Acid treatments on geothermal wells: Acid treatments on geothermal wells: first experiments and first experiments and modellingmodelling

at the at the SoultzSoultz EGS fractured reservoir.EGS fractured reservoir.

Sandrine PORTIER,

Laurent ANDRE,

and François-D. VUATAZ

Centre for Geothermal Research Centre of HydrogeologyUniversity of Neuchâtel

ENGINE Workshop3 CREGE June 29-30th, 2006

AcidAcid treatmentstreatments

Acidizing operation, 1932

Long and successful experience acquired from the oil industryLarge 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 usedConventional 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 acidizingFracture acidizing

ENGINE Workshop3 CREGE June 29-30th, 2006

12 000 - 25 000 L/m of open hole120 up to 6000 L/m of open holeTreatmentvolumes

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

Injectedfluid

properties

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

of packers, viscosity of acid, addition of solid particulates)

3 steps : injecting 15% HCl, thenan 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 acidAdditives

Fracture acidizingMatrix acidizingTypes of acidizing

processes

AcidizingAcidizing processesprocesses

ENGINE Workshop3 CREGE June 29-30th, 2006

CleaningCleaning of of geothermalgeothermal wellswells (1)(1)

High temperature geothermal fieldsNumerous 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 CREGE June 29-30th, 2006

CleaningCleaning of of geothermalgeothermal wellswells (2)(2)

511 544-

121.5 1874 25511 54

5Larderello (Italy)

2.64.7 12.11Salak (Indonesia)

2.2-1Beowawe (USA)24 wells

1.65 4.670.9 3.40.2 1.981.4 8.61.6 7.6

1.52 10.80.68 1.773.01 5.840.99 1.40.68 3.01

Injectivity Index (kg/s/bar)

4.42Bacman (Philippines)

1.4

2.67.1

successful30Coso (USA)

2.83.89.96.14.8

5Berlín

(El Salvador)

1.93Leyte (Philippines)

Improvement factor

Number of treated wells

Geothermal FieldsResults of HCl-HF treatments for scaling removal and connectivity development

ENGINE Workshop3 CREGE June 29-30th, 2006

CleaningCleaning of of geothermalgeothermal wellswells (3)(3)

EGS reservoirsOnly 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 CREGE June 29-30th, 2006

First acidification test at First acidification test at SoultzSoultz: GPK4 well: GPK4 well

Water injection test performed Water injection test performed beforebeforeacidification (February 22, 2005)acidification (February 22, 2005)

Water injection test performed Water injection test performed afterafteracidification (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).

ENGINE Workshop3 CREGE June 29-30th, 2006

RMA acid job at RMA acid job at SoultzSoultz: GPK4 well: GPK4 well

∆P ≈ 16 bars

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GPK4: RMA stimulation result

Dates/Time(GMT)

Dates/Time (GMT)

Inje

ctio

n ov

er p

ress

ures

in b

ars

(0 is

28

bars

)B

efor

e R

MA

test

Afte

r RM

A te

st

X Axis TitleIn

ject

ed F

low

rate

s (l/

s)B

efor

e R

MA

test

Afte

r RM

A te

st

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 CREGE June 29-30th, 2006

Simulation of acidification tests (1)Simulation of acidification tests (1)Use of the thermo-hydraulic-chemical coupled code : FRACHEMGeometrical 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 CREGE June 29-30th, 2006

Simulation of acidification tests (2)Simulation of acidification tests (2)

Assumptions on water-rock interactionsMajor circulation occurs in

hydrothermally altered granite. Acid interacts with carbonates, quartz, K-feldspars, sulfides and clays.

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

ENGINE Workshop3 CREGE June 29-30th, 2006

ResultsResults : : minerals behaviourminerals behaviourHigh 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 of 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

0 10 20 30 40 50distance (m)

min

eral

reac

tion

rate

(mol

.s-1.m

-3)

QuartzK-FeldsparAmSilica

-5.0E-03

0.0E+00

5.0E-03

1.0E-02

1.5E-02

2.0E-02

0 10 20 30 40 50distance (m)

carb

onat

es re

actio

n ra

te (m

ol.s-1

m-3

)

extended acidification

soft acidification

Calcite

Dolomite

ENGINE Workshop3 CREGE June 29-30th, 2006

Results: consequences on reservoir porosityResults: consequences on reservoir porosity

In FRACHEM code : double model of 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

0.000 100 200 300 400 500 600

distance (m)

Ove

r pre

ssur

e (P

(t=70

h)-P

init)

(bar

)

soft acidificationextended acidification

Over pressure

-10

0

10

20

30

40

50

60

70

80

90

0.1 1 10 100 1000distance (m)

amou

nt o

f min

eral

(kg) Calcite

DolomiteAmSilicaKfeldsparQuartz

Mass variation of minerals after extended acidification

.

0.09

0.1

0.11

0.12

0.13

0.14

0.15

0.16

0 10 20 30 40 50distance (m)

Poro

sity

extended acidification

soft acidification

Porosity

ENGINE Workshop3 CREGE June 29-30th, 2006

Modelling of acid stimulation experiments is carried out for the Soultz reservoirExpected 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 modelling the impact of acid jobsConclusion on modelling the impact of acid jobs

ENGINE Workshop3 CREGE June 29-30th, 2006

Thank Thank youyou for for youryour attentionattention