drilling hydraulics 1

8/12/2019 Drilling Hydraulics 1

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1. General Notes

2. Pore Pressure Prediction

3. Abnormal vs. normal Pressure

4. Fracture Gradient determination



1. Mud Weight

Planning

2. drilling hydrau

lics:

A.

the

hydrostatic

pressure



mud weight selection

Selecting the correct mud weightfor drilling the individual sections comprisesa key factor to realize ain-gauge hole and avoid various borehole problems.

Too low mud weight may result incollapse andfill problems (well cleaning),

while too high mud weight may result in

mud losses orpipe sticking

Fall

13

H. AlamiNia Drilling Engineering

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Course: 5



constant mud weight program

Practice has also shown thatexcessive variations in mud weight may lead toborehole failure (fatigue type effect), thus

a more constant mud weight program should be aimed.

Along with a more constant mud weight program,

the equivalent circulation density (ECD)as well asthe surge and swab pressuresshall be kept within limits.

Fall

13

H. AlamiNia Drilling Engineering

2

Course: 6



Benefits of higher mud weight

Washouts of the borehole aresometimes caused by jet actions of the bit nozzlesbut also sometimes by to low mud weightcausing a breakdown of the borehole wall.

A higher mud weight will thereforebalance the rock stresses better andtend to keep the borehole more in-gauge.

A decease in hole diameter is often due to swelling(clay swelling) requiring wiper trips or backreaming.This necessity is sometimes reduced by

higher mud weights.

Fall 13 H. AlamiNia Drilling Engineering 2 Course: 7



differential stuck

An increased mud weightincreases the danger of becoming differential stuckat permeable formations.Therefore mud weight shall not be chosen to be to high.

However, what is sometimes believed to bea differentially stuck drillstring is sometimesdue to a borehole collapse whichpacks the hole around the bottom-hole assembly.

A lower mud weight also causes

breakouts of shale layersleaving sand formations in-gauge, (next fig).This can increase the danger of

getting differential stuck at the exposed sand stringers.




Partial collapse in mixed lithology




lost circulation

Thus when consideringthe danger of differential sticking,

it is recommended to keep the mud weightbelow a certain value but

it shall not be as low as possible.

The same is true for lost circulation problems.

As long as the mud weight is kept below a critical value,lost circulation will not occur.




Mud weight vs. penetration rate

It is often argued that to havea as high as possible rate of penetration,the mud weight shall be kept as close as possible

to the formation pressure gradientplus a safety margin of around 100 [psi].

Although it is true that a small reduction in mud weightincreases the penetration rate,but this increases has to be weighted against the possible

induction of hole problems and additional lost time.

A higher mud weight requires the use of more mudadditives which makes the well more expensive,but it was found that

these extra costs are usually neglectable.




drilling for a kick

When drilling within areas wherethe subsurface pressure regimes are not well known,it is often argued that a lower mud weight

easies the detection of abnormal pressures.

In some locations,a practice called “drilling for a kick” was applied todetect overpressured formations.For this, a relatively low mud weight was applied

until a kick was detected(pressure gradient at this depth was equal to the used mudweight)

and handling the kick, the mud weight was increased.Therefore and since

a higher mud weight also suppresses high gas readings,the mud weight of exploration wells are often

designed to be lighter than the ones for development wells.




median line concept

Based on all these considerations,the “median line concept” is recommendedgenerally for mud weight planning.

Thereto, the mid-point betweenthe fracture pressure andthe pore pressuredefines the borehole pressure that isequal to the ideal in-situ stress.

Maintaining the mud pressure close to this levelcauses least disturbances on the borehole wall.

This principle is sketched in next two slides




Effects ofvarying the borehole pressure




average horizontal in-situ stress

Mentioned principle ismathematically foundwith followingequation:

= +

2 [psi] average

horizontal in-situ stress,

Pwf [psi] fracture stress,

Po [psi] pore pressure

Pressure gradients for a wellFall 13 H. AlamiNia Drilling Engineering 2 Course: 15



Changing mud weight

Experience had shown that

new drilling fluid exacerbates fracturing/lost circulationand

leakoff tests applying used drilling muds give higher

leak-off values than when carried out with new ones.

Therefore it is a good practice that,

when the mud weight has to be changedafter setting casing,

drilling is usually started with a lower mud weight.

After drilling about 100 [m] below the casing shoe,

the mud weight is then gradually increasedto the desired value.




Changing mud weight Cont.)

Furthermore it should be noticed thatwithin an open-hole section,

the mud weight shall only be increased and

not decreased since tight hole may result.

An increase of mud weight in steps of 0.05 [g/cm3]is good practice and in convenience of the mudengineer.




aspects of optimum hydraulics

To realize a safe, efficient andcost-effective drilling project,drilling hydraulics,

also known as rig hydraulics, play an important role.

The different aspects that make up optimum righydraulics are:1. Hydraulic energy impact on the bit,

2. Friction pressure losses through the surfaceequipment, drillstring, annuli and drill bit,

3. Efficient hole cleaning,

4. Nozzle selection and,

5. Produced pump pressure.




improper drilling hydraulicsconsequencesSome of the drilling problems that are

due to improperly designed drilling hydraulics arefailure of sufficient hole cleaning

leaving cuttings in the hole and

lead to stuck pipe,lost circulation causing kicks and slow penetration rates.

To understandthe various dependencies of efficient drilling hydraulics,following concepts should be known

the hydrostatic pressure inside the wellbore,types of fluid flow,

criteria of fluid flow and

commonly used fluid types for different drilling operations




Hydrostatic PressureInside the Wellbore

Hydrostatic definition:

relating to or denoting the equilibrium of liquids andthe pressure exerted by liquid at rest.

For oil well applications, the fluid may bemud, foam, mist, air or natural gas.

For a complex fluid column consisting of multiplefluids, the hydrostatic pressure is given in field units

by:

[ppg] mud weight of the ith fluid column




barometric formula

When gas is present in the well, the hydrostaticpressure developed by the gas column is calculatedwith:

z [1] ... real gas deviation factor

po [psi] ... surface pressure

D [ft] ... total depth (TVD)

Tf [F] ... bottom hole temperature of the formation

The molecular weight M of the gas is found as:

• where:

o [ppg] ... density of the gas

o T [F] ... average gas density




equivalent mud weight

For practical purposes, the hydrostatics due to acomplex fluid column are converted to anequivalent single-fluid hydrostatic pressure.

To do this,all individual hydrostatic pressures are summed up for aspecific depth pd andthen converted to an equivalent mud weight [ppg]that would cause the same hydrostatic pressure.

Therefore the equivalent mud weighthas to be always referenced to a specific depth.




average mud weight

As the mud is used to transport the cuttings fromthe bottom of the hole to the surface andpenetrated formations often contain a certainamount of formation gas, the mud column at the

annulus is usually mixed with solids and gas.This alters the weight of the mud at the annulus.

The new average mud weight m of a mixture containingmud and solids can be calculated as:

mi [lbm] mass of component i

Vi [gal] volume of component i

[ppg] density of component i

fi [1] volume fraction of component i




Notes about mud weight

only solids that are suspended within the muddo alter the mud weight.

Settled particles do not affect the hydrostatic pressure.

If gas is present in the mud column as well,the density of the gas component isa function of the depth andwill decrease with decreasing pressure.

In this way, the density of mud containing gas isdecreasing with decreasing depth.




over-balanced drilling techniquesOBD)well control and the safety of drilling operations are

strongly depended onthe maintenance of proper hydrostatic pressure.This pressure is needed to counterbalance the formation pressure.

In case the hydrostatic pressure in the borehole is

higher than the formation pressure,the situation is called “over-balanced”.This prevents kicks (fluid flow from the formation into the borehole)

and causes at permeable formations an intrusion of some mud(water component) into the formation. The intrusion is stopped by the built up of mud cake

that seals off permeable formations.

On the other hand, the hydrostatic pressure inside the boreholemust not be higher thanthe fracture pressure of the formations penetrated since this would fracture the formation artificially,

cause loss of circulation and lead to well control problems.




underbalanced drilling techniquesUBD)To obtain maximum penetration rates

the hydrostatic pressure should be keptas close as practical to the formation pressuresince a higher differential pressure

(hydrostatic pressure - formation pressure)

leads to worst cutting removal from the bottom of the well.Due to this circumstance,

UBD techniques have been developed that useair, foam or mist as drilling fluids.Here the formation pressure is higher than the hydrostatic

pressure caused by the mud and thus the well is constantly

kicking.With UBD techniques much higher penetration rates are

possible but well control can be a problem.Therefore UBD is prohibited

by some governments and/or in some areas.




1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling

Engineering.”

Master of PetroleumEngineering. Curtin University of Technology,

2001. Chapter 3 and 4

drilling hydraulics 1

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