drilling fluid dilution

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Koya University

Faculty of Engineering

Petroleum Engineering Department

Third stage

Drilling Lab.

Lab No.: 3

Drilling Fluid

dilution

Prepared by : Sarwar Salam

Group: B

Date of Experiment:- Nov 26th 2015

Supervised by: Mr. Pshtiwan –Mr. Mohammed

Drilling Lab. Mud Density

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The four-scale mud balance is used to determine the density or weight

of drilling fluids in two steps. At the beginning primary mud weight is

calculated and then measured by baroid, then fresh water is added to

the previous mud and then measured again and differences will be

obvious.


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Dilution is one of the primary methods used to control solids in a

drilling mud. When water added to the system Base Oil to make

water/oil ratio as per program, the total weight of the solids remains

the same, while the volume is increased. Dilution results in a lower

percent by volume of total solids.

All of the physical properties in a drilling fluid are affected when the

solid content increases when the increase in solids is due to inactive

solids, the viscosity increase (due to an increased resistance to flow).

Also, an increase in inert solids uses up the available water & brings

whatever active solids are already in the system closer together.

This gives an increase in viscosity, due to the attractive forces

between charged particles. Generally this viscosity rise (PV) will be

rise will be greater than any density increase. When the solids

increase is due to active solids, the available water is unused up again.

This brings the solids already present closer together.

That, plus the reaction of the newly introduced solids, will give a

marked rise in the funnel viscosity. Dilution has a dual effect when

used to control both of these situations. It not only will supply the

water needed to wet these solids, but also will provide the necessary

water to keep these particles dispersed. The further apart the active

solids are kept, the lower the marsh funnel viscosity will be.

Plastic viscosity in an indication of the mechanical friction among

particles in a drilling fluid. It is worthwhile to note that the major


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Factor affecting plastic viscosity is the amount of surface area. This is

due to not only the amount of solids present, but also to the size &

shape of these solids.

The smaller the particles, the greater the exposed surface area for a

given percent solids by volume. When a dilatant is added, the

percentage of concentration of solids in a given volume is reduced;

hence, the mechanical friction among particles is decreased.


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Dilution refers to the process of adding a liquid phase to a drilling

fluid to decrease the drilled-solids concentration. Dilution is used in

several ways.

If no solids-control equipment is used or if the equipment is used

ineffectively, dilution may be the principal method of keeping drilled

solids to a reasonably low level. This is an expensive solution to the

problem.

For example, to decrease drilled solids by 50% requires that

50% of the system he discarded and replaced with clean drilling fluid.

Usually dilution is used after processing by solids-removal equipment

to dilute drilled solids remaining in the drilling fluid. Dilution may be

added as a clean drilling fluid or as the liquid phase of a drilling fluid

with the other necessary drilling fluid ingredient, usually through a

chemical barrel and a mud hopper.

In this discussion, dilution will refer specifically

to the clean drilling fluid (i.e. Fresh water) necessary to decrease

drilled-solids concentration. Clean drilling fluid is the liquid phase

with all necessary additives such as barite, polymers, clay. etc.

As an example of dilution, consider a well in which the drilling-fluid

specifications suggest that the volume percentage of (%vol) drilled-

solids concentration should be. And is 6%vol. Assume that 10 bbl of

formation solids are brought to the surface and that no solids-removal

equipment is used. All 10 bbl would be retained in the drilling-fluid

system. These solids would require dilution to maintain the 6%

volume concentration in the new drilling fluid.


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The new drilling-fluid volume would consist of drilled solids (10 bbl)

and clean drilling fluid (dilution). If the drilled solids (lO bbl) are

6% volume of the new drilling fluid built, the volume of new drilling

fluid built would be 167 bbl (or lO bbl/0.06). The volume of clean

drilling fluid (dilution) required may be calculated from the statement

that the new drilling-fluid volume, 167 bbl, would consist of 10 bbl of

drilled solids plus the clean drilling fluid. Obviously the dilution, or

the clean drilling fluid, needed would be 157 bbl. This dilution would

increase the pit volume by 157 bbl.

When nothing is removed from the system, the pits would overflow if

they were originally full. The only volume available to accept dilution

would be the volume (liquid and solids) removed from the pit system.

The volume removed would include all fluid and solids removed by

the solids-removal equipment and any drilling fluid removed from the

system to be stored or discarded.

This is an important concept and is not trivial. Dilution calculations

are based

on simple material balances—addition and subtraction of volumes

added and removed. Before the dilution (clean drilling fluid) is added,

the pit levels would remain the same as they were before the

formation was drilled.

The solids that were drilled occupy the same volume as they did

before they were pulverized by the drill bit. The new hole volume is

exactly the volume of material added to the drilling fluid

system.(Drilling fluid processing Handbook, (Engineers, 2011)


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After preparing a desired mud with the aid of the mathematical

equations which relates each of the solid and continuous phase of the

mud and the final mud data is determined. We are about to add a

specific amount of barite as calculated before to get the final mud

requirement.

During the mud sample preparation it will be necessary to use some

extra tools, which they are:

-Weighting balance

- Syringe

Fresh water

- Mixing tube

-Beaker

-Bentonite

- Mixing device


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Figure 2 Test apparatuses-Lab. (Sarwar Salam 2015)

Every time when before measuring the density of the mud that we

have the pre-main test (i.e. calibration) is done to double check for the

most accurate reading during test in the following points:

Clean and dry the mud balance before doing a re-calibration.

Fit the knife edge of the balance arm into the fulcrum.

Set the rider on 8.3 pounds per gallon or 1.0 specific gravity.

Fill the cup with fresh water at around 70°F (21°C).


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Add or remove lead shot from the shotwell until the instrument is

in balance.

The main test procedure:.

1. Place the base (preferably in the carrying case) on a flat level

surface.

2. Fill the balance cup with the sample to be tested to the limit. Put

the lid onto the balance cup by pushing it downward with a slow

rotating motion until it is firmly seated. Make sure that some of the

test sample is forced out through the vent hole in the lid. This action

will also help to rid the sample of any entrained air of gas.

3. Cover the hole in the lid with a finger and wash all mud from the

outside of the cup and arm. Then thoroughly dry the entire balance.

4. Fit the knife edge of the balance arm into the fulcrum and balance

the assembly by moving the rider along the arm. The Mud Balance is


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horizontal when the level bubble fluctuates an equal distance to either

side of the centre line.

5. Read the sample density from the top of the rider nearest the

balance cup. The measurement reading is equivalent to density of the

sample.

6- Add about 325cm3 of fresh water to the previous mud slowly at

mixing device,

7- Make sure previous mud particles and fresh water are forming a

homogenous mixture.

8- Re-measure the new mud density, which represents the diluted mud

density.

What is the primary objective of diluting drilling fluid?

You sometimes need to reduce your mud weight in hole.

The method to reduce mud weight is dilution. The concept

of dilution is to add light weight fluid into heavy fluid and

when they mix together whole drilling fluid weight will

decrease. The light weight fluids frequently used are fresh

water, sea water, base oil, diesel, synthetic oil, etc. This is

normally your based fluid.

2- What are the drilling fluid additives?


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There are fundamental aspects that have to be controlled

in order to have an effective, successful, and purposeful

drilling fluid. These aspects can be categorized as:

-Weight Control

-Viscosity Control

-Fluid Loss Control

-Corrosion Control

3- What were the main factors which causes errors on your test

readings?

The main factors that left impact on the test readings, use of

normal water instead of fresh distillate water in both mud

preparations , weighting balance sensitivity(i.e. we were about to

prepare 25.06gm of Bentonite but the balance was not so

sensitive so Bentonite weight was 25.0gm) When using the mixer

in preparing the mud, there was a lot of powder from the clay that

was not mixed in the mixing cup and even some that blew away

onto the table (first mud preparation).

The actual amount of clay added may be less than prepared. An

alternative way of adding the exact amount of clay is by having

the clay in tabulate forms or pill forms so that the clay cannot be

blown away, also there might be having loss of some fresh water

droplets during weighting on the balance, adding into the

prepared mud as well.


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4- What is the mathematical equation used to determine the

diluting fluid quantity that should be added into previously

prepared mud?

You can calculate how much light weight fluid in barrel or

any corresponding volume unit is required reducing original

mud weight to new mud weight with this following formula:

Mud weight reduction by diluting with light weight fluid/

V2 = V1 x (W1 – W2) ÷ (W2 – Dw)

Where;

V1 = Intial volume in bbl

V2=Volume of light fluid

W1 = Initial drilling fluid weight in ppg

W2 = Final drilling fluid weight in ppg

Dw = Density of light weight fluid in ppg

5- Solve this example using above equation,

Example_] Determine the number of barrels of base oil

7.0 ppg (Dw) required to reduce 150 bbl (V1) of 12.0

ppg (W1) to 8.68 ppg (W2):

Solution:

Light weight fluid volume in bbl = V1 x (W1 – W2) ÷ (W2 – Dw)

Light weight fluid volume in bbl = 150 x (12 –8.68) ÷ (8.68 – 7.0)

Light weight fluid volume in bbl = 296.43 bblYou need to mix

196.43 bbl of base oil 7.0 ppg into 150 bbl of 12 ppg in order

to achieve final mud weight of 8.68 ppg.


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6- Why do we need to change mud density during drilling

operation?

Ans. As it is known by all, the earth’s crust has been made from

different formations and layers which they are differs in their specific

dependent physical properties, during drilling operation going down

in each formation based on its unique behaviour and formation fluid

content, it’s necessary to change in the drilling fluid properties as

well.

For instance, if we faced a low pressure formation which it’s a

function of pore fluid pressure, or having lost circulation due to high

density of mud, or even if the formation fracture gradient has passed..

It is crucial to decrease mud weight by dilution process.

7- What are the critical dilution factors?

Ans. Critical Dilution Factors Independent from the

previously mentioned list, critical dilution factors are only

those that are part of the math needed to calculate projected

dilution requirements. They are as follows: • Current and

target low gravity solids content • Specific gravity of drilled

solids • Current and target mud weight • Amount and specific

gravity of barite to be used • Amount and specific gravity of

each mud additive scheduled to be used • Rate of penetration •

Hole or bit size • Solids Removal Efficiency • Retained Low

Gravity Solids • Active pit volume • Cycle volume •

Percentage of total dilution from reserve pit or water-recovery

unit • Density and low gravity solids content of reserve pit

water.


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1- Engineers, S. S. (2011). Drilling Fluid Processing

Handbook . Bostun,USA: ASME Shale Shaker

Committee .

2- Drilling Stuff. (2010) Dump, Dilute and Picket Fences

[Online] Avialable from:

http://drillingstuff.com/ExpDumpandDilute.htm.[Accessed:

Dec 12th 2015]

3- VN Drilling Forum. (2008). What is the corrective way to

dilute a mud system while drilling [Online] Available

from:http:// http://vndrilling.com/forum.aspx?g=posts&t=68.

[Accessed: Dec 12th 2015]

4- Drilling Formula. (2009). Reduce mud weight by dilution

[Online] Available from :

http://www.drillingformulas.com/reduce-mud-weight-by-

dilution/. [Accessed: Dec 12th 2015]

5- Elliot, J, Cervantez, R., Estep, J., Sills, B. “New Inhibitive

Water-Based Fluid Provided Drilling Performance Comparable

to Invert Emulsion Systems in Reactive Shale Sections,”

AADE- 05-NTCE-03

http://vndrilling.com/forum.aspx?g=posts&t=68#ixzz3tqQzxasN