Elsevier

Earth-Science Reviews

Volume 185, October 2018, Pages 1157-1169
Earth-Science Reviews

Geothermal reservoir modeling in a coupled thermo-hydro-mechanical-chemical approach: A review

https://doi.org/10.1016/j.earscirev.2018.09.004Get rights and content

Abstract

Heat extraction from the geothermal reservoir is sensitive to reservoir properties, operating parameters and coupling among various processes. Due to complex reservoir structure, heat extraction performances and flow field behaviour in geothermal reservoirs are very different than the small scale laboratory model experiments. In recent past, significant progress in reservoir scale numerical modeling has been made for quantification of challenges in geothermal energy system development. In this paper, a comprehensive review of state of the art geothermal reservoir modeling for heat extraction is presented. Various numerical tools and approaches such as the finite difference method, finite element method, finite volume method, etc. to model the geothermal reservoir in the last four decades are discussed. The thrust of this paper is on a critical review of individual evaluation of coupling among all possible processes that are thermo, hydro, mechanical, and chemical processes on heat extraction performance. Future directions in developing better understanding on geothermal reservoir systems are proposed.

Introduction

Thermal energy stored in the earth's crust is known as geothermal energy. Geothermal heat originates within the earth by the decay of natural radioactive isotopes (uranium, thorium, potassium) trapped in magma during formation of the earth. Heat flows towards the surface by rising of the magmas and buoyantly circulating water. The energy usually heats the rock and fluids inside fractures and pores within the rock. However, heat flow and geothermal gradient are not uniform; regional and local variations are always there. The estimated average heat flow from the earth is 82 MW/cm2 and the total global output is over 4 × 1013 W (Uyeda, 1988).

Nowadays, geothermal energy has become a significant source of energy for heating/cooling of buildings and for electricity generation in several countries (New Zealand, Turkey, France, Germany, Norway, Iceland, Japan, Indonesia, USA, etc.). The worldwide installed capacity has exceeded 12GW, and annual energy production is around 70,000 GWh shown in Fig. 1 (Bertani, 2016). The application and utilization of georesource largely depend on resource temperature. For example, low temperature (30–90 °C) geothermal resources at shallow depths are utilized for direct applications such as heating/cooling of buildings, paper drying, chemical processing, fish farming, agriculture greenhouses, oil recovery from tar sands and water desalination (Stauffer et al., 2013; Noorollahi et al., 2017; Luo et al., 2015; Bundschuh et al., 2015; Gude, 2016; Focaccia et al., 2016; Goosen et al., 2010; Missimer et al., 2013; Holbein et al., 2016; Omer, 2008). At present, 82 countries utilize the low-temperature geothermal water for direct applications with an installed thermal power capacity of 70,885 MW and a thermal energy use of 164,635 GWh/year (Fig. 2) (Lund and Boyd, 2016). Fig. 3, Fig. 4 show the direct utilization of geothermal energy (MWt) and installed capacity of geothermal energy (MW) of top 10 countries. The moderate temperature geothermal resources in the range of 90 °C to 150 °C are useful for both direct use and electricity generation. Around 70% of the geothermal resource worldwide has a temperature <150 °C. Due to its low temperature, the thermal efficiency of plant is usually low (6–12%) (Zarrouk and Moon, 2014) and uneconomical for electricity production. For generation of electricity, the temperature of geothermal resources should be nearly 150 °C or more. The high-temperature geothermal resources are mostly located in active volcanic areas and adjoining the tectonic plate boundaries where active geothermal activity commonly happens in the form of hot springs, fumaroles, steam vents, and geysers. Beside these locations, high energy containing reservoir with a desirable temperature for electricity generation could also be possible if the rock is rich in radioactive minerals. Hot dry rock (HDR) or high heat producing granites can be a good source as these reservoirs store enormous amounts of heat energy. Such reservoirs are usually located at 3–10 km below the earth's surface. It is estimated that at a depth of 10 km across the world, HDR reservoir energy contains 1.3x1027J of energy which is 100–1000 times more than the quantity of fossil energy (Lu, 2018). However, HDR reservoirs are almost dry and impermeable in the natural state. The permeability enhancement of the reservoirs is required before actual production, by different techniques such as thermal, chemical or hydraulic stimulation. The hydro-fracturing method of reservoir enhancement is the most common method for the geothermal industry. In hydro-fracturing, fluid is injected into the reservoir rock at a pressure above the minimum in-situ principal stress at a given depth. This creates a fracture and opens up the pre-existing interlocking joints or new fractures. Once the reservoir permeability has been artificially enhanced, fluid can be circulated through the fractures/cracks. The fracture surface area acts as a heat exchanger between the host rock and the injected fluid. The engineered HDR is known as the enhanced geothermal system (EGS). In an EGS reservoir, three physical reservoir measurements are important for better exploitation of the resources. These are impedance, recovery factor and tracer-swept volume test (Grant, 2016).

This paper provides a state of art review of the various physical and chemical processes inside the geothermal reservoir that take place during fluid injection and heat extraction process. The review work has covered the numerical modeling approaches and development of various tools during last four decades. This paper also highlights the capability of various numerical tools to handle coupled processes. Based on the past studies, the paper is organized in four groups based on the coupled processes: thermo-hydro i.e. decoupled the mechanical and chemical effects, thermo-hydro-chemical i.e. decoupled the mechanical effect, thermo-hydro-mechanical i.e. decoupled the chemical effect, and fully coupled thermo-hydro-mechanical-chemical effects. The importance of these coupled processes and their influence on energy production or heat extraction are also highlighted.

Section snippets

Numerical modeling of geothermal reservoir

Numerical modeling of a geothermal reservoir is important for long-term understanding of the interaction of injected fluids with existing reservoir fluids and with the reservoir rocks. Relatively cold water injection into the reservoir disturbs the chemical, thermal and mechanical equilibrium of reservoir which modifies the porosity/permeability and other transport properties of the reservoir. The porosity/permeability the stress field changes largely depend on dissolution/precipitation

Geothermal heat extraction modeling

Porosity/permeability distribution, and presence of faults and fractures play an important role in fluid flow in a geothermal reservoir. The processes within the reservoir during relatively cold water injection are: convection (at the solid-fluid interface by the motion of fluid), advection (transport of heat as well as reactants products by bulk motions of fluids), heat conduction in the low permeable rock matrix, molecular diffusion, hydrodynamic dispersion and thermo-poro-elastic deformation

Governing equations of coupled THMC processes in geothermal reservoir

The governing equations for modeling a geothermal system include mass, momentum, energy, species transport, stress and displacement equations. Geothermal reservoirs can either be a porous medium or have permeable fractures inside a low permeable rock matrix. In case of latter, the modeling of various physical and chemical processes in fracture and rock matrix requires solving the partial differential equations separately but coupled at the fracture rock interfaces. The governing equations for

Thermo-hydro modeling

Hydro-thermal flow occurs when cold water is injected into the reservoir and hot water is pumped out from the reservoir after receiving the heat energy. In a thermo-hydro coupling, the effect of deformation (porosity/permeability evolution due to fluid pressure and chemical reaction are decoupled with deformation) is neglected but the temperature and pressure dependent fluids properties such as viscosity, density, heat transfer coefficient etc. are the main dominating variables that decide the

Thermo-hydro-chemical modeling

The injection of cold water into geothermal reservoirs enhances the water-rock reactions which initiates the dissolution/precipitation processes, alter the pore-geometry of the reservoir, and, as a consequence, their hydraulic and transport properties, such as porosity/permeability. A number of studies have focused on the evolution of the porosity/permeability by geochemical effects during heat extraction (Jing et al., 2002, Rabemanana et al. (2003), Kiryukhin et al., 2004, Bächler and Kohl,

Thermo-hydro-mechanical modeling

During heat extraction from a geothermal reservoir, the matrix and fracture/joints deform due to cooling and fluid overpressure. Cooling causes contraction while fluid overpressure results in the expansion of reservoir matrix. The spatial variation of thermal stress and pore pressure due to injection and production during operation can cause a non-uniform evolution of reservoir porosity/permeability. This creates a spatially varying reservoir transmissivity and may generate channelized flow

Thermo-hydro-mechanical-chemical modeling

The studies on coupled TH, THM and THC do not capture the complete evolution of permeability/porosity during geothermal heat extraction. In a real scenario, the porosity/permeability evolution is combined with effects of THMC processes. The coupling among the various processes is shown in Fig. 14. There are limited studies on modeling of fully coupled thermo-hydro-mechanical-chemical effects on geothermal reservoir evolution. Taron and Elsworth (2009) proposed a fully coupled THMC model in a

Conclusion and future directions

Long-term permeability change during geothermal heat extraction is influenced by complex interactions among fluid flow, reaction rates, mineralogy, thermo-mechanical properties of reservoir, residence time of fluid, joint stiffness and heterogeneity. However, detailed quantitative understanding remains to be developed for a field-scale geothermal system having a long-term sustainable operation. Based on previous numerical studies, it may be summarized that coupling among different physical

References (140)

  • A. Çelik et al.

    Experimental modeling of silicate-based geothermal deposits

    Geothermics

    (2017)
  • Y. Chen et al.

    Evaluation of Geothermal Development in Fractured Hot Dry Rock based on three Dimensional Unified Pipe-network Method

    Appl. Therm. Eng.

    (2018)
  • J. Craig et al.

    Hot springs and the geothermal energy potential of Jammu & Kashmir State, NW Himalaya, India

    Earth Sci. Rev.

    (2013)
  • R.A. Crooijmans et al.

    The influence of facies heterogeneity on the doublet performance in low-enthalpy geothermal sedimentary reservoirs

    Geothermics

    (2016)
  • G. Cui et al.

    Injection of supercritical CO2 for geothermal exploitation from single- and dual-continuum reservoirs: Heat mining performance and salt precipitation effect

    Geothermics

    (2018)
  • S. Focaccia et al.

    Shallow geothermal energy for industrial applications: a case study

    Sustainable Energy Technologies and Assessments

    (2016)
  • D.B. Fox et al.

    Sustainable heat farming: modeling extraction and recovery in discretely fractured geothermal reservoirs

    Geothermics

    (2013)
  • A. Franco et al.

    Numerical simulation of geothermal reservoirs for the sustainable design of energy plants: a review

    Renew. Sust. Energ. Rev.

    (2014)
  • D.L. Gallup et al.

    Control of Silica-Based Scales in Cooling and Geothermal Systems

  • Q. Gan et al.

    Production optimization in fractured geothermal reservoirs by coupled discrete fracture network modeling

    Geothermics

    (2016)
  • N. Garapati et al.

    Brine displacement by CO 2, energy extraction rates, and lifespan of a CO 2-limited CO 2-Plume Geothermal (CPG) system with a horizontal production well

    Geothermics

    (2015)
  • A. Ghassemi et al.

    Changes in fracture aperture and fluid pressure due to thermal stress and silica dissolution/precipitation induced by heat extraction from subsurface rocks

    Geothermics

    (2007)
  • A. Ghassemi et al.

    Thermo-poroelastic effects on reservoir seismicity and permeability change

    Geothermics

    (2016)
  • A. Ghassemi et al.

    A three-dimensional thermo-poroelastic model for fracture response to injection/extraction in enhanced geothermal systems

    Geothermics

    (2011)
  • A. Ghassemi et al.

    Effects of heat extraction on fracture aperture: a poro thermo elastic analysis

    Geothermics

    (2008)
  • M.A. Grant

    Physical performance indicators for HDR/EGS projects

    Geothermics

    (2016)
  • V.G. Gude

    Geothermal source potential for water desalination–current status and future perspective

    Renew. Sust. Energ. Rev.

    (2016)
  • T. Hadgu et al.

    Modeling of heat extraction from variably fractured porous media in Enhanced Geothermal Systems

    Geothermics

    (2016)
  • T. Heinze et al.

    A dynamic heat transfer coefficient between fractured rock and flowing fluid

    Geothermics

    (2017)
  • T.W. Hicks et al.

    A hydro-thermo-mechanical numerical model for HDR geothermal reservoir evaluation

    International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts

    (1996)
  • H. Hofmann et al.

    A hybrid discrete/finite element modeling study of complex hydraulic fracture development for enhanced geothermal systems (EGS) in granitic basements

    Geothermics

    (2016)
  • W. Huang et al.

    Heat extraction performance of EGS with heterogeneous reservoir: a numerical evaluation

    Int. J. Heat Mass Transf.

    (2017)
  • G. Izadi et al.

    Reservoir stimulation and induced seismicity: Roles of fluid pressure and thermal transients on reactivated fractured networks

    Geothermics

    (2014)
  • G. Izadi et al.

    The influence of thermal-hydraulic-mechanical-and chemical effects on the evolution of permeability, seismicity and heat production in geothermal reservoirs

    Geothermics

    (2015)
  • A.B. Jacquey et al.

    Thermo-poroelastic numerical modeling for enhanced geothermal system performance: Case study of the Groß Schönebeck reservoir

    Tectonophysics

    (2016)
  • J. Jamero et al.

    Mineral scaling in two-phase geothermal pipelines: two case studies

    Geothermics

    (2018)
  • P. Jeanne et al.

    A 3D hydrogeological and geomechanical model of an Enhanced Geothermal System at the Geysers, California

    Geothermics

    (2014)
  • P. Jeanne et al.

    Influence of injection-induced cooling on deviatoric stress and shear reactivation of preexisting fractures in Enhanced Geothermal Systems

    Geothermics

    (2017)
  • Z. Jing et al.

    A 3-D water/rock chemical interaction model for prediction of HDR/HWR geothermal reservoir performance

    Geothermics

    (2002)
  • S.N. Karlsdóttir et al.

    Corrosion behavior of materials in hydrogen sulfide abatement system at Hellisheiði geothermal power plant

    Geothermics

    (2017)
  • A. Kiryukhin et al.

    Thermal–hydrodynamic–chemical (THC) modeling based on geothermal field data

    Geothermics

    (2004)
  • J. Koh et al.

    A numerical study on the long term thermo-poroelastic effects of cold water injection into naturally fractured geothermal reservoirs

    Comput. Geotech.

    (2011)
  • T. Kohl et al.

    Coupled hydraulic, thermal and mechanical considerations for the simulation of hot dry rock reservoirs

    Geothermics

    (1995)
  • O. Kolditz

    Modelling flow and heat transfer in fractured rocks: Dimensional effect of matrix heat diffusion

    Geothermics

    (1995)
  • J. Li et al.

    Evaluation of mineral-aqueous chemical equilibria of felsic reservoirs with low-medium temperature: a comparative study in Yangbajing geothermal field and Guangdong geothermal fields

    J. Volcanol. Geotherm. Res.

    (2018)
  • S.M. Lu

    A global review of enhanced geothermal system (EGS)

    Renew. Sust. Energ. Rev.

    (2018)
  • J.W. Lund et al.

    Direct utilization of geothermal energy 2015 worldwide review

    Geothermics

    (2016)
  • F. Luo et al.

    Numerical investigation of fluid flow and heat transfer in a doublet enhanced geothermal system with CO2 as the working fluid (CO2–EGS)

    Energy

    (2014)
  • J. Luo et al.

    Heating and cooling performance analysis of a ground source heat pump system in Southern Germany

    Geothermics

    (2015)
  • S. Luo et al.

    The role of fracture surface roughness in macroscopic fluid flow and heat transfer in fractured rocks

    Int. J. Rock Mech. Min. Sci.

    (2016)
  • Cited by (122)

    View all citing articles on Scopus
    View full text