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 

  1. 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. 
  2. 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. 
  3. 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. 
  4. 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.
  5. Quiz: On geomechanical risk identification & in-situ stress measurement techniques. 


SESSION 2 / DAY 2: Geomechanics fundamentals: Equilibrium, stress & strain, tensile & shear failure

  1. 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. 
  2. 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. 
  3. 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. 
  4. 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. 
  5. 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.

  1. Why is bore hole stability important and how does the work plan look like, including data acquisition and interpretation, evaluation approach. 
  2. 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. 
  3. 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). 
  4. 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. 
  5. 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. 
  6. 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  

  1. 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. 
  2. Incorporating the right structural features (layers, offset, faults). What level of details is required, upscaling. 
  3. Estimation of the initial stress and associated uncertainties. Anisotropy of stress and relevance for risk assessment. 
  4. Determination of elastic and shear failure models and their properties of indentified formation from experimental results and using petrophysical logs and correlation functions, 
  5. Selecting relevant reservoir pressure and temperature scenarios. 
  6. 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). 
  7. Quiz: 3D geomechanical modelling approach and deployment of correlation functions.
  8. Exercise: calculating reservoir compaction and subsidence


SESSION 5 / DAY 5: Containment: top-seal integrity, fault reactivation & induced seismicity 

  1. 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. 
  2. 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. 
  3. 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. 
  4. 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.  
  5. 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.