Review of Generating Capacity Estimates for the Momotombo Geothermal Reservoir in Nicaragua

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Review of Generating Capacity Estimates for the
Momotombo Geothermal Reservoir in Nicaragua

• Grimur Bjornsson
• Geothermal Reservoir Engineer

http//ni.irias.biz/Volcanoes/Momotombo/
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Introduction

• 25 years of continuous generation
• Operation plagued with resource problems
• Installed capacity 77 MW, current output at 32 MW
  after a decline to 9 MW in 1999
• Early resorvoir models suggest substantially
  higher generating capacity (100-150 MW)
• Ormat asked author for a review on generating
  capacity estimates, to address discrepancy
  between predicted and actual field output

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Momotombo from Air
PP
N
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The Momotombo Wellfield
21 unsuccessful well 9 wells quenced in flow 7
wells for reinjection 11 wells in production
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Conceptual Reservoir Model
-200 0 200 400 600 800 1000 1200 1400 1600 1800 20
00
Depth (m u.s.l.)
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Wellfield Performance

• Generation peaked in 1988-1990
• Gradual decline after 1991
• Make-up drilling unsuccessful
• Stable generation after 2000
• Very low mean output per well

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Tracer Recoveries

• Napthalene disulfonate tracers into 4 wells at
  Eastern wellfield boundary in 2001-2002
• Recoveries monitored in 12 production wells
• Tracer recoveries indicate rapid fluid recharge
  from East field to all production wells -
  fracture dominated

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DAL 1994 Numerical Reservoir Model

• Accounts for shallow reservoir and vertical
  upflow zone
• Presumes extensive, deep reservoir
• 50-150 MW generating capacity
• Deep hypothetical reservoir sustains high
  generation

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Volumetric Reservoir Models
A possible resource study by Geothermex 2001
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Proven ResourceArea
500 mbsl
800 mbsl
White lines identify volumes hotter than 240
C From Porras et al., 2007
1300 mbsl
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Why Older Models Predict High Generating Capacity

• Calibrated against field data collected prior to
  rapid cold fluid invasion from East
• Deep reservoir layer below 1600 m u.sl. seems
  unlikely (1 well of 17 found permeability)
• Confirmed resource area appears smaller (2 km2 _at_
  240 C) than presumed in models
• Resource thickness also on high-side in old
  models
• If old models are adjusted for thickness and
  area, the current 32 MW output appears reasonable
  for the proven reservoir (15 MW/km2)

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A New 3-D Reservoir Model

• Calibrated in 2005-2006, TOUGH2 and iTOUGH2 EOS1
  (Porras et al.,2007)
• Accounts for new wells and much longer generation
  history than older models
• Captures losses in flowing enthalpies
• A 50 MW maximum 15 year generation capacity
  predicted by a single porosity model, presuming
  20 MW from 4 new wells
• Model is not considering new tracer data.

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Discussion

• The proven resource covers only 2 km2 and is
  1-1.5 km thick. Current generation of 15 MW/km2
  seems professional and reasonable
• Recent deep drilling has resulted in only 2
  MW/well mean output and flow declines by 5 per
  year
• Colder fluid recharge from East has led to
  chemical unstability and loss in enthalpies
• Mitigated by various efforts, such as deeper
  drilling, inhibition systems, acid jobs in
  wellbores and formation, mechanical cleaning,
  installing a binary unit and by revised
  reinjection strategy
• Cold recharge from East shows that heat reserves
  control production, not the mass reserves

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Means of Increasing Output

• Most available technologies for stabilizing
  production have already been implemented
• Manage cooling from East constant pressure
  boundary
• Increase distance between constant pressure
  boundary and production points
• Revisit the option of deeper drilling and
  locating favorable sectors outside current
  explotation concession boundary
• Statistically, the field developer should prepare
  for 2 MW mean well output and 5 decline per year

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Current Fluid Flow Model
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Conclusions

• The 25 years Momotombo development history has
  resulted in 32 MW stable production, despite
  drilling of nearly 50 wells and 77 MW installed
  capacity
• Year 1999 decline to 9 MW has been reversed by
  drilling of new wells, calcite scaling
  inhibitation, mechanical workovers, installation
  of 7 MW binary unit and revised injection
  strategy
• Older reservoir models suggest much higher
  generating potential than the current 32 MW
• These model were not supported by recent field
  data like loss in enthalpies, low success of deep
  drilling and limited proven resource area

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Conclusions, cont.

• If older reservoir models are revised in terms of
  area, thickness and include cold recharge from
  East, the current 32 MW stable generation looks
  professional and successful
• Proven reservoir generating capacity is more
  constraint by heat than mass reserves
• Wealth of field studies and reservoir
  understanding suggests that the field output can
  be raised
• Rapid flow decline of deep wells and low ouput
  calls for a resource contracting and leasing
  environement that allows for recovery of drilling
  costs

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Thank you!