1. Drilling Engineering 2 Course (2nd Ed.)

2. 1. General Notes
2. Pore Pressure Prediction
3. Abnormal vs. normal Pressure
4. Fracture Gradient determination

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

5. 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
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 5

6. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 6

7. Benefits of higher mud weight
(+HMW) Washouts of the borehole are
sometimes caused by jet actions of the bit nozzles
but also sometimes by too 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.
(+HMW) 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 7

8. differential stuck
(-HMW) An increased mud weight
increases the danger of becoming differential stuck
at permeable formations.
Therefore mud weight shall not be chosen to be too 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.
(+HMW) 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 8

9. Partial collapse in mixed lithology
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 9

10. lost circulation
(-HMW) 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.
(-HMW) 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 10

11. 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].
(-HMW) 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.
(-HMW) 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 11

12. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 12

13. 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
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 13

14. Effects of
varying the borehole pressure
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 14

15. average horizontal in-situ stress
Mentioned principle is
mathematically found
with following
equation:
𝜎 𝑎 =
𝑃 𝑤𝑓+𝑃𝑜
2
 𝜎 𝑎 [psi] average
horizontal in-situ stress,
Pwf [psi] fracture stress,
Po [psi] pore pressure
Pressure gradients for a well
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 15

16. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 16

17. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 17

20. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 20

21. 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
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 21

22. 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 [ppg] mud weight of the ith fluid column
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 22

23. 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
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 23

24. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 24

25. 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
𝜌𝑖 [ppg] density of component i
fi [1] volume fraction of component i
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 25

26. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 26

27. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 27

28. 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.
Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 28

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

30. 1. drilling hydraulics:
A. types & criteria of fluid flow
B. fluid Rheology and models
a. Bingham plastic & Power-law models