Scaling Issues Regarding The Formation of Fault Zones and Fluid Flow

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Scaling Issues Regarding The Formation of Fault
Zones and Fluid Flow

• Stephen J. Martel
• University of Hawaii
• Matthew dAlessio
• University of California
• Sponsored by U.S. Department of Energy

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Objectives

• To understand fault growth processes, their
  variety and effects, and how/if they change with
  scale
• Focus on low porosity, homogeneous, crystalline
  rocks

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The Problem of Fault Formation
Brace Bombolakis (1963)

• Scholz (1990)
• we are still left with the difficulty
  concerning the inability of shear cracks to
  propagate within their own planes. How, then, do
  faults form and grow to their often great
  lengths?

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Approach

• To investigate the faulting process by field
  observations and mechanical analyses
• To infer how the fracture connectivity develops
  and where the large aperture (high conductivity)
  fractures are likely to be

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Alternative Approaches
Fractal/Statistical Laboratory
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Faults as Slipped Joints
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Fault Linkages and Fluid Flow
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Fault Structure Affects Fault Linkage
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Faults from Cooling Joints, Hawaii
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Critique

• Several field studies have documented cases in
  which faults formed by the linking together of
  joints These cases, however, do not provide a
  satisfactory general explanation for fault
  formation. If this were always the mechanism of
  fault initiation, every case of faulting would
  have to be preceded by an early stress field that
  has an appropriate orientation to form the
  tensile fractures later reactivated in shear
  Furthermore, in these examples, the length of the
  reactivated joints is limited for general
  applicability the initial tensile fractures would
  have to be persistent enough to form the long
  fault systems observed. (Scholz, 1990)

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Fault as a Slipped Dike,Grimsel, Switzerland
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Fault as a Slipped Dike,Grimsel, Switzerland
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Echelon Fractures in a Dike,Sierra Nevada,
California
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Fault as a Slipped Dike, Sierra Nevada,
California
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The Great Dike, Zimbabwe
http//images.jsc.nasa.gov/images/pao/STS54/100654
88.htm
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Independence Dike Swarm, Sierra Nevada,California
http//www.geosci.unc.edu/Petunia/IDS_Web_Site/IDS
_14.html
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Implications

• Mode I can and does precede mode II over a broad
  range of scale
• Structural hydrologic heterogeneity increase
  as flaw size decreases (2-D representation of
  3-D mechanical process)

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Scaling of Secondary Fracturing Isolated
Faults (Ls/Lf0.01)
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Scaling of Secondary Fracturing Interacting
Faults (Ls/Lflt0.001)
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Conclusions (I)

• Even in homogeneous rocks, pre-existing
  heterogeneities control fault growth
• Heterogeneities vary with scale and location
• The distribution of flaws affect how they
  interact and link
• Although simple scaling rules might capture the
  gross behavior of faults, by themselves they can
  not capture the rich variety of structure and
  processes that occur along natural faults and
  that effect fluid flow

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Conclusions (II)

• Fault linkages affect fluid flow
• Conductivity high at edges of original flaws
• Structural and hydrologic heterogeneity should
  increase as original flaw size decreases