Evolution of permeability in fracture networks: Geophysical and geochemical effects in enhanced geothermal systems

J. Taron, D. Elsworth, O. Kolditz

Research output: Chapter in Book/Report/Conference proceedingConference contribution

2 Scopus citations

Abstract

Dominant mechanisms for permeability change in hydrothermal fracture networks are driven by the combined action of thermal, hydrologic, mechanical, and chemical forcings. Potential mechanisms include, but are not limited to, thermomechanical deformation, mineral reaction, shear dilation, and chemical-mechanical creep. While some effort has been devoted to examining each of these processes individually, magnitudes of relative interaction remain poorly constrained at small- and largescale. In this work, a numerical simulator is used to model these processes at reservoir scale. Permeability and porosity are modified as fractures dilate or contract under the influence of pressure solution creep, thermo-hydro-mechanical compaction/dilation, and bulk mineral reaction in a deformable, dual-porosity medium. Simulations focus on a prototypical enhanced geothermal system as cold (70°C) water is injected at geochemical disequilibrium within a heated reservoir (250°C). For an injector withdrawal doublet, separated by 500m, the results demonstrate the strong influence of mechanical effects in the short term (several days), the influence of thermal effects in the intermediate term (<1 month at injection), and the prolonged and long-term (>1 year) influence of chemical effects, especially close to injection. Differences are examined between small scale, frequent fractures and large scale, more widely spaced fractures. Permeability increases more for widely spaced fractures as the thermal strain is distributed on fewer fractures per unit volume of reservoir. This in turn results in a lowering of fluid temperature in the reservoir as fluid throughput increases (under constant pressure drive) and as thermal diffusion length to the fracture concomitantly increases. A contact area based model for pressure solution creep is retrofit into the simulator and results indicate potential importance for pressure solution at reservoir scale. However, an equilibrium simplification is incapable of examining long term compaction trends, and a kinetic based form may be necessary to reproduce these large scale behavior.

Original languageEnglish (US)
Title of host publicationGeothermal Resources Council Annual Meeting 2010, Geothermal 2010
Pages432-439
Number of pages8
StatePublished - 2010
EventGeothermal Resources Council Annual Meeting 2010, Geothermal 2010 - Sacramento, CA, United States
Duration: Oct 24 2010Oct 27 2010

Publication series

NameTransactions - Geothermal Resources Council
Volume34 1
ISSN (Print)0193-5933

Other

OtherGeothermal Resources Council Annual Meeting 2010, Geothermal 2010
CountryUnited States
CitySacramento, CA
Period10/24/1010/27/10

All Science Journal Classification (ASJC) codes

  • Renewable Energy, Sustainability and the Environment
  • Energy Engineering and Power Technology
  • Geophysics

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