SOLID CONTROL EQUIPMENT

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, %

ρ_m = mud density, ppg

Vw = water volume, %

ρ_w = water density, ppg

Vo = oil volume, %

ρ_o = oil density, ppg

Vs = solid volume, %

ρ_s = 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%

Solid = 35 %

Water = 5%

Base oil weight = 7.0 ppg

Water weight = 8.6 ppg

ρ_s = 21.06 ppg

Total density of solid in the drilling mud is 21.06 ppg.