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:

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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