4_SolidControl

Drilling Engineering – Fall 2012

Prepared by: Tan Nguyen

Drilling Engineering – PE 311

Chapter 2 – Drilling Fluids

Solid Control



Laboratory tests and practical field experience show that closely monitoring drilled solids in the

mud and minimizing their concentration can result in large savings of both money and time.

These savings manifest in three ways:

1. Improved drilling rate

2. Increased bit life

3. Reduced wear on mud pumps.

Solid Control in Drilling Operations

Introduction



Solids control methods are based on the average diameters of the particles being handled:

Coarse Particles: Greater than 2000 microns

Intermediate Particles: From 250 and 2000 microns

Medium Particles: from 75 to 249 microns

Fine Particles: from 45 to 74 microns

Ultra-fine Particles: from 2 to 44 microns

Collodial Particles: less than 2 microns


Introduction




Introduction



Treatment of solids-related mud problems may involve one or more of the following

mechanisms: settling, dilution, mechanical separation and chemical treatment.

Settling involves retaining mud in a nearly quiescent state long enough to allow the

undissolved solids, which are heavier than water, to "fall out" of the fluid. The relative success

of this method depends on several factors, including the size and shape of the particles, the

density of the particles, the density of the fluid, and the overall retention (settling) time.

The settling time can be reduced by using a flocculant to increase the particle size, or by

inducing centrifugal force to increase the gravitational effect.

Solid Control Methods

Settling



Dilution, unlike the other solids control methods, does not involve removing solid particles

from the mud; rather, it is a means of decreasing the solids concentration by adding base fluid

to the system. Dilution is most often used to correct mud properties that have been altered by

the accumulation of drilled solids. The drawback to this method is that as drilling progresses,

concentrations of drilled solids continue to increase, and undesirable mud properties eventually

reappear. Also, dilution is often expensive for the following reasons:

The consumption of the products required to maintain desired mud properties is

continually increasing.

Lack of storage space for the increased mud volume often leads to the discarding of

hundreds of barrels of valuable drilling mud.

Extra cleanup and transportation costs are incurred in environmentally sensitive areas.


Dilution



Mechanical separation devices are available in two basic types: vibrating screening devices

(shakers) and systems that use centrifugal force to increase settling rate. Mechanical

treatment of solids buildup is often the most practical and cost effective of the four available

methods—it does not alter essential mud properties and it decreases the need for dilution.

Generally speaking, the greater the cost per barrel of a given mud, the greater the savings in

using mechanical equipment to rectify mud properties.

The equipment used to mechanically remove solids from the mud must be designed to fit the

requirements of a given drilling operation; not every piece of equipment is appropriate in every

situation. Furthermore, the equipment specifically selected to aid in mechanical removal of

solids must be rigged up and maintained to ensure that the units operate at peak performance.


Mechanical Separation



Shale Shakers: The double-decker

shale shaker has two screens

mounted on a flat-bed construction.

The screens can range down to 100

mesh with the mesh cross section

varying from square to an exaggerated

rectangle. Drilled solids down to 177

microns are removed by 80-mesh

screens, and 840-micron size particles

by 20-mesh screens.


Mechanical Separation – Shale Shaker



Desilters and DesandersThe desilters/desanders must be equipped with centrifugal pumps

capable of providing sufficient pressure to the hydrocyclones to allow them to operate in the

desired pressure range. When correctly installed and operating in the design range, desilters

and desanders are capable of removing up to 95% of solid particles larger than 15 microns.


Mechanical Separation – Desilters and Desanders



Mud Cleaner: The mud cleaner is designed for

intermediate mud weight ranges of 11.0 to 14.0

ppg. It consists of an eight-cone desilter bank

mounted over a small high-speed shaker. The

mud cleaner combines the advantages of solids

separation by means of centrifugal force and

solids removal by screening.

The screen sizes vary, but the size most

commonly used is 200 mesh, which can remove

fines down to 75 microns in size. It is impractical

to use screen sizes much below 200 mesh

because of excessive loss of barite over the

shaker screen.


Mechanical Separation – Mud Cleaner



Centrifuge: In weighted mud systems it is often desirable to reduce mud maintenance costs

by methods other than dilution. Since it is not practical to use desilting equipment in these

systems, a centrifuge is often used.

Mud centrifuges work on the decanting principle. The mud flow enters a chamber rotating at a

high speed, and centrifugal force separates the mud stream into three components: fluid

phase, low-specific-gravity solids, and high-specific-gravity solids. Following separation of the

low-gravity solids, the high-gravity solids are returned to the active mud system.

In unweighted mud systems, a high-volume decanting centrifuge removes low-specific-gravity

drilled solids most efficiently and economically. The centrifuge can be operated on unweighted

muds at speeds up to 2200 to 2400 rpm, creating centrifugal forces greater than 1500 G-force.

The high-volume centrifuge can remove fine solids down to two microns (e.g., bentonite and

clays) .


Mechanical Separation - Centrifuge



The separation efficiency of hydrocyclones depends on four general factors:

1. Fluid properties;

2. Particle properties;

3. Flow parameters;

4. Hydrocyclone parameters.


Separation Efficiency




Mechanical Separation - Hydrocyclone











Barium sulfate (barite) is the primary additive used to increase the density of clay/water muds.

Densities ranging from 9 – 19 lbm/gal can be obtained using mixtures of barium sulfate, clay,

and water. The specific gravity of pure barium fulfate is 4.5, but the commercial grade used in

drilling fluids (API barite) has an average specific gravity of about 4.2.

Recently, alternative density control agents such as hematite (Fe2O3) with specific gravity

ranging from 4.9 to 5.3 and ilmenite (FeO.TiO2), with specific gravity ranging from 4.5 to 5.1

have been introduced. Because of their hardness, there is a concern about the abrasive of

these materials in the circulating system.

Solid Control in Drilling Fluids

Density control



The mixture density is given by

If the storage capacity is available, to increase the density of the drilling fluid, we simply add

barite to the mud. Therefore, the known and unknown variables in this case are:

Known: V1, 1, B, 2

Unknown: V2, mB


Density control – Unlimited V2

1, V1, V2

B, VB



For ideal mixing the volume of mud, V1 and weight material, VB, must sum to the desired new

volume, V2

Likewise, the total mass of mud and weight material must sum to the desired density-volume

product

Solving these equations simultaneously for unknowns V2 and mB yields


Density control – Unlimited V2



When excess storage capacity is not available, the density increase will require discarding a

portion of the mud. In this case the proper volume of old mud should be discarded before

adding weight material.

Known: V2, 1, B, 2

Unknown: V1, mB


Density control – Limited V2

1, V1, V2

B, VBDiscarded

mud



When excess storage capacity is not available, the density increase will require discarding a

portion of the mud. In this case the proper volume of old mud should be discarded before

adding weight material.

Ideal mixing

Mass balance

Solving these two equations for V1 and mB gives

Then the volume of fluid need to discard: Vd = Vi – V1 ; With Vi is the initial mud volume.


Density control – Limited V2



The addition of large amounts of API barite to the drilling fluid can cause the drilling fluid to

become quite viscous. The finely divided API barite has an extremely large surface area and

can absorb a significant amount of free water in the drilling fluid. This problem can be

overcome by adding water with the weight material to make up for the water adsorbed on the

surface of the finely divided particles. It is often desirable to add only the minimum water

required to wet the surface of the weight material. The addition of approximately 1 gallon of

water per 100 lbm of API barite is usually sufficient to prevent an unacceptable increase in fluid

viscosity.

Mass balance


Density control – wetted barite



Solving these equations for unknowns V1 and mB gives

Note that VwB is the volume of water need to add with one pound of barite. VwB = 0.01

For mB pounds of barite, VwB = 0.01 mB.


Density control – wetted barite – limited V2




Density control



Example: Compute the volume and density of a mud composed of 25 lbm of bentonite clay, 60

lbm of API barite, and 1 bbl of fresh water

Solution:

The total volume

Mixture density


Density control



Example: is desired to increase the density of 200 bbl of 11-lbm/gal mud to 11.5 lbm/gal using

API barite. The final volume is not limited. Compute the weight of API barite required.

Solution:

The final volume is given

The weight material barite required


Density control



Example: it is desired to increase the density of 800 bbl of 12-lbm/gal mud to 14-lbm/gal. one

gallon of water will be added with each 100-lbm sack of API barite to prevent excessive

thickening of the mud. A final mud volume of 800 bbl is desired. Compute the volume of old

mud that should be discarded and the mass of API barite to be added.


Density control



For a final volume of 800 bbl. V1 is given

Thus, 99.47 bbl of mud should be discarded before adding any API barite. The mass of API

barite needed is given by

The volume of water to be added with the barite

0.01mB = 1,083 gal or 25.79 bbl.


Density control

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