SETTLING PITS

Drilling fluid enters the removal-tank section after it passes through the
main shale shaker. Immediately below the main shaker is the first pit,
called a settling pit or sand trap. Fluid passing through the shaker screen
flows directly into this small compartment. The fluid in this compartment
is not agitated. This allows solids to settle. The fluid overflows from
the sand trap into the next compartment, which should be the degasser
suction pit. The sand trap is the only compartment not agitated in the
mud tank system.


The sides of a sand trap should slope at 45 or more to a small area in
front of a quick opening discharge valve. When the solids are dumped,
the valve can be closed quickly when drilling fluid begins to flow from
the trap. The purpose of the quick-opening valve is to allow only settled
solids to leave the compartment, with minimal loss of drilling fluid.
In many cases during periods of fast drilling, with coarse or damaged
shaker screens in use, the sand trap will fill several times per day.
An effective sand trap requires an overflow weir of maximum length
to create a liquid column as deep as possible. A common, and recommended,
practice is to utilize the full length of the partition between the
sand trap and the degasser suction pit.

1. SETTLING RATES
The rate at which solids settle depends on the force causing the settling,
the dimensions of the solid, and the fluid viscosity in which the solid is settling. Analysis of forces acting on irregularly shaped objects is
extremely complicated. Analysis of forces acting on spheres is not as complicated
and is addressed here: For simplicity, the solid will be assumed
to be spherical and settling in a quiescent fluid. The forces acting on the
sphere would be the gravitational force causing it to fall and the buoyant
force tending to prevent settling. The force causing settling could also
be centrifugal, from a device such as a hydrocyclone or a centrifuge. This
section will develop the equation relating to solids settling through a
drilling fluid in the sand trap.
Settling rates of spherical particles in liquid can be calculated from
Stokes’ law:

F = 6πμvR                                                                             (1)

where
. F is the force applied to the sphere by the liquid, in dynes
. μ is the fluid viscosity, in Poise
. v is the particle velocity, in cm/sec
. R is the radius of the sphere, in cm.
Stokes’ law was developed when the centimeter/grams/second (cgs) unit
system was popular with scientists. Viscosity is defined as the ratio
of shear stress in a liquid to the shear rate. One Poise has the units of
dynes-sec/cm2 in absolute units, or [g/cm-sec] in cgs units. The unit of
dyne also has the units of gcm/sec2.
A sphere falling through a liquid experiences a downward force of
gravity and an upward force of the buoyancy effect of the liquid. The
buoyancy force is equal to the weight of the displaced fluid:

buoyant force=4π/3(R^3)ρ1                                             (2)

The downward force is mass times acceleration, or the weight for gravity
settling. The mass of the sphere is the volume of the sphere times the
density of the sphere (ρs):

mass of a sphere=4π/3(R^3)ρs                                       (3)

Equation 1 now becomes

4π/3(R^3)ρs – 4π/3(R^3)ρ1 = 6πμvR                          (4)

4/18[R^2](ρs – ρ1) = μv                                                        (5)

Solving this equation for velocity and changing the radius R to diameter
d, in cm:

v=d^2/18μ(ρs – ρ1)g                                                             (6)

v:cm/sec=(d:cm)^2/18(μ:(poise))[(ρs – ρ1):gm/cm^3](gm:cm/sec^2)   (7)

v:cm/sec=(d:micron*10^-4)^2/18(μ:(cP=100)[(ρs – ρ1):gm/cm^3]*(980:cm/sec^2)                                                                                          (8)

v:ft/sec=(d:micron*10^-4)^2/18(μ:(cP=100))[(ρs – ρ1):gm/cm^3]*(980:cm/sec^2)(ft/30.48 cm)(60 sec/min)                                               (9)

v:ft/min=1.07*10^-4 (d:micron)^2 / μ:(cp) [(ρs – ρ1):gm/cm^3]           (10)

where
. v=settling or terminal velocity, in ft/min
. D=particle equivalent diameter, in microns
. ρs=solid density, in g/cm3
. ρ1=liquid density, in g/cm3
. μ=viscosity of liquid, centipoises (cP)
A 2.6-g/cm^3 drilled solid passing through an API 20 screen (850-
micron diameter) would fall through a 9.0-ppg, 100-cP drilling fluid
with a terminal velocity of 1.6 ft/min. This could be calculated from equation 10:

v:ft/min=[1.07*10^-4 (d)^2/ μ ]*(ρs – ρ1)

v:ft/min=[1.07*10^-4 (micron)^2 /100cp] (2.6-[9.0ppg/8.34ppg]) = 1.62 *10^-2 ft/min

If the rig is circulating 500 gpm through a 50-bbl settling tank or sand
trap, the fluid remains in this tank for a maximum of 4.2 min. If the sand
trap holds 100 bbl of drilling fluid, the retention time is 8.4 min. Solids
can settle about 6 inches during the 4.2-min retention time or 1 foot
during the 8.4-min retention time.
The selection of a viscosity to use in the equation is complicated.
On drilling rigs, normally the lowest viscosity measurement made is
with the 3-rpm viscometer reading. Some drilling rigs using polymer
drilling fluids use Brookfield viscometers, which measure very low shear
rate viscosities. Drilling-fluid viscosity is a function of shear rate, as
discussed in Chapter 2 on Drilling Fluids. As particles settle, the fluid
viscosity impeding the settling depends on the settling rate. As the
velocity decreases, the viscosity of the fluid increases. The K viscosity is
the viscosity of a fluid at one reciprocal second, which is within the shear
rate range of a small solid falling through a drilling fluid and can be
determined on most drilling rigs. Some drilling fluids are constructed to
have very large low-shear-rate viscosities, to facilitate carrying capacity
as the solids are moved up the borehole. Many drilling-fluid systems
have K viscosities in the range of 1000 effective cP instead of the 100 cP
used in the example above. Solids settling will be greatly hindered in
these fluids because they are designed to prevent settling.

2. COMPARISON OF SETTLING RATES OF BARITE
AND LOW-GRAVITY DRILLED SOLIDS

Stokes’ law can be used to describe the anticipated settling rate for
spheres of barite or low-gravity drilled solids:

stokes' law

Equations 11 and 12 can be used to solve for the ratio of diameters that
will cause the settling velocity of barite to be equal to the settling velocity
of low-gravity solids:

DB = 0.65Dlg                                                                 (13)

Equation 13 indicates that a 20-micron barite sample settles at the same
rate as a 30-micron low-gravity solid; or a 48-micron barite sample
settles at the same rate as a 74-micron low-gravity solid. Note that this
is true regardless of the viscosity of the fluid in which these particles are
settling.

3. COMMENTS

Linear motion and balanced elliptical motion shale shakers permit
the use of finer screens than were used in the past. Consequently, sand
traps are frequently ignored in a system using them. Considering the
inescapable fact that screens regularly tear and wear out, sand traps offer
the ability to capture some of the solids that would normally be left in
the drilling fluid.
When API 80 screens were used on shale shakers and represented the
smallest openings possible for processing drilling fluid, sand traps were a
very important component of the surface drilling-fluid system. Normally,
screens as coarse as API 20 to API 40 (850 microns to 425 microns) were
used in the upper part of a borehole. The solids that passed through
these screens settled quite rapidly. When API 200 screens are installed
on the main shakers, the largest solid presented to the fluid in the tank
is 74 microns. These solids settle much more slowly than the larger
850-micron (API 20) solids that were separated earlier.
The sand trap is still used in a system to provide backup for failures in
the main shaker screen. These screens sometimes break, and the failure
may go unnoticed for a long period of time. The sand trap offers the
possibility of capturing some of the solids that pass through the torn
screen.
Although not intended to be used as an insurance shield, scalping
shakers also provide the opportunity to remove solids larger than API 20
to API 40 before the fluid reaches the main shaker. This provides some relief from large solids reaching the sand trap if the finer wires of the main shaker break.

4. BYPASSING THE SHALE SHAKER
One rule cited frequently is

 ‘‘Do not bypass the shale shaker.’’

Cracking the bypass valve at the bottom of the shaker back tank allows a rig hand to mount fine screens on the shaker. However, the solids that are not presented to the shaker screen are not removed and cause great damage to the drilling fluid. Sand traps provide some insurance against this activity; but they do not capture all of the larger solids that bypass the
screen—so it is still a very bad practice. This is frequently the reason
hydrocyclones are plugged.
Another activity common on drilling rigs bypasses the shaker screen
more subtly. Before making a trip, the ‘‘possum belly,’’ or back tank, is
dumped into the sand trap to clean the shaker. Drilling fluid left on a
shaker screen dries during a trip and causes the screen to flood. In an
effort to prevent any screen plugging, the possum belly is also cleaned
and all of the settled solids are dumped into the sand trap. All of the
dumped solids, however, do not settle. When circulation is restored,
these suspended solids migrate down the removal system until they reach
the apex of a hydrocyclone. These solids plug many cones on drilling
rigs. Possum bellies should be dumped into a waste pit, NOT into the
drilling-fluid system.

(writed by Leon Robinson).

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