The most familiar combination separator is the Mud Cleaner or Mud Conditioner (Figure 1). In many cases, combinations of vibratory screening and settling/centrifugal force are used together to provide an effective separation.
Mud cleaners were developed in the early 1970s to remove fine drilled solids from weighted mud without excessive loss of barite and fluid. They have also proved valuable tools in closed systems and other “dry” location” applications. These devices use a combination of desilting hydrocyclones and very fine mesh vibrating screens (120–400 mesh) to remove fine drilled solids while returning valuable mud additives and liquids back to the active mud system.
Traditional mud cleaners use multiple 4” or 5” cyclones, mounted over a vibrating screen, and are able to effectively process 400–600 GPM. The process capacity is limited by screen capacity and its ability to discard “dry” solids. With the introduction of linear motion vibrating screens, the capacity of the mud cleaner screen has been greatly increased. This, in turn, allows the use of additional hydrocyclones and higher, overall process capacities.
The combination of hydrocyclones and linear-motion vibrating screens is called a Mud Conditioner to differentiate these machines from earlier mud cleaners. Mud conditioners often combine both desander and desilter cones mounted above the screen deck to take full advantage of the higher process capacity, usually 1000–1500 GPM,
and reduce the overall size and weight of the unit, when compared to mud cleaners.
After removal of large cuttings with a shaker, feed mud is pumped into the mud cleaner/conditioner’s hydrocyclones with a centrifugal pump. The overflow from the cyclones is returned to the mud system. Instead of simply discarding the underflow, the solids and liquid exiting the bottom of the cyclones are directed onto a fine screen. Drilled solids larger than the screen openings are discarded; the remaining solids, including most barite in a weighted system, pass through the screen and are returned to the mud system.
The cut point and amount of mass solids removed by a mud cleaner/conditioner depends primarily on the mesh of the fine screen used, Figure 2. Since there are many designs of mud cleaners/conditioners available, performance and economics will vary with machine and drilling variables.
Mud cleaners/conditioners should be considered in these applications:
- Whenever the application requires finer screens than the existing shaker can handle
- Unweighted OBM
- Expensive polymer systems
- When the cost of water is high
- Unweighted WBM with high disposal costs and/or environmental restrictions
- When use of lost circulation material requires bypassing the shaker
- Workover and completion fluid
Mud cleaners/conditioners are simply a bank of hydrocyclones mounted over a fine-mesh screen. In many instances (even with modern fine screen shakers), a finer separation is required than can be provided with existing shakers. The question to answer becomes how to achieve the necessary level of screening at the lowest cost. The alternatives are:
- Add additional similar shakers to handle the flow rate;
- Replace the existing shakers with more efficient units;
- Add a mud cleaner/conditioner downstream from the existing shakers.
Any of these may be correct, but a thorough study of the capital cost (the actual cost of new equipment,
plus transportation, rig modifications, and installation) and the operating cost (screens and other expendables, plus fuel) is necessary to make the proper choice. Also, because of the cut points produced by some “modern” layered screens, the use of mud cleaner/conditioners may be indicated downstream of linear motion shakers.
Salvage of the liquid phase of an unweighted drilling mud often costjustifies use of a mud cleaner/conditioner when the fluid phase of the mud or disposal is expensive. Compared to desanders and desilters, whose cyclone underflow may be as much as 15 bbl/hr or more, mud cleaners/conditioners can achieve efficient solids removal while returning most liquid back to the active mud system. Use of ultrafine screens (200 to 325 mesh) significantly improves solids control in any high-value fluid system.
An increasingly important application of mud cleaners/conditioners is the removal of drilled solids from unweighted water-base mud in semi-dry form. This system is commonly used in areas where environmental restrictions prohibit
the use of earthen reserve pits, and expensive vacuum truck waste disposal from steel pits is the alternative. The mud cleaner/conditioner is used to discard drilled solids in semi-dry form which is classified as legal landfill in most areas and is subject to economical dry-haul disposal techniques (dump truck or portable waste containers).
When used for this purpose, the screen underflow from the mud cleaner/conditioner is often diverted to a separate steel waste pit for vacuum truck disposal. This may seem counterproductive, but since a vacuum truck can only carry a limited amount of sand because of the over-the-road weight restrictions, whenever a vacuum truck must haul normal full-flow desilter waste, the waste must be diluted with rig water to reduce density. The operator is then billed for the haulage of a vacuum truck load comprised largely of rig water. On the other hand, since most of the solids are removed in semi-dry form by the mud cleaner/conditioner screen, the remaining solids in the screen underflow are dilute enough to be hauled away without watering them back. Vacuum truck loads often can be reduced to a small fraction of those required with full-flow desilting.
This approach to dry-solids disposal can be carried further by using a centrifuge with a mud cleaner/conditioner to form a “closed” system which eliminates discarding of any fluid. These systems are being used increasingly in areas where liquid mud waste must be hauled a significant distance and is subject to a high disposal fee.
In a closed system, underflow from the mud cleaner/conditioner screen is diverted to a holding tank and then centrifuged, which results in disposal of very fine, semi-dry solids and return of liquid to the active system. Such a system virtually eliminates the need for reserve pits, minimizes dilution, eliminates vacuum truck services for disposal of liquid mud, and meets environmental constraints when drilling within ecologically sensitive areas.
One special mud cleaner/conditioner application is the use of a double-deck unit for salvage of coarse lost circulation material (LCM). Usually when running LCM, the shale shaker is bypassed and drilled solids build up rapidly in the mud, necessitating a high level of dilution and new mud. Use of a two-deck mud cleaner/conditioner allows salvage of the LCM while minimizing the increase in solids content.
Within the mud cleaner/conditioner, a coarse top screen is used to pre-screen the mud and remove the lost circulation material. This material is discharged back into the active system for recirculation downhole. The drilled solids, mud additives and liquid phase pass through the top screen onto the lower, finer mesh screen, where the drilled solids are separated out and discarded. The cleaned mud then flows back into the mud system and is re-blended with the salvaged lost circulation materials.
Another mud cleaner/conditioner application is the clean up of workover and completion fluids. In order to reduce costs associated with this expensive task, a mud cleaner running one or two ultrafine screens (200 over 325 mesh) can be used to remove most of the solids before they reach cartridge type filters.
This application can significantly reduce filter replacement costs, reduce downtime in changing filters, and allow larger volumes of fluid to be cleaned at a faster rate.