Geomechanical Model and Report

Key Knowledge Document   |    NS051-SS-REP-000-000012

This technical report focuses on the Northern Endurance Partnership’s approach to building a subsurface geomechanical model for CO2 sequestration in the Endurance structure in the southern North Sea.

What is a geomechanical model and why is it important?

Geomechanics focuses on the mechanical behaviour of rocks. It involves studying the physical and mechanical properties of rocks, such as their strength, elasticity, and deformation behaviour, under different loading conditions. This information can be acquired through a variety of tests that take place during drilling, as well as laboratory testing of core samples which provide data on the mechanical properties of the rock.

Geomechanics is crucial for carbon capture and storage (CCS) operations. During the CO2 injection process into a saline aquifer, the pressure of the reservoir will increase due to the poor compressibility of the original saline water in the formation. This pressure increase will be greatest close to the wellbore and decrease away from the wellbore. The magnitude and behaviour of this pressure decrease away from the wellbore relates to the injection rate and reservoir properties, such as porosity and permeability, as discussed in the dynamic model review. How the increase in pore pressure will impact the mechanical behaviour of the reservoir rocks and possible surrounding formations is examined in the geomechanical model. As CO2 is injected, it will cause changes in the pore pressure and stress distribution within the rock mass, leading to possible deformation, fracturing, and potential damage to the overlying rock formations. 

Understanding the mechanical behaviour of rocks in the storage formation and the overlying rocks is essential for predicting and preventing potential hazards that may arise during and after CO2 injection. By analyzing the properties of the rocks and the stresses applied by the injected CO2, geomechanical models can help predict the behaviour of the reservoir and surrounding rock, including their deformation and potential failure mechanisms. This information can be used to design safe and efficient storage sites and to develop monitoring and mitigation strategies to prevent or mitigate any adverse impacts on the associated rock formations.  In CO2 storage projects, the risks of increasing pore pressure in the subsurface are possible fracturing of the reservoir, possible fracturing of the overlying seal rocks, changes in ground or seafloor elevation and ground slope angle due to increased pressure at depth. These factors are generally detrimental to a CO2 storage project and a geomechanical model attempts to model what will happen at different injection scenarios so these problems can be mitigated before injection takes place.  

This report relates to other key documents as reviewed here. The Primary Store Geological Model and Report presents a field scale geological model which is built based on the substantial geophysical data The Primary Store Geophysical Model and Report. Then, the geological model is integrated into the dynamic model with a series of engineering data, for instance fluid properties, relative permeabilities, well design information. Finally, the dynamic model The Primary Store Dynamic Model and Report lays out the foundation for simulating reservoir fluid flow, which allows the geomechanical model to be effectively coupled to validate the containment of the stored CO2 and assess the possibilities of rock failure within the Endurance structure during injection.

Modelling of geomechanical properties in the subsurface and calibrating this to dynamic models is evolving quickly and due to the amount of data and size of the area studied the models become large and complicated. It should be noted the actual models were not available for review, but the extensive discussion of workflow and results presented in this study is a thorough and comprehensive analysis of the geomechanics in the Endurance structure.

Technical summary

This document focuses on applying geomechanical models and simulations to the static and dynamic models discussed in previous reports. The geomechanical workflow uses SLB’s Petrel Reservoir Geomechanics software and VISAGE finite-element geomechanics simulator. The workflow investigates the stress/strain changes within and above the Bunter Sandstone storage formation resulting from injection-induced pressure increases. The primary focus is to assess the potential impacts on four key areas due to the pore pressure increase caused by CO2 injection:

  • Failure of the Röt Halite and Röt Clay sealing units through tensile or shear failure.
  • Tensile or shear reactivation of faults mapped in the overburden of Endurance down to Top Röt Halite.
  • Uplift and tilt of the seabed.
  • Tensile or shear failure of the Bunter Sandstone.

In poro-elasticity theory, the effective stress applied on reservoir rock skeleton is equal to the total stress minus the pore pressure multiplied by a correction factor (Biot coefficient). The total stress is normally denoted as “S” and the effective stress is normally denoted as “σ”. For geomechanical studies we are concerned with rock failure which is often visualized Using Mohr Circle theory. The rock failure envelope and Mohr circle theory are based on the effective stresses “σ“ on the rock matrix.

The reservoir fluid flow simulation results are exported from NEXUS software (Halliburton) and then imported into the Petrel Sim Grid to get ready for coupling with VISAGE (SLB). Three key pressure cases relating to 3.5, 5 and 10 Mtpa of CO2 injection from the NEXUS dynamic model were simulated in VISAGE for 25 years injection, utilising different combinations of fault and matrix properties. The conclusion from all the work outlined in this report was that none of the simulations, using these key pressure cases, display any failure or reactivation of faults. Therefore, demonstrating probable effective CO2 containment.

It should be noted that since this modelling has been carried out as a one-way coupled process, VISAGE uses the outputs from the reservoir simulator to calculate the stresses and strains in the discrete time step. In geomechancial modelling there are one-way and two-way coupling solutions between reservoir fluid flow and geomechanics modules. In one-way coupling, pressure and temperature changes are passed from the reservoir code to the geomechanics module, but no information is passed back. In two-way coupling, iteration is carried out between the reservoir and stress solution at every timestep until the pore volumes and permeabilities calculated from the stress model and those used by the reservoir model agree.

However, the two-way coupled method requires much larger computing resources, longer simulation time, and higher costs. In early modeling stages, one-way coupling is often used to solve the problem and understand the risk severities. In general, a two-way coupled method is only required when there is a high possibility that the changes in porosity and permeability are pronounced, or the rock is close to its failure conditions.  In this report, one-way coupling methodology is considered sufficient but two-way coupling may be incorporated for further work.

The geomechanical grids and properties are built over the Endurance structure and outcrop area including the target Bunter Sandstone reservoir and all the overburden sequence up to the seabed. The geomechanical model is derived from that property grid but only built over the Endurance structure (also called the Phase 1 area in this report). As noted in the geological and geophysical review, no faults were observed from the available seismic data to indicate there were any faults penetrating through the top seal into the Bunter Sandstone.  For containment of CO2 the lack of visible faulting on seismic sections is encouraging, as faults can be potential leak points.  However, to the east of the Endurance anticline nearer to the subsea outcrop of the Bunter Sandstone, faults extending down into the upper Bunter Sandstone are observed. Therefore, the potential for sub-seismic faulting in the Endurance anticline was investigated. Five of the imported faults were copied and manually edited to extend down into the upper few layers of the Bunter Sandstone Z6 unit in the Phase 1 area. This methodology models the potential occurrence of faults, that can not be detected on the available seismic data.

As part of building the geomechanical model the mechanical properties of the geological formations of the Bunter Sandstone and cap rocks were calculated. This study integrates seismic interpretations of horizons and faults, well logs, geomechanical core data and fracture tests from the Endurance area for the whole stratigraphic section from seabed down to the base Zechstein salt (underlying the Bunter Sandstone unit). These data were used to create the geomechanical grid and properties in the 3D Petrel model. It is important to note that:

  • There are six wells located within the Endurance structure. The three within the spill points contain key stress and geomechanical property data used during the modelling.
  • P wave or compressional sonic data (used to calculate Young’s modulus for rock mechanical properties) is present in most wells but only Well 42/25d-3 contains S wave or shear sonic data (for shear modulus calculation). S wave sonic and density are created using correlations to P wave sonic.
  • Well 42/25d-3 drilled in 2013 is a dedicated appraisal well for this CO2 storage project and includes a significant amount of geological, reservoir engineering and geomechanical data specifically acquired for CO2 storage appraisal. The key elements of the geomechanical data acquisition and analysis program are:
  • Multiple confined core tests of static elastic parameters, static compressive strength and tensile strength plus acoustic velocities.
  • Openhole logs, image logs and advanced sonic logs to determine in-situ dynamic elastic and strength parameters and in-situ stress azimuths and horizontal stress anisotropy.
  • Formation Integrity Test (FIT) in the Röt Halite to determine minimum halite stress.
  • MicroFracture tests in the Röt Clay and Bunter Sandstone to obtain the minimum principal total stress, regarded as Shmin, which is in the horizontal minimum stress direction.

The three principal stress directions (Sv, Shmin and Shmax) also need to be measured or estimated to initialize the model, these estimates were based on the following data.  The bulk of the dedicated in-situ stress tests were taken in well 42/25d-3, Formation Integrity Test (FIT) and Leak-off Test (LOT) data are also available in several other wells. FIT and LOT data indicate that an Shmin value of 0.80 psi/ft or 0.182 bar/m is a reasonable estimate at the base of the Lias Group which is approximately 500m above the Bunter Formation at levels of -530 to -713 mTVDss. Density log derived estimates of the vertical principal total stress (Sv) indicate values of 0.99 to 1.03psi/ft from the base of the Lias Group to the Base of the Bunter Sandstone Formation respectively.

From the Sonic Scanner analysis in well 42/25d-3 and regional considerations, the Shmax/Shmin ratio within the Bunter Sandstone is estimated at approximately 1.05. In general, shales have higher Shmax/Shmin anisotropies than sandstones so a Shmax/Shmin of 1.10 is regarded as more reasonable for the Röt Clay. The Röt Halite and Zechstein halites are regarded as lithostatic where Sv = Shmax = Shmin.

The reservoir simulation grid (Sim Grid) and associated properties are imported into the working Petrel project for generating geomechanics grid. This geomechanics grid includes data for all overburden above the Bunter sandstone to the sea floor. The ‘Overburden All’ grid is derived from the Sim Grid with the addition of extra surfaces in the overburden sequence. Three separate grids were created to incorporate the overburden section above Endurance. This is required to accurately model the stresses, strains and displacements occurring in the matrix and on the mapped faults from the top of the Bunter Sandstone, which is the upper limit of the modelled injection pressures, to the seabed. The grid of the Phase 1 modelling area is created by extracting a subset area of the coarser Overburden All Grid using the Phase 1 area. Then, the model is upscaled to create a new larger grid with that Phase 1 area embedded within it.

For the geomechanical property modelling process, sonic and density logs were upscaled to and distributed within the Overburden Grid and then upscaled into the Geomechanical Grids. These are then used to create geomechanical properties. The log derived, elasticity and strength, geomechanical properties are matched to core data and/or in-situ stresses at Wells 42/25d‐3, 42/25‐1 and 43/21‐1 before populating the whole model. The sideburden and underburden outside of the phase 1 grid are created to stabilize the initial stresses after initializing boundary conditions. The geometric expansion of cells in the sideburden and the underburden minimizes large changes in cell dimensions.

The geomechanical properties of salt are quite different to the geomechanical properties of sandstones and shales and must be correctly accounted for in any biomechanical model. Salt layers are identified in the original geomodel so they can be assigned correct properties in the geomechanical model.  The distributions of elastic and Mohr Coulomb properties including Young’s modulus, Poisson ratio, shear modulus, bulk modulus, unconfined compressive strength, tensile strength cut-off, friction angle, dilation angle, are populated in clastic rocks and salts. Salts are difficult materials to model as they are typically less dense than surrounding rocks and deform by creep mechanisms on geological timeframes leading to lithostatic stress states. By using an equivalent elastic medium approach, a Poisson’s ratio of 0.495 is assigned to ensure a near lithostatic stress state and low shear stresses. A Young’s Modulus value of 0.75 GPa is calculated from the measured bulk modulus of approximately 25 GPa, from logs, with the assigned Poisson’s ratio of 0.495. Since salt is effectively self-sealing, pore spaces will tend to be isolated and surrounded by creeping salt with lithostatic pressures. The pore pressures are therefore set as not to contribute to the effective stress calculation in VISAGE.  

The imposed boundary conditions group of methods were the primary choice used to generate predictions of the initial in-situ stresses for assessing how well these initial stresses match with the available data. This is considered a normal geomechanical workflow if there is sufficient data.  Setting Young’s Modulus of 0.75 GPa and Poisson’s ratio of 0.495 attains the lithostatic stresses within the salt units. This leads to relatively large negative strains in the overlying units, particularly above Röt Halite and very low Shmin values (particularly in the shallow sequence).  This negative strain may lead to erroneous results in the model. Therefore, two separate imposed boundary condition initializations were created to mitigate this situation. The two boundary conditions were then merged to create a more robust stress initialization used in the subsequent simulations. The results indicated a good match between measured data and the initialized stresses from the model.

A 2D Mohr circle diagram of a notional failure envelope and Mohr circles for stress were developed to help understand the simulation results. In summary, as the pore pressure increases, this poroelastic coupling will tend to shrink the Mohr circles and decrease the principal effective stresses during CO2 injection but not enough to cause mechanical failure.

A series of model scenarios were simulated with different combinations of injection pressures, fault properties and extents, and matrix properties. The initial pressure plus injection pressures for five steps between years 2025 to 2050 and one post injection monitoring pressure at year 2500 were simulated in all cases.  Simulated injection schemes are 3.5 Mtpa with no brine production, 5.0 Mtpa with brine production and 10.0 Mtpa with brine production. None of these reference cases had any failure during injection. The highlights are summarized below.

  • The pressures equilibrate rapidly in the high permeability Bunter Sandstone. This means that differences in the number and placement of injectors are less important to Endurance Bunter Sandstone reservoir pressures than the total material balance of CO2 injected compared to the brine produced.
  • The Bunter Sandstone unit displays a clear poroelastic response with the total horizontal principal stresses increasing during CO2 This reduces the likelihood of failure in this unit by reducing the differential stress and keeping it below the modelled failure envelopes despite the effective stresses decreasing.
  • Modelled maximum uplift at the seabed occurs over the Endurance structure crest and ranges from 0.17m to 0.19m, which are toward the high end of expectation. It is likely some uplift will be absorbed within the overburden.
  • Horizontal in-situ stress reductions above the Bunter Sandstone are expected from the elastic inflation and stretching of the Bunter Sandstone during injection. The VISAGE modelling indicates a slight decrease in the Röt Clay Shmin (-0.01 to -0.03 psi/ft) and a maximum change of -0.078 psi/ft in the Quaternary over the Endurance crest. These shallow stress reductions are not regarded as a significant issue for Endurance, as they are likely to be absorbed by the overburden.
  • Modelled maximum tilt values of the seabed in all cases reported here are below 0.002° and generally found on the flanks of the structure. This is unlikely to cause significant issues with the planned Hornsea 4 windfarm and other infrastructure.

Technical comments for possible future work on the geomechanics model

This study presents an advanced workflow by coupling geomechanics and reservoir simulation resulting in a thorough analysis of the effects of increased pressure caused by injection CO2 on the geomechanical properties of the Endurance structure. The geomechanical model provides a useful exploration of the possible rock mechanics properties and in-situ stresses expected within and above the Endurance structure including the overburden fault system. After integration with a comprehensive data gathering and monitoring program, it is concluded that risks of seal breach or adverse seabed uplift and tilting effects are considered low in the planned CO2 injection schemes of up to 10 Mtpa with brine production where necessary. This reduces the likelihood of failure in this unit by reducing the differential stress and keeping it below the modelled failure envelopes despite the effective stresses decreasing. However, the elastic strain estimates reported here can be used as input to surface facility designs, data gathering and monitoring program design or for further modelling to provide more detailed characterisation.

The one-way coupling is a reasonable geomechanical workflow for an initial review of the field.  To investigate possible failure scenarios in more detail two approaches could be taken.

  1. Röt Clay failure could be investigated by dual porosity/dual permeability models that explicitly couple the geomechanical effects with the potential for fluid ingress from the Bunter Sandstone to the Röt Clay via joints or small faults. This is a classic analytical approach for seal breach analyses and a conservative assessment of cap rock integrity.
  2. Two-way coupled models could be used where some criteria for changes in the Bunter Sandstone reservoir pressures and stresses lead to a revised permeability and pore pressure in the adjacent Röt Clay unit. Using this methodology if a scenario was run where failure did occur the failure and the resulting fluid flow could be modelled more accurately.
Document information

Document name: Primary Store Geomechanical Model & Report

Reference number: NS051-SS-REP-000-00012

Document length: 78 pages

Topic area: Geomechanical model for CO2 sequestration

Project: Net Zero Teesside / Northern Endurance Partnership

Original report date: August 2021

Original author: BP Exploration Operating Company

Link to all original reports

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