Simulation Modeling of Coupled THM Processes in Geo-energy Applications

29/10/2024

The Pyrenees Mountains, Huesca, Spain | Anas Sidahmed

Doctoral Candidate: Anas Sidahmed (School of Geosciences, University of Edinburgh)

Principal supervisor: Christopher McDermott (Chair in Hydrogeology and Coupled Process Modelling, School of Geosciences, University of Edinburgh)

Host institution: University of Edinburgh

Output type: literature review and summary research topic

Summary research:

1. Introduction

Engineering and geo-energy projects, such as geothermal energy, CO2 storage, and underground heat storage, hold promise in achieving the goals of the Paris Agreement, which aims to mitigate climate change by keeping the global average temperature increase well below 2°C above pre-industrial levels.

Efficient, safe, and economically feasible application and utilization of these geo-energy technologies require a comprehensive understanding of the thermal, hydraulic, mechanical, and chemical (THMC) processes induced by thermal energy extraction and fluid injection from subsurface geological formations.

Consequences of implementing these geo-energy projects include surface perturbations such as induced earthquakes and surface deformations (uplift or subsidence), resulting from strain transfer from deep-seated geological layers. Such deformations are primarily induced by changes in THM processes in the subsurface. While these perturbations raise safety concerns, they also serve as reliable indicators for understanding, analyzing, and quantifying THM changes in the system.

The importance of this research stems from its focus on modeling THM coupled processes in such reservoirs, contributing to the overall technical understanding of how these coupled processes behave in reservoirs encountered in various subsurface engineering projects. Research outcomes are expected to establish useful tools and frameworks for developing generic design criteria and predictive models for geo-energy projects while ensuring safety, environmental, and economic considerations. Moreover, the design criteria should be tailored to specific systems.

2. Reserach Aims

This research aims to achieve two primary objectives: identifying key processes impacting deformation signals received at the surface during fluid injection, and identifying key geological facies-related controls on the transfer of subsurface reservoir strain to the surface. Furthermore, the research will explore different THM models upscaling methods and implement the geomechanical facies approach to upscale THM models from laboratory to field scale.

3. Brief Literature Review

3.1 THMC Processes

Thermal, hydraulic, mechanical, and chemical processes coexist and are interconnected in various natural and human-made settings such as geothermal reservoirs, nuclear waste disposal repositories, carbon capture and storage reservoirs, and underground energy storage facilities (geobattaries). These processes are described by mass balance, energy balance, and momentum balance equations.

3.2 Solution Methods

Analytical methods may provide exact solutions but are limited to simple geometries and idealized linear problems. However, numerical methods have the ability to model complex problems encountered in the real world. They can handle non-linearities, complex boundary conditions, and geometries. The most commonly used numerical methods are the Finite Difference Method (FDM), Finite Element Method (FEM), and Finite Volume Method (FVM).

3.3 Simulation Modeling

Lab-scale Modelling

Different types of testing cells and equipment, also known as standard rock failure testing cells 1 have been designed over the years to map and understand the key parameters that impact the behavior of rock mechanics 2–4. However, the designed cells were only able to model two specific triaxial stress conditions, which have rare occurrence in nature (when all or two principal stresses are equal) 5.

The SMART cell was developed to apply rotatable stress fields on small cylindrical samples (38 mm diameter and 76 mm length) 6–8. The GREAT cell (Geo-Reservoir Experimental Analogue Technology cell) was designed to recreate the deep subsurface stress conditions that take place on larger lab-scale cylindrical rock samples. Compared to the SMART cell, more improvements and advancements were applied to the GREAT cell in terms of sample size, stress control, and monitoring technology.

To test the capabilities of the new apparatus (GREAT cell), three different experiments were designed using different samples. To validate the experimental results, an open-source THMC code Finite Element based (FEM) 9  simulator by OpenGeoSys 10 was used. Gmsh software by 11 was utilized to create the mesh and the cylindrical geometry of the sample model. More details of the construction and modeling of the experiments can be found at 12.

Field-scale Modeling

CO2 Storage Formations

The stability and capacity of CO2 storage formations can be assessed with reasonable accuracy using the concept of geomechanical facies approach. This approach can be viewed as a set of geological facies that have been grouped and classified based on certain thermal, hydraulic, and mechanical properties that are essential in designing the storage system from an engineering perspective 13,14) and by coupling the geomechanical models with expected fluid pore pressure resulting from CO2 injection.

Heat Storage Mines

The work in 15,16 focused on modeling THM coupled processes to evaluate the key processes impacting the geomechanical stability of underground mining systems due to cyclical heat injection/extraction and set a framework for establishing regulatory limits on maximum safe operating conditions of mine water heat schemes. Results have shown that cyclic heat injection/production impacts both displacement and the stability of underground mines. Additionally, the results have concluded that the studied mine water heat schemes are more sensitive to operational parameters (e.g., injection temperature, pressure, and changes in water level) rather than the natural underlying geological conditions. Furthermore, it was found that stress buildup increases around the workings and decreases with distance from those workings, emphasizing the importance of understanding the lithological properties of the rocks above and below any workings.

3.4 THM Coupled Processes Models Upscaling

Spatial variations in hydrological, thermal, and mechanical properties of fluids and rocks are inherent in THM processes across different engineering and geological systems. Therefore, upscaling is an essential step towards understanding how THM processes may behave at different scale levels and to capture those spatial variations. The broad goal of upscaling is to build fast and cost-effective large field-scale models that are capable of accurately predicting the safety, environmental and economic performance of different systems such nuclear waste disposal repositories, carbon capture & storage fields, geothermal reservoirs and so on. Some examples of various THM upscaling methods include geomechanical facies 17,18, dimensional analysis, homogenization, and multi-scale homogenization, geostatistical modelling and data-driven modeling.

3.5 Previous Studies on THM Models Upscaling

The scale-dependent parameters in subsurface geological heterogeneities significantly impact THM processes, making it essential to model the effects of heterogeneities at different scales. Several researchers have conducted small-scale analyses based on field data. For instance, researchers in 19 have analyzed data collected from the Sellafield site 20 and concluded two relationships between FLM in-situ stress and permeability in fractured rocks. Additionally, 21 proposed a new method based on the theory of the modified crack tensor 22, utilizing THM properties in fractured rocks. Testing the new method on DECOVALEX III BMT2 23 showed a reasonable match between the predicted and measured values of Young’s modulus and permeability 24.

Key References:

  1. Natau, O. P. & Mutschler, T. O. Suggested method for large scale sampling and triaxial testing of jointed rock. International journal of rock mechanics and mining sciences & geomechanics abstracts 26, 427–434 (1989).
  2. Hoek, E. & Franklin, J. A. A simple triaxial cell for field and laboratory testing of rock. Trans. Instn Min. Metall. (1968).
  3. Handin, J., Heard, H. C. & Magouirk, J. N. Effects of the intermediate principal stress on the failure of limestone, dolomite, and glass at different temperatures and strain rates. J Geophys Res 72, (1967).
  4. Heard, H. C. Effect of Large Changes in Strain Rate in the Experimental Deformation of Yule Marble. J Geol 71, (1963).
  5. Zoback, M. D. Reservoir Geomechanics. Reservoir Geomechanics (2007). doi:10.1017/CBO9780511586477.
  6. Smart, B. G. D., Somerville, J. M. & Crawford, B. R. A rock test cell with true triaxial capability. in Geotechnical and Geological Engineering vol. 17 (1999).
  7. A true triaxial cell for testing cylindrical rock specimens : B. G. D. Smart, International Journal of Rock Mechanics & Mining Sciences, 32(3), 1995, pp 269–275. International journal of rock mechanics and mining sciences & geomechanics abstracts 33, A67–A67 (1996).
  8. Crawford, B. R., Smart, B. G. D., Main, I. G. & Liakopoulou-Morris, F. Strength characteristics and shear acoustic anisotropy of rock core subjected to true triaxial compression. International Journal of Rock Mechanics and Mining Sciences and 32, (1995).
  9. Thomas, H. R. The Finite Element Method in the Static and Dynamic Deformation and Consolidation of Porous Media. Second Edition, by R. W. Lewis and B. A. Schrefler, Wiley, Chichester, 1998. ISBN O-471-92809-7. GB£75.00. Communications in Numerical Methods in Engineering vol. 16 377 Preprint at https://doi.org/10.1002/(SICI)1099-0887(200005)16:5<377::AID-CNM329>3.0.CO;2-6 (2000).
  10. Kolditz, O. et al. OpenGeoSys: An open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes in porous media. Environ Earth Sci 67, (2012).
  11. Geuzaine, C. & Remacle, J.-F. Gmsh: A 3-D finite element mesh generator with built-in pre- and post-processing facilities: THE GMSH PAPER. Int J Numer Methods Eng 79, 1309–1331 (2009).
  12. McDermott, C. et al. New Experimental Equipment Recreating Geo-Reservoir Conditions in Large, Fractured, Porous Samples to Investigate Coupled Thermal, Hydraulic and Polyaxial Stress Processes. (2018).
  13. Edlmann, K., Edwards, M. A., Qiao, X. J., Haszeldine, R. S. & McDermott, C. I. Appraisal of global CO2 storage opportunities using the geomechanical facies approach. Environ Earth Sci 73, (2015).
  14. McDermott, C. et al. Screening the geomechanical stability (thermal and mechanical) of shared multi-user CO2 storage assets: A simple effective tool applied to the Captain Sandstone Aquifer. International Journal of Greenhouse Gas Control 45, (2016).
  15. Todd, F., McDermott, C., Harris, A. F., Bond, A. & Gilfillan, S. Coupled hydraulic and mechanical model of surface uplift due to mine water rebound: Implications for mine water heating and cooling schemes. Scottish Journal of Geology 55, (2019).
  16. Todd, F. Modelling the thermal, hydraulic and mechanical controlling processes on the stability of shallow mine water heat systems. (The University of Edinburgh, Edinburgh, 2023).
  17. McDermott, C. I. et al. Investigation of coupled hydraulic-geomechanical processes at the KTB site: pressure-dependent characteristics of a long-term pump test and elastic interpretation using a geomechanical facies model. Geofluids 6, 67–81 (2006).
  18. McDermott, C. I. et al. GEOMECHANICAL FACIES CONCEPT AND THE APPLICATION OF HYBRID NUMERICAL AND ANALYTICAL TECHNIQUES FOR THE DESCRIPTION OF HTMC COUPLED TRANSPORT IN FRACTURED SYSTEMS. PROCEEDINGS, Thirty-Second Workshop on Geothermal Reservoir Engineering (2007).
  19. Liu, H.-H., Rutqvist, J., Zhou, Q. & Bodvarsson, G. S. Upscaling of Normal Stress-Permeability Relationships for Fracture Networks Obeying Fractional Levy Motion. in vol. 2 263–268 (Elsevier, 2004).
  20. Jackson, C. P., Hoch, A. R. & Todman, S. Self-consistency of a heterogeneous continuum porous medium representation of a fractured medium. Water Resour Res 36, (2000).
  21. Guvanasen, V. & Chan, T. Upscaling the Thermohydromechanical Properties of a Fractured Rock Mass Using a Modified Crack Tensor Theory. in Elsevier Geo-Engineering Book Series vol. 2 (2004).
  22. Oda, M. An equivalent continuum model for coupled stress and fluid flow analysis in jointed rock masses. Water Resour Res 22, (1986).
  23. Liu, H.-H., Zhou, Q., Rutqvist, J. & Bodvarsson, B. Understanding the Impact of Upscaling THM Processes on Performance Assessment (DECOVALEX III BMT2) First Draft Final Report from DOE/LBNL Team. (2002).
  24. Swedish Nuclear Power Inspectorate, S. (Sweden). DECOVALEX III/BENCHPAR PROJECTS Approaches to Upscaling Thermal-Hydro-Mechanical Processes in a Fractured Rock Mass and Its Significance for Large-Scale Repository Performance Summary of Findings Report of BMT2/WP3. (2005)