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Geothermal Modelling

Geothermal energy is a sustainable energy that is increasingly being used to produce both geothermal heating and geothermal power as the world minimizes usage of carbon-based energy. There are several different types of geothermal energy, although they all share the same basic principle: fluid at relatively high temperature is produced from the subsurface in a producer well while a fluid (usually water) is injected at low/ambient temperature through an injector well to generate pressure gradients and facilitate fluid flow.  By contrast, in household systems, the inflow and outflow may be accommodated in the same wellbore.

At the industrial-scale, the three most common classes of system are:

  • Enhanced Geothermal Systems (EGS),  which exploit high temperature of rocks buried at depths of a few kilometres as a source of heat. The injection of fluid is carefully performed at appropriate conditions to facilitate re-opening of pre-existing fractures and enhance the permeability of the system thus allowing the circulation of the fluid. The circulating fluid transports the heat by advection to the surface. This type of geothermal energy may also be produced from Enhanced Gas Recovery (EGR) operations due to the suitability and similarity of the system. Furthermore if CO2 is used as an injection fluid during the EGR the (partially) depleted gas reservoir may be also used for geological storage (sequestration) of CO2.

  • The potential of abandoned mine workings for geothermal installations is also being investigated, for example at  UK Geoenergy Observatory Glasgow (UKGEOS).  These take advantage of the high permeability pathways provided by excavated and collapsed abandoned flooded hot mine shafts, galleries, etc, to circulate the injected and produced water.

  • Aquifer Thermal Energy (ATE) which exploits the thermal energy from high permeability sedimentary aquifers which facilitate the circulation of water. The energy may be directly obtained from Hot Sedimentary Aquifers (HSA) or stored and recovered seasonally in relatively low temperature shallow (< 200 m deep) aquifers (Aquifer Thermal Energy Storage, ATES). The most simple systems consist of a single injector and single producer wells (called a doublet) but a system may contain many doublets.

Evolution of the 110 ºC isotherm and temperature distribution in a 3D model of geothermal energy production in an Enhanced Geothermal System. Yellow layers have higher permeability than brown layers.

Initial and final temperature distributions due to geothermal energy production in a mine-workings

The success/feasibility of all of the above techniques rely on a quantitative assessment of the hydrodynamic system. As with any geological/geotechnical application there are inherent uncertainties and many challenges to be resolved to optimize production; e.g.: 

  • Is the considered scenario geological system technically suitable/economically profitable for geothermal energy production?

  • Which is the optimal injection/production rate to maximize energy supply while avoiding thermal breakthrough (i.e. when the produced fluid temperature becomes strongly influenced by the injected fluid temperature)?

  • What is the life cycle of the considered geothermal system?

  • If fracture stimulation is required, what is the required injection rate to facilitate growth of an appropriate fracture system, and what will be the potential environmental impact of this process?

  • What are the potential consequences of the stress field perturbation due to injection/production operations? How will the permeability of the system be affected by the pore pressure increase and temperature decrease near the injector.  Will the geology and operating conditions lead to the potential for fault reactivation?

  • Optimal design of ATE systems requires optimisation of the doublet spacing and flow rates to ensure longevity of the installation?

The problem thus requires an understanding of the coupled effects between geomechanical, fluid flow and thermal fields within the considered geological scenario. ParaGeo coupling capabilities enable the construction and calibration of numerical models for simulation and prediction of all classes of geothermal systems. This facilitates understanding and provides insight to aid assessment and decision making. There are many features in ParaGeo which are essential for geothermal modelling applications:

  • Thermo-Hydro-Mechanical coupling schemes

  • Well models for appropriate definition of the boundary conditions

  • Parallel solvers for multi-million element 3D models

  • Mechanical, fluid flow and thermal contact models for proper representation of faults and fractures

  • FracMan integration for importing discrete fracture networks (DFN) in the model

  • Embedded fracture models for treatment of fractures in a continuum manner (facilitating usage of coarser meshes, less CPU intensive)

  • Multiphase flow for CO2 (under development)

Simulated temperature distribution after 30 years of thermal energy production in a 3-doublet system injecting/producing from a Hot Sedimentary Aquifer

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