Acid treatm ents on geotherm al w ells: Acid treatm ents on geotherm - - PowerPoint PPT Presentation

Acid treatm ents on geotherm al w ells: Acid treatm ents on geotherm al w ells: first experim ents and first experim ents and m odelling m odelling at the at the Soultz Soultz EGS fractured reservoir. EGS fractured reservoir.

Sandrine PORTIER, Laurent ANDRE, and François-D. VUATAZ

Centre for Geothermal Research Centre of Hydrogeology University of Neuchâtel ENGINE Workshop3

CREGE

June 29-30th, 2006

Acid Acid treatm ents treatm ents

Acidizing operation, 1932

• Long and successful experience acquired from the oil industry

Large number of methods and experiences set up for oil and gas wells. Procedures partially adapted to the needs for geothermal reservoirs.

• Aims

enhancing well productivity; reducing skin factor by removing near-wellbore damage; dissolving the scales in fractures.

• Reactants used

Conventional acid systems

• HCl acid and HCl-HF mud acid
• Mixture containing organic acid and HF

Chelatants (EDTA family) Retarded acid systems

• Addition of retardants to prolong the effect of the reactive agent

further in the fractures

• Types of acidizing processes

Matrix acidizing Fracture acidizing ENGINE Workshop3

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June 29-30th, 2006

12 000 - 25 000 L/m of open hole 120 up to 6000 L/m of open hole

Treatment volumes

injection of a viscous fluid chemical formulation of mud acid depends on the rock composition

Injected fluid properties

acid is injected in natural/created fractures by fluid-loss control (use

• f packers, viscosity of acid,

addition of solid particulates) 3 steps : injecting 15% HCl, then an HCl-HF mixture, followed by a sufficient afterflush of water to clear all acid from well tubulars

Procedure

performed above fracturing flow rate and pressure performed below fracturing flow rate and pressure

Process

cracking of the rock; farthest penetration of acid along the fracture enhancing well productivity; reducing skin factor by removing near-wellbore damage

Aims

corrosion inhibitor to protect tubulars during exposure to acid

Additives

Fracture acidizing Matrix acidizing

Types of acidizing processes

Acidizing Acidizing processes processes

ENGINE Workshop3

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June 29-30th, 2006

Cleaning Cleaning of

• f geotherm al

geotherm al w ells w ells ( 1 ) ( 1 )

• High temperature geothermal fields

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

ENGINE Workshop3

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June 29-30th, 2006

Cleaning Cleaning of

• f geotherm al

geotherm al w ells w ells ( 2 ) ( 2 )

5 11 54 4

• 12

1.5 18 7 4 25 5 11 54 5 Larderello (Italy) 2.6 4.7 12.1 1 Salak (Indonesia) 2.2

• 1

Beowawe (USA) 24 wells 1.65 4.67 0.9 3.4 0.2 1.98 1.4 8.6 1.6 7.6 1.52 10.8 0.68 1.77 3.01 5.84 0.99 1.4 0.68 3.01 Injectivity Index (kg/s/bar) 4.4 2 Bacman (Philippines) 1.4 2.6 7.1 successful 30 Coso (USA) 2.8 3.8 9.9 6.1 4.8 5 Berlín (El Salvador) 1.9 3 Leyte (Philippines) Improvement factor Number of treated wells Geothermal Fields

Results of HCl-HF treatments for scaling removal and connectivity development ENGINE Workshop3

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June 29-30th, 2006

Cleaning Cleaning of

• f geotherm al

geotherm al w ells w ells ( 3 ) ( 3 )

• EGS reservoirs

Only two chemical stimulation were performed on past EGS reservoirs : Fenton Hill (USA) and Fjällbacka (S). The Soultz EGS has probably the best experience on soft HCl / RMA stimulation.

ENGINE Workshop3

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June 29-30th, 2006

First acidification test at First acidification test at Soultz Soultz: GPK4 w ell : GPK4 w ell

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

∆P ≈ 40 bars

Drop of the wellhead pressure : possibly due to minerals dissolution

(From Gérard et al., 2005)

February 2005: 5200 m3 of HCl acid solution at 2 g/L and a flow of 27 L/s. A total of 11 tons of HCl were injected.

• 35% reduction of the wellhead pressure due to acidification
• Decrease of the reservoir impedance by a factor 1.5 (0.2 to 0.3 L/s/bar).

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June 29-30th, 2006

RMA acid job at RMA acid job at Soultz Soultz: GPK4 w ell : GPK4 w ell

∆P ≈ 16 bars

5 10 15 20 25 30 35 40 45 50 55

5 10 15 20 25 30 35 40 45 50 55

GPK4: RMA stimulation result

Dates/Time (GMT) Dates/Time (GMT) Injection over pressures in bars (0 is 28 bars) Before RMA test After RMA test X Axis Title Injected Flowrates (l/s) Before RMA test After RMA test

Impact of the RMA acidification test on the wellhead measured by comparison before and after the acidification test on GPK4 well (May 2006). (GEIE, 2006). May 2006 : acid treatment performed in four stages. Main flush : injection of 200 m3 of Regular Mud Acid (RMA), (12 % HCl - 3 % HF acid mixture treatment), with addition of a corrosion inhibitor, at a flow rate of 22 l/s during 2,5 hours. During this test, 98 tons of HCl were injected. Estimation of the increase of GPK4 injectivity due to acidification : from 0.3 to 0.4 L/s/bar.

Recent acid jobs on GPK4 were rather successful, but new chemical treatments should connect this well to major fractures.

ENGINE Workshop3

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June 29-30th, 2006

Sim ulation of acidification tests ( 1 ) Sim ulation of acidification tests ( 1 )

• Use of the thermo-hydraulic-chemical coupled code : FRACHEM
• Geometrical model is intended to represent Soultz site
• Well configuration and data for mineralogical composition were taken from the

EGS at Soultz.

2-D simplified geometrical model 1250 fractured zones Matrix

• Kmatrix = 10-15 m2/Pa
• Porositymatrix = 0

Fractured zones

• Kfrac = 7.4 10-8 m2/Pa
• Porosityfrac = 10 %

Qfrac = 0.02 L/s Tinjection = 65 °C and Tinitial = 200 °C Pinjection = 8 MPa and Pproduction = 0 MPa ENGINE Workshop3

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June 29-30th, 2006

Sim ulation of acidification tests ( 2 ) Sim ulation of acidification tests ( 2 )

Assumptions on water-rock interactions

• Major circulation occurs in

hydrothermally altered granite.

• Acid interacts with carbonates, quartz,

K-feldspars, sulfides and clays. Characteristics of injected fluids

• Injection of HCl acid solutions at two different concentrations
• Soft acidification: 2 g/L during 60 hours
• RMA treatment: 15 g/L during 70 hours
• Temp. injection = 65°C
• Total injection flow fixed at 25 L/s

Characteristics of formation fluid

• NaCl brine, Temp. = 200°C, pH = 4.9
• TDS ~ 100 g/kg (ionic strength ~ 1.8 m)
• Chemical composition : analysis of GPK2 (1999)

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June 29-30th, 2006

Results Results : : m inerals behaviour m inerals behaviour

• High reactivity of carbonates and

massive dissolution of these compounds near the injection well.

• The rapid reaction means the

acid does not penetrate very far into the formation before it is spent.

• Soft acidification: 11 tons of

HCl

20 % of the carbonates are

dissolved in in the first 3.5 m.

• Extended acidification: 98 tons
• f HCl

70 % of the carbonates are

dissolved in a radius of 7.5 m.

Weak impact on other

minerals: low precipitation of K-feldspar and amorphous silica, quartz is not affected by the HCl acidification.

• 1.0E-06
• 5.0E-07

0.0E+00 5.0E-07 1.0E-06 10 20 30 40 50 distance (m) mineral reaction rate (mol.s

• 1.m-3)

Quartz K-Feldspar AmSilica

• 5.0E-03

0.0E+00 5.0E-03 1.0E-02 1.5E-02 2.0E-02 10 20 30 40 50 distance (m) carbonates reaction rate (mol.s

• 1m-3)

extended acidification soft acidification

Calcite Dolomite

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June 29-30th, 2006

Results: consequences on reservoir porosity Results: consequences on reservoir porosity

• In FRACHEM code : double model
• f fracture and grain.
• Porosity increases near the

injection well due to carbonates dissolution (calcite and dolomite).

• Porosity increase within the first 10

metres from injection well.

• Pressure decrease near the

injection well.

• Impact on the reservoir injectivity.
• 5.00
• 4.00
• 3.00
• 2.00
• 1.00

0.00 100 200 300 400 500 600 distance (m) Over pressure (P(t=70h)-Pinit) (bar)

soft acidification extended acidification

Over pressure

• 10

10 20 30 40 50 60 70 80 90 0.1 1 10 100 1000

distance (m) amount of mineral (kg)

Calcite Dolomite AmSilica Kfeldspar Quartz

Mass variation of minerals after extended acidification .

0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 10 20 30 40 50 distance (m) Porosity

extended acidification soft acidification

Porosity

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June 29-30th, 2006

Modelling of acid stimulation experiments is carried out for the Soultz reservoir

Expected increase of porosity due to the dissolution of calcite and dolomite

present in the fractures.

Mixing an acid solution with the formation brine instead of fresh water prevents

a weak precipitation processes of pyrite, quartz and amorphous silica.

Due to the high reactivity of HCl, all these simulated processes occur in a very

limited zone around the injection well. Improvement of the simulation of the acidification processes

by increasing the injection times of low concentration solutions or by

augmenting the acid concentration of the injected fluids;

by increasing the injection flow to allow a farther acid transport trough the

fractures;

by increasing the acid injection pressure to simulate fracture acidizing.

Conclusion on m odelling the im pact of acid jobs Conclusion on m odelling the im pact of acid jobs

ENGINE Workshop3

CREGE

June 29-30th, 2006

Thank Thank you you for for your your attention attention