Q921 de2 lec3 v1

Drilling Engineering 2 Course (1st Ed.)

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General Notes
Pore Pressure Prediction
Abnormal vs. normal Pressure
Fracture Gradient determination

1. Mud Weight Planning
2. drilling hydraulics:
A. the hydrostatic pressure

mud weight selection
Selecting the correct mud weight
for drilling the individual sections comprises
a key factor to realize a
in-gauge hole and avoid various borehole problems.
Too low mud weight may result in
collapse and
fill problems (well cleaning),
while too high mud weight may result in
mud losses or
pipe sticking

Fall 13 H. AlamiNia

Drilling Engineering 2 Course:

5

constant mud weight program
Practice has also shown that
excessive variations in mud weight may lead to
borehole 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 as
the surge and swab pressures
shall be kept within limits.

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Benefits of higher mud weight
Washouts of the borehole are
sometimes caused by jet actions of the bit nozzles
but also sometimes by to low mud weight
causing a breakdown of the borehole wall.
A higher mud weight will therefore
balance the rock stresses better and
tend 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.
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differential stuck
An increased mud weight
increases the danger of becoming differential stuck
at permeable formations.
Therefore mud weight shall not be chosen to be to high.
However, what is sometimes believed to be
a differentially stuck drillstring is sometimes
due to a borehole collapse which
packs the hole around the bottom-hole assembly.

A lower mud weight also causes
breakouts of shale layers
leaving sand formations in-gauge, (next fig).
This can increase the danger of
getting differential stuck at the exposed sand stringers.
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Partial collapse in mixed lithology

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lost circulation
Thus when considering
the danger of differential sticking,
it is recommended to keep the mud weight
below 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.

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Mud weight vs. penetration rate
It is often argued that to have
a as high as possible rate of penetration,
the mud weight shall be kept as close as possible
to the formation pressure gradient
plus a safety margin of around 100 [psi].

Although it is true that a small reduction in mud weight
increases 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 mud
additives which makes the well more expensive,
but it was found that
these extra costs are usually neglectable.
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drilling for a kick
When drilling within areas where
the 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 to
detect 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 mud
weight)
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.

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median line concept
Based on all these considerations,
the “median line concept” is recommended
generally for mud weight planning.
Thereto, the mid-point between
the fracture pressure and
the pore pressure
defines the borehole pressure that is
equal to the ideal in-situ stress.

Maintaining the mud pressure close to this level
causes least disturbances on the borehole wall.
This principle is sketched in next two slides
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Effects of
varying the borehole pressure

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average horizontal in-situ stress
Mentioned principle is
mathematically found
with following
equation:
𝜎 𝑎 =

𝑃 𝑤𝑓 +𝑃 𝑜
2

 𝜎 𝑎 [psi] average
horizontal in-situ stress,
Pwf [psi] fracture stress,
Po [psi] pore pressure

Pressure gradients for a well
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Changing mud weight
Experience had shown that
new drilling fluid exacerbates fracturing/lost circulation
and
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 changed
after 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 increased
to the desired value.
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Changing mud weight (Cont.)
Furthermore it should be noticed that
within 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 mud
engineer.

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aspects of optimum hydraulics
To realize a safe, efficient and
cost-effective drilling project,
drilling hydraulics,
also known as rig hydraulics, play an important role.

The different aspects that make up optimum rig
hydraulics are:
1. Hydraulic energy impact on the bit,
2. Friction pressure losses through the surface
equipment, drillstring, annuli and drill bit,
3. Efficient hole cleaning,
4. Nozzle selection and,
5. Produced pump pressure.
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improper drilling hydraulics
consequences
Some of the drilling problems that are
due to improperly designed drilling hydraulics are
failure of sufficient hole cleaning
leaving cuttings in the hole and
lead to stuck pipe,

lost circulation causing kicks and slow penetration rates.

To understand
the 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
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Hydrostatic Pressure
Inside the Wellbore
Hydrostatic definition:
relating to or denoting the equilibrium of liquids and
the pressure exerted by liquid at rest.

For oil well applications, the fluid may be
mud, foam, mist, air or natural gas.
For a complex fluid column consisting of multiple
fluids, the hydrostatic pressure is given in field units
by:
𝜌

𝑚𝑖

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[ppg] mud weight of the ith fluid column

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barometric formula
When gas is present in the well, the hydrostatic
pressure developed by the gas column is calculated
with:
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 𝜌 𝑔 [ppg] ... density of the gas
o T [F] ... average gas density
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equivalent mud weight
For practical purposes, the hydrostatics due to a
complex fluid column are converted to an
equivalent single-fluid hydrostatic pressure.
To do this,
all individual hydrostatic pressures are summed up for a
specific depth pd and
then converted to an equivalent mud weight 𝜌 𝑒 [ppg]
that would cause the same hydrostatic pressure.

Therefore the equivalent mud weight
has to be always referenced to a specific depth.
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average mud weight
As the mud is used to transport the cuttings from
the bottom of the hole to the surface and
penetrated formations often contain a certain
amount 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 containing
mud and solids can be calculated as:
mi [lbm] mass of component i
Vi [gal] volume of component i
𝜌 𝑖 [ppg] density of component i
fi [1] volume fraction of component i
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Notes about mud weight
only solids that are suspended within the mud
do 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 is
a function of the depth and
will decrease with decreasing pressure.
In this way, the density of mud containing gas is
decreasing with decreasing depth.

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over-balanced drilling techniques
(OBD)
well control and the safety of drilling operations are
strongly depended on
the 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 borehole
must not be higher than
the fracture pressure of the formations penetrated
 since this would fracture the formation artificially,
 cause loss of circulation and lead to well control problems.

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underbalanced drilling techniques
(UBD)
To obtain maximum penetration rates
the hydrostatic pressure should be kept
as close as practical to the formation pressure

since 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 use
air, 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.

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1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling
Engineering.” Master of Petroleum
Engineering. Curtin University of Technology,
2001. Chapter 3 and 4

Q921 de2 lec3 v1

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Q921 de2 lec3 v1