overloaded. The desilter removes the majority of the solids down to the 15-micron range.
Important keys for effective solids control:
- Obtain solids removal equipment for your operation.
- Remove as many drilled cuttings as possible before being pumped back down the borehole.
- Do not bypass the shale shaker or other solid control equipment while drilling.
- Use the smallest mesh screen possible on the shale shaker. This will change from formation to formation.
- Maintain an adequate inventory of recommended spare parts.
- Train and assign rig personnel to be responsible for equipment operation and maintenance.
