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OVERPRESSURE PREDICTION BY MEAN TOTAL STRESS ESTIMATE USING
WELL LOGS FOR COMPRESSIONAL ENVIRONMENTS WITH STRIKE-SLIP OR
REVERSE FAULTING STRESS STATE
A Thesis
by
ASLIHAN OZKALE
Submitted to the Office of Graduate Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
December 2006
Major Subject: Petroleum Engineering
OVERPRESSURE PREDICTION BY MEAN TOTAL STRESS ESTIMATE USING
WELL LOGS FOR COMPRESSIONAL ENVIRONMENTS WITH STRIKE-SLIP OR
REVERSE FAULTING STRESS STATE
A Thesis
by
ASLIHAN OZKALE
Submitted to the Office of Graduate Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
Approved by:
Chair of Committee, Jerome Schubert
Committee Members, Jerry L. Jensen
Brann Johnson
Head of Department, Steve Holditch
December 2006
Major Subject: Petroleum Engineering
iii
ABSTRACT
Overpressure Prediction by Mean Total Stress Estimate Using Well Logs for
Compressional Environments with Strike-Slip or Reverse Faulting Stress State.
(December 2006)
Aslihan Ozkale, B.S., Middle East Technical University,Turkey
Chair of Advisory Committee: Dr. Jerome Schubert
Predicting correct pore-pressure is important for drilling applications. Wellbore stability
problems, kicks, or even blow-outs can be avoided with a good estimate of pore-
pressure. Conventional pore-pressure estimation methods are based on one-dimensional
compaction theory and depend on a relationship between porosity and vertical effective
stress. Strike-slip or reverse faulting environments especially require a different way to
determine pore-pressure, since the overburden is not the maximum stress.
This study proposes a method which better accounts for the three-dimensional nature of
the stress field and provides improved estimates of pore-pressure. We apply the mean
total stress estimate to estimate pore-pressure. Pore pressure is then obtained by
modifying Eaton’s pore-pressure equations, which require either resistivity or sonic log
data.
The method was tested in the Snorre Field in the Norwegian North Sea, where the field
changes from strike-slip to reverse stress state. Eaton’s resistivity and sonic equations
iv
were used to predict pore-pressure in this region by replacing the vertical stress by the
mean total stress estimate. Results suggest that the modified Eaton method with
resistivity log data gives better results for the area than the conventional method. The
ratio of maximum horizontal stress to minimum horizontal stress throughout each well
should be known for best results.
v
DEDICATION
To my mother, Sevim, and my father, Ali.
vi
ACKNOWLEDGEMENTS
I would like to express my deepest gratitude and appreciation to my committee chair, Dr.
Schubert, and my committee members, Dr. Jensen and Dr Johnson, for their guidance
and support throughout the course of this research.
Sincere thanks are extended to Knowledge Systems Inc. for allowing me to use their
Drillworks Predict software for this study. I also would like to thank the Knowledge
Systems Inc. staff who answered my all questions throughout the project. I also express
my sincere appreciation to Steve Hobart an experienced researcher for his interest in my
work.
vii
TABLE OF CONTENTS
Page
ABSTRACT .................................................................................................................iii
DEDICATION .............................................................................................................. v
ACKNOWLEDGEMENTS ......................................................................................... vi
TABLE OF CONTENTS ............................................................................................vii
LIST OF FIGURES...................................................................................................... ix
LIST OF TABLES ..................................................................................................... xiv
CHAPTER
I INTRODUCTION ................................................................................. 1
Overview ............................................................................................... 1
Background ........................................................................................... 1
Need for Solution .................................................................................. 4
Description of the Solution .................................................................. 5
Objective ............................................................................................... 7
Data Required........................................................................................ 7
Method .................................................................................................. 8
II LITERATURE SURVEY..................................................................... 9
What Is Overpressure? .......................................................................... 9
The Main Overpressure Generating Mechanisms ................................ 9
Undercompaction .................................................................... 11
Fluid Volume Increase ............................................................ 15
Fluid Movement and Buoyancy .............................................. 17
Tectonics ................................................................................ 18
State of Art for Pore Pressure Determination...................................... 22
Direct Methods ........................................................................ 24
Vertical Methods ..................................................................... 25
Horizontal Methods................................................................. 33
Other Methods......................................................................... 37
Conclusions ......................................................................................... 49
III MEAN TOTAL STRESS METHOD................................................. 51
viii
CHAPTER Page
Introduction ........................................................................................ 51
Conclusions ......................................................................................... 61
IV MINIMUM AND MAXIMUM HORIZONTAL STRESS
DETERMINATION............................................................................ 63
Leak- Off Test Inversion ..................................................................... 65
Wellbore Break-out Analysis .............................................................. 69
World Stress Map................................................................................ 71
Conclusions ......................................................................................... 72
V APPLICATION OF MEAN TOTAL STRESS METHOD ................ 73
Geology of the Snorre Field ................................................................ 73
Horizontal Stress Boundaries for Snorre Field ................................... 75
Results ................................................................................................. 85
Well 1 ...................................................................................... 93
Conclusions ......................................................................................... 97
VI SUMMARY, CONCLUSION AND SUGGESTIONS FOR
FUTURE WORK .............................................................................. 100
Summary ........................................................................................... 100
Conclusions ....................................................................................... 107
Suggestions for Future Work ............................................................ 108
NOMENCLATURE.................................................................................................. 110
REFERENCES.......................................................................................................... 113
APPENDIX A ........................................................................................................... 121
APPENDIX B ........................................................................................................... 143
VITA ......................................................................................................................... 158
ix
LIST OF FIGURES
FIGURE Page
1 Definition of overpressure............................................................................... 10
2 Representation of compaction process ............................................................ 11
3 Porosity vs. depth and porosity vs. vertical stress relationships.
Arrows indicate porosity increase. .................................................................. 12
4 Example of Terzaghi’s example on actual formations. Povb is overburden
pressure inserted on sediment. Notice that in each case, same overburden
is applied. But in the case where overpressure is observed, the
permeability was low. In normal pressured case, the permeability was
higher, fluid escapes freely under increasing overburden. ............................ 14
5 Overpressure due to hydraulic head mechanism. If the reservoir is
interconnected to a higher level fluid head, overpressure will be
observed. ........................................................................................................ 18
6 Stress regimes. ................................................................................................. 19
7 Tectonically influenced overpressure distribution on earth. Shaded
regions represent where tectonically induced pore pressure is observed,
lines represent where Cenozoic folding occurred, and triangles represent
where mud volcanoes are observed. ................................................................ 20
8 Example of Foster and Whalen study to determine overpressure from net
overburden which is vertical effective stress. ................................................. 27
x
FIGURE Page
9 Determination of pore pressure from equivalent depth method of Foster
and Whalen. Fsh is the formation factor of clay rich shale formations............ 28
10 Pore pressure estimation nomograph example for equivalent depth
method used by Foster and Whalen. ............................................................... 28
11 Resistivity ratio trend for Eaton’s equation. .................................................... 36
12 Virgin compaction curve and unloading curve example, Bowers’ study. ...... 39
13 Example of fluid expansion from Indonesia, Bowers’ study. ......................... 40
14 Representation of unloading by velocity vs. density plot, Bowers’ study.
If unloading is present as an overpressure generating mechanism, pore
pressure calculated by existing methods will underestimate the pore
pressure. Effective stress vs. velocity plot should be checked for
unloading before any computation. ................................................................ 42
15 Pressure vs. depth plot to demonstrate centroid effect, Traugott’s study. ....... 46
16 Holbrook study results for effective stress constants for different
lithologies. ....................................................................................................... 47
17 Representation of Anderson Faulting Theory. ................................................. 54
18 Goulty’s representation for relationship between unloading and normal
compaction curves. ........................................................................................ 59
19 Typical example for LOT. Leak off pressure and minimum stress is
illustrated. ........................................................................................................ 66
xi
FIGURE Page
20 Example for a hydraulic fracturing test. Same test stages are required by
ELOT .............................................................................................................. 67
21 Borehole breakouts and horizontal stress generating them. ............................ 70
22 Illustration for four-arm caliper responses to borehole anomalies like
breakout, washout and key seat....................................................................... 72
23 North Sea stress map from world stress map project. ..................................... 73
24 West-East tectonics and topography profile of Snorre Field. Seismic data
are used to identify fault. ............................................................................... 74
25 Norwegian North Sea stress map. Snorre Field stress data set is
indicated. ......................................................................................................... 76
26 East-west minimum stress profiles of Snorre and Visund Fields over
depth. Leak off test results are illustrated by Grollimund et al. ...................... 77
27 West-East pore pressure profile of Snorre and Visund Fields normalized
by vertical stress. ............................................................................................. 78
28 Leak-off test data results for Snorre Field over depth. Inclination is the
borehole inclination......................................................................................... 80
29 Minimum horizontal and maximum horizontals stress magnitudes for
Visund Field. The data are used to set boundaries on mean total stress
for the Snorre Field. Horizontal stress ratio for Visund Field is obtained.
from this figure. The minimum horizontal stress and the maximum
horizontal stresses are indicated on the figure by Wiprut and Zoback. At
xii
FIGURE Page
2900 m the maximum horizontal stress is 70MPa and the minimum
horizontal stress 54 MPa. The ratio of max horizontal stress to minimum
horizontal stress is 1.3. There is a wide range of error bars for maximum
horizontal stress. When these error boundaries include in the analysis,
horizontal stress ratio changes between 1.2 and 1.4 for deeper sections. ....... 81
30 Leak off test stress data in specific gravity plotted against depth. The
equation of the trend is also indicated. ............................................................ 82
31 Sonic log compaction trends used for pore pressure prediction for well 1,
well 2, well 3, respectively.............................................................................. 86
32 Resisitivity compaction trends used for pore pressure prediction for well
1, well 2, well 3, respectively.......................................................................... 87
33 Eaton sonic method results for well 1, results are plotted in result
gradient part..................................................................................................... 89
34 Eaton resistivity method results for well 1, results are plotted in result
gradient part..................................................................................................... 90
35 Pore pressure difference between mean total stress used pore pressure
prediction and overburden based pore pressure prediction using sonic
log data for Well 1........................................................................................... 91
36 Pore pressure difference between mean total stress used pore pressure
prediction and overburden based pore pressure prediction using
resistivity log data for Well 1. ......................................................................... 91
xiii
FIGURE Page
37 Input data for well 1, Gamma ray, resistivity, sonic and bulk density log
data are plotted. ............................................................................................... 95
38 Resistivity log data based pore pressure comparison for each method to
RFT.................................................................................................................. 96
39 Sonic log data based pore pressure comparison for each method to RFT. ..... 97
40 Sonic pseudo RFT vs. pore pressure predicted analysis. ................................ 98
41 Resistivity pseudo RFT vs. pore pressure predicted analysis. ........................ 99
xiv
LIST OF TABLES
TABLE Page
1 Summary of the vertical methods.................................................................... 33
2 Summary of the horizontal methods. .............................................................. 37
3 Summary of the other methods. ...................................................................... 50
4 Mean Square Error Analysis for the pore pressure estimation methods
using sonic log data is compared. For Well 1 PP OBG has the smallest
error. ................................................................................................................ 92
5 Mean Square Error Analysis for the pore pressure estimation methods
using resistivity log data is compared. For Well 1 PP M 1,3 has the
smallest error. .................................................................................................. 92
6 Mean Absolute Error Analysis for the pore pressure estimation methods
using sonic log data is compared. For Well 1 PP OBG has the smallest
error. ................................................................................................................ 92
7 Mean Absolute Error Analysis for the pore pressure estimation methods
using resistivity log data is compared. For Well 1 PP M 1.3 has the
smallest error. .................................................................................................. 92
8 Number of RFT calibration point used for normalization to evaluate
accuracy of each method. ................................................................................ 93
1
CHAPTER I
INTRODUCTION
Overview
Determining overpressured zones is quite important for the petroleum industry. Drilling
success, safety and reservoir depletion history are all affected by the presence of
overpressured strata.1 This research will focus on the drilling part of the problem.
For a successful drilling design it is extremely important to know or estimate the pore
pressure for a given area. Casing design and mud weight designs are planned according
to the estimated pore pressure. If the mud weight is not adjusted for the correct pore
pressure, unwanted events like “kicks” can occur, which may result in lost time, or even
blowouts. A good estimate of pore pressure is also essential to avoid wellbore stability
problems like borehole breakouts or stuck pipes. Avoiding problems related to pore
pressure determination can save lives and money. This research project will investigate
the pore pressure determination in tectonic environments where strike slip or reverse
faulting is present.1
Background
There are many mechanisms for the generation of overpressure. The important ones are:
1. Undercompaction,
This thesis follows the style of SPE Drilling and Completion.
2
2. Fluid volume increase,
3. Fluid movement and buoyancy,
4. Tectonics.2,3
For undercompaction and fluid volume increase mechanisms, there are methods to detect
overpressure using well log data.4-7 Present day methods to determine overpressure using
well logs are related to undercompaction and fluid volume increase. This thesis reports
on the extension of the existing methods to include the influence of tectonics on
compaction and resultant fluid pressures.
The industry practice to determine pore pressure or overpressure from well logs depends
on important assumptions. One assumption is that the shale pore pressure is the same as
pore pressure in sand-rich nearby formation (e.g. sands). Another assumption is the
relationship between porosity and effective stress is an indicator for pore pressure. A
consequence of the last assumption is that excess porosity is an indicator of overpressure
due to undercompaction or fluid volume increase.
Excess pore pressure determination methods that use well logs can be classified as direct
methods, vertical methods, horizontal methods and others.4-8
Direct methods and some of other methods use porosity indicators such as resistivity and
sonic logs. During compaction, porosity decreases with depth as effective stress
increases. Deviation is defined by the normal trend observed for hydrostatically
3
pressured strata above the overpressured zone. When an overpressured lithology is
encountered, there is a deviation observed from the normal compaction trend of porosity
indicators. The amount of deviation is correlated with the observed pore pressure
increase. In this method pore pressure from previously drilled wells from the same field
can be related to the previously known porosity indicator deviations.
Regardless of the method used, either horizontal or vertical, porosity trend lines are used
for pore pressure prediction. The methods that use a trend line project the value of the
porosity indicator for a hydrostatic pressure throughout the well. With the help of shale
analysis from the gamma ray log, applied to a porosity indicator log, shale points are
obtained for the well. The calculated pore pressure then is obtained by relating the
deviation of the shale points from the trend line with an empirical equation.
Other methods eliminate the explicit use of a trend line, however, the existing methods
are generally based on either directly or indirectly establishing an effective stress-
porosity relationship.9 Interestingly, the empirical or semi-empirical methods used by
the industry for pore pressure prediction were developed for the Gulf of Mexico. The
Gulf of Mexico is known to be a tectonically relaxed, normal fault region. Notice that
the effective stress defined for these methods is based on the vertical stress so that the
effective stress used for these methods is the vertical effective stress. The method
proposed in this thesis use mean effective stress instead of vertical effective stress.
4
Need for Solution
Vertical effective stress for overpressure determination can only be applied to normal-
tectonically relaxed environments. Often we see the horizontal stresses affecting the
predictions where compressional lateral stresses are present near the target area.10 In
normal faulting or passive environments, the relationship between the observed stress
magnitudes is Sv > SH > Sh, where Sv is the vertical or overburden stress, SH is the
maximum horizontal principal stress, and Sh is the minimum principal horizontal stress.
The existing techniques of estimating pore pressure are successfully applied in normal
faulting environments because the effect of horizontal principal stresses is negligible and
porosity changes are mainly related to overburden. Thus, detection methods based on
vertical effective stress are successful in where maximum compressive stress is vertical
(extensional tectonic regimes) regions.
In tectonic compressional environments, however, the overburden stress may not be the
component controlling pore pressure. The observed stress distribution in these
environments changes according to the type of the faulting system present. In strike-slip
faulting environments, the observed stress state is SH > Sv > Sh; while in reverse faulting
environments, SH > Sh > Sv.11 In strike-slip and reverse faulting environments, the pore
pressure arises not only because of overburden stress, but horizontal (lateral) stresses
also play an important role in overpressure development. There is evidence that where
observed porosity does not result simply from overburden change, overpressured zones
can not be adequately estimated by the existing methods of pore pressure prediction.12
5
Since the stress distribution is not the same everywhere, the magnitude of the principal
horizontal stresses may vary spatially compared to stress created by the overburden
alone. In the areas where we see the effect of large lateral (horizontal) principal stresses,
the existing methods for predicting pore pressure tend to incorrectly estimate pore
pressure. This error could result in inefficient well designs and unmanageable borehole
stability issues.
Description of the Solution
This study proposes to incorporate estimates of maximum and minimum horizontal
principal stresses into an overpressure analysis technique. We calculate the mean of all
three principal stresses and use this mean total stress in the calculation of pore pressure
estimation
Sm = (SH + Sh +Sv) /3 ....................................................................................................... (1)
The technique can be applied to three faulting regimes; normal, strike slip, and reverse,
for estimation of the pore pressure. Where lateral stresses are higher than vertical stress,
such as in strike-slip or reverse faulting systems, the excess pore pressure cannot be
explained by use of overburden stress alone. As shown later, incorporation of the mean
total stress approach provides better estimates. To evaluate Sm, several quantities have to
be measured or estimated.
1. Vertical stress Sv is calculated from density logs. Vertical stress is
S v = g ( z ) ρ ( z )dz ........................................................................................................... (2)
Vertical stress is the easiest of the three principal stresses to estimate.
6
2. The magnitude of the minimum horizontal stress is calculated from leak off and
hydraulic fracture test data.13 Caliper, image logs, and mud weight can be used to
obtain the azimuth and magnitude of minimum horizontal stress around the
wellbore.14
3. It is difficult to calculate the magnitude of maximum principal horizontal stress. But
the direction of maximum principal horizontal stress is perpendicular to both Sv and
Sh directions. It is also possible to bound the magnitude of SH by the Anderson’s
Faulting theory if Sv and Sh are known.15 Recent techniques used in the petroleum
industry suggest that the magnitude of SH can be calculated using sonic log
readings.16 It is also possible to use World Stress Map Data base to obtain an
estimate of the ratio SH /Sh.
Thus, while some characteristics are directly available from well logs, the direct
determination of maximum horizontal stress and minimum horizontal stress by well logs
is not possible. Until recently, four or more arm caliper logs have been used to determine
the orientation of minimum horizontal stress and to estimate its magnitude. Another
petrophysical tool is the use of image logs from which the orientation of the minimum
horizontal stress can be interpreted. The Anderson Faulting Theory combined with
Mohr-Coulomb failure analysis can provide bounds on a quantitative solution for
maximum principal horizontal stress in tectonic environments where pore pressure,
vertical stress and minimum horizontal stress are known.15 The knowledge of these
stresses allows us to calculate the mean total stress, which is necessary to more
7
accurately estimate pore pressure in environments where lateral stress magnitudes are
greater than vertical stress.
Objective
The objective of this study is to explore and develop a mean total stress technique for
estimation of pore pressure. This study will give a better understanding of the pore
pressure distribution over the areas where faults and lateral stresses are present.
The difference between industry practice for pore pressure determination, which uses
only overburden stress, and pore pressure determination using mean total stress (or mean
effective stress) will be demonstrated. The difference in estimation of pore pressure
between the two techniques may significantly affect well drilling design.
To validate this work, pressure test readings, like a Repeat Formation Test (RFT) or
Modular Formation Dynamic Test (MDT), from the well in concern will be used to
evaluate the improvement in pore pressure estimation using a mean stress approach
compared to vertical stress approach.
Data Required
The data required for the analysis can be categorized into three categories. First one is
well logging data for shale porosity determination as sonic log, gamma ray log,
Resistivity log. Second data category is well test data as MDT, RFT and Leak off Test
8
(LOT) data. LOT data is used as minimum horizontal stress. MDT and RFT data is used
to evaluate the results of pore pressure determination techniques. The final and third
category is stress state data for a given field. The stress state data is needed to indicate
what kind of stress state the field is in and any information related to horizontal stress
ratio boundaries is also needed.
Method
In this research I am going to calculate pore pressure using mean total stress as a
replacement for overburden in the existing pore pressure evaluation techniques. Data
from the Snorre Field from North Sea will be used to validate the Mean Total Stress
Pore Pressure Prediction Method. This field is known to be in a strike slip to reverse
faulting stress regime.17 Three wells from this region will be evaluated. The result of the
two methods will be compared to actual pore pressure readings from the RFT data.
Results will presented as percent changes between pore pressure calculated from mean
total stresses and overburden stress normalized to measured RFT data.
9
CHAPTER II
LITERATURE SURVEY
Overpressure estimation methods generally use the compaction theory. In order to be
able to understand the methods the following concepts should be understood.
What Is Overpressure?
Overpressure is often also called abnormally high pore pressure. It is known as the
excess pore pressure observed for the area. In this context it will be defined as any pore
pressure which is higher than the hydrostatic water column pressure extending from
surface to the drilling target. Figure 1 shows the definition of overpressure.
The Main Overpressure Generating Mechanisms
In the literature, there are many mechanisms proposed to explain the development of
overpressure. In this text, four main categories will be mentioned. It is important to
differentiate the porosity changes with respect to these different mechanisms. Every
mechanism has its own method to relate porosity change to pore pressure change. Recent
surveys of overpressure mechanisms have been published by Swarbrick et al.18 and
Bowers4. Swarbrick classified three mechanisms, and Bowers classified four.
10
Bowers4classified the mechanisms as; undercompaction, fluid expansion, lateral transfer,
and tectonic loading. However, Swarbrick et al.18 classified the mechanisms as: stress
related, fluid volume increase, and fluid movement including buoyancy.
Figure 1 Definition of overpressure.
For this study, it is critical to differentiate the tectonic mechanisms in a separate heading
since Mean Total Stress Pore Pressure Prediction Method proposed is mainly interested
in porosity change due to tectonics.
So this text will explain mechanisms under four categories; undercompaction, fluid
volume increase, fluid movement and buoyancy, and tectonics.
11
Undercompaction
Undercompaction is also known as disequilibrium compaction. In this type of
mechanism vertical loading stress (overburden stress) is the main agent on shaping the
pore space and pore throat systems. Hydrostatically pressured (normally pressured) areas
usually have a characteristic rate of compaction. The reflection of this characteristic rate
of compaction on well logs is a trend. Figure 2 shows the compaction process. During
compaction the fluid in the systems will flow into upper sediments under the influence
of vertical stress (overburden) if there is sufficient permeability. As the fluid escapes
grains continue to support the applied vertical stress.
Vertical Stress
Grain Grain Grain
Grain Grain Grain
Figure 2 Representation of compaction process.
12
In hydrostatically pressured areas, with increasing depth, a porosity decrease is
observed. Porosity is related to depth vs. vertical stress, therefore porosity is also related
to increasing vertical stress. Figure 3 illustrates porosity vs. depth and porosity vs.
effective stress relationships for normally pressured-shales.
Porosity
Porosity
Figure 3 Porosity vs. depth and porosity vs. vertical stress relationships. Arrows indicate porosity increase.
However, in some cases where the sedimentation rate is fast and low permeability is
observed, fluid is trapped and supports a part of the increased vertical load. Overpressure
results where this kind of phenomenon is observed in the target area. This mechanism is
known as undercompaction or disequilibrium compaction.
13
Understanding compaction mechanism is critical for overpressure detection. Compaction
can be defined as porosity reduction as pore water escapes under loading. Loading
causes the formation to get squeezed and expel water as much as the vertical
permeability will allow in establishing a stress equilibrium.
19
Terzaghi & Peck conducted some experiments in 1948 to show the effect of
permeability which is directly related to the ability of the pore fluid to escape. They
showed this using a closed system where they control the ability of the water to escape
by taps while they applied vertical stress. They used springs to model the resistance of
pore grains in reaction to the load applied. When the tap was open, fluid was free to
escape. Fluid e


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