Tag: drilling mud systems

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DRYER SHAKERS

The dryer shaker, or dryer, is a linear motion shaker used to minimize the volume of liquid associated with drilled cuttings discharged from the main rig shakers and hydrocyclones. The liquid removed by the dryers is returned to the active system.

Why use dryer shaker

Dryers were introduced with the closed-loop mud systems and environmental efforts to reduce liquid-waste haul-off. Two methods, chemical and mechanical, are available to minimize liquid discharge. The chemical method uses a system called a dewatering unit, while the mechanical method takes place through linear motion shakers. These systems may be used separately or together.

The main function of drying shaker

The dryer shaker deliquifies drilled cuttings initially separated by another piece of solids-separation equipment. These drilled solids can be discharged from the main shaker or a bank of hydrocyclones. Dryers recover liquid discharged with solids in normal liquid/solids separation that would have been previously discarded from the mud system. This liquid contains colloidal solids, and the effect on drilling-fluid properties must be considered since dewatering systems are frequently needed to flocculate, coagulate, and remove these solids.

The dryer family incorporates pieces of equipment long used as independent units: the main linear motion shaker, the desander, and the desilter, which are combined in several configurations to discharge their discard across the fine screens (e.g., API 200) of a linear motion shaker to capture the associated liquid. These units, formerly used as mud cleaners, are mounted on the mud tanks, usually in line with the main linear motion shaker. They can be tied into the flowline to assist with fine screening when not being used as dryers. Their pumps take suction from the same compartments as desanders and desilters and discharge their overflow (effluent) into the proper downstream compartments.

How to use it

A linear motion dryer may be used to remove the excess liquid from the main shaker discharge. The flow rate across a linear motion dryer is substantially smaller than the flow rate across the main shaker. The lower flow rate permits the removal of the excess fluid by the linear motion dryer by using a finer screen. The dryer is usually mounted at a lower level than the other solids-separation equipment to use gravity to transport solids to it. Whether by slide or by conveyor, the cuttings dump into a large hopper, located above the screen, that replaces the back tank, or possum belly. As the cuttings convey along with the screen, they are again liquefied. This excess fluid, with the fine solids that passed through the screens, is collected in a shallow tank that takes the place of a normal sump. The liquid is pumped to a catch tank that acts as the feed for a centrifuge or back to the active system.

A dryer unit can be used to remove the excess fluid from the underflow of a bank of hydrocyclones (desanders or desilters). This arrangement resembles a mud cleaner system. In this configuration, the dryer unit may be used on either a weighted or an unweighted mud system. The liquid recovered by the linear motion shaker under the hydrocyclones can be processed by a centrifuge, as previously described.

How to select a proper one for your mud solution

The perfection of the linear motion shaker for drilling-fluid use, coupled with advanced fine-screen manufacturing technology, has made these dryers very efficient. In most configurations, the dryers use the same style of screens, motors, and/or motor/vibration combinations as do other linear motion shakers by the same manufacturer.

Depending on the fluid, saving previously discarded liquid may be financially advantageous. The dryer discard is relatively dry and can be handled by backhoe and dump truck rather than by vacuum truck.

Drilling-fluid properties must be monitored properly when the recovered liquid is returned to the active system. Large quantities of colloidal solids may be recovered with the liquid. This could affect the PV, YP, and gel strengths of a drilling fluid.

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Separation of Drilled Solids from Drilling Fluids

The types and quantities of solids (insoluble components) present in drilling mud systems play major roles in the fluid’s density, viscosity, filter-cake quality/filtration control, and other chemical and mechanical properties. The type of solid and its concentration influences mud and well costs, including factors such as drilling rate, hydraulics, dilution rate, torque and drag, surge and swab pressures, differential sticking, lost circulation, hole stability, and balling of the bit and the bottom-hole
assembly. These, in turn, influence the service life of bits, pumps, and other mechanical equipment. Insoluble polymers, clays, and weighting materials are added to drilling mud to achieve various desirable properties.
Drilled solids, consisting of rock and low-yielding clays, are incorporated into the mud continuously while drilling. To a limited extent, they can be tolerated and may even be beneficial. Dispersion of clay-bearing drilled solids creates highly charged colloidal particles (<2 μm) that generate significant viscosity, particularly at low shear rates, which aids in suspension of all solids. If the clays are sodium montmorillonite, the solids will also form thin filter cakes and control filtration (loss of liquid phase) into the drilled formation. Above a concentration of a few weight percent, dispersed drilled solids can generate excessive low-shear-rate and high-shear-rate viscosities, greatly reduced drilling rates, and excessively thick filter cakes. As shown in Figures 2.3 and 2.4, with increasing mud density (increasing concentration of weighting material), the high-shear-rate viscosity (reflected by the plastic viscosity [PV]) rises continuously even as the concentration of drilled solids (low-gravity solids [LGSs]) is reduced. The methylene blue test (MBT) is a measure of the surface activity of the solids in the drilling fluid and serves as a relative measure of the amount of active clays in the system. It does not correspond directly to the concentration of drilled solids, since composition of drilled solids is quite variable. However, it is clear that, in most cases, drilled solids have a much greater effect than barite on viscosity and that the amount of active clays in the drilled solids is one of the most important factors. Thus, as mud density is increased, MBT must be reduced so that PV does not reach such a high level that it exceeds pump capacity or causes well-bore stability problems.


As shown in Figure 2.4, increasing the mud density from 10 lb/gal to 18 lb/gal requires that the MBT be reduced by half [M-I llc]. Different mud densities require different strategies to maintain the concentration of drilled solids within an acceptable range. Whereas low mud densities may require only mud dilution in combination with a simple mechanical separator, high mud densities may require a more complex strategy:
(a) chemical treatment to limit dispersion of the drilled solids (e.g., use of a shale inhibitor or deflocculant like lignosulfonate).
(b) more frequent dilution of the drilling fluid with base fluid,
(c) more complex solids removal equipment, such as mud cleaners and centrifuges [Svarovsky].
In either case, solids removal is one of the most important aspects of mud system control, since it has a direct bearing on drilling efficiency and represents an opportunity to reduce overall drilling costs. A diagram of a typical mud circulating system, including various solids-control devices, is shown in Figure 2.5 [M-I llc].


While some dilution with fresh treated mud is necessary and even desirable, sole reliance on dilution to control buildup of drilled solids in the mud is very costly. The dilution volume required to compensate for contamination of the mud by 1 bbl of drilled solids is given by the following equation:

where Vsolids is the volume of drilled solids expressed in volume percentage. As discussed earlier, drilled solids become less tolerable with increasing mud density. For drilling-fluid densities less than 12 lb/gal, Vsolids<5% is desirable, whereas for a density of 18 lb/gal, Vsolids<2 or 3% is best. When Vsolids=5%, the equation above gives Vdilution=19 bbl
drilling fluid/bbl drilled solids. The cost of this extra drilling fluid (neglecting downhole losses) is the sum of the cost of the drilling fluid itself plus the cost to dispose of it. This dilution cost is generally so high that even a considerable investment in solids-control equipment is more economical.
Solids removal on the rig is accomplished by one or more of the following techniques:
. Screening: Shale shakers, gumbo removal devices
. Hydrocycloning: Desanders, desilters
. Centrifugation: Scalping and decanting centrifuges
. Gravitational settling: Sumps, dewatering units
Often these are accomplished using separate devices, but sometimes these processes are combined, as in the case of the mud cleaner, which is a bank of hydrocyclones mounted over a vibrating screen. Another important hybrid device is the cuttings dryer (also called a rotating shaker), which is a centrifuge fitted with a cone-shaped shaker; this apparatus is used to separate cuttings from NAF-based muds and strip most of the mud from the cuttings’ surfaces before disposal. Additional devices
can help to enhance solids-removal efficiency. For example, a vacuum or atmospheric degasser is sometimes installed (before any centrifugal pumps, typically between the shakers and desanders) to remove entrained air that can cause pump cavitation and reduction in mud density. Refer to Chapter 5 on Tank Arrangements for more details.
With the advent of closed loop systems, dewatering of WBMs has received strong impetus, and it has been found useful to add a dewatering unit downstream of a conventional solids-control system [Amoco]. Dewatering units usually employ a flocculation tank—with a polymer to flocculate all solids—and settling tanks to generate solidsfree liquid that is returned to the active system. Dewatering units reduce waste volume and disposal costs substantially and are most economical
when used to process large volumes of expensive drilling fluid.
Solids-control equipment used on a rig is designed to remove drilledsolids—not all solids—from a drilling fluid. As such, the equipment has to be refined enough to leave desired solids (such as weighting material) behind while taking out drilled solids ranging in size from several millimeters to just a few microns. Although such perfect separation of desired from undesired solids is not possible, the advantages offered by the solids-control equipment far outweigh their limitations. Each
device is designed to remove a sufficient quantity and size range of solids. The key to efficient solids control is to use the right combination of equipment for a particular situation, arrange the equipment properly, and ensure that it operates correctly. This, in turn, requires accurate characterization of the drilled solids, along with optimal engineering and maintenance of the drilling fluid.