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

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

4. Introduction
The knowledge of the formations to penetrate,
their strength properties as well as their behaviour
when in contact with various drilling fluids is
essential to properly plan and complete a
successful drilling project. Parameters like pore
pressure and formation strength determine aspects
like:
Choice of mud weight profile,
Determination of casing setting depths,
Design of optimal casing strings,
Selection of the drill bit,
Cementing additives and procedures.
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Drilling Engineering 2 Course: Geomechanics
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5. pressures as gradients
The way how the formations react to drilling mud
influences the selection of mud additives, borehole
stability and therefore well control aspects.
Within drilling, it is common to express pressures
as gradients.
With this concept, the hydrostatic pressure can be given
as equivalent density which is independent of the depth
and thus makes its comprehension and correlations of
various concepts easier.
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Drilling Engineering 2 Course: Geomechanics
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6. reference depth
On the other hand, when gradients are applied, it
has to be always kept in mind that they are referred
to a specific depth.
Knowing this reference depth is essential to compute
back the corresponding downhole pressures.
Within drilling engineering, the drilling floor or rotary table
(RKB) is the most often used reference depth.
Geologists and geophysicists generally prefer to use their data
in reference to ground floor or mean sea level (MSL).
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Drilling Engineering 2 Course: Geomechanics
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7. Geology Prediction
Normally when a well is to be drilled, the drilling
engineer is supplied from the geology department (or
the geologist within the project team) with a sequence
of predicted subsurface formations, their characteristics
and markers, as well as knowledge about where special
care has to be taken.
Geologists draw this information from studying
the local geology (deposition history),
seismic mappings (2D or 3D surveys) and
perform well to well correlations (geological maps).
Whenever new information is gained (due to drilling
and evaluation of a new well or further geophysical
measurements) these maps are updated.
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Drilling Engineering 2 Course: Geomechanics
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8. Typical geological profile
to plan a well
Typical geological profile
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Seismic record to determine the subsurface structure
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10. local subsurface pressure regimes
To understand the local subsurface pressure
regimes, the geologic processes along with the
depositional history and tectonic abnormalities
have to be studied.
When the well is located within shallow sediments
that were laid down slowly within a deltaic
depositional environment, the subsurface
formation pressures can be assumed to be
hydrostatic.
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Drilling Engineering 2 Course: Geomechanics
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11. Hydrostatic Pressure
By definition, a hydrostatic pressure is developed
due to the own weight of a fluid at a certain depth.
This relationship is expressed as:
 𝑝 = 𝜌. 𝑔. ℎ = 9.81. 𝜌. ℎ
Or in field units:
𝑝 = 0.052. 𝜌 𝑓𝑙 . 𝐷
where:
•
•
•
•
•
•
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𝜌 𝑓𝑙 [ppg]
𝜌 [kg/m3]
D [ft]
h [m]
p [psi]
g [m/s2]
density of the fluid causing hydrostatic pressure
average fluid density
depth at which hydrostatic pressure occurs (TVD)
vertical height of column of liquid
hydrostatic pressure
acceleration due to gravity
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12. hydrostatically pressured formation
When the weight of the solid particles buried are
supported by grain-to-grain contacts and the
particles buried water has free hydraulic contact to
the surface, the formation is considered as
hydrostatically pressured.
As it can be seen, the formation pressure, when
hydrostatically pressured, depends only on
the density of the formation fluid (usually in the range of
1.00 [g/cm3] to 1.08 [g/cm3]) and
the depth in TVD.
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Drilling Engineering 2 Course: Geomechanics
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13. overburden stress
When the burial depth
increases, the overlaying
pressure (overburden stress)
increases.
This decreases the pore space
between the grains and thus the
porosity of the formation.
The overburden stress can be
calculated assuming an average
bulk density b of the overlaying
formations as:
Porosity profile
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Drilling Engineering 2 Course: Geomechanics
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14. average bulk density
The average bulk density is normally found by
integration of the density log readings.
When density logs were not run (e.g. at shallow
formations),
sonic log correlation methods,
together with lithology and mineralogical evaluations
are applied to determine 𝜌 𝑏
During burial of the sediments, formation water is
constantly expelled due to the reduction of
formation porosity, as see in next slide.
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15. Volume of fluid expelled during
compaction of an argillaceous
sediment
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16. abnormally pressured
As long as formation water can be expelled, the
formations are hydrostatic (or normally) pressured.
When drilling a well, formations are often
encountered that are under a different pressure
regime.
These formations are named to be “abnormally
pressured”.
Abnormal pressures can be positive
• (actual formation pressures are higher than hydrostatic pressure)
 or negative “subnormal pressure”
• (actual formation pressures are lower than hydrostatic pressure).
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Drilling Engineering 2 Course: Geomechanics
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17. Abnormally Mechanisms
Some mechanisms that lead to abnormally pressured
formations are:
 1. Compaction effects,
 2. Aquathermal expansion,
 3. Diagenetic effects,
 4. Differential density effects (Osmosis),
 5. Fluid migration effects,
 6. Evaporite Deposits,
 7. Organic matter transformation,
 8. Tectonics,
 9. Connection to depleted reservoirs,
 10. Others.
From the various effects mentioned above, the compaction
one is considered to be often the governing one.
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Drilling Engineering 2 Course: Geomechanics
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20. normally pressured formations
while burying of the sediments, formation water is
expelled with increasing depth and temperatures
due to reduction in pore space and diagenesis of
the rock materials.
As long as the permeability and the effective
porosity of the rock is high enough so that the
formation water can escape as quickly as the
natural compaction takes place, the formations are
normally pressured.
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Drilling Engineering 2 Course: Geomechanics
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21. Modelling vertical Pressures
The (vertical) pressures acting inside formations
can be modelled as:
𝜎 𝑜𝑏 = 𝜎 𝑧 + 𝑝
where:
𝜎 𝑜𝑏 [psi]
𝜎 𝑧
[psi]
connections
p
[psi]
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overburden stress
vertical stress supported by the grain-to-grain
formation pore pressure
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22. abnormally pressured formations
When the formation water can not escape as quickly as
the pore space is reduced, it is trapped inside the
formations.
In this scenario, the increasing overburden stress will
pressurize the formation water and the formation will
become abnormally pressured.
In this situation, the porosity of the formation will not follow
the natural compaction trend (porosity at abnormally
pressured formations will be higher than at normally
pressured ones).
Along with the higher porosity, the bulk density as well as the
formation resistivity will be lower at abnormally pressured
formations.
These circumstances are often applied to detect and estimate
the abnormal formation pressures.
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Drilling Engineering 2 Course: Geomechanics
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23. formation pore pressures
The actual measurement of formation pore
pressure is very expensive and possible only after
the formations have been drilled.
In this respect, pore pressures have to be estimated
before drilling to properly plan
the mud weights,
casing setting depths,
casing design, etc.
as well as being closely monitored during drilling.
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Drilling Engineering 2 Course: Geomechanics
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24. pore pressure estimation
To estimate the pore pressure and most important,
define where abnormal pore pressures are to be
expected,
porosity logs and
seismic measurements are applied most often.
shale formations tend to follow a defined porosity
reduction trend with increasing depth.
When this trend is interrupted, abnormally pressured
formations are to be expected.
The knowledge of its depths are important since they may
lead to a necessary setting of casing and weighting up the
mud system.
The amount of how much the mud weight has to be increased
depends on the amount of abnormal pressure expected and the
contingency of the well.
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Drilling Engineering 2 Course: Geomechanics
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25. Abnormal pressure detection
while drilling
When the well is in progress
and abnormal formation
pressures are expected,
various parameters are
observed and cross-plotted.
Some of these while drilling
detection methods are:
 (a) Penetration rate,
 (b) “d” exponent,
 (c) Sigmalog,
 (d) Various drilling rate
normalisations,
 (e) Torque measurements,
 (f) Overpull and drag,
 (g) Hole fill,
 (h) Pit level – differential flow
– pump pressure,
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 (i) Measurements while
drilling,
 (j) Mud gas,
 (k) Mud density,
 (l) Mud temperature,
 (m) Mud resistivity,
 (n) Lithology,
 (o) Shale density,
 (p) Shale factor (CEC),
 (q) Shape, size and
abundance of cuttings,
 (r) Cuttings gas,
 (s) X-ray diffraction,
 (t) Oil show analyzer,
 (u) Nuclear magnetic
resonance.
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26. Abnormal pressure evaluation
After an abnormal pressure is detected or the well is
completed, various wireline log measurements are used
to evaluate the amount of overpressures present.
Among the most common ones are:
(a) Resistivity, conductivity log,
(b) Sonic log,
(c) Density log,
(d) Neutron porosity log,
(e) Gamma ray, spectrometer,
(f) Velocity survey or checkshot,
(g) Vertical seismic profile.
With these log measurements trend lines are
established and the amount the values deviate at the
abnormally pressured formations from the trend line
are applied to determine the value of overpressure.
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27. Schematic responses of wireline logs
in an undercompacted zone
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29. Leak-off data
Normally, after a casing is set and cemented, a so
called leak-off test (LOT) is performed.
The main issue of a LOT is to check the strength of the
formation at the casing shoe.
With this knowledge, the maximum kick pressure
allowed that does not fracture the formation is
determined.
It is also the key parameter in stress modelling and
borehole integrity evaluation.
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Drilling Engineering 2 Course: Geomechanics
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30. formation integrity test
Sometimes the LOT test is not continued until leakoff (especially when oil based muds are used) and
the formation is only pressured up until a certain
value.
This test is called formation integrity test (FIT).
In this way, when fracture strength is evaluated, it
is important to distinguish LOT data and FIT data.
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Drilling Engineering 2 Course: Geomechanics
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31. fracture gradient
The pressure where fractures are initiated is
commonly called leak-off pressure and when
referred to the individual depth, named fracture
gradient.
The determination of fracture gradients for shallow
depth is often difficult since very little data exists.
This is due to the circumstance that at shallow depth,
blowout preventers are often not installed and thus no
pressure testing can be carried out.
Especially at offshore wells, the knowledge of shallow
fracture gradients are important since the margin
between pore pressure and fracture gradient is narrow
and the danger of shallow gas pockets exists.
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32. 1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling
Engineering.” Master of Petroleum
Engineering. Curtin University of Technology,
2001. Chapter 3