1.0 Executive Summary
Overview: UT GeoFluids will study the state and evolution of pressure, stress, deformation and fluid
migration through experiments, theoretical analysis, and field study. The
Bureau of Economic Geology (BEG) at the Jackson School of Geosciences will
partner with the Department of Civil and Environmental Engineering at MIT: BEG
will lead the consortium.
Experimental: We will analyze
the fabric, acoustic, electrical, and mechanical properties of mudrocks over
effective stresses from 0.1-100 MPa. We will study 1) real geologic materials
(Gulf of Mexico mudrock, Boston Blue Clay) using resedimentation techniques and
2) core from a range of depths in the subsurface. Analysis of resedimented
material allows us to examine material properties of a consistent material at a
range of in-situ stresses. Uniaxial consolidation experiments will measure
vertical and lateral stress, resistivity, permeability, and velocity (Vp &
Vs) during compression. Triaxial experiments will measure strength parameters.
We will describe and quantify mudrock fabric at various effective stresses with
mercury porosimetry, and x-ray goniometry and image mudrock fabric using
electron beam techniques. We will 1) illuminate the origin and evolution
anisotropy in mudrocks, 2) document how composition (e.g. clay/silt fraction
and clay composition) controls geomechanical properties, and 3) develop a
geomechanical model for mudrocks that will better allow us to predict
compaction behavior, pore pressure, and borehole stability at geologic
Modeling: We will develop and apply models that link realistic rheologies, deformation,
stress (shear and normal), and pore pressure. Pressure prediction techniques
and basin modeling generally assume uniaxial deformation. In areas of central
interest to the petroleum industry, deformation is not uniaxial and stress and
pressure are coupled in more complex ways. In thrust belts, the lateral stress
is greater than the vertical stress and sediment that was originally buried in
a basin under uniaxial strain is later deformed in plane strain. In the
sub-salt regime, the interaction of isostatically stressed salt with sediment
that bears differential stresses results in complex stress and deformation near
the salt-sediment interface. To understand and ultimately predict pressures,
stresses, and rock properties in these regimes, a new level of understanding,
modeling, and analysis must be applied.
Field Study: We will
analysize pore pressure in thrust belts and in salt provinces. In the subsalt,
we will analyze pressure and stress in and near the Mad Dog field to understand
how pore pressure couples with salt advancement and to study the present state
of stress and pressure in sub-salt systems. We will study fold thrust systems
through ongoing work in the Nankai Trough. We look forward to working with
industry data sets in deepwater fold-thrust belts.
extensive previous work exploring mudrock compressional behavior, there are
still major gaps in our understanding of mudrock evolution. We do not yet have
an understanding of stress and strength behavior over a large range of effective
stresses (0.1-100 MPa) and we do not fully understand the evolution of
anisotropy. Our work will lay the groundwork for a new generation of basin
modeling algorithms and pressure/stress prediction techniques. This new
generation of modeling techniques will go beyond the restrictive assumption
that deformation is uniaxial. Instead, we will couple reasonable assumptions
about far field stress state with rheological models to predict stress and
pressure. The results will allow us to better predict pressure, stress,
borehole stability, and hydrocarbon migration in environments critical to
exploration: thrust belts and sub-salt.
Reporting: The Consortium
will 1) hold Annual Meetings, 2) provide Annual Reports, 3) develop an on-line
database of experimental results, 4) provide Company Visits, and 5) provide
Notice of Papers Submitted for Publication.
Team: The Consortium Team will include Peter Flemings (University of Texas at Austin), Ruarri Day-Stirrat (University of Texas at Austin), John Germaine (Massachusetts Institute of Technology), and Derek Elsworth (Pennsylvania State University). We envision
supporting 10 graduate students and 3 post-doctoral scientists at 3
Date: June 1, 2009
Duration: 10 years
Director: Peter B. Flemings, Geoscientist and Professor
2.0 UT GEOFLUIDS PLAN:
techniques and basin models generally assume uniaxial deformation (Figure 1A). Pressure
is predicted from a single effective stress-porosity relationship (Figure 1B,C)
and the least principal effective stress (fracture gradient) is modeled as a
fraction the vertical effective stress (parameterized by the stress ratio, Ko) (Figure
In practice, the
specific nature of the porosity-effective stress relationship and the effective
stress ratio (Ko) are empirically derived, have significant variation, and
we do not understand well what controls this variability. At geologic stresses
(0.1 -100 MPa) we do not have a clear understanding of compaction or strength
behavior. We do not fully understood how the fabric of a compacting mud rock
evolves. Thus, the evolution of anisotropy (strength, permeability,
resistivity, velocity) is poorly undertood.
Figure 1: A) Uniaxial
strain is generally assumed in pressure predictiona and basin modeling. B)
Under these conditions, there is a single relationship between vertical effective
stress and strain. D) In complex stress regimes, vertical and lateral strain
occurs. E) Here, the rock that is subject to both vertical and lateral
shortening behaves more stiffly (red dashed line) than the same rock undergoing
uniaxial strain (solid black line). F) Predicted pore pressures are lower, yet
horizontal stresses are higher in this example relative to the uniaxial strain
In settings of critical
interest to industry, strain is not uniaxial. Under these conditions, the
porosity-effective stress relationship will be different (red dashed line vs.
solid black line, Fig 1E). There has been little effort to incorporate this
behavior in the study of mudrock consolidation. We illustrate that rocks with
the same observed porosity will have different pressures because one region was
subject to uniaxial strain (top) whereas the other has both vertical and
lateral strain imposed (bottom) (Figure 1).
The example above
illustrates the importance of this problem for predicting pressure. However,
the problem also applies to basin modeling. Most basin modeling software
assumes uniaxial strain (e.g. IFP’s Temispack, IES’s Petromod, or our own (Dugan, 2000). By definition these
modeling packages cannot capture the physical processes that occur when strain
is not uniaxial. As a result they will incorrectly simulate pore pressures and
resultant flow and they cannot capture variations in the stress field.
This consortium will pursue
three directions. 1) We will perform a broad range of experimental measurements
to analyze the evolution of mudrocks through mechanical compaction. 2) We will
develop geological models that incorporate a full soil model and thus the
effects of both mean and shear stress: we will be able to predict both stress
and pressure. 3) We will pursue field studies in two environments where the
uniaxial strain assumption is not appropriate: salt environments and fold-thrust
environments. The goal is to develop, through analysis of data sets and
theoretical modeling, a fundamental understanding of the evolution of pressure,
stress, and rock properties in these systems. From this understanding we will
develop specific pressure prediction and basin modeling techniques that will be
2.1 Mudrock Properties: Experimental
There are a wide range of
porosity-effective stress relationships that have been developed (Figure 2).
Burland et al. (1990) summarized the dependency of compression behavior on clay content. Karig and Hou (1992) used mixtures of material to illustrated striking differences in compression behavior between mudstones and sandstones. Fukue et al. (1986) explored the variation in compressibility for a range of clay-silt mixtures. Aplin and Yang (1995) used well cuttings to illustrate how compression behavior varies with clay content. Dewhurst and Aplin (1998, 1999) experimentally examined the evolution of permeability, porosity, specific surface area, and pore throat distribution within mudstones and siltstones of London Clay mudstone. Yang and Aplin (2004) presented compressibility behavior of well data emphasizing the importance of clay content on compressibility. Most recently, Mondol et al. (2007) described the compression and velocity behavior of synthetic mixtures ranging from pure smectite to pure kaolinite.
Figure 2: Illustration of the many porosity vs. depth relationships for
mudrocks (Mondol et al., 2007).
There are still major gaps in our
understanding of the evolution of mudrocks. We do not yet have a predictive
understanding of stress and strength behavior over a large range of effective
stresses (0.1-100 MPa). How does anisotropy evolve in mud rock successions?
Kwon et al. (Kwon et al., 2004a; Kwon et al., 2004b) demonstrate permeability anisotropy in Gulf Coast Wilcox formation samples at 4km, but the question of when and how anisotropy develops remains (Figure 3).
Figure 3: Shale fabric changes during
consolidation. (O'Brien and Slatt, 1990).
We will analyze the fabric,
acoustic, electrical, and material properties of mudrocks over effective
stresses from 0.1-100 MPa. We will study 1) synthetic mixtures and real
geologic materials (Gulf of Mexico mudrock) using resedimentation techniques
and 2) intact cores from a range of in-situ effective stresses (Ursa Region,
Figure 4). We will use these materials to study the evolution of mudrocks
during consolidation. We will use mercury injection porosimetry, X-ray
diffraction, and Scanning Electron Microscopy and high resolution X-ray texture
goniometry to characterize the evolution of pore throat geometry, quantify
composition and clay fabric intensity, and provide images. We will characterize
permeability, and compressibility. Analysis of resedimented material will allow
us to examine material properties of a consistent material at a range of
in-situ stresses. Uniaxial consolidation experiments will measure vertical and
lateral stress, resistivity, permeability, and velocity (Vp & Vs) (Figure
5). Triaxial experiments will measure strength parameters. The experimental
suite will be grounded and augmented by extensive whole cores from 0-600 meters
below mudline in the Ursa Region of the Gulf of Mexico (Figure 5). These cores
are largely composed of mudrock of a relatively homogenous composition that
have been consolidated from a surface porosity of approximately 70% to a
minimum porosity of 35% at a depth of 600 mbsf.
Figure 4: Hole U1324 from the Ursa
Basin, Gulf of Mexico (Flemings et al.,
2008). A broad range of data including intact core, logging data, and other physical
property data are available.
The primary goal of the
experimental program is to provide the underpinnings for more complex and
comprehensive mudrock models than are currently used in the energy industry.
Through this analysis we will 1) illuminate the origin and evolution of
anisotropy in mudrocks, 2) illuminate how composition (e.g. clay/silt fraction
and clay composition) impacts geomechanical properties, and 3) develop a
geomechanical model for mudrocks that will better allow us to predict
compaction behavior, pore pressure, and borehole stability at geologic stresses
The long term motivation is to:
- Improve geophysical imaging by
having a better understanding of propagation velocity, elastic anisotropy,
and resistivity anisotropy as a function of mudrock density.
- Design drilling programs by
having a better understanding of the stress profile and mechanical
properties of the mudrock.
- Provide parameters for forward
modeling of basin evolution
- Develop tools to predict pore
pressure and stress from rock properties
We will measure the following
1. Velocity: vertical compressional (Vp) and Shear (Vs) velocity at various
stress levels during uniaxial consolidation.
2. Stress (horizontal and vertical) and undrained compression strength will
be measured by triaxial measurements at various stages of consolidation.
3. Resistivity: Vertical and horizontal resistivity will be measured at
various stress levels during uniaxial compression.
4. Permeability: vertical permeability will be measured as a continuous
function of compression. Horizontal permeability will be measured at multiple
stages of the experiment.
5. Fabric: At multiple stages of the deformation, samples will be extracted
from the experimental cell and analyzed for pore structure, clay alignment,
Figure 5: A full suite of
experiments will be run on resedimented material (upper left). These
experiments will lead to a detailed understanding of material behavior for each
property over a range of stresses (lower left). In turn, measurements will be
made on intact core (upper right). We will compare results from resedimented
material (left side) to results from intact core (right side) and from this
analysis develop a broad model of behavior of intact mudstone (lower right).
2.2 Coupled Poromechanical Modeling:
We will develop and
apply coupled poromechanical models of sedimentary basin evolution. This type
of modeling is used routinely in the geotechnical community. We envision two
approaches; 1) we will use static stress modeling to expore the present day
state of stress and pressure in meaningful geological scenarios; and 2) we will use coupled poromechanical models to describe the evolution of geological
A static stress
model is shown below. We have reproduced results presented by Fredrich et al (Fredrich
et al., 2003) to illustrate the approach (Figure 6). We assume visco-elastic
salt behavior and elastic soil behavior. The necessity for both isostatic
stresses in the salt and continuity of deformations at the salt/formation
interface can induce stress perturbations and stress rotations in the near-salt
formations (Figure 6) (Fredrich et al., 2003; Rohleder et al., 2003). We observe signficant stress concentrations at the top and base of the salt body (Figure 6, bottom Left). Large differences in stresses can be generated that, if sufficient, induce shear failure of the formation near salt. This approach provides fascinating insight into several important issues including: 1) how to estimate vertical stress in salt systems; 2) the behavior of least principal stress at the salt-sediment interface; 3) where high shear
is encountered and failure might be expected.
We will further
this approach in the following ways. First we will consider realistic
geometries as encountered in the case studies that we will purse (see Section
3.3). Second, we will incorporate a more realistic rock rheology into the
model. We will most likely start with a Cam Clay soil model to simulate the
in-situ stress state. This step is a significant one as during the relaxation
process, there may be significant inelastic strains. Finally, we will couple
fluid flow and stress behavior in the static stress models.
Figure 6. A: Two dimensional static
stress model of circular salt body encased in elastic solid. Gravity (rgh)is included as
body force. Salt is assumed to be viscoelastic (E=30GPa, v= 0.25,m= 1e19 PaS) and
sediment is assumed to behave elastically (E=30GPa, v= 0.25). Boundary conditions:
lateral stress increases linearly from 0 on the surface by the equation F=brgh: b is assumed to equal 0.7 to simulate a passive margin. B: maximum shear stress. Stress
concentration occurs in the surrounding sediments on top and beneath the salt. C:
evolution of differential shear stress along a centerline through the salt
sphere during relaxation. Shear stresses in the salt gradually disappear
during the processes of viscous stress relaxation.
Our second step in poromechanical
modeling will be to simulate the evolution of geological systems. At the
broadest level, we will incorporate stress modeling within basin modeling
codes. Thus, we will go beyond current basin models (e.g. Petromod or Temis) to models that calculate stress, and deformation beyond uniaxial. We will work with the
Applied Geophysics Laboratory in this development and we present one example of
previous work done by AGL.
In Figure 7, a salt sheet has
advanced over the underlying material. The result is concentrated zones of
shear in front of the salt body. The models will require significant
complexity. In particular, we envision significant problems of re-gridding
around zones of large displacement.
Figure 7: Finite Element model of salt sheet advance by Dan Schultz-Ela and provided courtesy of the AGL (http://www.beg.utexas.edu/indassoc/agl/agl_if.html). Finite-element
models suggest that basal shear is facilitated by very weak (overpressured)
subsalt sediments. The structural style of the sheet toe may therefore be
providing an indication of pressure conditions beneath the sheet. Weak
sediments also permit subsalt deformation, including a footwall wedge.
We will use existing FEM codes
(e.g. Plaxis, Comsol, Abacus) to simulate pore pressure, stress, and strain
evolution in salt advance systems. We will expand on this effort by applying
realistic soil properties and coupling fluid flow to this system. We will
examine the evolution of the state of stress and pressure immediately
underneath the advancing thrust sheet. We look forward to exploring how
increases in pore pressure beneath the salt control salt advancement.
We will study this behavior in both
salt environments and thrust environments. Outboard of the prism and beneath
the decollement (Figure 8 inset, green boxes), sediment packages undergo
vertical uniaxial strain: the horizontal stress is some fraction of the
vertical stress (Ko conditions). In contrast, above the decollement (Figure 8
inset, red box), biaxial strain is present: the horizontal stress is now
greater than the vertical stress and it is is limited by the failure envelope
of the rock.
Initially, we will consider
steady-state solutions where the accretionary wedge migrates in a self-simlar
form. These simple models will still include the effects of lateral stress to
predict pressure above and below the decollement. Ultimately, we will pursue a
forward model that describes the time dependent evolution of the prism.
Figure 8. Mohr-Columb diagram
illustrating stress conditions above and below the decollement. Above and
outboard of the decollement (green circle), ‘K0’ conditions are present where
the sediments are undergoing vertical uniaxial strain and the horizontal stress
is a fraction of the vertical stress. Within the prism (red circle), the
sediments are at failure: the horizontal stress is greater than the vertical
stress and is limited by the failure envelope (red circle).
2.3 Field Studies
We will study pressure, stress, and
rheology beneath salt through characterization and theoretical analysis. A
fundamental challenge is to predict pore pressure and stress state beneath
salt. One of the earliest examples where this problem was encountered was the
Mahogony Field (Harrison and Patton, 1995) (Figure 8). They found high pore
pressure and a low fracture gradien beneath salt (Figure 8B). Whitson et al. (2001) also reported lower least principle stresses than expected immediately beneath salt at the Loyal Prospect whereas they encountered normal minimum stresses in the Catahoula prospect (Walker Ridge, deepwater Gulf of Mexcio). At the Spa Prospect on Walker Ridge (Blk 285), Rohleder et al. (2003) describe drilling through almost 10,000 feet of salt. They encountered relatively low pore pressures beneath the salt and subsequent pressure increases almost 5,000 feet below the salt. Other workers also reported suprisingly weak least principal stresses below salt (e.g. (Whitson and McFadden, 2001)). There is debate as to the nature of the material beneath salt. Rubble zones have been reported (Harrison and Patton, 1995)) and described as zones of broken or pre-fractured rock analogous to diffuse semi-brittle fault zones (Rohleder et al., 2003). Alsop et al. (2000) describe drag zones adjacent to salt. These zones may have little cohesion, and thus little intrinsic strength and they may be quite permeable.
Figure 9: A) Cross section of the Mahogany Field; B) Pore pressure profile through sub-salt wells
at Mahogany. The mud weights are greatest immediately beneath salt and decline
beneath salt. The fracture gradient is low (i.e. the fracture gradient
collapses toward the pore pressure gradient) beneath the salt. ((Harrison and
We will initially
analyze the region including the Mad Dog Field (Figure 10). We will work with
the Applied Geodynamics Laboratory (AGL) to pursue this project.
Figure 10. Interpreted section across the Mad Dog Salt Sheet. courtesy of Mike
Hudec, AGL http://www.beg.utexas.edu/indassoc/agl/agl_if.html
We will investigate Mad Dog in three
manners. 1) Geology: With AGL, we will build a geological model that
describes both the present geometry and the evolution of the Mad Dog structure.
2) Pressure and Stress: we will measure the distribution of pressure and
stress based on direct pressure measurements, leak off tests, fracture
completions, and other drilling data. 3) Rock Properties: we will pursue
petrophysical analysis with wireline and core data. We will integrate these
data to understand the current pressure distribution in and around Mad Dog.
Deepwater fold-thrust belts are a
focus for hydrocarbon exploration. In this regime, many of the traditional
assumptions in pressure prediction do not hold because large lateral tectonic
stresses play a role in deformation and pore pressure. In these regimes, it is
necessary to account for the difference in stress state above and below the
decollement in order to correctly predict pressure (Fitts, 2003; Goulty, 2004).
Figure 6 illustrates the complexity of
considering the stress path of material that enters thrust belt systems.
Material above the decollment is subject to large horizontal stresses that
exceed the vertical stress. The result is both a high mean stress and a large
increase in shear stress for material inside the thrust belt (Figure 11, upper
left). In this enviroment, a consolidation model must consider both the effect
of normal stress (green line, lower left, Fig. 11) and shear stress (red line,
Figure 11. Relation between stress state
and porosity above and below the decollement based on a modified Cam Clay (MCC) soil model. Upper Left: p’-q plot (mean vs. differential stress). Sediments
that are subducted beneath the prism will follow the Ko line (green) and in
this example go from the open cicle to the green circle. Sediments that enter
the accretionary prism will follow the blue line (open circle to red circle).
Lateral stresses increase until slip occurs and no higher lateral stress can be
achieved. Lower left: stress-porosity behavior for 3 stress paths: isotropic
(black line), Ko (green line), and failure (red line). Right: The
porosity-stress relationship for the Ko line is contrained from porosity
measurements at Site 1173, outside the accretionary prism.
We will initially examine the pore
pressure evolution in the Nankai Accretionary prism (Figure 12) and we will
then extend that work to areas of active exploration. We will analyze the
petrophysics, the geology, the pore pressure, and the stress within the Nankai
accretionary prism. We have available a 3D seismic survey, multiple
penetrations that have been logged and cored through the thrust belt system,
and in-situ monitoring of pore pressures. We will perform deformation
experiments on Nankai sediments in order to better constrain in-situ pressures from pre-consolidation stresses, and also to determine material
properties to better constrain our soil models.
Figure 12. Basemap of Nankai Region. Left: Japan and surrounding bathymetry. Right: Bathymetry at the toe of the Nankai Accretionary
Prism. ODP Sites 808 and 1174 are in the tip of the accretionary prism. ODP
Site 1173 is in the sediments in front of the accretinary prism.
Figure 13. Seismic cross-section through
the Nankai Accretionary Prism. The Lower Shikoku Basin Facies is a high
porosity mudstone that is used in this example to predict pressure. The
decollement seperates lower porosity mudstone above from higher porosity
We will extend and formalize our
pressure prediction techniques in fold-thrust belt systems. We will explore
multiple soil models and will constrain these soil models with the experimental
results. We will place error bounds on our pressure prediction. Ultimately we will
derive general practices for pressure prediction and drilling in thrust belt
We are currently negotiating with
industry partners to extend our field studies to a deepwater fold thrust belt
where this active exploration.
3.0 INDUSTRIAL ASSOCIATES (I.A.) STRUCTURE
3.1 Annual Meeting
(1) Annual Meeting, covering the period from June 1 - May 31, will be held each
year in May and employees of any member company will be invited to attend.
(1) Annual Report, covering the period from June 1 - May 31, will be produced in
digital (pdf) format. The Annual Report will be delivered at the time of the
Annual Meeting to each member company.
On-line Rock Properties Database
An on-line rock properties database
will be developed. Experimental data will be posted on-line and access to
annual reports will also be available.
3.4 Company Visits
(1) day per year will be spent by the I.A. Principal Investigator or his
designated representative at individual Industrial Associate member sites if
the member desires. The purpose will be to communicate the technical results of
the I.A. effort. Travel costs to member sites are to be born by the member company.
Notice of Papers Submitted for Publication
members will receive copies of any work submitted for publication.
Peter B. Flemings: (Professor, University of Texas at Austin)
Prof. Flemings will direct the
consortium. He has skills in reservoir characterization, flow modeling, and
pressure and stress analysis.
Ruarri Day-Stirrat: (Research
Associate, University of Texas at Austin):
Dr. Day-Stirrat works on mudstone
fabrics and clay mineralogy. He has an extensive background in mudstone
John T. Germaine: (Senior
Research Scientist, Massachusetts Institute of Technology)
Jack Germaine is a Senior Research
Scientist at the Massachusetts Institute of Technology. He is a world leader
in experimental methods in geotechnical analysis.
Derek Elsworth: (Professor, Pennsylvania State University):
Professor Elsworth has interests in the
areas of computational mechanics, flow and transport in porous media,
geomechanics and the roles of fluids in mechanical processes. In recent years
his main interests have been in describing the fluid flow, fluid transport, deformation
and failure behavior of porous and fractured geological media.
We currently have four (4) M.S.
students and (4) Ph.D. students working on related research projects. These students have experience analyzing pressure, stress,
and production data and have interests in the interactions between pressure,
stress, and fluid flow. We envision supporting 10 graduate students and 3
post-doctoral scientists at 3 universities over multiple years.
The UT-Austin Geomechanics laboratory and the
MIT Geotechnics laboratory have the facilities in place to do this research. At
UT-Austin, our subsurface mapping is accomplished with Landmark software (we have a $5
million dollar software grant from Landmark). We pursue multi-phase modeling
with our own software, VIP (Landmark) and Eclipse (Schlumberger). We work with
a large seismic and wireline database with both Landmark and Geolog software.
We have 15 Linux workstations, over 300 GB of online disk space, and 2
TB of offline robotic tape storage. Geotechnical experiments are ongoing at
both the University of Texas at Austin and MIT. UT-Austin has invested in a large new geomechanics facility for
Flemings. Penn State has long capacity in place for extensive numerical
6.0 BUDGET AND COST
The UT GeoFluids I.A.
program will last 10 years with an annual commitment of $45,000 per year. The
cost is indexed for inflation. The primary cost will be the support of research
scientists, graduate students, post-docs, and experimental apparatus. Other
significant costs include computer equipment, database maintenance, and travel.
7.0 BENEFITS TO COMPANIES
Benefits to Industry
Associates include: 1) Annual Meetings; 2) Access to experimental
data; 3) Access to online database; 4) Reports on field
characterizations prior to publication; 5) Contributing to the
development of, and having access to, a new generation of geoscientists skilled
in overpressure problems.
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