Obtaining Permeability in Geothermal Wells by Targeting Fault Damage Zones

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SINCLAIR KNIGHT MERZ

• Obtaining Permeability in Geothermal Wells by
  Targeting Fault Damage Zones
• Ian Bogie, P.J. White, J.V. Lawless, G.N. Ussher
  and P.R. Barnett

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Introduction

• Good permeability is essential for a successful
  geothermal well
• Blind drilling and limited coring means that the
  nature of high permeability is often speculative
• Formation imaging tools have revealed that many
  zones of high permeability correspond to
  fracture/fault networks, or fault damage zones
• These networks by their nature are associated
  with master faults, even if the master faults are
  not intersected by the well
• This paper aims to apply this information to
  improving the targeting structural permeability
  in geothermal wells

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Fault Damage Zones

• Occur around various faults
• including the strike-slip and normal faults in
  geothermal systems that structural permeability
  is most commonly attributed to
• Are fractal
• forming similar patterns at a wide variety of
  scales
• Previous work (e.g. Kim et al. (2004)) identify
  three different types of zones on strike slip
  faults
• tip-,
• wall-, and
• linking-damage zones
• These can also be generally applied to normal
  faults, which are more often the target in
  geothermal reservoirs

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Tip Damage ZonesInitial opening
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Wall Damage Zones

• Develop with ongoing slip upon the fault
• extension fractures, antithetic faults, synthetic
  faults and rotated blocks with associated
  triangular openings
• As a fault progressively moves,
• the damage zones evolve through varying
  proportions of tip and wall damage zones.
• the damage zone gets wider and more intense

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Wall Damage Zones
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Link Damage Zones

• when en echelon faults propagate towards each
  other, or a master fault bends
• can be extensional or contractional.
• Extensional link zones include pull-aparts,
• Contractional zones include antithetic and
  synthetic faults
• common to both types are rotated blocks and
  strike slip duplexes

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Link Damage Zones
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Permeability requires interconnected open space

• Host lithology capable of brittle fracture.
• Otherwise the damage zone lacks permeability and
  the fault overall acts as a permeability barrier.
  
• Intense shearing can create a fine pug
• there may still be good permeability in the
  surrounding damage zone

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Hydrothermal Alteration Effect on Permeability

• generally produces clays, which make the rock
  less competent and less permeable.
• Vein minerals - silicification and, where boiling
  is occurring, adulariasation,
• May provide brittle rock
• Enable re-establishment of stress
• The overall effect is to enhance the fracturing
  process,
• providing that a competent lithology was present
  in the first place and it is resistant to
  alteration to clays
• Andesites are the most common lithology in which
  permeable damage zones develop in geothermal
  systems.
• Only narrow zones of illitic alteration form
  around fractures, and much of the alteration is
  propylitic

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Fractal Scaling

• For a single fault, the damage zone is at a scale
  of metres or tens of metres
• On geothermal field scale
• The entire productive part of a geothermal field
  may lie within a fault link damage zone, with a
  sweet spot in which highly productive wells are
  drilled.
• The master faults might never be intersected by a
  well, and if they are, they are not particularly
  productive.
• The fault related nature of permeability may
  never be recognised

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Implications for Well Targeting 1

• Fault targets are not necessarily simple planar
  features
• With a link damage zone,
• the target will be the entire competent
  lithological unit
• should not be confused with stratigraphic
  permeability.
• In targeting a well, orientations that provide
  the longest intersection with the target zone are
  favoured
• For a single fault,
• the target zone will occupy a zone around the
  fault where it intersects a competent lithology.
  
• For young volcanic piles in extensional tectonic
  environments this is typically where steeply
  dipping normal faults intersect andesitic flows,
  intrusives or basement rocks

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Implications for Well Targeting 2

• The orientation of the master fault/faults
  need not have any simple relationship to the
  orientation or distribution of permeable features
  in the damage zone
• A well can intersect the master fault BUT cross a
  fault damage zone parallel to the permeable
  features.
• Less likely in footwall directional wells,
  vertical wells and wells that are oblique to the
  fault strike,
• more information is needed to accurately target
  footwall directional and vertical wells
• Where a link zone can be recognised at the
  surface (e.g. a pull apart basin),
• the orientation of open space fractures in the
  link zone can be predicted from the sense of
  movement on the master faults

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A Pull-apart basin in South Sumatra Suoh
Geothermal Field
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Well Targeting
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Implications for Well Targeting 3

• Detailed data is required to determine where
  competent lithologies intersect fault damage
  zones, and the orientation of permeable features
  in the damage zones
• Requires detailed structural mapping
• Judicious use of formation imaging tools.
• Little information will be available for an
  initial exploration well, but data from early
  wells will greatly assist with accurate targeting
  of later production wells

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Conclusions

• Structural permeability is found where fault
  damage zones intersect competent lithologies
• Volumes of permeability around individual master
  faults and
• Larger scale features in link zones between
  master faults
• Permeable fractures can have a variety of
  orientations,
• requiring wells to be oriented correctly to
  maximise the number and quality of intersections
  with them
• Can be achieved utilising vertical wells and
  oblique footwall directional wells
• Information from downhole formation imaging tools
  early in an exploration program can greatly
  assist with accurate well targeting

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