SOLID CONTROL EQUIPMENT
Recall mud is made up of fluid (water, oil
or gas) and solids (bentonite, barite etc).The aim
of any efficient solids removal system is
to retain the desirable components of the mud
system by separating out and discharging
the unwanted drilled solids and contaminants.
Solids in drilling fluids may be
classified in two separate categories based on specific gavity,
(or density) and particle size.
Solids, classified by specific gravity,
may be divided into two groups:
• High Gravity Solids (H.G.S.) sg = 4.2
• Low Gravity Solids (L.G.S.) sg = 1.6 to
2.9
The solids content of a drilling fluid
will be made up of a mixture of high and low gravity
solids. High gravity solids (H.G.S) are
added to fluids to increase the density,e.g. barytes,
whilst low gravity solids (L.G.S) enter
the mud through drilled cuttings and should be
removed by the solids control equipment.
Mud solids are also classified according
to their size in units called microns (). A micron is
0.0000394 in or 0.001 mm. Particle size is
important in drilling muds for the following
reasons:
• The smaller the particle size, the more
pronounced the affect on fluid properties.
• The smaller the particle size, the more
difficult it is to remove it or control its
effects on the fluid.
The API classification of particle sizes
is:
Particle Size () Classification Sieve
Size (mesh)
> 2000 Coarse 10
2000 - 250 Intermediate 60
250 - 74 Medium 200
74 – 44 Fine 325
44-2 Ultra Fine
2-0
Colloidal
7.1 SOLIDS CONTROL EQUIPMENT
Solids contaminants and gas entrapped in
mud can be removed from mud in four stages:
• Screen separation: shale shakers,
scalper screens and mud cleaner screens.
• Settling separation in non-stirred
compartments: sand traps and settling
pits.
• Removal of gaseous contaminants by
vacuum degassers or similar equipment
• Forced settling by the action of
centrifugal devices including hydrocyclones
(desanders, desilters and micro-cones) and
centrifuges.
SCREEN SEPARATION DEVICES
Figure 7.6 shows a layout for solids control equipment for a weighted mud system.
Shale shakers and scalper screens (Gumbo
shakers) can effectively remove up to 80% of all
solids from a drilling fluid, if the
correct type of shaker is used and run in an efficient
manner. Mud laden with solids passes over
the vibrating shaker (Figure 7.7) where the liquid
part of mud and small solids pass through
the shaker screens and drill cuttings collect at the
bottom of the shaker to be discharged.
There are two types of shaker operation: elliptical
and linear motion. Field experience
indicate that elliptical shakers work better with
water based muds and linear motion shakers
are more suited to oil based muds.
An absolute minimum of three shale shakers is
recommended and that these shakers are
fitted with retrofit kits to allow quick and simply
replacements.
The shakers should also be in a covered, enclosed
housing with a means of ventilation and
each shaker fitted with a smoke hood.
REMOVAL OF GASEOUS CONTAMINANTS
Gas entrapped in mud must be removed in order
to maintain the mud weight to a level
needed to control down hole formation pressures.
Gas is removed from mud using a
vacuum degasser, see Figure 7.8. The latter is a simple equipment
containing a vacuum
pump and a float assembly. The vacuum pump
creates a low internal pressure which
allows gas-cut mud to be drawn into the degasser
vessel and it then flows in a thin
layer over an internal baffle plate. The combination
of low internal pressure and thin
liquid film causes gas bubbles to expand
in size, rise to the surface of the mud inside the
vessel and break from the mud. As the gas
moves toward the top of the degasser it is
removed by the vacuum pump. The removed
gas is routed away from the rig and is then
either vented to atmosphere or flared.
FORCED SETTLING BY CENTRIFUGAL DEVICES
Desanders and
desilters are
hydrocyclones and
work on the principle of separating solids
from a liquid by reating centrifugal forces inside the hydrocyclone. Mud is
injected tangentially into the hydrocyclone and the resulting centrifugal
forces drive the solids to the walls of the hydrocyclone and finally discharges
them from the apex with a small volume of
mud, Figure 7.9.
The fluid portion of mud leaves the top of
the hydrocyclone as an overflow and is then sent to the active pit to be pumped
downhole again.
(a) Desanders
Desanders are hydrocyclones with 6 in ID
or larger.The primary use of desanders is in the
top hole sections when drilling with water
based mud to help maintain low mud weights. Use of desanders prevents overload
of the desilter cones and increases their efficiency by
reducing the mud weight and solids content
of the feed inlet. Desanders should be used if the sand content of the mud
rises above 0.5% to prevent abrasion of pump liners.
Desanders should never be used with oil
based muds, because of its very wet solids
discharge. The desander makes a cut in the
40 to 45 micron size range. With a spray discharge, the underflow weight should
be between 2.5 to 5.0 ppg heavier than the input
mud.
(b) Desilters
Desilters, in conjunction with desanders,
should be used to process low mud weights used to drill top hole sections, Figure 7.10. If it is required to raise the mud
weight this must be done with the additions of barytes, and not by allowing the
build up of low gravity solids.
Desilters should never be used with oil
based muds.
The desilter makes a cut in the 20 to 25
micron size range.
Typical throughput capacities are as
follows:
Desanders 12"cone 500 gpm per cone.
6" cone 125 gpm per cone.
Desilters 4"cone 50 gpm per cone.
2" cone 15 gpm per cone.
As a visual check to see that the
hydrocyclone
operations are at an optimum, the
discharge should be in the form of a fine spray and a suction should be felt at
the apex when covered with the hand. A rope
discharge means than the mud has lost its circular motion and the cone is not working properly.
(c) Mud Cleaners
A mud cleaner consists of a battery of
hydrocyclones placed above a high energy vibrating
screen, Figure
7.11. Mud cleaners must only be used when it becomes
impossible to
maintain low mud weights by use of the
shale shakers alone. It is far more efficient to use
desilters and process the underflow with a
centrifuge than to use the screens of a mud
cleaner.
The use of mud cleaners with oil based muds
should be minimised since experience
has shown that mud losses of 3 to 5
bbls/hr being discharged are not uncommon,
coupled with the necessity to adhere to strict
environmental pollution regulations.
(d) Centrifuges Centrifuges use
centrifugal forces to remove heavy solids from the liquid and
lighter components of the mud. A decanting
centrifuge consists of a horizontal conical
steel bowl rotating a high speed, see Figure 7.12. The bowl contains a double-screw type
conveyor which rotates in the same direction as the steel bowl, but at a slightly lower speed. When
mud enters the centrifuge, the centrifugal
force developed by the bowl holds the mud in a pond
against the walls of the pond. In this pond the silt and sand particle settle
against the walls and the conveyor blade scrapes and pushes the settled solids towards the narrow end of
the bowl where they are collected as damp particles with no free liquid. The
liquid and clay particles are collected as as overflow from ports at the large end of the bowl.
It is recommended to have at least one centrifuge on
the rig site during all drilling
operations. For expensive muds or long term drilling
operations, two centrifuges may prove
economical.
When dealing with low weight muds, the solids
underflow is discarded as a means of solids
control to obtain desirable particle size distribution
and reduce mud weight. Processing
capacity of the centrifuge may limit its use for this
purpose to lower hole sections where the
circulation rates are low as the bowl speed must be at
a maximum, so lower capacities can be
dealt with. It can also be used to process the
underflow from desilters, returning an expensive
or environmentally harmful liquid phase to the active
mud system, and discarding relatively
dry solid fines.
With weighted muds, the solids underflow containing
barytes may be returned to the mud
system and the liquid phase containing viscosity
building colloids discharged. However it is
unlikely to be used for this purpose with oil based
muds for both economic and
environmental reasons.
Centrifuge efficiency is affected predominantly by the
feed flow rate, but it is also affected
by the following operating parameters:
• Bowl speed (rpm).
• Bowl conveyer differential speed (rpm).
• Pool depth.
Solid Density From Retort
Analysis Mud Calculation
Retort
analysis is the method to determine solid and liquid components in the drilling
fluid. In this article, we will adapt mass balance and retort analysis data to
determine solid density in the mud.
Mass balance
for mud is listed below;
We can
rearrange the Euqation#1 in order to determine the solid density
In the report analysis, the
volume is presented in percentage and summation of solid and liquid fraction
equals to one.
The unit of
each parameter is described below;
Vm = mud
volume, %
ρm = mud density, ppg
Vw = water
volume, %
ρw = water density, ppg
Vo = oil
volume, %
ρo = oil density, ppg
Vs = solid
volume, %
ρs = solid density, ppg
Note: this is not only cutting
weight but it includes all weights of solid (cutting and weighting material).
Example: Mud weight used for the
report is 12.0 ppg and the result from the analysis showing in the following
percentage;
Base oil =
60%
Water = 5%
Base oil
weight = 7.0 ppg
ρs = 21.06 ppg
Total density of solid in
the drilling mud is 21.06 ppg.
