DRILLING FLUIDS

Drilling fluid -mud – is usually a mixture of water, clay, weighing material and a few chemicals. Sometimes oil may be used instead of water, or oil added to the water to give the mud certain desirable properties. Drilling fluid is used to raise the cuttings made by the bit and lift them to the surface for disposal.

But equally important, it also provides a means of keeping underground pressures in check. The heavier or denser the mud, is the more pressure it exerts. So weighing materials -barite – are added to the mud to make it exert as much pressure as needed to contain formation pressures. The equipment in the circulating system consists of a large number of items. The mud pump takes in mud from the mud pits and sends it out a discharge line to a standpipe. The standpipe is a steel pipe mounted vertically on one leg of the mast or derrick. The mud is pumped up the standpipe and into a flexible, very strong, reinforced rubber hose called the rotary hose or kelly hose.
The rotary hose is connected to the swivel . The mud enters the swivel the swivel:goes down the kelly, drill pipe and drill collars and exist at the bit. It then does a sharp U-turn and heads back up the hole in the annulus. The annulus is the space between the outside of the drill string and wall of the hole. Finally the mud leaves the hole through a steel pipe called the mud return line and falls over a vibrating, screen like device called the shale shaker. Agitators installed on the mud pits help maintain a uniform mixture of liquids and solids in the mud. If any fine silt or sand is being drilled, then devices called desilters or desanders may be added. Another
auxiliary in the mud system is a device called degasser.

SHAKER SCREEN CLOTHS

Shale shakers remove solids by processing solids-laden drilling fluid over
the surface of a vibrating shaker screen. Particles smaller than the shaker screen openings pass through the screen along with the liquid phase of the drilling fluid.

derrick48-30 shaker screen

 

Larger particles are separated into the shaker overflow for discard.
The shaker screen acts as a ‘go no-go’ gauge. That is, particles larger than
the screen openings remain on the screen and are discarded. Particles
finer than the screen openings go through the screen with the drilling
fluid. The criterion for early shale shaker screens was a long screen
life. This demand for screen life was consistent with the shaker designs
and solids-removal philosophies of the time period. Early shale shakers
could remove only large solids from the drilling fluid. The sand trap,
reserve and settling pits, and downstream hydrocyclones (if utilized)
removed the bulk of drilled solids. Today’s shale shakers are capable
of utilizing finer screens that remove more solids. Desirable characteristics
for a shaker screen are:
1. Economical drilled-solids removal
2. Large liquid flow rate capacity
3. Plugging and blinding resistance
4. Acceptable service life
5. Easy identification
For any particular shale shaker, the size and shape of the shaker screen openings have a great effect on solids removal. This means that the performance of any shaker is largely controlled by the screen cloth used.
The first four items in the preceding list are largely controlled by
choice of screen cloth and by the screen panel technology. Large gains
in shale shaker performance are a direct result of improved screen
cloth and panel fabrication. shaker Screens used on shale shakers have evolved into complex opening patterns.

 

SHALE SHAKER USER’S GUIDE

Every solids-removal system should have enough shale shakers to process
100% of the drilling-fluid circulating rate. In all cases, consult the
owner’s manual for correct installation, operation, and maintenance
154 Drilling Fluids Processing Handbook procedures. If an owner’s manual is not available, the following general guidelines may be helpful in observing proper procedures.

1 Shale Shaker Installation

1. Low places in the flowline will trap cuttings. Flowline angle should
be such that settling of solids does not occur, that is, a 1-inch drop for
every 10 feet of flowline seems to be a good rule of thumb.
2. When a back tank (possum belly), is used, the flowline should enter
at the bottom to prevent solids from settling and building up. If the
flowline enters over the top, it should be extended to within one pipe
diameter of the flowline from the bottom.
3. Rig up with sufficient space and approved walkways around the
shaker(s) to permit easy service and maintenance.
4. Branched tees (Figure 7.21) should be avoided. Solids preferentially
travel in a straight path, resulting in uneven solids distribution to the
shale shakers.
5. Ensure equal fluid and solids distribution when more than one shaker
is used, as shown in Figure 7.22.
6. The options shown in Figures 7.22 and 7.23 are better than the
distribution system shown in Figure 7.21.
7. An optional top delivery (Figure 7.23) prevents cuttings settling in the
back tank.
8. A cement bypass that discharges outside the active system is desirable.
9. Mount and operate the shale shaker where it is level. Both the solids
and fluid limits will be reduced if this rule is not followed.
10. Motors and starters should be explosion-proof. Local electrical
codes must be met. Be sure that starter heaters are the proper size.
11. Provide the proper electrical voltage and frequency. Low line voltages
reduce the life of the electrical system. Low frequency reduces
the motion and lowers the capacity of the shale shaker.
12. Check for correct motor rotation.
13. Check for correct motion of the shale shaker deck.
14. Check drive belts for proper tension according to manufacturer’s
instructions.
15. Screens should be installed according to manufacturer’s instructions.
16. Provide a wash-down system for cleaning.
17. Water-spray bars, if installed, should provide only a mist of water—
not a stream.

7.21 shale shaker branched flowline

7.22 Equal distribution flowline.

7.23 Top feed tank distribution flowline

2 Shale Shaker Operation

1. For double-deck shale shakers, run a coarser-mesh shaker screen on the top deck and a finer-mesh shaker screen on the bottom. The coarser shaker screen should be at least two API sizes coarser than the finer-mesh shaker screen. Watch for a torn bottom screen. During normal drilling operations, cover at least 75–80% of the bottom screen with drilling fluid to maximize utilization of available screen area. Properly designed flowback pans may improve shaker performance. (Gumbo shakers mounted above as an integral part of linear shale shakers are not called double-deck shale shakers, although the operation guidelines above still apply.)
2. For single-deck shale shakers with multiple screens on the deck, try to
run screens all of the same mesh. If coarser screens are necessary to
prevent drilling fluid loss, run the finer screens closest to the possum
belly. All screens should have approximately the same-size openings.
For example, use a combination of MG API 100 (140 microns) þ MG
API 80 (177 microns), but not MG API 100 (140 microns) þ MG API
50 (279 microns). Under normal drilling conditions, cover at least 75–80% of the screen area with drilling fluid to properly utilize the
screen surface area.
3. Water-spray bars (mist only) may be used for sticky clay to aid
conveyance that reduces whole drilling fluid loss. High-pressure
washers should not be used on the screen(s) while they are circulating,
as solids will be dispersed and forced through the screen openings.
Water-spray bars are not recommended for weighted fluids or oilbased
NAFs.
4. Do not bypass the shale shaker screens or operate with torn screens;
these are the main causes of plugged hydrocyclones. This results in
a build-up of drilled solids in the drilling fluid. Dumping the back
tank into the pits (to clean the screen or for whatever reason) is a
form of bypassing the shale shaker and should not be done.
5. All drilling fluids that have not been processed by solids-removal
equipment and are intended to be added to the active system should
be screened by the shale shakers to remove undesirable solids. This
specifically includes drilling fluid delivered to a location from remote
sources.
6. Do not dump the back tank, or possum belly into the system before
trips. These solids do not settle and will plug hydrocyclones downstream.

3 Shale Shaker Maintenance

1. For improved screen life with nontensioned screens, make certain that
the components of the screen tensioning system, including any rubber
supports, nuts, bolts, springs, etc., are in place and in good shape.
Install screens according to the manufacturer’s recommended installation
procedure.
2. For improved life pretensioned screens, ensure the deck rubber
support seals are not worn or missing.
3. Lubricate and maintain the unit according to the manufacturer’s
instructions. (Some units are self-lubricating and should not be
‘‘relubricated’’).
4. With screens that are not pretensioned, check the tension of screens
at 1, 3, and 8 hours after installation and hourly thereafter.
5. Check the tension of and adjust drive belts according to the manufacturer’s
instructions.
6. If only one deck of a multiple-deck shaker is used, be sure that other
tension rails are secured.
7. Wash screens at the beginning of a trip so as not to allow fluid to dry
on them. We repeat: Do not dump the possum belly into the active
system or the sand trap below the shaker. The result will be plugging
of hydrocyclones downstream and/or an increase in drilled-solids
concentration in the drilling fluid.
8. Check the condition of vibration isolator members and screen
support rubbers and replace them if they show signs of deterioration
or excessive wear.
9. Check the fluid bypass valve and other places for leaks around the
shaker screens.
10. Remove drilling-fluid buildup from the vibrating bed, vibrators, and
motors. Caution: Do not spray electrical equipment or motors with
oil or water.
11. Make certain that no hose, cables, etc., are in contact with the
vibrating bed.

4 Shale Shaker Operating Guidelines

Shale shakers should run continuously while circulating. Cuttings cannot
be removed if the shaker is not in motion
1. Drilling fluid should cover most of the screen. If the drilling fluid covers
only one fourth or one third of the screen, the screen is too coarse.
2. A screen with a hole in it should be repaired or replaced at once.
Holes in panel screens can be plugged. Install screens according to
manufacturer’s recommended installation procedures. Cuttings are
not removed from the drilling fluid flowing through the hole.
3. Shaker screen replacements should be made as quickly as possible.
This will decrease the amount of cuttings remaining in the drilling
fluid because the shale shaker is not running.
4. Locate and arrange tools and screens before starting to make the
replacement. If possible, get help.
5. If possible, change the screen during a connection. In critical situations,
the driller may want to stop (or slow) the pumps and stop
drilling while the screen is being replaced.
6. For improved screen life with nonpretensioned screens, make certain
the components of the screen tensioning system, including any rubber
supports, nuts, bolts, springs, etc. are in place and in good shape.
7. Check condition of vibration isolators members and screen support
rubbers and replace if they show signs of deterioration or wear.
8. Water should not be added in the possum belly (or back tank) or
onto the shale shaker screen. Water should be added downstream.
9. Except in cases of lost circulation, the shale shaker should not be
bypassed, even for a short time.
10. Wash screen(s) at the beginning of a trip so fluid will not dry on the
screen(s).
The possum belly (or back tank), should not be dumped into the sand
trap or mud tank system just before making a trip. If this is done,
cuttings will move down the tank system and plug desilters as the next
drill bit starts drilling.

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. 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.

dryer shaker 3d

The dryer shaker deliquifies drilled cuttings initially separated by another
piece of solids-separation equipment. These drilled solids can be the discharge from a 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.

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 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 the screen,
they are again deliquified. 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.

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.