Geomechanical Assessments at Well and Field scale
Peter van den Bogert
5 * 4 hours online
Business context
Various subsurface project risks can be reduced significantly by
proper understanding of the geomechanical response of the
subsurface along well trajectories and at field scale.
Non-productive drilling time caused by stuck-pipe incidents is
regularly attributed to bore hole instability, sand failure and
sand production may cause erosion of equipment and filling-up of
separation facilities, while unintended and uncontrolled fracturing
may create permeable pathways and loss of containment of reservoir
fluids. But also, field-scale geomechanical assessments provide
important input to safe operating envelopes for the reservoir
pressure as well as production & injection wells. Reservoir
compaction (and pore collapse) may cause a reduction of reservoir
permeability, mechanical well failure and unacceptable (seabed)
subsidence. Fault reactivation may create permeable pathways and
loss of containments - e.g. in waterflood operations and Carbon
Capture Storage (CCS) projects - and even induced-seismicity.
Proper geomechanical understanding of the subsurface response to
our operations is essential, both at well scale and on field scale,
to manage pressure and temperature of fluids (gasses) in wells and
reservoirs.
Who should attend
This course provides a starting point for anyone who needs to
understand what geomechanical risks are relevant for oil & gas
development and CCS projects, and how to assessment them.
Therefore, this training is aimed at staff involved in field
development projects and are responsible for managing, integrating
or using the results of geomechanical risks assessments. This could
be well engineers, production staff, geologists, reservoir
engineers and petro-physicists. This training is also suitable for
junior geomechanical specialists that want to develop their
competencies integrating field and experimental data with
model-based risks assessments on both well and field scale.
Finally, subsurface team leads, or staff/managers that oversee
externally conducted geomechanical studies will find benefit in
understanding, challenging and assuring the geomechanical
evaluation results relevant to their development
projects.
Academic thinking level and practical MS Excel skills are
desired for optimal learning benefits.
Course content
This course covers data acquisition and interpretation,
geomechanical modelling and interpretation, and integrated risk
assessment in conjunction with other subsurface disciplines to
reduce project risks. Data acquisition includes field data, such as
(X)LOT, InSAR, and petrophysical log data to estimate in-situ
stress and formation properties using correlation functions. Also,
attention is paid to rock mechanical experiments and
interpretation. Geomechanical modelling is applied at material
point level (poro-elasticity theory, shear & tensile failure),
at wellbore scale (stress around the bore hole) and at field-scale
(compaction & subsidence, fault reactivation & induced
seismicity, containment). Workflows for geomechanical risks
assessment are discussed using (simple) analytical approaches (e.g.
Geertsma) and using Finite-element techniques, highlighting their
advantages and limitations.
Objectives of this Virtual Instructor Led Training (VILT):
On completion of this VILT course, participants will be able
to:
v Identify potential project risks that may need a
geomechanical evaluation, and develop a work plan to assess
them
v Understand (the uncertainties associated with) the
pressure-depth plot
v Construct and interpret a Mohr-circle for shear and
tensile failure
v Describe log-based correlation function to estimate
mechanical properties
v Identify potential lab experiments to measure required
formation properties
v Calculate the mud weight that leads to shear and tensile
failure (fracturing conditions)
v Understand the impact of well orientation on the
recommended mud weight / bore hole instability
v Describe the workflow and data to develop a field-wide
fit-for-purpose geomechanical model
v Estimate reservoir compaction and (seabed) subsidence
using simplified analytical approaches
v Describe the failure mechanisms and leakage pathways
that could cause loss of containment
v Explain the difference between fault reactivation and
induced seismicity
v Recognise when a Mohr-circle analysis and 3D
finite-element models are inappropriate to assess fault
reactivation
v Outline the workflow and key elements of any
geomechanical risk assessment.
Learning, methods and tools
In this 5 half-day Virtual Instructor Led Training (VILT)
course, the data, methods and fundamental background for managing
geomechanical projects risks are addressed with a focus on
wellbore stability, compaction & subsidence and fault
reactivation. The participants will be provided with a
general approach how to identify geomechanical project risks as
well as the re-occurring elements of any geomechanical assessment.
Various case studies and examples will be discussed to demonstrate
concepts, theory and application.
This VILT course provides a number of quizzes which are made
individually, and a number of exercises that are made in small
groups or individually as appropriate. Exercises will involve some
calculations for which hand calculators (not provided) or simple MS
Excel spreadsheets are used. The VILT course provides a venue for
discussion, raising questions and sharing of experience.
Participants are encouraged to bring their own work issues and
challenges and seek advice from the expert course leader and other
participants.
Day by day programme
SESSION 1 / DAY 1: Geomechanical risk identification &
Stresses in the Earth
- Introduction: A brief introduction to what the VILT will cover
and the methods and media that will be utilised over the next 5
sessions. Learning objectives of participants are shared to address
individual needs.
- An overview is given of potential project risks that might need
a geomechanical assessment. This overview can be used as check list
to risks in your oil & gas, geothermal or CCS
project.
- The three principal stresses in the earth are introduced in
relation with the (geological) structural setting. Special
attention is paid to various field measurement techniques, e.g.,
FIT and XLOT, as well as the stress orientation near salt bodies,
folds and faults.
- The pressure-depth plot is introduced by means of some
examples. Field data and other sources are discussed to contruct
it. Main elements: pore pressure, overburden stress, minimum total
horizontal stress, maximum total horizontal stress and it's
orientation and effective stress.
- Quiz: On geomechanical risk identification & in-situ stress
measurement techniques.
SESSION 2 / DAY 2: Geomechanics fundamentals: Equilibrium,
stress & strain, tensile & shear failure
- The concept of stress and strain are introduced. Notation,
meaning and use of total, effective, principal, deviatoric and
isotropic stress are given. The intricacies of Mohr's circle as a
graphical representation of the stress in any point in the
subsurface is explained in detail.
- One-, two- and three-dimensional deformation is described using
linear elasticity theory, providing a relationship between stress
and strain. Elasticity parameters (Young's modulus & Poisson's
ratio) are introduced and how to convert them to (among others)
uniaxial compressibility and bulk compressibility.
- Rock mechanical experiments, such as uni-axial compression
test, are described and how to derive the linear-elastic
parameters. Drained- and undrained behaviour is linked to
poro-elasticity and experimental results. Discussion: when to
invest in lab experiments.
- Shear and tensile failure are explained using Mohr's circle.
The Mohr-Coulomb failure criterion and its parameters are
introduced and supported by experimental techniques and evidence.
Different metrics to express the onset of shear failure (such as
the Shear Capacity Utilisation and the Coulomb failure stress) are
discussed, explained and used.
- Exercise: Construct a Mohr stress circle and evaluate the onset
of shear failure based on the Mohr-Coulomb shear failure criiterion
(Coulomb Failure Stress, Shear Capacity Utilisation).
SESSION 3 / DAY 3: Stuck-pipe prevention and Bore-hole stability
evaluation.
- Why is bore hole stability important and how does the work plan
look like, including data acquisition and interpretation,
evaluation approach.
- The different causes of stuck-pipe incidents are discussed,
including hole cleaning, differential sticking, lost circulation
and bore hole break-outs. Video showing bore hole failure
(break-outs);examples that show the issues and options to prevent
bore hole instability.
- Kirsch equations are given that describe the stress
distribution around the bore hole.Radial and tangential stress
components and the role of mudweight are explained, The difference
between OBM and WBM for maintaining bore hole stability is shown;
The impact of well orientation and well deviation is explained;
Mudweight that causes break-outs and the mud weight window and
calculated; Mudweight that causes fracturing and lost circulation
are calculated; Break-out angle and other failure metrics are
introduced; Exercise: Determine the mudweight window (bring your
own case).
- Break-outs and lost-circulation are linked to shear and tensile
failure at the bore hole wall using linear-elasticity theory
(Kirsch equations), highlighting the difference between material
failure and operation (bore hole) failure. The use of FMI and
calliper logs is outlined in the determination of the potential
failure mode.
- The approach how to arrive at the optimum mud weight is
presented, and the relation of the mud weight window to the
Fracture Gradient is explained.
- Practical eamples of bore hole stability issues in
over-pressured environments and in-fill drilling in depleted
reservoir are given.
SESSION 4 / DAY 4: Constructing and evaluating 3D geomechanical
field models
- The purpose and approach of 3D geomechanical modelling is
described by a number of examples. Risk assessment objective;
Available & required data; Description of the work
plan.
- Incorporating the right structural features (layers, offset,
faults). What level of details is required, upscaling.
- Estimation of the initial stress and associated uncertainties.
Anisotropy of stress and relevance for risk assessment.
- Determination of elastic and shear failure models and their
properties of indentified formation from experimental results and
using petrophysical logs and correlation functions,
- Selecting relevant reservoir pressure and temperature
scenarios.
- Change of in-situ stress due to reservoir depletion &
subsequent reflation. The latter occurs if reservoirs are used for
storing CO2 for example. Understanding the horizontal
and vertical stress-path coefficient, stress arching, and its
relation to the elasticity parameters (notably: Poisson's ratio);
The influence of stiffness contrasts; The influence of the
geological structur; How stress change impact field development
(and geomechanical risk assessment).
- Quiz: 3D geomechanical modelling approach and deployment of
correlation functions.
- Exercise: calculating reservoir compaction and subsidence
SESSION 5 / DAY 5: Containment: top-seal integrity, fault
reactivation & induced seismicity
- An overview of the containment risks are given and how they can
be evaluated using 2D and 3D geomechanical models, with particular
attention to CO2 storage projects.
- Different mechanisms that may cause loss of top-seal integrity
(i.e. leakage of hydrocarbons or CO2 through the cap
rock) are discussed with special attention to the change of stress
in the top seal and the likelihood of shear and tensile
failure.
- Fault reactivation is explained using the Mohr circle and the
Mohr-Coulomb shear failure criterion. The difference between fault
reactivation and induced seismicity is explained, based on the
results of dynamic rupture simulations and observed seismicity in
the Groningen gas field. It is shown that reservoir depletion may
cause a-seismic fault slippage over extensive length thereby
potentially creating permeable pathways and loss of containment.
The following aspects are discussed and explained The impact of
fault orientation and elastic parameters; The impact of reservoir
thickness and reservoir offset; The maximum allowable depletion or
injection pressure.
- The (mis)use of 3D geomechanical models and Mohr-circle
analysis to estimate allowable reservoir and well injection
pressures to prevent fault reactivation is discussed. Alternative,
analytical approaches are demonstrated.
- Before course close-out, a practical approach is presented how
to identify the project risks that may need a geomechanical risk
assessment, and develop the work plan for their evaluation.