|
The Drillers Club
One of the primary uses for fluid is to carry
unwanted drilled solids from the borehole
Solids are essentially contaminant and, if
left in the mud, will lead to numerous operational problems.
Three options are available to maintain acceptable drilling
fluid properties:
1.
Do nothing and
let the solids build up. When the mud no longer meets
specifications, throw it away and start with fresh mud.
2.
Dilute the mud
and rebuild the system to keep the properties within acceptable
ranges, while dumping excess mud to the reserve pit.
3.
Lower the solids
content of the mud through solids removal to minimize the
addition/dilution necessary to maintain acceptable properties.
Increased public awareness of environmental
issues has provided both regulatory and economic incentives to
minimize drilling waste. In many instances, the first two
choices have become very expensive and unacceptable.This has
served to stress the importance of the third option, efficient
solids control. Using
solids removal to minimize addition/dilution volumes is normally
most effective and provides the following benefits:
Benefits of controlling solids:
Increased penetration
rates
Reduced mud costs
Lower water requirements
Reduced torque and drag
Less mixing problems
Reduced system pressure losses
Lower circulating density (ECD)
Better cement jobs
Reduced instances of lost circulation
Reduced formation damage
Less differential sticking
Reduced environmental impact
Less waste, lower disposal costs
Mechanical solids removal:
As we can see from the list of benefit, the
controlling of the drilled solids is essential options 1 and 2
combined with the environmental issues
leave us with mechanical removal (solid control
equipment)
Solids control equipment has been standard
hardware on most rotary drilling rigs since the early 1960s. In
the early years, many of the solid/liquid separation devices
were borrowed from other industries and applied directly to
oilfield rotary drilling.
Although the basic operating principles and
technology associated with mechanical solids removal have not
changed significantly over the years, refinements in design
specifically for drilling applications have yielded considerable
improvements in performance and reliability.
Gumbo
Shakers
For decades the young shale's of the world
have created problems when drilled with water-based muds.
Like a sponge the
weakened shale absorbs water, this caused an unstable borehole
resulting in high torque, drag, hole fill, stuck pipe and poor
cementing. These shale’s are often referred to as “gumbo”
the sponge effect causes the shale to swell and become
extremely sticky plugging of flow lines and blinding of shale
shaker screens.
conveyors
Older
vibrating shakers often designed as duel shaker
would use what are known as scalping screen to convey the
sticky mass over the side, however this meant a lot of work for
the people working at the sand traps and the constant changing
of damaged screens. Modern day gumbo shakers are often installed
as close to the return flow line as possible, they do not
vibrate but convey cutting, dumping them overboard
The Header Tank
Also known as the possum belly, it is from here the returning
fluid is evenly distributed to the Shale shakers, included in
the design are gates allowing one or more of the shakers to be
closed off or the fluid directed back to the mud pit room
Shale Shaker
As the first step in the mud
cleaning/solids-removal chain, the shale shaker represents the
first line of defense.
Many potential problems can be avoided by observing and
adjusting the shale shakers to achieve maximum removal
efficiency for the handling capacity, by using screens the
finest mesh to remove as many drill solids as possible on the
first circulation is the most efficient method of solids
control. A 250-square-mesh shale shaker screen will remove 100%
of the solids greater than 74 microns,
De-sanding Cyclones
A de-sander is needed to prevent overload on
the de-silters.
Generally, a 6-in. ID or larger hydroclone is used, with a unit
made up of two 12-in. hydro-clones, rated at 500 gpm per
hydro-clone, being common. Large de-sander hydroclones have the
advantage of a large volumetric capacity (flow rate) per
hydroclone, but have the disadvantage of making wide
particle-size cuts in the 45- to 74-micron range. To obtain
efficient results, a de-sander must be installed with the proper
“head” pressure.
De-silting Cyclones
to achieve maximum efficiency and prevent overloading the
desilter, the entire flow should be de-sanded before being
de-silted. Generally, a
4-in” ID hydroclone is used for de-silting, with a unit
containing 12 or more 4”, hydroclones,
rated at 75 gpm per hydroclone, being common. The proper
volumetric capacity for de-silters and de-sanders should be
equal to 125 to 150% of the circulation rate.
A well designed and
properly operated 4” hydroclone will have a D50 cut point of 15
to 35 microns. Since barite falls into the same size range as
silt, it also will be separated from the mud system by a de-silter.
For this reason, de-silters
are rarely used on weighted mud’s above 12.5 lb/gal. Both de-silter
and de-sander are used primarily while drilling surface hole and
where un-weighted, low-density muds are used.
Mud Cleaner
Basically, a mud cleaner is a de-silter
mounted over a vibrating-screen shaker generally 12 or more
4-in. hydroclones above a very fine-mesh screen, high-energy
shaker. A mud
cleaner will remove sand-size drill solids from the mud, yet
retain the barite.
It first processes the mud through the de-silter, then screens
the discharge through a fine-mesh shaker
Centrifuges
Decanting Centrifuges were first introduction to the oilfield in
the early 1950s, and have become an increasingly common addition
to the solids control system. Centrifuges have the ability of
removing the very fine solids that cannot be removed by any
other mechanical separation device. In un-weighted mud’s, the
centrifuge can greatly improve the separation efficiency of the
solids removal system and reduce liquid discharge volumes when
used in conjunction with hydrocyclones.
|
