A Critical Review of Cerrently Pore Pressure Methods
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A
critical
review of
currently
available pore
pressure
methods
and their input
parameters.
Glaciations and compaction of North Sea
sediments.
A
copyright of this
thesis
rests
with the author. No quotation
from it should be published
without his prior written
consent
and information derived from it
should be acknowledged.
By
Carl Fredrik Gyllenhammar
0
NOV
2003
This
thesis
was submitted as the
fu l f i lmen t
of the requirements for the
degree
o f
Doctor
of Philosophy
Car l
l-redrik
Gyl lenhammar
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Abstract
Historically
pore
pressure
evaluation in exploration
areas
was based on
empirical
relationships between d r i l l i n g parameters,
wireline
logs and the mud weight.
Examples include Eaton's Ratio and the Hottman & Johnson Methods,
which
were
based on data
from
the G u l f of
Mexico.
These methods are not readily transported to
other
areas,
such as the North Sea Basin, where the sediments are different in
character and where
burial
and temperature histories are
distinctly different.
Data f r o m
several offshore
North
Sea
wells,
w i t h
high quality wireline
and associated
data have been analysed to determine the most appropriate method to estimate pore
pressure
in mudrocks. The data have led to an understanding of the key parameters
for
successful pore
pressure
estimation. The most
effective
method is shown to be the
Equivalent
Depth
Method,
but
only
where
disequilibrium
compaction is the source of
the overpressure in the mudrocks.
Core samples
f r o m
576
Br i t i s h
Geological Survey sites in the offshore area o f the
B r i t i s h
Islands were compared w i t h
>
10,000 porosities collected
f r o m
the
deep oceans
(DSDP/ODP sites), which show that the porosities in the shallow section in the North
Sea are anomalously low. The shallow section of the
North
Sea includes large
volumes of Pleistocene-Recent sediments deposited as glacial and inter-glacial
deposits. Frequency analysis
(Cyclolog)
of the
wireline
data covering this
interval
in
several
North
Sea wells revealed a pattern in the relative featureless
original
data.
Comparison w i t h
the
global
signature fo r oxygen isotopes for the
same
time period
suggests
that there have been ten cycles of ice
sheet b u i l d
up
(Glacial
period)
followed
by
melting
(Interglacial
period)
during
the last one
m i l l i o n
years.
Glacial
deposits
from
10
individual
glacial cycles have therefore been
identified
in several
exploration wells in the North Sea. Implications of
loading/unloading
o f ice for the
migration
and trapping o f hydrocarbons in the
North
Sea Basin are
assessed.
Carl
Fredrik Gvi l en h a m m a r
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Acknowledgements
First I must thank my supervisor, Richard Swarbrick ( D i c k ) . I met Dick f i rs t time in
December 1995 in London. It was the
f i r s t
GeoPOP meeting I attended representing
Norske Conoco. When a year later I expressed interest in doing a PhD at the
university of Durham, his enthusiasm, despite my age, made what was
laying ahead
possible. My w i f e M a r i t and I l e f t our jobs late 1998, we sold our house in Stavanger
and moved to Durham w i t h our two children,
Elen-Martine
and Fredrik.
A t the university I found Dic k' s continues support and interest i n my subject as w e l l
as the requirement to deliver regular reports to GeoPOP an
assurance
for continued
progress. N e i l
Goulty
was never fa r away to discuss any d i f f i c u l t equation. Both his
and Dick' s enthusiasm for my "ice theory" made this thesis what it is. I had also help
f r o m
Fred Wollard's
long
experience w i t h using principle component analysis. The
last year in Durham I was lucky to share an office
w i t h
M a r t i n Traugott. He shared his
long
experience in pore
pressure
predic tion as w e l l as his own developed software
PresGraf w i t h me.
M y thanks also goes to Norske Conoco, in particularly James Middleton. They
supplied
the w e l l data I used as w e l l as providing financial support fo r the project.
Thanks go to all
those f r o m
GeoPOP who were there to help and exchange ideas,
Toby, Paul, Daniel, Neville, Gareth Yardley, Andy A p l in and Yunlai Yang.
Finally many thanks to my new colleges at BP, for their support during the last year,
Nigel Last, Mark Alberty and M i k e McLean.
Call Freclrik Gyl ienhamrnur
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Table of Contents
Abstract ii
Acknowledgements ii i
Table of Contents iv
List of Tables vi
Lis t of Figures
v
i i
Declaration x i
Chapter 1 Introduction 1
1.1 Background 2
1.2 Data 3
1 3 Introduction 7
1.4
Pressure,
the basic concepts 11
1.5 Aims and layout of thesis 14
Chapter 2 Pore Pressure Evaluation Concepts and definitions
16
2.1 Def in i t i on 17
2.1.1 Mudrock porosity 18
2.1.2 Different porosity evaluation equations 18
2.1.3
Normal compaction curve and trend lines 23
2.1.3.1
Athy-type re lationship
24
2.1.3.2 Soil mechanics relationship 25
2.1.3.3 A t hy
- Soil mechanics: how are they
different
25
2.1.4
Vertical versus
mean
effective stress
28
2.2
Pore Pressure
Calculation Methods 29
2.2.1 Vertical
Methods 30
2.2.1.1 Equivalent depth method 30
2.2.1.2
Harrold
method 32
2.2.1.3 Exp l i c i t
method using the
resistivity
log 33
2.2.2
Horizontal
methods 35
2.2.2.1 Eaton Method 35
2.2.2.2 The pore
pressure
calculation program; PresGraf 37
2.2.2.2.1
PresGraf
normal
compaction trend 37
2.2.3 Seismic 40
2.2.4 ShaleQuant 41
2.2.5 Principle Component Analysis 41
2.3
Dr i l l ing parameters
46
2.3.1 Real time
data
48
2.3.2 D'exponent 48
2.3.3 Torque 54
2.3.4 Hydraulics 55
2.3.5 Bit type and wear 55
2.3.6 Lagged data 56
2.3.6.1 Gas 56
2.3.6.2 Cuttings and Cavings 58
2.3.6.3 Mud temperature in and out 58
2.3.6.4 Mud
resistivity
in and out 58
2.3.7 Mud chemistry and mud-formation chemical reactions 59
Chapter 3 Comparison of
different
pore
pressure
methods using a North Sea wel l 60
3.1 Introduction 61
3.2
Pore pressure
evaluation of
w e l l
1/6-7 in the
North
Sea, Norwegian sector 61
3.2.1 Pore
pressure
evaluation while dr i l l ing
(wellsite)
62
3.2.2 The Post-well analysis • 63
3.2.2.1 Tertiary 67
3.2.2.2 Chalk 67
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Fredrik GvHenhainmar iv
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3.2.2.3
Jurassic
67
3.3 Normal Compaction in the North Sea 69
3.3.1 Palaeocene and Lower Eocene 75
3.3.2 Normal compaction f r o m resistivity data 76
3.4 Wireline log
pore
pressure calculation 78
3.5 Comparing the North Sea
w i th
the
G u l f
of Mexico Basin 86
3.6
Vimto#land#2
88
3.7
Summary
and conclusions 93
Chapter
4 The impact of the Glaciation on the Normal compaction in the North Sea 96
4.1
Introduction 97
4.2 Glacial history 99
4.2.1
The
Neogene
-
Pleistogene sedimentary succession
102
4.3
Ti l l s
103
4.4 Mudrock
porosities 104
4.5 Oxygen
isotope
data
106
4.6 Time series frequency
analysis
(CycloLog) 108
4.7 Ice loading and
pore pressure 112
4.8 Subglacial water f l o w
.....118
4.8.1 Resistivity log response 121
4.9 Hydrocarbon migration 122
4.10
Erosion of the Scandinavia during
Quaternary
123
4.11
Conclusions 124
References: 126
Appendix 1
136
Appendix 2
145
Appendix 3
155
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Fiednk
Gvll enliamma '
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L i s t of Tables
Table 2-1 Eigenanalysis of the correlation matr ix 42
Table 2-2 A
l ist
of
measurements
that can be
used
to interpret the
shale
pore
pressure.
47
Table 2-3 Eigenanalysis of the correlation matr ix 52
Table 4-1 BGS sits and exploration wells 105
Table 4-2 The f l o w rates are calculated assuming hydrostatic
pressure
in the aquifer
underlying
the aquitard 120
Curl l-reclrik
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L i s t of Figures
Figure 1.1 The wireline log plot of w e l l 1/6-7
f r o m
seabed to 2000m 4
Figure 1.2 The wireline log plot of
w e l l
1/6-7
f r o m
1900 to 4000m 5
Figure
1 3
The wireline log plot o f
w e l l
1/6-7
f r o m
3000
m
to
4995
m
(TD) 6
Figure
1.4
Schematic of
wireline
logging. The
lithological
column to the right is a
schematic o f a pressure probe (RFT) being
used
to measure the pore pressure in
permeable sandstone
9
Figure 1.5
Pressure
plotted
against
depth in a
fictional w e l l .
The effective
stress
is
equal to the overburden pressure minus the pore pressure and the
overpressure
is
equal to the pore pressure minus the hydrostatic pressure 12
Figure 1.6 The Figure to the right shows how a pressure
versus
depth plot
( l e f t ,
Figure
1.5)
becomes
presented
as
pressure
gradient
versus
depth 14
Figure 2.1
a,b,c,d. la
porosity variation as a function of sonic velocity. B, porosity
versus
bulk density. C, the sensitivity to pore water density. D, the sensitivity to
matrix density 20
Figure 2.2 Log derived porosities in
w e l l Nor-1/6-7
Norway. The low porosity
interval
f r o m
3261m to 4346m depth is the
Cretaceous
Chalk. The
values
are
listed in Appendix 1 23
Figure 2.3 Comparison o f porosity w i t h effective stress for the
A t h y
and the SM
equations. I n i t i a l porosity (sea floor porosity) for
A t h y
is 0.7 (70 %) whi le the
porosity at 100 kPa (approximately 100
meters
below sea
f l o o r )
is 0.66 (66 %)
The compaction factors a re for
A t h y ; 0.00008
and SM; 0.74 26
Figure 2.4 The relation ship between porosity, solidity and
v o i d
ratio is shown. The y-
axis is the compaction as a length reduction. It is
assumed
that a confined
volume is
compressed
beginning
w i t h
a
v o i d
ratio of four 27
Figure 2.5. Porosity
f r o m
a
pseudo w e l l
is plotted
versus
depth. Integrating the density
log and subtracting the hydrostatic pressure calculate the effective
stress.
The
two
normal compaction curves are coming
f r o m
Figure 2.3 30
Figure 2.6 Porosity
f r o m
a
pseudo
w e l l is plotted
versus
mean effective stress. The
two
normal compaction
curves
are coming
f r o m
Figure 2.3. See text for an
explanation for the equivalent effective stress method 32
Figure 2.7 Comparing the porosity derived
f r o m
the sonic, density and neutron log
w i t h the resistivity derived using the equation 1.37 proposed by
A l ix a n t
and
Desbrandes(1991)
34
Figure 2.8 The PresGraf normal compaction trend to the
l e f t
compared w i t h the
A t h y
normal
compaction curve (Figure 2.3) to the right 39
Figure 2.9 Scree plot of the eigenvalue of
each
principal component of the PCA of the
wireline
logs 43
Figure
2.10
a, b, c and d. The plots to the
l e f t
(a and c) are cross
sections
through
data
perpendicular to
PCI,
PC2 and PC3. b and d show the loading values w i t h
respect
to the different principal components 44
Figure
2.11
a and b.
PCI versus
depth in blue and the sonic travel time in green 46
Figure 2.12 The plot
shows
the
d'exponent
and the corrected
d'exponent versus
depth.
The straight lines are trend lines representing one particular pressure gradient
(one mud weight) 49
Figure 2.13 Cross plot of the d'exponent
versus
the sonic travel time, w e l l N 1/6-7. It
suggests
a good relationship between the sonic log and the
d'exponent
in the
Tertiary
section (pink
squares).
The correlation is
less
obvious in the
Jurassic
(yellow
triangles). There is no correlation in the Chalk 50
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Frecii
ik
Gvl l en h a m m a r
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Figure
2.14 Cross
plot of the
d'exponent versus
the resistivity log. The plot show no
correlation 51
Figure
2.15
Scree plot of the eigenvalue of
each
principal component of the PCA of
the
dri l l ing
parameters
53
Figure 2.16 a, b, c, d. The plots to the left (a and c) are cross
sections
through
data
perpendicular to
PCI,
PC2 and PC3. b and d show the loading
values
with
respect
to the
different
principal
components
53
Figure
2.17
a, b and c.
PCI
versus
depth
wi th
the standardized log porosity overlaid in
Figure b and the normalized sonic travel time in Figure c 54
Figure 3.1 The corrected d'exponent plotted
versus
depth
wi th
a normal trend line
overlaid
in green.
Normally
new trend lines
w i l l
be
added
paralleling the green
line
each time a new bit is put on the drill string. The new line w i l l represent the
actual
M W .
An alternative trend line is
suggested
in red. That trend line
w i l l also
result in a
reasonable
calculated pore
pressure
at the target of interest (i.e. 4200
to 4800
m)
63
Figure 3.2 The sonic velocities in the
shale sections
plotted
against
depth on a
semilogaritmic graph. The yellow line is the best visual f i t trend line 65
Figure 3.3 A comparison of the calculated pore
pressure
f r o m the
d'exponent
(blue
dots) and the sonic velocity calculated pore
pressure
66
Figure 3.4
Shale
velocities f r o m the wells in Figure 3.2 compared
wi th
the wells
used
by
Hansen (1996). The Figure to the left has a logarithmic X-axis. At such a plot
the A t h y type normal compaction trend
become
a straight
line.
On the plot to the
right
it is much more obvious that the
wel l used
in this study are different f r o m
the one
used
by Hansen
(1996)
70
Figure 3.5 Porosity
data
f r o m
10000
DSDP/ODP mudrock
samples.
ODP site #336 is
in
the
deep
water Norwegian Sea. The yellow curve is the
suggested
normal
compaction trend drawn through the
data
set 71
Figure 3.6 Compaction
trends
as plotted as vo id ratio
versus
effective stress.
A l l
the
different trends
that
have been tested
in this
chapter
are overlaid. The PresGraf
(in
blue) is an approximation as the real curve is proprietary
data
to BP 72
Figure 3.7.
Different
wireline methods to calculate the volume of clay are compared
in wel l
N 1/6-7. The red dot are V c l f o rm using the GR log, the blue
dots
f r o m
the neural network (ShaleQuant and the green
dots
the neutron and density log.
74
Figure 3.8 Twenty-six wells in 5 offshore
areas
and 4
onshore
fields (MacGregor,
1965). The pink line is a
suggested
normal trend for the
G u l f
of Mexico 77
Figure 3.9
Pore pressure
in
mega-Pascal versus
depth in
meters.
The green solid line
is the overburden and the blue solid line is the hydrostatic pressure. The
equivalent depth method calculated pressure is in blue
dots
while the
orange
is
the University of Durham method. The
dashed
black curve is the
operator's
interpretation while the olive solid line is the mud weight. The red crosses are the
R FT
direct pore pressure
measurements
79
Figure 3.10
Pore pressure
in
mega-Pascal versus
depth in meters. The green solid line
is the overburden and the blue solid line is the hydrostatic
pressure.
The Eaton
equation
wi th
the sonic log as input (red dots) compared
wi th
the Equivalent
depth method
wi th
the porosity as input (blue dots). The red
crosses
are the RFT
direct pore
pressure measurements.
The
values
are listed in Appendix 2 82
Figure
3.11 Pore pressure
in
mega-Pascal versus
depth in
meters.
The green solid line
is the overburden and the blue solid line is the hydrostatic pressure. The Eaton
equation in red
dots
compared
wi th
the Equivalent depth method in solid blue,
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both w i t h the sonic log as input. The red
crosses
are the RFT direct pore
pressure
measurements.
The values are listed in Appendix 2 83
Figure
3.12 Pore
pressure
in mega-Pascal versus depth in meters. The green solid line
is the overburden and the blue solid line is the hydrostatic
pressure.
The
Equivalent
depth method tested w i t h two
different
normal trends. The
A t h y
equation used by the operator of
w e l l
Nor-1/6-7
(blue dots)
versus
the DSDP-
ODP
based trend (red dots).The red crosses are the RFT direct pore
pressure
measurements. The values are listed in Appendix 2 83
Figure 3.13 The
shale
porosity (red solid curve to the right) and the
shale
travel time
(green
solid line
to the
right) versus
depth in the
Jurassic
section. The x axis is in
for porosity,
u,sec/ft
for the sonic log. The curves to the l e f t of the overburden
(strait solid green line) is in MPa. Between the overburden and the hydrostatic
pressure ( l e f t
most solid blue) are
f r o m l e f t
the pore
pressure
calculated using the
sonic log as input (blue curve) then w i t h porosity as input (orange curve). The
values are lis ted in Appendix 2 84
Figure 3.14 Figure 3.13, the
pressure
transition
zone
f r o m
4850- 4890 meters. The
values are listed in Appendix 2 84
Figure
3.15 Pore pressure in mega-Pascal versus depth in meters. The green solid line
is the overburden and the blue solid line is the hydrostatic pressure. The Eaton
equation w i t h the resistivity log as input (green dots) compared w i t h the Eaton
sonic (red dots). The red
crosses
are the RFT direct pore
pressure measurements.
The values are listed in Appendix 2 85
Figure 3.16 Depiction of salt features in the area around the basin. The
local
depocenteres
are termed mini-basins (Yardley and Couples, 2000) 87
Figure
3.17
Comparison of the NRG derived
shale
porosity
versus
permeability
curves,
w i t h
some
basin modell ing default curves. A
range
o f
clay-fractions
are
shown,
f r o m
20% to 80% (Yang and A p l i n , 2000) 89
Figure
3.18
Comparison o f the GeoPOP derived
shale
compaction curves w i t h
some
basin modell ing default curves. A
range
o f clay fractions are shown,
f r o m
20%
to 80% (Yang and A p l i n , 1999) 90
Figure 3.19 Pore
pressure
prediction for
V i m t o
#2. The blue curve is the pore
pressure
calculated using the Eaton method and the
shale
sonic
velocity
as input.
The red
line
is pore
pressure
using the equivalent depth method. The
pink
diamonds are the M D T
pressure
points. The red line to the l e f t is the overburden.
The values are listed in Appendix 3 91
Figure 3.20 The reservoir section for
w e l l
Vimto#2.
The MDT
pressures
are generally
50 to
100
psi higher than the calculated
shale pressures 91
Figure 3.21
The red curve is the pore
pressure
calculated f r o m a 2-D model alowing
for lateral transfere (Yardley and Couples, 2000) 92
Figure 4.1 A curve showing the variation in oxygen isotope composition of the sea-
water for the last 6 m i l l i o n years. The oxygen isotope
data
are
based
on
foraminifera f r o m three boreholes near the coast of Ecuador (Shackleton et al .,
1990; Shackleton et a l . , 1995 98
Figure 4.2 Reconstruction o f the Scandinavian and
B r i t i s h
Ice
Sheet
during a glacial
stadial (after Hughes,
1998).
The maximum ice thickness onshore was 2600 m
and the maximum thickness i n the offshore
North
Sea was approximately 1600
m
101
Figure 4.3 Porosity
versus
depth. A compilation of core
measurements
and
wireline
calculated porosities 106
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I-relink Gvl lenhammar
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Figure 4.4 To the l e f t is the oxygen isotope data shown in Figure
4 . 1 .
To the
right
is
the GR log followed by the filtered GR log f r o m w e l l
Nor-1/3-2.
The t h i r d curve
is picks representing sudden
changes
i n the c y c l i c i t y of the filtered GR curve.
The t h i r d curve show peaks, positive or negative representing sudden transitions
f r o m
high
to low GR value, shown as a negative peak (to the l e f t ) . The opposite
results in a positive peak. The last curve is the integration of previous curve.
These curve were output f r o m CYCLOLOG*. This curve represents the
cumulative
difference between the predicted log values and the actual log values.
Breaks in the c y c l i c i t y succession may be related to missing sections or abrupt
changes
in sedimentation
rates.
A large positive peak
could
be a condensed
section
109
Figure
4.5 Correlation of
five
wells
d r i l l e d
in the southern part of the Norwegian
sector using
CycloLog
software. The distance
f r o m Nor-1/3-2
to
Nor-2/11-7
is
about 100 km (60
m i l s ) .
The cycles are compared on the
l e f t w i t h
oxygen isotope
signature (See text) 111
Figure
4.6 The
four maps
above present the palaeo-coastline for each
subsequent
crustal motion
model. It is important to note that large parts of the
North
Sea
were dry land after the last deglaciation, in each case for a period of several
1000
years 113
Figure 4.7 The figure to the l e f t show a
typical
pore pressure profile in the
North
Sea
w i t h no seawater just prior to a
glaciation.
The sand at 2000 meters subcrope to
seafloor and has therefore hydrostatic pressure.
During
glaciation of the
North
Sea the overburden pressure and the hydrostatic pressure increase w i t h a pressure
equivalent to the weight of the ice-sheet. I f the sand subcrops under the ice-sheet
the pore
pressure
w i l l
also increase in the sand. But
i f i t
subcrops outside the ice-
sheet,
its pore pressure
w i l l only
vary as much as the sealevel
changes
114
Figure 4.8 The profile
B - B '
shown on Figure 4.7. It show that the maximum
subsidence was i n the centre o f the
Baltic
Sea of more than 400 metres,
while
it
was
potentially
u p l i f t in the
North
Sea ( M i l n e et
a l . ,
1999, Mitrovica et
a l . ,
1994,
Tushingham,
A . M . ,
1991) 116
Figure
4.9
Resistivity
curves f r o m the
North
Sea compared w i t h G u l f of
Mexico.
The
graph to the
l e f t
is raw data
while
the raw
resistivity
curves have been
temperature corrected on the graph to the
right 122
C a r l
Fredr ik Gyllenhammar
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Declaration
The content of this
thesis
is the original work of the author (other
people's
work,
where included, is acknowledged by reference). It has not
been
previously submitted
for
a
degree
at this or any other university.
Carl
Fredrik
Gyllenhammar
Durham
September 2003
Copyright
The copyright of this thesis rests w i t h the author. No quotation f r o m it should be
published without his prior written
consent
and information derived
f r o m
it should be
acknowledged.
Cur
Fredrik
Gyl lcnhammar
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Chapter 1 Introduction
Carl
Fredrik Gvilenhummur
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h i i - o ; H k ' ; i o n
1.1 Background
Fifteen years ago, most pore pressure studies were undertaken solely for safety
aspects in the design and
d r i l l i n g
o f exploration
wells.
As the need for accurate pore
pressure
evaluation is
growing
due to its general application in exploration studies
such as hydrocarbon
migration
studies, more accurate methods founded on sound
physical
principles, and not just empirical observations, are needed.
Pore
pressure
estimation is a particular challenge i n the North Sea on account of the
complex tectonic and sedimentological history of the region, where the highest
overpressure (pore pressures above the normal , hydrostatic pressure) are found in
Jurassic
and Triassic reservoir
sandstones.
The
presence
o f a thick Chalk section as
w e l l as a variety of
mudrock
types, including a kerogen-rich petroleum source rock,
challenge standard practices fo r pore pressure evaluation which were, in many cases,
developed
in the G u l f o f
Mexico
where the rocks are younger and exclusively
siliciclastic
(sandstone, siltstones and shale mudrocks). The late history (Pleistocene-
Holocene) o f the
North
Sea has
involved
ice loading and the deposition of
glacially-
derived
sediments which add a further component of complexity to the stress and
f l u i d
history of
North
Sea sediments.
The availability of a very high quality set of w e l l data f r o m the Norwegian North Sea
(Central
Graben) provided impetus for this project
which
was designed to test current
methods of pore
pressure prediction,
assess the impact of a late ice-loading and
unloading
history and apply new technology on mudrock compaction (being
concurrently developed in the GeoPOP research group - see below).
There are a number o f complementary data which can be used for pore
pressure
evaluation including basin modelling, seismic velocities, wireline logs and
d r i l l i n g
parameters. Each requires
different
data
input
and interpretation requirements. In this
thesis the emphasis is fo r pore
pressure
evaluation using
wireline
logs. The
response
f r o m
the
d r i l l i n g
parameters was used as an independent
control.
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Chapter nirouui'i i o n
The thesis was funded by Norske Conoco in Norway and the work was included as
part of GeoPoP. GeoPoP (GEOsciences Project
into
OverPressure) was a j o i n t
research project involving University of
Durham,
Newcastle University, Heriot Watt
University
and
industrial sponsors
such as major
o i l
companies
l i k e
BP, Amoco,
Statoil,
Norsk
Hydro,
Phillips, Conoco, etc. The aim of GeoPOP was to expla in how
pore
pressures
evolve in mudrocks and to evaluate and develop new methods to
predict and calculate the pore
pressure
in
these
sediments.
1.2 Data
Norske Conoco made most of the data available, consisting of
wireline
data f r o m
exploration wells.
The most important
w e l l
was
1/6-7, d r i l l e d
by Norske Conoco in
1989, which is classified as a
high
pressure
( >
10,000 psi) and
high
temperature
(>350°F) (HPHT) w e l l . In addition to w e l l 1/6-7 were a number of offset well s in the
southern part o f the Norwegian shelf. The data set included also
some
wells
f r o m
Haltenbanken and the Barents sea.
W e ll
1/6-7 has
high quality wireline
and mud
logging data particularly w i t h respect to testing pore pressure prediction and
calculation
methods (Figure 1.1, 1.2 and 1.3).
GeoPOP provided the data f r o m the G u l f of
Mexico.
Data f r o m the shallow coring
project
by
B r i t i s h
Geological Survey (BGS) were provided by BGS. Data
f r o m
the
Ocean D r i l l i n g Project (ODP) are freely available on the Internet and were
downloaded free o f any charge.
Frednk
Gvlk 'n luunmar
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Chapter
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Figure 1.1 The wireline log plot of we ll 1/6-7 fr o m seabed to 2000m.
C a r l Fredrik
Gyllenhammar
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Chapter
I t tw
U N
Figure 1.2 The wireline log plot of well 1/6-7 f ro m 1900 to 4000m.
Carl
Fredrik Gyllenhammur
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W E L L
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Figure
1.3
The wireline
log
plot
of
well
1/6-7
from
3000 m to
4995
m
( T D ) .
( ai l Fredrik Gvllenhumniur
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1.3 Introduction
I n exploration d r i l l i n g operations pressure f r o m the
circulating
d r i l l i n g f l u i d (mud) is
used to prevent the pore
f l u i d
in the porous rock entering the borehole. The pressure
f r o m the mud at a particular depth is a function o f the average density (MW = Mud
Weight)
and the vertical height of the column
f r o m
that depth to the surface. In low
permeability formations, such as mudrocks, the formation can cave into the wellbore
through
tensile failure i f the pore pressure is higher than the counter pressure f r o m the
mud. The industry has a long history of establishing empirical relationships between
d r i l l i n g
parameters
such as the rate of penetration and the gas measured in the
returning
d r i l l i n g
f l u i d
to the pore
pressure
in the mudrocks. The
uses
o f
d r i l l i n g
parameters are very subjective and prone to large uncertainties. The pressure can also
be calculated
indirectly
f r o m petrophysical
measurements.
Petrophysical
data
can be
acquired while d r i l l i n g or after d r i l l i n g a section. In the former
case
the petrophysical
sensors are placed behind the d r i l l bit in operations known as Logging While D r i l l i n g
( L W D )
or Measurement
While
D r i l l i n g
( M W D ) .
When
data
is acquired
once d r i l l i n g
has been completed, the petrophysical sensors are lowered down the hole
suspended
f r o m
a
wire (wireline
logging) and readings taken by the tools while being reeled back
up. The pore pressures in the reservoir rocks w i t h high permeability are measured
directly
using a
wireline
tool w i t h a pressure
gauge.
A cylindrical probe w i t h a small
aperture is hydraulically forced into the formation (Figure 1.4) and the
tool
remains at
the location u n t i l the pressure stabilizes between the inside of the tool (where the
pressure gauge is located) and the
formation
(where the probe has
been
extended).
The pressure is recorded as pressure vs time. The most common trade acronyms for
these tools are RFT (Repeat Formation), FMT (Formation multi-tester) or MDT
(Modular Dynamics Tester). In mudrocks where permeability is very low, this
tool
cannot be used due to the time it w i l l take for pressure to stabilize. Direct pressure
measurements are also recorded when a hydrocarbon zone is tested, called a D r i l l
Stem Test (DST).
The accompanying petrophysical
measurements
collected at the same time as the
pressure
tests
include sonic, velocity, neutron porosity, density, and resistivity (unless
you intended to l i s t something else). These sensors are all calibrated for the porous
formation
and w i l l tend to give erroneous reading i f any clay minerals are
present.
C'ai
h e d n k
Gvilenlnuriinai'
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Inli oduciion
The challenge is therefore to use these
measurements
in mudrocks
w i t h
low
permeability and high clay content. During compaction of compressible sediment,
such as mudrock, water is expelled and the porosity decreases. I f the free water which
needs
to be expelled to maintain equilibrium
w i t h
the imposed
stresses
cannot drain
out of the system, the porosity w i l l not decrease,
w i t h
the result that the pore pressure
increases
above
the hydrostatic pressure. Porosity cannot be
measured
directly in a
borehole. The porosity is calculated indirectly f r o m the sonic velocity, neutron
porosity, density or the resistivity
measurement,
or a combination of
these
measurements.
The effective or inter-granular
stress
is then calculated using a
relationship between the porosity, the normal compaction trend and the total
lithostatic stress
(overburden
stress).
A variety of empirical relationships
have been
developed for calculating mudrock
porosity
f r o m
different log
responses.
Typically, a stress-porosity relationship is not
used
directly, but instead porosity is compared
against
a normal compaction trend,
which would be the porosity
against
depth for the location in question assuming a
'normal'
pressure
profile
equivalent to the hydrostatic
head
of a water column. In this
work it w i l l be shown that the normal compaction trend often yields the biggest
uncertainties in calculating the pore pressure in mudrocks.
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lenhann 1
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Figure 1.4 Schematic of wireline logging. The lithological column to the right is a schematic of a
pressure probe
( R F T )
being used to measure the pore pressure in permeable sandstone.
Having inferred mudrock porosity f r o m logs and computed or established a normal
compaction trend of expected porosity for normal pressure, the f i n a l step is to f i n d a
relationship
quantifies the pore pressure magnitude associated w i t h a mismatch
between the estimated mudrock porosity f r o m log response and the normal
compaction trend.. This transform or equation might be based on physical principles,
such as the equivalent depth (or effective stress) method, or empirical relationships,
such as the Eaton's method. It
w i l l
be shown that the transform method used for
calculating the pore pressure is
less
important than the choice of normal compaction
trend.
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Chapter i
The
i n i t i a l
goal for this research was to establish a new method to calculate the pore
pressure
in mudrocks as a function of petrophysical
measurements. During
the
course
o f
this research it became
apparent
that the classical equivalent depth method is a
reliable equation and it
would
be of
l i m i t e d
value to attempt an improvement to it.
Also,
the porosity of the mudrocks can be reliably calculated f r o m a combination of
the available wireline logs. A sensitivity study shows clearly that the biggest
uncertainty is the normal compaction curve. Eaton (1975) summarized it
best:
"The
methods used
to establish normal
trends
vary as much as the number o f people who
do it".
A
normal compaction curve
represents
the
reference
trend describing the compaction
behaviour of
sediments which
are normally
pressured.
The compaction (porosity loss
involving expulsion of fluids) is caused by increases in vertical and /or horizontal
stress. Conventional pore
pressure
prediction uses the normal compaction curve to
estimate
the magnitude of
overpressure.
Data f r o m
which
normal compaction
curves
are derived include shallow buried
sediments
o f the same age and lithology, or
published compaction relationships. For example,
Hansen
(1996) examined
three
wells
in the
North
Sea where he
assumed
that the mudrocks have normal pore
pressure. He established a relationship between the sonic travel time and the mudrock
porosity
used
in this research. Other approaches are based on laboratory
measurements
of compaction such as by Skempton (1970) where he showed a
relationship between compaction and the volume of fine-grained material in the
samples. The shortcoming o f that approach is that the relationship does not take into
account the different compaction behaviours of clay minerals such as montmorillonite
versus
fine-grained quartz, (K.
Bjorlykke (2001)
personal oral cornmun.).
This research
shows
that it is unlikely that any useful normal compaction trend can be
established in the
North
Sea due to
recent glacial
events. The glacial
t i l l s l e f t
by a
earlier glacial event have been overlooked for many years. The nature of these
sediments
is found to be very different from normal marine and non-marine shale
mudrocks. This
suggest
that the previous method of establishing a normal trend by
overlaying
a number of porosity
curves
f o r m offset wells
w i l l
give wrong
results
i f
used
in basins such as the North Sea.
C a r l Frednk Gyllcnliammar
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imroiiuaiijn
1.4 Pressure, the basic concepts
Fluids differ
f r o m solids in that they are unable to support
shear stress.
When a body
is
submerged in a
f l u i d
such as water, the
f l u i d
exerts a force perpendicular to the
surface at all locations around the surface of the body. I f the body is small enough so
we can neglect any differences in the vertical water column, the force (F) per
unit area
( A )
is the
same
in all directions. This force per
unit
area
is called the
pressure
P of the
f l u i d :
=
F/A [ E l . l ]
The SI
unit
of
pressure
is Newton per
square
meter ( N / m
2
) ,
which
is called
Pascal
(Pa). The equivalent imperial
unit
is pounds per
square inch
(psi =
l b / i n
).
Liquids found in rocks in the subsurface are relatively incompressible. This means
that the ratio of
mass
to volume, called density is approximately constant. For a
l i q u i d
whose density is constant, the
pressure increases
linearly
w i t h
depth. The
pressure
P at any point in a
l i q u i d
column is:
=P
0
+
pxgxh
[E1.2]
P
is the
pressure
at the surface and h is the vertical
l i q u i d
column. The Greek letter
p
(rho) is the density. Density has the
unit
mass/volume
(kg/m
3
= g/cm
3
).
g
is the
acceleration due gravity at the earth surface and equal to
9.81
m/s
2
.
Figure
1.5 shows a
simplified
diagram of how pore
pressure
may
increase
in a
w e l l .
The hydrostatic
pressure
(often called the normal
pressure)
in sediments underlying
the ocean often
follows
a gradient equal to 0.0101
MPa/m.
That is the
increase
in
hydrostatic
pressure
in water
w i t h
an average density of 1.03 g/cm
3
. The overburden
pressure
is the
pressure
exerted by all
overlying
material, both
solid
and f l u i d . Below
the water bottom, this
line
approximates 0.0226 MPa/m (1
psi/ft)
in a clastic
sedimentary environment. The pore
pressure
is the
pressure
of the
f l u i d
in the pore
space of the rock. It may be equal to or higher than the hydrostatic
pressure,
but not
higher than the overburden
pressure
(Figure 1.5). I f the pore
pressure approaches
the
overburden
pressure
the rock
w i l l
fracture and
release fluids.
However, often
fracturing w i l l occur at a lower pressure, equivalent to the least principal stress,
which
C a r l r iedi ik (Jyllenhammiir
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Chapter I
Introduction
i n an extensional basin is
less
than the overburden (the vertical
stress).
I f at a specific
depth of
burial
the
mudrock
permeability
becomes
so low that the excess water
f r o m
normal compaction can no longer
f l o w
out of the system as fast as the
rate
of new
sediments, the pore
pressure
w i l l increase. The maximum
increase
o f pore
pressure
by
this
mechanism called disequilibrium compaction (Swarbrick and Osborne, 1997)-
and is often found to be parallel to the lithostatic gradient (Clayton and Hey, 1994),
indicating,
at depth, transfer of most/all of the load onto the pore f l u i d ,
w i t h
very
little/no increase w i t h
vertical effective stress..
S E A
sun
nidi)
1500
SHALE
s
\
4J
g 2500-
ressure
.
Q
3000 -
-
•
•
3500-
\
\
/J
4000-
SAND
3>\ ^
4500
I I I I I I
5000
70
-lo
l l
id
100
U
Ml
90
Pore
pressure
(MPa)
Figure 1.5 Pressure plotted against depth in a fictional well. The effective stress is equal to the
overburden pressure minus the pore pressure and the overpressure is equal to the pore pressure
minus the hydrostatic pressure.
I n a borehole, the pressure exerted by the
d r i l l i n g
f l u i d to either prevent
i n f l u x
of
pore
fluids
f r o m the formation or prevent hole caving
instability
is equivalent to
C a r l F r e d r i k Gyllenhammar
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density of the
d r i l l i n g
f l u i d and its column height. Therefore, the formation pore
pressures are often converted into
d r i l l i n g
f l u i d density equivalents so it is clear as
what
d r i l l i n g
f l u i d
density just
balances
the pore
pressures.
Figure
1.6
shows how a
typical
pore
pressure
profile
can be displayed as
pressure
gradient
versus
depth.
I f
one
follows
the
change
in the
pressure
gradient of the pore
pressure
(red curve), every
point
on the curve
represents
a
pressure
gradient and a corresponding
average
f l u i d
density that particular
pressure
at that depth
represents.
The maximum pore
pressure
gradient is reached at the top of the reservoir (3200 meters) equal 0.016
MPa/m.
That
is
equivalent to the
pressure
at the bottom of a 3200-meter vertical f l u i d column
w i t h
an
average f l u i d
density of 1.64 g/cm
3
. In exploration
d r i l l i n g
a
d r i l l i n g
mud is used
where materials such as barite is mixed to f o r m a
l i q u i d
(called
d r i l l i n g
mud)
w i t h
such
high average
density. The terminology used is equivalent mud weight ( E q M W ) .
The
pressure
gradient
plot
illustrates a big challenge
while
d r i l l i n g
these wells.
The
EqMW
has to be
high
enough to
hold
back the f l u i d f r o m the depth where the
formation
has the highest-pressure gradient. However, in
some
formations,
typically
the shallower
ones,
this mud density
would
apply a
pressure
significantly
greater than
the pore
pressures
in
these
formations. This
excess
pressure
may lead to
fracturing
of
the rock and losses o f the
d r i l l i n g
f l u i d .
A
confusing
aspect
in the oil industry
w i t h
regard to
pressure
terminology is the
mixing the terms;
pressure
gradient and density ( E q M W ) . This
becomes
particularly
d i f f i c u l t
and confusing when
working w i t h
a mixture of both imperial and the SI
units.
It has already
been
shown that the
pressure
gradient
equals
density
multiplied
by
the acceleration due to gravity. In the imperial system, the norm is to use
weight
density
rather than density. Weight density is defined as the ratio of the weight of an
object to its volume. The units are pounds per gallon (ppg). As the weight is equal to
the
mass multiplied
w i t h
gravity, both weight density and
pressure
gradient
have
the
same units. The imperial
unit
system has historically been the norm in the oil industry
and the people
involved
has become used to converting directly f r o m weight density
(ppg)
to
pressure
gradient
(psi/ft)
and to
pressure
(psi).
The
word
weight density is
often
shortened to density. This has created a problem when converting to the SI
system. Too often,
while
converting f r o m density (g/cm
3
) to
pressure
gradient
(MPa/m),
density is not
multiplied
by gravity (9.81 m/s
2
). A
typical
example is a
recent paper
t i t l e d
"Pore Pressure terminology" in the Leading Edge
written
to explain
C ' a r i
I redrik Gyllenhammar
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Chapter i
Introduction
the problem, but f a i l i n g to explain the difference between weight density and density
(Bruce and Bowers, 2002).
ijrn
Pore p rrnurr
gnoient
Effective stress
I.
O 3000
ft
4500
I—I-
J
I I
I I I I I I I I
L-LJ
000
n
j ;
.014
0.016
0 70 80 90 100 0.01 0.012
i l l
20 30
.i.
50
Pore pressure (MPa) Pore pressure gradient (MPa/m)
Figure 1.6 The Figure to the right shows how a pressure versus depth plot (left, Figure 1.5)
becomes
presented
as pressure gradient versus depth.
1.5 Aims and layout of
thesis
The aims and objective of this thesis are to:
1. Develop a critical review of current methods used to calculate the pore
pressure in mudrocks.
2 .
Establish the uncertainties o f the input variables using in principle
component analysis, applied to the wireline measurements w i t h reference
to the mudrock porosity calculated and the d r i l l i n g parameters w i t h
reference to the calculated
d r i l l i n g
exponents.
3. Identify the variables that have the biggest impact on the estimation of
pore pressure, and how they can be improved.
C a r l F r e d r i k Gyllenhammar 14
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Chapter 1
In i roduct ion
4. Compare the wireline signature of overpressured shales in the North Sea
basin
w i t h
those f r o m the
G u l f
of
Mexico.
5. Examine why the resistivity
measurements
of the mudrocks can be used as
input
parameter
to calculate pore
pressure
in the
G u l f
of
Mexico,
while
this has proved d i f f i c u l t to apply in the estimation of pore pressure in the
North
Sea.
Following the introduction comes Chapter 2 where the pressure concepts w i t h
respects to pore pressure in shallow
sediments
are
discussed.
That is followed by a
discussion of mudrock porosity and normal compaction in mudrocks. Then the
different pressure
calculation methods,
first w i t h
wireline logs as input, then
those
using d r i l l i n g parameters.
Chapter 3 discusses the results f r o m using these different pore
pressure
estimation
methods
on a test
w e l l ,
Nor 1/6-7 in the North Sea. The sensitivity of the input
parameters
are
discussed.
The
results
f r o m the North Sea are then compared
w i t h
the
mudrocks
f r o m
a mini-basin in the
G u l f
of
Mexico,
Chapter 4
examines
the glacial history of the North Sea to explain the
nature
of the
shallow
sediments,
and their physical and petrophysical properties. Use of a novel
application of the software Cyclolog has helped in characterising the glacial
sediments. Finally the relevance o f the glacial history of the North Sea is reviewed in
relation to the petroleum system which has
generated
productive oil and gas fields .
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Ch apter 2 Pore Pressure Evaluation
Concepts
and
definition;-
Chapter 2 Pore Pressure Evaluation
Concepts
and
definitions
C a r l F r e d r i k Gvlienharnmar
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C i K i p i c r
2
Poi
v. Pi'fssm'c- )_• \. Ina ion Concep t and
ck'iinilions
2.1 Definition
Underpinning
pore pressure interpretation is the effective stress equation for porous
media (Terzhagi, 1936):
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Chapter 2
Pore Pressure
.Evaluation
Conce pts and d eii niii ons
2.1.1 Mudrock porosity
Blatt
(1970) has defined mudrock
based
on grain size, where
mud
is sediment
composed of clay sized particles. Typically mudrocks contain some
s i l t .
A
mudstone is a sedimentary rock composed of
l i t h i f i e d
mud, and
shale
is a fissile
mudstone. The term porosity has a different meaning in various disciplines as
w e l l
as
being different for
coarse
grain
sandstone
when compared
w i t h
a mudrock. The
porosities
discussed
here w i l l be l i m i t e d to the physical or total porosity, which is the
ratio
of v o i d volume to total volume.
The preferred method of obtaining the porosity in a rock is to carry out laboratory
experiments on
core
extracted
f r o m
the
w e l l
during
d r i l l i n g
operations. The porosity
o f
low permeability rocks such as mudrocks is
measured f r o m
the bulk density, then
drying
the sample,
followed
by
measurement
of the dry density in the laboratory.
This
procedure ideally must be commenced prior to the
samples
drying after reaching
the surface. On
research
vessels
such
used
during the
Ocean D r i l l i n g
Program (ODP),
these measurements
are
done
just after the
samples
are recovered at surface. Mudrock
is generally not cored during exploration
d r i l l i n g .
I f it is cored, the
samples
are waxed
at the wellsite so the water content is
preserved.
Mudrock porosity as
w e l l
as general rock porosity
f r o m
exploration wells is in most
cases
calculated
f r o m
wireline measurements such as the sonic log, the density log or
the neutron log. None of these measurements are a direct
measurement
of porosity.
They are referred to as log-derived porosity to indicate that their
o r i g i n
is
f r o m
wireline
log
responses.
For all
these
instruments, the
tool
response
is affected by the
formation
porosity, f l u i d and matrix. I f the f l u i d and matrix effects are known, the
porosity can be derived
f r o m
the tool response.
I n addition to the above tools, the resistivity response can also
used
to determine
porosity. However the resistivity is greatly influenced by the
f l u i d
saturation.
2.1.2 Different porosity evaluation
equations
Sonic derived porosity
C a r l
I
reclrik Gvllenhammar 18
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W y l l i e et
a l .
(1958) demonstrated that
there
was an approximate linear relationship
between sonic velocity and porosity in
sandstone.
The porosity is calculated f r o m a
linear interpolation between the zero porosity matrix sonic velocity (in principle
slowness
when using |isec/ft unit) and the
100
% porosity
fluid
sonic velocity.
f
t,
-t
log
ma
[E2.5]
where; t
ma
is the matrix velocity (67 (isec/ft in mudrock, 47.5 |isec/ft in chalk, 55.5
iisec/ft in sandstone) and // the fluid velocity equal 189 (isec/ft in fresh water
(Schlumberger, 1989). t\„
is the measured sonic velocity.
Another equation was suggested by
Raiga-Clemenceau
et al. (1988):
0
=
1-
f
At
^
ma
At
[E2.6]
The matrix velocity
t
ma
and x are both constants that are basin specific. Raiga-
Clemenceau called " x " the acoustic formation factor exponent.
Issler (1992) developed this relationship using data f r o m the Beaufort-Mackenzie
Basin, Northern Canada where the shales are quite uniform in their composition. The
matrix transit time is the same as for mudrocks in the W y l l i e equation (67 |isec/ft)
( W y l l i e , 1958) and the x was calculated to 2.19.
, , 6lY
2A9
p =
1
- — i
A t
J [E2.7]
Hansen
11 (1996), using
shale densities
measured on sidewall and cuttings
samples
f r o m the Nor th Sea, modified this equation. He
suggests
using the
f o l l o w i n g
equation
where the shale matrix velocity is 76.5 |isec/ft and x=
1.17:
0 = 1-
^ 7 6 . 5 ^
1 7
At J
[E2.8]
C u r l I icdrik Gvllenh amnia r 19
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Cha pter 2 i'u;e
i ' l v ^ u r c i . wduaia
at
Coincpiv a Vi
dc : retaat--
Figure 2.1a show how the porosity in a mudrock
w i l l
change as a func tion of
sonic
velocity. In the shallow section where the velocities often are
150 L i s e c
/ f t the
Hansen
(1996)
model suggests 44 % porosity versus the W y l l i e
(1958)
equation estimation of
68 %. The
W y l l i e
equation, although
based
on an empirical relationship in
sandstones
is used to calculate mudrock porosity in
several
publications
(e
.g.
Hermanrud
et
al.,
1998).
Iss er
ii 0
Hansen
Wyllie 67
i
:
u
41
Wyllie 76.5
0
fO
Wyllie
(68.8)
0 30 5
P 0.40
u . 0
0
20
0 10
) 00
o c o
2
15
75
5
90 100 110 120 130 140 150 160 170
:
Si
Bulk
Dens i t y
S o n i c u sec / f t )
0.315
o
)?
J
J13
(I 10
0.J11
o ;'5
a-
0 26
)00
4
n ir,7
i: 2.
0 20
tot
2
S
0=
1
04 • 06
1 J?
: OS
01
Matrix
Dens i t y
ater
Dens i t y
Figure 2.1 a,b,c,d. la porosity variation as a function of sonic velocity. B, porosity versus bulk
density. C , the sensitivity to pore water density. D, the sensitivity to matrix density.
Density derived porosity
C a r
F r e d r i k GvUenhammai
20
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Chapier 2
Pore
Pressure Evalua tion Concepts
and definitions
Density log-derived porosity is calculated f r o m the log
bulk
density, the matrix
density and the pore f l u i d density (equation 2.9).
0:
\
Pm a
~
Pb
Pn,a P f
[E2.9]
where p
ma
is the matr ix density, Pf is the pore water density and ph is the
bulk
density
measured by the density
t o o l .
In mudrocks
f r o m
the
North
Sea,
2.715 g/cm
was used
as an
average
matrix density. This value is used by
B r i t i s h
Geological Survey (BGS)
and is
based
on their shallow coring program in the
North
Sea. The pore water density
is
assumed
to
increase
f r o m
1.03
to
1.08
g/cm
3
w i t h
depth. As the
shale
compacts, the
released water has a lower salt concentration than the remaining pore water. Figure
2.1b shows the porosity variation, as a function of
bulk
density. The matr ix density
and the fluid density are kept constant at 2.715
g/cm
3
and 1.03 g/cm
3
while
the
measured
bulk
density increases f r o m 1.75 to 2.75
g/cm
3
.
The porosity varies linearly
w i t h
the measured
bulk
density and an
increase
of 0.1
g/cm
3
changes the porosity by
5.8 %. The second sensitivity
plot
(Figure 2.1c) shows that increasing the f l u i d
density f r o m
1.03
to
1.08 g/cm
3
only
increases the porosity by
1
%. Figure 2.
I d
shows
that the porosity
changes
by 4 % if the matr ix density
changes
by
0.1
g/cm . The
relationship
between porosity and matrix density is not linear, but
near
linear. The
porosity increases slightly
faster at lower matrix densities that at the higher end. The
matrix
density is a
function
o f
mineralogy.
Smectite has a low matrix density
(2.21
to
2.71 g/cm
3
)
while
chlorite matrix density can be as
high
as 2.94
(Fertl
and
Chilingarian,
1989). In the
North
Sea the mudrock compositions vary and therefore
so do the dry densities. Using a constant dry density w i l l therefore result in 10 to 20%
error in calculated the porosity using the density log.
Neutron derived porosity
Neutron-derived
porosity is related to the hydrogen index,
which
is an indication of
the amount of hydrogen in the sediment. As most of the hydrogen in a
formation
is in
the water and hydrocarbon molecules it is for all practical
purposes
a
measure
of the
water and/or hydrocarbon content. In formations
w i t h
phyllosilicates, the bound water
w i l l
be counted as water, and
hence v o i d space,
by the neutron log. When comparing
C a r l Predrik Gvlle nha mma r 21
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Chapter 2
Fore Pressure
(-.valuation
Concepts and definitions
i or.
Sonic
{Wy t l i e } -
porosity
:i
9 0
S. Hansen
-
porosity
Density
-
porosity
i.«0
Neutron
-
porosity
L O G
-
porosity
n
7f
1
S
0.60
i
i
it
•
0.50
1
.
i
J
i
S
0.30
0 20
0 10
L
C no
500 1000 1500
2000 2500 3000 3500 4000 4500 5000
D e p t h m R K B )
Figure 2.2 Log derived porosities in well Nor-1/6-7 Norway. The low porosity interval from
3261m
to
4346m
depth is the Cretaceous Chalk. The values are listed in Appendix 1.
Based on Figure 2.2 it is important to realize the uncertainties that exit in the porosity
estimates
in
shales
based on wireline logs. This put limitations on the conclusions we
may wish to draw concerning the
mechanisms
underlying the porosity reduction or
compaction. I f one only has available a wireline log-derived porosity profile, one can
clearly
not attribute the porosity change solely either to a mechanical
process
or to a
chemical process.
2.1.3 Normal compaction curve and trend lines
During
normal compaction, a mudrock
undergoes
a monotonic
increase
in effective
stress,
which
causes
an elastoplastic reduction in porosity. Compaction is a result of
grain
reorientation and
breakage.
Mudstone
consists
of clay minerals, fine grained
quartz, feldspar and mica. As the compressibility is dif ferent for different minerals as
w e l l
as for different clay minerals, the mudrock compressibili ty becomes very
d i f f i c u l t
to predict. The resultant relationship between effective
stress
and porosity is
known
as the normal compaction curve (Harrold et
a l . ,
1999). In this
case
equilibrium
is reached such that:
Pf (pore
pressure)
= ph
yc
j (hydrostatic pressure).
Curl
Fredrik Gyllenhammar
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Chapter 2
Pore
Pressure b\
alum io n Concept : and definitions
Although
many porosity - depth
data have been
published, details of age, lithology or
effective
stress are generally
absent.
In this study it was chosen to evaluate and
compare two different relationships: (1) A t h y type and (2) Soil-mechanical type.
These compaction curves
assume
mechanical compaction
only,
and are suitable
only
to describe siliciclastic sediments. Below 2-3 km depth ( 7 0 - 1 0 0 ° C ) , mineral
dissolution
and precipitation becomes important (Bjorlykke, 1999). At these
temperatures hydrocarbon generation also comes into play. There have been many
publications on attempts to assess the potential overpressure generated by these
reactions. The results are conflicting in the
sense
that for the same reaction, some
suggest
that no overpressure is generated
while
others
suggest
generation of large
overpressure. The
conflict
lies to a large degree in the assumed permeability. For
many of
these
reactions to
generate
overpressure, the permeability
w i l l
not be low
enough for overpressure to be retained over geological time (Osborne and Swarbrick,
1999).
It is also evident that w i t h all the uncertainties w i t h regard to chemical
compaction or chemical reactions in mudrocks, it would be quite impossible to predict
the normal compaction trend and hence impossible to calculate the pore
pressure.
Chemical compaction w i l l therefore not be taken into account in the
present
study.
2.1.3.1
Athy-type relationship
The exponential curve to describe compaction was introduced by A t h y
(1930).
It was
based on curve f i t t i n g a particular data set and is given as:
Where 0 is porosity at depth of interest, 00 is porosity at sea bed, c the compaction
coefficient and z the depth. Variations of the compaction curve result
f r om
substituting
depth w i t h mean or vertical effective
stress.
The A t h y compaction curve
was later
modified
by Hubbert and Rubey
(1959),
who recognised that porosity is
controlled
by
effective
stress and not by depth:
0 —
0oe
(-«)
( A t h y , 1930)
[E2.10]
0 = K 0
o
e
c
[ E 2 . l l ]
Carl Predrik Gvllenhammar 24
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Chapter 2
Pore
Pressure
kvaluation
Concepts and
ueiimhuns
Where
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Chap
Pore Pressure Evaluation Concepts and definitions
reach
0 % porosity, only at
i n f i n i t y .
The sea
floor
porosity is a factor that can be
related to
samples.
The compaction coefficient is a function of rock compressibility,
which
in theory could be
measured
in the laboratory ( A t h y , 1930). But the
compressibility w i l l vary
w i t h
depth as the rock
becomes
more consolidated. In
practice what is being
done
is to use a
w e l l
where the pore
pressure
is
assumed
hydrostatic
based
on the M W
used
and RFT
pressure
points. Then calculate the
mudrock porosity to calibrate the compaction coefficients in the normal compaction
equation.
Figure 2.3
shows
how different the two
equations express
the change in porosity
w i t h
increased
total
stress.
The
soil
mechanical function
suggests
a larger
rate
of porosity
reduction in the shallow section and
w i l l
always end up w i t h zero porosity i f the total
stress gets
large enough. The
A t h y
function suggests a more
moderate
change of
porosity in the shallow section.
W i t h
increasing stress, the porosity
w i l l
move
asymptotically to zero porosity, but never
become
zero.
Normal
compaction
Lo g
derived porosity
0.1
0.4
5000
10000
15000
20000
L0
25000
30000
Athy, 0.7 0.00008
35000
40000
SM , 2 (0.66), 0.74
45000
50000
Figure 2.3 Comparison of porosity with effective stress for the Athy and the SM equations. Initial
porosity (sea floor porosity) for Athy is 0.7 (70 ) while the porosity at 100 kPa (approximately
100 meters below sea floor) is 0.66 (66 ) The compaction factors a re for Athy; 0.00008 and
SM ; 0.74.
Ca r l Fredr ik
Gyllenhammar
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Chapter 2
I'orc Pressure valuation Concepts and definitions
100 (porosity in
% )
0
40
60 80
Porosity
0
Solidity
°
1.5
Void Ratio
•
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Void ratio
Figure 2.4 The relation ship between porosity, solidity and void ratio is shown. The y-axis is the
compaction as a
length
reduction. It is assumed that a confined volume is compressed beginning
with a void ratio of four.
Figure 2.4
shows
the
three
different frames of
reference
to
describe
the
loss
of pore
space. This is a theoretical model
bu i l t
in
EXCEL based
on the
definition
of porosity,
solidity
and v o i d ratio. The compaction as length reduction on the y-axis
represents
the proportional thickness reduction and the corresponding porosity,
solidity
and vo id
ration. Solidity
is the volume of
solid
grains as a
percent
of the total volume of
sediment, i.e. (1- ())). The complement to
solidity
is porosity as the volume of pore
space as a
percent
of the total volume of sediment. This is the opposite of what has
been suggested
by
Baldwin
and Butler (1985). The t h i r d
parameter
is v o i d ratio,
which
is the ratio of the volume of pore space and the volume of solids. Figure 2.4
shows
that
there
is a linear relationship between v o i d ratio and compaction
while
the
relationship is non-linear between porosity as
w e l l
as
solidity
to compaction. It
would
therefore be mathematically
easier
to
describe
the compaction as a function of
vo i d
ratio
rather than porosity. The
reason
why the oil industry
uses
porosity is that it has
become
the convention in the reservoir section. By comparison the convention in the
soil mechanics
environment is to use v o i d ratio as compaction is of interest.
Ca r l
Fredr ik Gyllenhammar
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2.1.4 Vertical versus mean effective stress
The mean effective
stress
(cr
m
)
is defined as the difference between the mean
�