Centre for Geothermal Research Centre of Hydrogeology University of Neuchâtel Acid treatm ents on geotherm al w ells: Acid treatm ents on geotherm al w ells: first experim ents and m odelling m odelling first experim ents and at the Soultz Soultz EGS fractured reservoir. EGS fractured reservoir. at the Sandrine PORTIER, Laurent ANDRE, and François-D. VUATAZ CREGE June 29-30 th , 2006 ENGINE Workshop3
Acid treatm ents treatm ents Acid � 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; Acidizing operation, 1932 � 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 CREGE June 29-30 th , 2006 ENGINE Workshop3
Acidizing processes processes Acidizing Types of acidizing Matrix acidizing Fracture acidizing processes performed below fracturing flow performed above fracturing flow Process rate and pressure rate and pressure Aims enhancing well productivity; cracking of the rock; reducing skin factor by removing farthest penetration of acid along near-wellbore damage the fracture 3 steps : injecting 15% HCl, then acid is injected in natural/created Procedure an HCl-HF mixture, followed by a fractures by fluid-loss control (use sufficient afterflush of water to of packers, viscosity of acid, clear all acid from well tubulars addition of solid particulates) Injected chemical formulation of mud acid injection of a viscous fluid depends on the rock composition fluid properties Treatment 120 up to 6000 L/m of open hole 12 000 - 25 000 L/m of open hole volumes Additives corrosion inhibitor to protect tubulars during exposure to acid CREGE June 29-30 th , 2006 ENGINE Workshop3
Cleaning of of geotherm al geotherm al w ells w ells ( 1 ) ( 1 ) Cleaning � 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. CREGE June 29-30 th , 2006 ENGINE Workshop3
Cleaning of of geotherm al geotherm al w ells w ells ( 2 ) ( 2 ) Cleaning Geothermal Fields Number of Injectivity Index Improvement Results of HCl-HF treated wells (kg/s/bar) factor treatments for 0.68 � 3.01 4.4 scaling removal Bacman (Philippines) 2 0.99 � 1.4 1.4 and connectivity development 3.01 � 5.84 1.9 0.68 � 1.77 Leyte (Philippines) 3 2.6 1.52 � 10.8 7.1 4.7 � 12.1 Salak (Indonesia) 1 2.6 11 � 54 5 4 � 25 7 1.5 � 18 Larderello (Italy) 5 12 - 4 11 � 54 5 1.6 � 7.6 4.8 1.4 � 8.6 6.1 Berlín 0.2 � 1.98 5 9.9 (El Salvador) 0.9 � 3.4 3.8 1.65 � 4.67 2.8 Beowawe (USA) 1 - 2.2 Coso (USA) 30 24 wells successful CREGE June 29-30 th , 2006 ENGINE Workshop3
Cleaning of of geotherm al geotherm al w ells w ells ( 3 ) ( 3 ) Cleaning � 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. CREGE June 29-30 th , 2006 ENGINE Workshop3
First acidification test at Soultz Soultz: GPK4 w ell : GPK4 w ell First acidification test at February 2005: 5200 m 3 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). ∆ P ≈ 40 bars Water injection test performed after after Water injection test performed Water injection test performed before before Water injection test performed acidification (March 13, 2005) acidification (March 13, 2005) acidification (February 22, 2005) acidification (February 22, 2005) Drop of the wellhead pressure : possibly due to minerals dissolution (From Gérard et al., 2005) CREGE June 29-30 th , 2006 ENGINE Workshop3
ENGINE Workshop3 RMA acid job at Soultz RMA acid job at GPK4 injectivity due to acidification : from 0.3 to 0.4 L/s/bar. 2,5 hours. During this test, 98 tons of HCl were injected. Estimation of the increase of mixture treatment), with addition of a corrosion inhibitor, at a flow rate of 22 l/s during Main flush : injection of 200 m 3 of Regular Mud Acid (RMA), (12 % HCl - 3 % HF acid May 2006 : acid treatment performed in four stages. before and after the acidification test on GPK4 well (May 2006). (GEIE, 2006). Impact of the RMA acidification test on the wellhead measured by comparison Injection over pressures in bars (0 is 28 bars) Before RMA test After RMA test (GMT) Dates/Time 10 15 20 25 30 35 40 45 50 55 0 5 "29.05.2006 04:09:59" RMA stimulation result GPK4: "29.05.2006 08:22:27" "29.05.2006 12:37:48" "24.04.2006 18:13:04" "29.05.2006 16:47:48" "24.04.2006 22:23:04" "29.05.2006 20:57:48" "25.04.2006 02:33:04" "30.05.2006 01:07:48" "25.04.2006 06:43:04" "30.05.2006 05:17:48" "25.04.2006 10:53:04" "30.05.2006 09:27:38" "25.04.2006 15:03:04" "30.05.2006 13:37:38" Soultz: GPK4 w ell "25.04.2006 19:13:04" "30.05.2006 17:47:38" "25.04.2006 23:23:04" X Axis Title "30.05.2006 21:57:38" "26.04.2006 03:33:04" "31.05.2006 02:07:38" "26.04.2006 07:43:04" "31.05.2006 06:17:38" "26.04.2006 11:53:04" "31.05.2006 10:27:28" "26.04.2006 16:03:04" "31.05.2006 14:37:28" "26.04.2006 20:13:04" : GPK4 w ell "31.05.2006 18:47:28" "27.04.2006 00:23:04" CREGE "31.05.2006 22:57:28" "27.04.2006 04:33:04" "01.06.2006 03:07:28" "27.04.2006 08:43:04" "01.06.2006 07:17:28" "27.04.2006 12:53:04" "01.06.2006 11:27:17" "27.04.2006 17:03:04" "01.06.2006 15:37:17" "27.04.2006 21:13:04" "01.06.2006 19:47:17" "28.04.2006 01:23:04" "01.06.2006 23:57:17" "28.04.2006 05:33:04" "02.06.2006 04:07:17" "28.04.2006 09:43:04" (GMT) Dates/Time 0 5 10 15 20 25 30 35 40 45 50 55 Injected Flowrates (l/s) Before RMA test After RMA test ∆ P ≈ 16 bars June 29-30 th , 2006 major fractures. this well to should connect treatments new chemical successful, but were rather jobs on GPK4 Recent acid
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 • K matrix = 10 -15 m 2 /Pa • Porosity matrix = 0 � Fractured zones • K frac = 7.4 10 -8 m 2 /Pa • Porosity frac = 10 % � Q frac = 0.02 L/s � T injection = 65 °C and T initial = 200 °C � P injection = 8 MPa and P production = 0 MPa CREGE June 29-30 th , 2006 ENGINE Workshop3
Sim ulation of acidification tests ( 2 ) Sim ulation of acidification tests ( 2 ) 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) 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 Assumptions on water-rock interactions � Major circulation occurs in hydrothermally altered granite. � Acid interacts with carbonates, quartz, K-feldspars, sulfides and clays. CREGE June 29-30 th , 2006 ENGINE Workshop3
Results : : m inerals behaviour m inerals behaviour Results 2.0E-02 � High reactivity of carbonates and -1 m -3 ) massive dissolution of these carbonates reaction rate (mol.s 1.5E-02 compounds near the injection well. extended acidification 1.0E-02 � soft acidification The rapid reaction means the acid does not penetrate very far Calcite 5.0E-03 into the formation before it is Dolomite spent. 0.0E+00 � Soft acidification: 11 tons of HCl -5.0E-03 � 20 % of the carbonates are 0 10 20 30 40 50 dissolved in in the first 3.5 m. distance (m) 1.0E-06 � -1 .m -3 ) Extended acidification: 98 tons of HCl mineral reaction rate (mol.s 5.0E-07 � 70 % of the carbonates are dissolved in a radius of 7.5 0.0E+00 m. � Weak impact on other Quartz minerals: low precipitation of K-Feldspar -5.0E-07 AmSilica K-feldspar and amorphous silica, quartz is not affected -1.0E-06 by the HCl acidification. 0 10 20 30 40 50 distance (m) CREGE June 29-30 th , 2006 ENGINE Workshop3
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