Centre for Geothermal Research (1)
Centre de Géochimie de la Surface (2)
Neuchâtel - Switzerland
Strasbourg - France
Overview of chemical stimulations
for EGS and non EGS reservoirs
François-D. VUATAZ 1,
Bertrand FRITZ 2
and Laurent ANDRE
ENGINE Launching Conference
1
BRGM - Orléans, February 14th, 2006
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
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
Matrix and fracture acidizing
Technology overview
Matrix acidizing
Fracture 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
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 >2000 L/m for extended matrix acidizing.
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.
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 fracture’s 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.
Chemical compounds
Reactive agents for carbonates and silicates
Hydrochloric acid (HCl) and hydrochloric-hydrofluoric (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.
Strong acids
Solution of inhibited HCl or HCl-HF mud
Chemical formulation of mud acid depends on the rock composition
Dilute mud acid:
HCl < 7.5 % and
HF < 1.5 %
Regular mud acid: 7.5 % < HCl < 12 % and 1.5 % < HF < 3 %
Super mud acid: 12 % < HCl < 16 % and
3 % < HF < 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.
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.
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.
Acidification of high temperature geothermal wells
Results of HCl-HF
treatments for
scaling removal in
geothermal wells
Geothermal Fields
Number of
treated
wells
Bacman (Philippines)
2
Leyte (Philippines)
3
Injectivity
Index
(kg/s/bar)
Improvement
factor
0.68  3.01
4.4
0.99  1.4
1.4
3.01  5.84
1.9
0.68  1.77
2.6
1.52  10.8
7.1
Tiwi (Philippines)
1
2.52  11.34
4.5
Mindanao (Philippines)
1
12.4  21.5
1.7
Salak (Indonesia)
1
4.7  12.1
2.6
1.6  7.6
4.8
1.4  8.6
6.1
0.2  1.98
9.9
0.9  3.4
3.8
1.65  4.67
2.8
Berlín (El Salvador)
5
Beowawe (USA)
1
–
2.2
Coso (USA)
30
24 wells
successful
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.
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.
Impact on wellhead pressure drop :
probably due to dissolution in the
vicinity of the well.
Estimation of the increase of GPK2
injectivity due to acidification :
(From Gérard et al., 2005)
from 0.3 to 0.5 L.s-1.bar-1
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.
Drop of the wellhead
pressure of GPK3 : possibly
due to effect of acid on the
minerals.
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)
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)
DP  40 bars
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.
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.