In the Cerro Prieto Geothermal field, where the predominant fluid in the reservoir is water, they have had problems with silica and other deposits in the first exploration wells as well as in production wells. Scaling problems have also been encountered in silencers, cyclone separators, drains, water pipes, etc. Some scale problems have also been encountered in the turbine blades of the geothermal electric plant. Most of these problems have been solved by corrective procedures which, in some cases, have turned into routine. Scale deposition is a problem that certainly diminishes the useful capacity of geothermal fluids with water predominance, but it does not actually endanger the installations, since this problem is under control

Guiza, J.

Mercado, S.

UNT Digital Library

PROBLEMS OF SILICA SCALING AT CERRO PRIETO GEOTHERMAL POWER STATION S. Mercado and J. Guiza Federal Commission of E l e c t r i c i t y  Mexico Abstract  I n  t h e  Cerro P r i e t o  Geothermal f i e l d ,  where the predominant f l u i d  i n  t h e  r e se rvo i r  i s  w a t e r ,  w e  have had problems with s i l i c a  and other depos i t s  i n  t h e  f i r s t  exploration w e l l s  as w e l l  as i n  production w e l l s .  Scaling problems have a l s o  been encountered i n  s i l ence r s ,  cyclone separa tors ,  d ra ins ,  water pipes,  e t c .  i n  t h e  tu rb ine  blades of t h e  geothermal electric p lan t .  problems have been solved by co r rec t ive  procedures which, i n  some cases, have turned i n t o  rout ine .  diminishes t h e  use fu l  capac i ty  of geothermal f l u i d s  wi th  water predominance, but i t  does not ac tua l ly  endanger t h e  i n s t a l l a t i o n s ,  s ince  t h i s  problem is under cont ro l .  Some s c a l e  problems have a l s o  been encountered Most of these Scale depos i t ion  i s  a problem that c e r t a i n l y  ermal sur face  mani- , f e s t a t i o n s  i n  an  area of about 30 square kilometers,  where t h e  production zone i s  loca ted '  (Fig. 1 ) .  f i e l d  t o  a depth ranging from 700 m. t o  2,000 m. through pre-slotted casing o r  through gun perforated casing, a f t e r  cement- where t h e  hot strats are located. t hese  per fora t ions  a n d . i  duct ion casing. As t h e  water ascends through t h e  production casing, the hydros ta t ic  pres- s u r e  diminishes and it  p a r t i a l l y  f l a s h e s  i n t o  steam. The flow through t h e  va lve  tree is  a water-steam mixture containing from 20 t o  40% of steam. separa tor ,  54" in  d separated w a t e r  t o  an evap From 20 t o  80 tons  r hour of steam are obtained from each w e l l  a t  a pressure  of 100 ps To da t e ,  13 w e l l s  are used t o  supply t h e  necessary steam t o  maintain a t  f u l l  load two u n i t s  of 37.5 MW each, a t  a steam rate of 9.4 Kg/KWh. Thi r ty  deep w e l l s  have been d r i l l e d  i n  t h i s  Flow is  obtained ing the production casing, except 150 m. t o  300 m. of t h e  bottom p a r t ,  - High enthalpy water flows through onducted t o  t h e  sur face  through the  7-5/8" pro- This mixture i s  admitted t o  a cen t r i fuga l  Webre type cen t r i fuga l  steam i s  s e n t  t o  t h e  power p l an t  and the 59 DISCLAIMER  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER  Portions of this document may be illegible in electronic image products.  Images are produced from the best available original document.  L.. Composition of t h e  Brine and its Relationship t o  S i l i c a  and Other Scaling A t y p i c a l  a n a l y s i s  of t h e  b r i n e  ex t rac ted  by t h e  w e l l s  is shown i n  Table I. A s  i t  can be seen, the following r e l a t ionsh ip  can be established: C a t  ions : Anions : C1- > HC03 > SO, - - According t o  t h e  c l a s s i f i c a t i o n  of Chebotarey (1956) t h e  b r i n e  i s  of sub- terranean o r i g i n  and formed under r e s t r i c t e d  c i r c u l a t i o n - o r  stagnation. By means of i so top ic  a n a l y s i s  of the geothermal f l u i d  it has been es tab l i shed  t h a t  the water from the geothermal r e se rvo i r  is of m e  o r i g i n  and proceeds mainly from t h e  Colorado River. The high concentration of dissolved s o l i d s  is  t h e  r e s u l t  of t h e  s o l u b i l i t y  ac t ion  of t h e  hot water on t h e  sedimentary rocks (see t h e  sandstone ana lys i s  i n  Table 11) ac t ing  f o r  thousands of years,  and reaching a n  equilibrium state o r  s a tu ra t ion  with respect t o  some elements such as s i l i c o n ,  sodium and potassium. The geothermal f l u i d s  a l s o  conta u b s t a n t i a l  amounts of dissolved gases,  mainly C02 and H2S and i n  minor amounts NH3, H2, and CHq. The high content of dissolved s i l i ca  i n  t h e  w e l l  b r i n e  i s  t h e  r e s u l t  of d i s so lu t ion  of rock a t  temperature, a f a c t  experimentally v e r i f i e d  by d i f f e r e n t  inves t iga tors .  wi th  a sandstone core sample obtained from a w e l l  a t  a depth of 1,090 m. and using b r i n e  of t h e  w e l l s  are shown i n  Fig. 2. s i l i c a  i n  t h e  b r ine  was prec ip i t a t ed  and separated from t h e  br ine .  expected, t h e  concentration of s i l ica  i n  so lu t ion  increases  wi th  the t e m -  perature.  S i l i c a  concentrations determined i n  these tests are shown i n  Fig. 2 by (+). Kennedy (1950) and Morey and Fournier (1962). It has been poss ib le  t o  e s t a b l i s h  that t h e  f l u i d  discharge by the Cerro P r i e t o  w e l l s ’ i s  sa tura ted  w i t h  respec t  t o  s i l i ca  a t  t h e  temperature a t  which t h e  b r ine  is  found. This f a c t  has been u t i l i z e d  t o  determine t h e  temperature and enthalpy of t h e  hydrothermal b r ines  with acceptable accuracy. Mahon (1966) f o r  t h e  same purpose. Results of tests conducted i n  our Laboratory P r i o r  t o  t h e  test ,  As A s  can be seen, they follow t h e  s o l u b i l i t y  curve obtained by This method has been employed by Fournier and Rowe (1966) and 60 . Table I Chemical Analysis of t h e  Separated Water of Well M-5 After Eeing Exposed t o  Atmospheric Pressure b, Element N a  K L i  C a  Mg ppm 9062 2287 38 520 1 B 14 Si02 1250 c1 16045 B r  31 F 2 so4 6 co3 2 HCO3 74 N a C l  (Cl) 26442 A s  ' 0.5 Fe 0.3 PH 7.7 Cond. '(mhos) 32200 Table I1 Chemical Composition of Sandstone Obtained from A Depth  of 1090 M. a t  "Cerro P r i e t o "  I ppm N a  4875 K 11200 72 28000 8505 244 2100 ' 520 B r  65 siop 266000 Element 61 1 T . .  '5 * . .  . , 1* I n  a similar manner, t h e  b r ine  is sa tura ted  with respect t o  carbonates a t  t h e  temperature of t h e  br ine .  Due t o  t h e  s a t u r a t i o n  condition of some of i t s  components, deposit ion of scale occurs on t h e  i n s i d e  of t h e  w e l l  casing as t h e  geothermal f l u i d  undergoes physical changes on i ts  way t o  t h e  surface.  phys ica l ly  when t h e  high enthalpy w a t e r  ascends through t h e  production casing and t h e  pressure a t  which it is subjected is reduced causing p a r t i a l  f lash ing  of t h e  water i n t o  steam. change of phase occurs depends on t h e  o r i g i n a l  enthalpy of t h e  f l u i d s .  3 shows t h e  r e l a t i o n  between t h e  enthalpy measured i n  each w e l l  and t h e  depth a t  which t h e  depos i t s  are found. Chemically, depos i t ion  occurs when the  b r i n e  ( o r i g i n a l l y  sa tu ra t ed  with respec t  t o  s i l ica  and carbonates a t  t h e  i n i t i a l  temperature) becomes supersaturated as a por t ion  of t h e  water f l a shes  i n t o  steam and t h e  t e m -  pera ture  of t h e  b r ine  is reduced by t h i s  change of phase. The sca l ing  occurs The zone of t h e  casing where t h i s  Fig. C02 o r i g i n a l l y  present i n  the  b r ine  is t r ans fe r r ed  t o  t h e  s t e a m  when bo i l ing  is i n i t i a t e d .  p rec ip i t a t ed  as carbonates according t o  t h e  following reac t ion :  The i n i t i a l  equilibrium is d is turbed  and bicarbonates are Ca(HC03)2 -t CaCO + H20 + C02+. 3 Scaling problems of t h e  production cas ing  are more or  less severe,  depending on t h e  zone where scale is  produced and t h e  depth of t h i s  zone. I f ,  f o r  instance,  t h e  depos i t  is  formed i n  t h e  production casing, rimming of t h e  w e l l  recovers t h e  w e l l  output. I f ,  however, t h e  depos i t  is formed i n  t h e  s l o t t e d  por t ion  of t h e  casing, f l u i d  flow is  r e s t r i c t e d  and t h e  s l o t s  can- not be cleaned by rimming. Scale may a l s o  occur i n  t h e  surrounding ground seals and diminishing permeability with no means of cleaning i t .  Deposits similar t o  those formed i n  t h e  production casing keep on forming i n  t h e  separa tors ,  water piping, s i l e n c e r s  and dra ins .  inso luble  depos i t s  are formed because water containing these  s a l t s  is ca r r i ed  over t h e  steam. i n  condensate t r a p s  and on turb ine  blades. Si l ica  and o the r  This carryover i s  respons ib le  f o r  scales formed Conclusions Scaling i n  t h e  production pipes from w e l l s  and i n  other i n s t a l l a t i o n s  has caused problems and r e s t r i c t e d  the  i n t e g r a l  use of t h e  geothermal f lu ids .  However, it has been poss ib le  t o  so lve  sca l ing  problems as they occurred and operation of t h e  w e l l s  and t h e  geothermoelectric p l an t  has been con- t inuous s ince  i t s  inauguration i n  Apr i l ,  1973. use preventive systems t h a t  w i l l  make t h e  use of geothermal energy from water-predominant r e se rvo i r s  much easier. In  t h e  fu tu re ,  w e  hope t o  62 Table I11 Chemical Composition of Scale i n  Production Pipe of Wells: IA, M7, M9, M10, M11, M29 and M39 W e l l  1A: CaC03 N a C l  w 0 3  Well M7: C a  Ms c1 Fe203 s io2 sp4 94.6 % 4.2 % 1.1 % 29 % 6.1 % 1.213% 1.23% 6.0 % 0 %  . W e l l  M9: (750731) a t  147-170 m. NaC1.  FeS Well M l O :  FeS 75.56% 12.51% 0.82% 9.46% 75.67% 3.19% 19.52% 1.62% 63 Well M11: A. sio2 F e  c u  C a  Mg N a  K Zn Mn c1 Au Ag co3 C.L a t  4OOOC ‘c.L a t  8OOOC B. Depth m 27 74 121 167 214 260 307 344 399 482 585 600 659 715 % (w) c 25.1 3.7 0.1 24.4 12.0 -0.4 0.3 0.2 0.9 1.4 0.0001 0.0061 14.2 1.4 22.9 % (w) CaC03 sio2 F eS N a C l  30.53 35.0 48.92 43.9 35.93 44.44 54.7 52.00 33.89 38.85 38.55 39.31 55.54 79.74 61.10 60.05 44 34 51.6 56.96 49.54 39.5 42.28 59.46 54.10 53.91 52.52 37,44 12.87 5.73 4.31 4.59 3.67 4.36 4.03 5.08 5.08 5.81 6.08 6.52 3.74 1.64 1.25 0.71 0.64 0.43 0.46 0.54 0.59 0.64 0.64 0.84 0.97 1.02 1.63 0.49 0.28 64 Well M29: (Nov.-Dec. 1975) % (w) Depth CaC03 sio2 N a C l  FeS m 500 78.00 8.02 0.81 507 90.18 2.31 0.33 554 91.55 0.59 0.07 6 01 92.3 1.34 0.07 648 92.95 1.18 0.07 694 94.07 0.52 0.08 7 50 93.59 1.54 6.12 7 97 93.72 1.55 0.1 8 67 87.69 5.27 0.12 890 85.47 8.37 0.26 1030 18.00 25.63 0.27 12.59 0.69 0.59 2.16 0.80 0.71 1.21 0.5 2.16 1.54 9.78 (1) (1) MgC03 41.53% Well M39.(Aug.-Sept. 75) % (w) Depth CaC03 s io2 N a C l  FeS m 250 84.43 6.09 0.11 928 61.24 23.34 0.27 964 87.78 2.89 0.03 992 71.11 15.10 0.28 1057 73.25 17.95 0.26 1015 70.26 15.98 0.17 107 6 90.56 5.18 0.14 1085 71.28 17.08 ~ 0.20 1095 73.30 17.60 0.25 1116 78.94 9.10 0.19 1140 76.55 18.43 0.15 65 3.18 1.86 0.082 1.46 4.64 0.94 0.98 2.27 3.07 3.97 1.37 Chemical Analysis of Scale i n  Water Discharge Pipes i n  Wells M3 and M34 W e l l  M3 (whit e) x (w) Well M34 .(white, gray and brown) CaC03 9.52 s i02  3.6 _ _  Fe203 0 Mu0 , '0 co 0.005. N i  0.0015' cu 0.013 - Au 0.00002 Ag - 6.5 85.1 4.29 0.04 0.003 0 0.003 0.00013 0.002 Chemical Analysis of Scale Ins ide  of Cyclone Separator - of WellM34. Color of Sample: Black s i02  Fe203 MnO co N i  cu Au Ag si02 Fe203 MnO co cu Au Ag 0.2 0.05 0.013 0.049 0.0005 0.0043 Chemical Analysis of Scale i n  Si lencer  of W e l l  M5 and W e l l  M8 x (w) Well M 5  Well M 8  (gray) (white) 91.7 1.4 0.06 0.0069 0.015 0.00002 0.0021 93.2 0.86 0.05 0.0245 0.0086 0.001 0.0021 66 e .  t/ Chemical Composition of Scale i n  Turbine Rotor Blades and Fixed Blades Phases detected . i n  X-ray ana lys i s :  Magnetite P y r i t e  Pyr rohot i te  Calcite H a l i t e  KC1 ’ A. 2nd Maintenance Unit No. 1, Nov. 1975 Fixed Blades Rotor Blades 1 4 5 6 1 si02 55.4 1.0 0.8 2.0 0.5 F e0 1.3 35.2 26.6 29.7 34.9 CaO cr04 MgO so4 MnO S c1 B. 3rd Mai Fixed Blades 1 Si02 14.1 0.5 0.5 0.7 1.1 CaO 24.0 0 FeO 25.7 4 S - 0.5 3.1 2.4 0.3 cog 14.5 0 K20 0.2 - N a q O  9.4 0 L, 67 I \\ -7 ' * >  $ 1 Km.  .E t ! Hardy R i v e r  9 S p r i n g s  \ . fells  /y Volcano P o u n d  d c FIG. 1. - CERRO PRIETO GEOTHERMAL FIELD 68 I 69 70 TS IN WEL CHEMICAL VARIATIONS VS DEPTH S 71 100 O 1  DEPOSITS IN WEL L-M-39 I \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ #  \ \ \ \ \ \ \ \ \ Si 02 . .  _ -  .I 1 I I I O  20 3'0 40 50 60 l o  80 % Lid 72 EL .. . i . * .. -006 -0OL -009 too1 62-W 773M NI SLISOd30 1 - C A L I B R A T I O N  , . - - . . - ,.. . i . . .  - 6 .  L :  . I' I I I I I t I I t m 2 1 1 . 9 0  2 2 8 . 2 0  1 1 / 2  1 229 .50  74 S M WJLSM 760706 , -  I 4 + <  t b, 75 Li C A L I B R A T I O N  ~ I I O O  ,- 4 p o o  - ' 1 76 SMGAJLSM 760707 J TT 0. D.?g/g I J  # 1.0.6.97" m 1 C A L I B R A T I O N  S M WJLSM 760700 M - 13 0 k$ ~ I  I r I t  I"' i. D. 792 'I I " 1  I I 1 78 7 9  T a a E LI $4 's I 9 80 I N  WELL 81 I I I 0.0. 51/p V I .  B. 4. 9" I P J C A L I B R A T I O N  I I 82 3 . 4 . -  Deposits inside of silencer: a) Bottom; and b) Test Plate. 83 3.5.- Si l i ca  deposit (99% Si02) i n  drainage. 84 3.5.- Sil ica deposit (99% Si021 in  drainage. 85 3.6 . -  Scale i n  turbine f ixed blades. F ir s t  and second steps.  86 3.6 . -  Scale i n  turbine blades of 6th step.  87 

Problems of Silica Scaling at Cerro Prieto Geothrmal Power Station

https://digital.library.unt.edu/ark:/67531/metadc892533/m2/1/high_res_d/886861.pdf

Problems of Silica Scaling at Cerro Prieto Geothrmal Power Station

Abstract

Similar works

Full text

Available Versions

UNT Digital Library