Fluid pressure drops during stimulation of segmented faults in deep geothermal reservoirs

Guillem Piris (Corresponding Author), Albert Griera, Enrique Gomez Rivas, Ignasi Herms, Mark McClure, Jack H. Norbeck

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Abstract

Hydraulic stimulation treatments required to produce deep geothermal reservoirs present the risk of generating induced seismicity. Understanding the processes that operate during the stimulation phase is critical for minimising and preventing the uncertainties associated with the exploitation of these reservoirs. It is especially important to understand how the phenomenon of induced seismicity is related to the pressurisation of networks of discrete fractures. In this study, we use the numerical simulator CFRAC to analyse pressure drops commonly observed during stimulation of deep geothermal wells. We develop a conceptual model of a fractured geothermal reservoir to analyse the conditions required to produce pressure drops and their consequences on the evolution of seismicity, fluid pressure, and fracture permeability throughout the system. For this, we combine two fracture sets, one able to be stimulated by shear-mode fracturing and another one able to be stimulated by opening-mode fracturing. With this combination, the pressure drop can be triggered by a seismic event in the shear-stimulated fracture that is hydraulically connected with an opening-mode fracture. Our results indicate that pressure drops are not produced by the new volume created by shear dilatancy, but by the opening of the conjugated tensile fractures. Finally, our results reveal that natural fracture/splay fracture interaction can potentially explain the observed pressure drops at the Rittershoffen geothermal site.
Original languageEnglish
Article number24
Pages (from-to)1-21
Number of pages21
JournalGeothermal EnergyScience – Society – Technology
Volume6
DOIs
Publication statusPublished - 16 Nov 2018

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fluid pressure
pressure drop
induced seismicity
dilatancy
simulator
seismicity
permeability
hydraulics
well

Keywords

  • enhanced geothermal reservoirs
  • pressure drops
  • reservoir simulation
  • induced seismicity
  • fracture networks

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Fluid pressure drops during stimulation of segmented faults in deep geothermal reservoirs. / Piris, Guillem (Corresponding Author); Griera, Albert; Gomez Rivas, Enrique; Herms, Ignasi; McClure, Mark; Norbeck, Jack H.

In: Geothermal EnergyScience – Society – Technology, Vol. 6, 24, 16.11.2018, p. 1-21.

Research output: Contribution to journalArticle

Piris, Guillem ; Griera, Albert ; Gomez Rivas, Enrique ; Herms, Ignasi ; McClure, Mark ; Norbeck, Jack H. / Fluid pressure drops during stimulation of segmented faults in deep geothermal reservoirs. In: Geothermal EnergyScience – Society – Technology. 2018 ; Vol. 6. pp. 1-21.
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note = "Acknowledgements The Institut Cartogr{\`a}fic i Geol{\`o}gic de Catalunya is acknowledged for their support in our investigation of Geothermal resources. G. Piris was supported by an AGAUR grant of the Industrial Doctorate programme 2016-DI-031. EGR acknowledges the support of the Beatriu de Pin{\'o}s programme of the Government of Catalonia’s Secretariat for Universities and Research of the Department of Economy and Knowledge (2016 BP 00208). The authors would like to thank three anonymous reviewers and the editors Dr. Carola Meller and Prof. Olaf Kolditz for their helpful comments that improved this manuscript.",
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N1 - Acknowledgements The Institut Cartogràfic i Geològic de Catalunya is acknowledged for their support in our investigation of Geothermal resources. G. Piris was supported by an AGAUR grant of the Industrial Doctorate programme 2016-DI-031. EGR acknowledges the support of the Beatriu de Pinós programme of the Government of Catalonia’s Secretariat for Universities and Research of the Department of Economy and Knowledge (2016 BP 00208). The authors would like to thank three anonymous reviewers and the editors Dr. Carola Meller and Prof. Olaf Kolditz for their helpful comments that improved this manuscript.

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N2 - Hydraulic stimulation treatments required to produce deep geothermal reservoirs present the risk of generating induced seismicity. Understanding the processes that operate during the stimulation phase is critical for minimising and preventing the uncertainties associated with the exploitation of these reservoirs. It is especially important to understand how the phenomenon of induced seismicity is related to the pressurisation of networks of discrete fractures. In this study, we use the numerical simulator CFRAC to analyse pressure drops commonly observed during stimulation of deep geothermal wells. We develop a conceptual model of a fractured geothermal reservoir to analyse the conditions required to produce pressure drops and their consequences on the evolution of seismicity, fluid pressure, and fracture permeability throughout the system. For this, we combine two fracture sets, one able to be stimulated by shear-mode fracturing and another one able to be stimulated by opening-mode fracturing. With this combination, the pressure drop can be triggered by a seismic event in the shear-stimulated fracture that is hydraulically connected with an opening-mode fracture. Our results indicate that pressure drops are not produced by the new volume created by shear dilatancy, but by the opening of the conjugated tensile fractures. Finally, our results reveal that natural fracture/splay fracture interaction can potentially explain the observed pressure drops at the Rittershoffen geothermal site.

AB - Hydraulic stimulation treatments required to produce deep geothermal reservoirs present the risk of generating induced seismicity. Understanding the processes that operate during the stimulation phase is critical for minimising and preventing the uncertainties associated with the exploitation of these reservoirs. It is especially important to understand how the phenomenon of induced seismicity is related to the pressurisation of networks of discrete fractures. In this study, we use the numerical simulator CFRAC to analyse pressure drops commonly observed during stimulation of deep geothermal wells. We develop a conceptual model of a fractured geothermal reservoir to analyse the conditions required to produce pressure drops and their consequences on the evolution of seismicity, fluid pressure, and fracture permeability throughout the system. For this, we combine two fracture sets, one able to be stimulated by shear-mode fracturing and another one able to be stimulated by opening-mode fracturing. With this combination, the pressure drop can be triggered by a seismic event in the shear-stimulated fracture that is hydraulically connected with an opening-mode fracture. Our results indicate that pressure drops are not produced by the new volume created by shear dilatancy, but by the opening of the conjugated tensile fractures. Finally, our results reveal that natural fracture/splay fracture interaction can potentially explain the observed pressure drops at the Rittershoffen geothermal site.

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