Prйsentation PowerPoint

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Centre for Geothermal Research (1) Neuchâtel -
Switzerland
Centre de Géochimie de la Surface (2) Strasbourg
- France
Overview of chemical stimulations for EGS and
non EGS reservoirs
François-D. VUATAZ 1, Bertrand FRITZ 2 and
Laurent ANDRE 1
ENGINE Launching Conference BRGM - Orléans,
February 14th, 2006
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Plan of the presentation

• Acid treatments of reservoirs
• Methodology
• Different types of acidizing processes
• Chemical compounds
• Short inventory of reactive agents
• Mostly used compounds and their properties
• Examples of geothermal reservoir acidification
• Acidification of high temperature geothermal
  wells
• Chemical stimulation of EGS reservoirs
• The case of Soultz

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Acid treatment of reservoirs

• Aims
• Enhancement of well productivity
• Reduction of skin factor by removing
  near-wellbore damage
• Dissolution of scaling deposits in fractures.

• Technology mainly developed and applied for the
  development of oil reservoirs.
• Technology frequently applied for the cleaning
  and the stimulation of high temperature
  geothermal reservoirs.

Acidizing operation, 1932
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Matrix and fracture acidizing
Technology overview
Fracture acidizing
Matrix acidizing
Performed below fracturing rate and pressure
Performed above fracturing rate and pressure
Acid reacts with minerals present in existing
pores and natural fractures
Etching of sealed fractures providing well
stimulation, not just damage removal
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Matrix acidizing process (1)

• This technology is normally used for the removal
  of skin damage associated with work-over, well
  killing or fluids injection as well as to
  increase formation permeability in undamaged
  wells.

• Followed protocol
• Adequate preflush with hydrochloric acid (HCl)
  to dissolve associated carbonates
• Calcite CaCO3 HCl ? Ca2 Cl- HCO3-
• Dolomite CaMg(CO3)2 2 HCl ? Ca2 Mg2
  2 Cl- 2 HCO3-
• Mainflush with a correct HCl-HF mixture
  formulation
• Overflush with weak HCl or freshwater.

• Acid concentrations and amounts
• Acid concentrations vary from 6 to 12 for HCl
  and from 0.5 to 3 for HF.
• Acid amounts vary from 200 L/m of open hole
  section for wellbore cleanouts to gt2000 L/m for
  extended matrix acidizing.

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Matrix acididizing process (2)

• Role played by HCl during preflush
• Rapid dissolution reaction with carbonates rocks.
• Avoids further reaction of carbonates with HF in
  the next stage (no precipitation of calcium
  fluoride CaF2).

• Role played by HCl-HF mud acid during mainflush
• Reaction with associated minerals of sandstones
  (clays, feldspars and micas), rather than with
  quartz.
• Reactions of HF with clays or feldspars are 100
  to 200 times faster than the one with quartz.
• Use of HCl allows to keep a low pH and prevents
  precipitation of HF reaction products.

• Disadvantages of this method
• Acids dissolve the rock when reaching the grain
  surface, creating new pathways and/or wormholes
  (no connectivity).
• Si and Al have a strong affinity with F and
  silicium or aluminum complexes (SiF62-, AlF2,
  AlF2, AlF3, AlF4-). If they precipitate, the
  formation can be damaged by plugging.

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Fracture acidizing

• Also called acid fraccing, two main techniques
  could be used
• The fluid-loss control contains the acid in
  natural or newly opened fractures (use of
  packers).
• A viscous fluid is injected at a rate higher than
  the reservoir matrix will accept leading to
  cracking of the rock. Continued fluid injection
  increases the fractures length and width and
  injected HCl acid reacts all along the fracture
  to create a flow channel that extends deep into
  the formation.

• The key to success is the penetration of reactive
  acid along the fracture.

• The treatment volumes for fracture acidizing are
  much larger than for matrix acidizing treatment,
  being as high as 12 000 -25 000 L/m of open hole.

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Chemical compounds

• Reactive agents for carbonates and silicates
• Hydrochloric acid (HCl) and hydrochloric-hydrofluo
  ric (HCl-HF) mud acid
• Acetic acid (CH3COOH) and chloroacetic acid
  (ClCH2COOH)
• Formic acid (HCOOH)
• Sulfamic acid (H2NSO3H)
• Chelatants (EDTA)
• Reactive agents for quartz
• Sodium carbonate (Na2CO3).

• Additives
• Corrosion inhibitor to protect casings
• Anti-sludge agents
• Iron chelating agents
• Retardants to prolong the effect of the reactive
  agent further in the fractures
• Different solvents according to the treated
  formation.

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Strong acids

• Solution of inhibited HCl or HCl-HF mud

• Chemical formulation of mud acid depends on the
  rock composition
• Dilute mud acid HCl lt 7.5 and
  HF lt 1.5
• Regular mud acid 7.5 lt HCl lt 12 and
  1.5 lt HF lt 3
• Super mud acid 12 lt HCl lt 16 and
  3 lt HF lt 6
• Corrosion inhibitor (MEXEL, )

• Role and advantages
• Reaction with the carbonates and siliceous
  minerals
• Rapid reaction rates.

• Disadvantages
• Corrosion risks of the casing (to be evaluated)
  it can be strongly limited by the use of
  appropriate inhibitor, or by injection through a
  coil tubing.
• Precipitation risks of insoluble compounds formed
  between the fluoride from HF and the cations from
  the brine (mitigated by the use of HCl).
• High reactivity prevents a deep penetration into
  the formation. This drawback can be limited by
  retardants.

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Weak acids

• Mixture containing organic acid and HF

• Chemical formulation
• 9 formic acid (or 10 acetic acid)
• Corrosion inhibitor (MEXEL, )

• Role and advantages
• Reaction with the carbonates and siliceous
  minerals
• Dissolving capacity 25 higher than HCl
• pH higher than for strong acids, limiting the
  corrosion risks and the amounts of corrosion
  inhibitor
• Reaction rate slower than for HCl, allowing a
  better penetration into the formation.

• Disadvantages
• Corrosion risks on the casing (to be evaluated)
  risks can be strongly limited by the use of
  appropriate inhibitor, or by injection through a
  coil tubing.

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Chelating agents

• These solutions are used as formation cleanup and
  for stimulating oil and gas wells especially in
  formations that may be damaged by strong acids.

• Chemical formulation
• EDTA (Ethylenediaminetetraacetic acid), HEDTA
  (Hydroxyethylenediaminetriacetic acid), HEIDA
  (Hydroxyethyliminodiacetic acid)
• HCl
• Corrosion inhibitor (MEXEL, )

• Role and advantages
• Acting as a solvent, increasing the water-wetting
  properties and dissolving (entirely or partially)
  minerals containing Fe, Ca, Mg and Al
• Dissolving capacity 50 higher than HCl
• pH higher than HCl, limiting corrosion risks and
  amounts of corrosion inhibitor
• Reaction rate slower than for HCl allowing a
  better penetration into the formation.

• Disadvantages
• Corrosion risks are more limited than with strong
  or weak acids.
• Environmental problems in case of fluid discharge.

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Acidification of high temperature geothermal wells
Results of HCl-HF treatments for scaling removal
in geothermal wells
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Chemical stimulation of EGS reservoirs
Attempts to increase the reservoir connectivity

• Fenton Hill, Los Alamos Scientific Laboratory
  (USA)
• In November 1976, an attempt was carried out to
  reduce the impedance of the existing system by a
  chemical leaching treatment. The base Na2CO3 was
  used to dissolve quartz from the formation.
• 190 m3 of 1 N Na2CO3 solution were injected. A
  considerable amount of quartz (about 1000 kg) was
  dissolved and removed from the granitic reservoir
  but no reduction impedance resulted.

• Fjällbacka (Sweden)
• The granitic reservoir contains abundant
  fractures and minor fractures zones which showed
  an evidence of being hydraulically conductive and
  which were filled with calcite, chlorite and clay
  minerals.
• 2 m3 of HCl-HF acid were injected in Fjb3 to
  leach fracture filling. Qualitatively, the
  results showed the efficiency of acid injection
  in returning rock particles.

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Acidification tests at Soultz GPK2 well

• 23/01/03 injection of HCl acid solution at a
  concentration of 1.8 g.L-1 and a flow of 30
  L.s-1.
• 12/02/03 injections of HCl acid solution at
  concentrations of 1.8 g.L-1 and 0.9 g.L-1 for
  flows about 15 and 30 L.s-1, respectively.
• During this test, 1.5 tons of HCl were injected.

Estimation of the increase of GPK2 injectivity
due to acidification from 0.3 to 0.5 L.s-1.bar-1
(From Gérard et al., 2005)
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Acidification tests at Soultz GPK3 well

• June 2003 Acidification run during a
  circulation test between the injector GPK3 and
  the producer GPK2.
• 950 m3 of an acid solution at a concentration of
  about 3.2 g.L-1 injected at a flow of 21.3 L.s-1.
• During this test, 3 tons of HCl were injected.

Difficult to estimate the real increase of GPK3
injectivity no water injection test was
performed in similar conditions before and after
acidification.
(From Gérard et al., 2005)
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Acidification tests at Soultz GPK4 well

• 23/02/05 5200 m3 of HCl acid solution at a
  concentration of about 2 g.L-1 and a flow of 27
  L.s-1.
• During this test, 11 tons of HCl were injected.

Water injection test performed before
acidification (February 22, 2005)
Water injection test performed after
acidification (March 13, 2005)

• 40 reduction of the wellhead pressure due to the
  acidification treatment(under evaluation).
• Decrease of the reservoir impedance by a factor 2
  (0.2 to 0.4 L.s-1.bar-1).
• Due to a leak in the casing, there are still
  doubts on the effect of acid in GPK4.

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Preliminary conclusions on the chemical
stimulation

• Long and successful experience acquired from the
  oil industry
• Large number of methods and experiences set up
  for oil and gas wells.
• Effect of acid stimulation is usually limited to
  the first metres around the wells.
• Procedures are partially adapted to geothermal
  reservoirs.

• High temperature geothermal fields
• Numerous wells in geothermal fields have been
  chemically stimulated, mostly by strong acids
  (Philippines, El Salvador, USA, Italy).
• Mineral deposits on casings and around the well
  are treated successfully several times per year
  at Heber geothermal field, California.
• Corrosion damage can be mostly avoided by using
  adequate inhibitors.

• EGS reservoirs
• Old projects only a few chemical stimulations
  were realised (Fenton Hill, Fjällbacka).
• The Soultz EGS has probably the best experience
  on soft HCl stimulation so far.
• Modelling the effect of acid stimulation for the
  Soultz reservoir is under progress.
• New experiments are planned to stimulate GPK4
  well and to connect it to major fractures.

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• Thank you for
• your attention