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					                     Geothermal Resources Council
                           2007 Annual Meeting



 Intermittent recharge processes at the Los
   Humeros (Mexico) geothermal reservoir
      indicated by analysis of gas data

      Rosa María Barragán R.1, Víctor Manuel Arellano G.1,
            Magaly Flores A.2, Rigoberto Tovar A.2

1Instituto   de Investigaciones Eléctricas, Gerencia de Geotermia, Reforma 113,
                 Col. Palmira, 62490, Cuernavaca, Morelos, México

2Comisión    Federal de Electricidad, Gerencia de Proyectos Geotermoeléctricos,
                           Morelia, Michoacán, México
     Objective



   to investigate the occurrence of recharge
    processes in the Los Humeros reservoir by the
    analysis of gas data through FT-HSH2 equilibria
    in two representative wells
           The Los Humeros geothermal field
 Located in the Mexican Volcanic Axes,
average elevation of 2800 m a s l                     CERRO PRIETO
                                                      (720MWe)



 35 MWe installed capacity

 Two reservoirs: a shallower liquid-      TRES
                                           VIRGENES
dominated (300 - 330°C) and a deeper       (10MWe)                    PATHE

“low water saturation” (300-400°C)
                                                                        LOS HUMEROS
                                                                        (37MWe)

                                               LA PRIMAVERA

 High enthalpy fluids at wellhead (but                 LOS AZUFRES
                                                        (98MWe)
H-1, 1D)


                                           The liquid phase produced by
                                          most of the wells is diluted and
                                          contains     condensed     steam
                                          which is rich in SO4 and HCO3

                                           Gas   data provide                reliable
                                          reservoir information
Composition of steam phase
                                                                                    80
                                                                                                                                (A)




                                      Gas/steam
                                                                                    60




                                                               (mol ‰)
                                                                                    40

             H-1                                                                    20


             H-1D
                                                                          0
                                                                       1000




                                          (mol ‰, dry basis)
                                                                                                                                (B)

             H-7
                                                                                960




                                    CO2
                                                                                920
                                                                                880
 H-1 is representative of the                                                  840

shallower reservoir and H-7 is
                                                                              800
                                                                              150




                                                  (mol ‰, dry basis)
                                                                                                                                 (C)
representative of the deeper                                                  100




                                    H2S
stratum                                                                             50


 Relatively high gas/steam ratio                                                    0
                                                                                    30
and CO2 in wells H-1, 1D



                                                               (mol ‰, dry basis)
                                                                                                                                 (D)


compared with values in H-7
                                                                                    20




                                      H2
                                                                                    10
 Relatively high H2S and H2 in                                                      0
well H-7 compared with values                           (mol ‰, dry basis)
                                                                                    2.5

in wells H-1, 1D
                                                                                                                                (E)
                                                                                    2.0
                                     CH4

                                                                                    1.5

 Since 2000 H2S increased and                                                      1.0
                                                                                    0.5
CO2 decreased in most wells                                                         0.0
                                                                                      1980   1985   1990   1995   2000   2005    2010
                                                                                                           Year
                          Methodology

   FT-HSH2 method (D’Amore, 1988) considers the reactions:
        FT:    CH4 + 2H2O = 4 H2 + CO2

and the pyrite-magnetite-hematite combined equilibrium:
HSH2: 5/4 H2 + 3/2 FeS2 + ¾ Fe2O3 + 7/4 H2O = 3 H2S + Fe3O4
   More local oxidant conditions are assumed (high H2S and low H2
    and NH3 concentrations in the fluid) compared with the FT-HSH
    method that is based on pyrite-magnetite buffer.

   FT-HSH2 method best fits mature and magmatic systems (Siega et
    al., 1999).

   FT and HSH2 are obtained from the                 gas   composition,
    concentrations of gas are taken in total fluid:

        FT = 4 log (H2/H2O) + log (CO2/H2O) -log (CH4/H2O)

        HSH2 = 3log (H2S/H2O) – 5/4 log (H2/H2O)

   Results are given in a FT-HSH2 diagram
 Interpretations given by D’Amore and Truesdell, (1995):
1.     Increase    T,    decrease     y:
     contribution of fluid from a hotter
     and deeper source with high
                                            -5


     liquid saturation.

2. Increase T, increase y: apparent
     increase in T and y due to lateral    -10
     source of steam, with practically                VAPOR GAIN (Y>0)

     zero liquid saturation and with a                1.0
     strong local accumulation of gas.            0.50
                                                                                           4
                                                      0.25


3. Decrease T, decrease y: local
                                                            0.10



                                      FT
                                           -15                                                  1                               350
                                                  0.05                                                  4
   source    of     pure    and   low
                                                                          2
   temperature water with no gas                                0.01                                                      325
   content as in the case of                                    0.005
                                                                                  3                                       T in oC
   reinjection fluids or fast meteoric                          1
                                                                                  =0
                                                                                       )
                                           -20
   recharge.                                           0.001                uid
                                                                                (Y                             300
                                                                         liq
                                                                    le
                                                                  ng
                                                                Si
4. Decrease T, increase y: caused by                        0                                   -0.05
                                                                                                        275
   either recharge from peripheral                              -0.001       -0.01
                                                            VAPOR LOSS (Y<0)
   fluids rich in gas; or sulfides         -25

   precipitation caused by local over-           -8                                        -6                        -4               -2
                                                                                                        HSH2
   production with blockage of main
   fractures.
                    Methodology

 Limitations of this method apply when the gas phase produced
by the well comes from different sources. When a deep hot zone
of the reservoir located below the exploited reservoir rich in
reactive gas species and CO2 becomes an important fraction of
the total produced gas, an overestimation of temperature and
steam fraction values could be obtained (D’Amore, 1998).


 This seems to be the case for Los Humeros, however the
reservoir temperature estimations obtained by FT-HSH2 is
similar compared with measured temperatures for static
conditions of the wells (Arellano et al. 2006).


 At Los Humeros very high temperatures (> 400°C) have been
recorded, (Truesdell, 1991).
       Gas behavior for pre-exploitation conditions
                                   -5

                                               Lineal tendency: mixing of two fluids
                                               approximately from shallow well H-1 and
The case of well H-16:                         deep well H-23    Fitting line, r=0.82
                                                                                                                                           23
 Well H-16 located at the        -10
“Colapso Central” zone shows                 VAPOR GAIN (Y>0)
                                                                                                                  19
                                                                                                                           17
                                                                                                                                  18

                                                                                                                                         16    14/07/87
anomalous behavior regarding
                                                                                                                                         16
                                                                                                                             12
                                             1.0                                                                  7
fitting. The well produced from          0.50                                                                 8
                                                                                                                           6
                                                                                                                               11


both reservoirs depending on                 0.25
                                                                                                                      10
                                                                                                                                    16R 13/10/89
                                                                                                                                    16R

                                                   0.10
wellhead conditions
                            FT
                                  -15                                                                                                   16R
                                                                                                                                        16R 4/05/94       350
                                         0.05


                                                       0.01                                                                                    325


 Acidic fluids were produced                          0.005                                           1
                                                                                                                                                T in oC

by well H-16 in the past. In      -20                                                   =0
                                                                                             )
                                                                                   (Y                                             300
1989 the well was cemented                    0.001
                                                                      liq
                                                                          u   id
                                                                gle
to avoid production from the                              Sin

deeper reservoir
                                                   0                                                  -0.05
                                                                                                              275
                                                       -0.001       -0.01
                                                   VAPOR LOSS (Y<0)
                                  -25
                                        -8                                                       -6                                       -4                    -2
                                                                                                              HSH2
                                                    Gas behavior for pre-exploitation conditions
                                                          Oxygen-18 is more depleted at                                                                                                       CO2 is more concentrated at
                                                          shallower levels                                                                                                                    shallower levels
                                     1600                                                                                                                       1600

                                                                                                 1
                                                                                                 1
                                                                                                       H-1 is not in the                                                                      1
                                                    16R
                                                           H-16R follows the                           tendency                                                                                                        16R




                                                                                                                           Elevation of producing zone (masl)
Elevation of producing zone (masl)




                                     1200                  convective process                                                                                   1200
                                                                                                                                                                                                                  CO2 is more
                                                                                                                                                                                                                  concentrated in
                                                                                                                                                                                                                  H-16R than it is
                                                                                                                                                                                                                  in well H-1
                                     800                       16                16                                                                              800                                      16
                                                                         10           y=0.92                                                                                   10
                                                                y=0.84
                                                                                 17                                                                                              17
                                                                                                                                                                                19
                                                                                      19
                                                                                         7                                                                                            7
                                                                                      11 8                                                                                     11         8

                                     400                                                                                                                         400
                                                                                             6                                                                                        6

                                                                                                        23                                                                     23


                                       0                                                                     18                                                    0            18

                                                Oxygen-18 is more                                                                                                                                 CO2 is more depleted
                                                enriched at deeper                         Total discharge   12                                                           12
                                                                                                                                                                                                  at deeper levels
                                                levels                                                                                                          -400
                                     -400
                                            -9.00           -7.00        -5.00             -3.00             -1.00                                                     0.000          0.002       0.004        0.006         0.008   0.010
                                                                                                                                                                                CO2 in reservoir fluid (mol fraction)
                                                            18O in reservoir fluid (‰)

                                                                                                                       Well H-16R produced a condensate,
CO2 and oxygen-18 profiles                                                                                             this suggests a preferential flow of
indicate a convective process                                                                                          condensate toward the “Colapso
that enriches volatile species                                                                                         Central” zone.
(CO2) at shallower levels and
concentrate       non-volatile                                                                                         The acidity observed in the past
species (oxygen-18) at deeper                                                                                          could   be   a   consequence     of
levels.                                                                                                                convection and preferential flow of
                                                                                                                       condensate.
                    Reinjection at Los Humeros
 Since 1995 condensed steam, separated
                                                                                      200
                                                                                                                                   H-29R

water and air mixture is reinjected.




                                                 Mass flow rate reinjected (ton/hr)
                                                                                      160

Condensed steam is saturated in volatile
species (CO2, H2S, etc.)                                                              120



                                                                                      80

 The reinjection fluid in contact with hot
reservoir rock is heated and boils, volatile                                          40


species are released together with the new
vapor phase and flow to the production zones
                                                                                       0
                                                                                            1995           2000            2005        2010

of wells                                                                                                           Year



                                                                                      120
 The new steam phase has poor liquid                                                              H-38R

saturation (comes from condensed steam)




                                                 Mass flow rate reinjected (ton/hr)
                                                                                       80
 The air injected is a tracer for the vapor
phase, provides “peripheral” characteristics
to well discharges                                                                     40



 This is identified in the FT-HSH2 diagram as
temperature decrease and excess steam                                                   0
increase.                                                                                   1995            2000
                                                                                                                    Year
                                                                                                                            2005           2010
    Studied wells                                    ro
                                                  tre e
                                               Po aps
                                                       s

                                                                     31
                                                                                  21
                                                                                                                                             N
                                 2179000     os oll
                                            L C             15




                                                                                                      Imán Fau
                                           22         37
                                                9                     30
 H-1 (shallower reservoir).                          3516     38
                                                                                  17 32




                                                                                                               lt
                                 2178000
Drilled in 1981 to a depth of                   3           33 29                         28
                                                                                         11




                                                                 Humero
1458 m, produced until July      2177000
                                                                          4
                                                                                       36
                                                                                           34
                                                                                              20

1995 when it was deviated




                                                                                       Nue
                                                                   s Fault
                                                                                             19




                                                                                          va F
                                                                                  10
because of scaling.
                                                                                                                                                     25
                                                                  8 7




                                                                                               ault
                                 2176000                                                                                 27       Las Papas Fault
                                                5                   40
 After   repaired,  H-1D                                                                             26                     23
                                                                                                                                  Las Cruces Fault




                                                            An
reached a depth of 1850 m        2175000




                                                               tig
                                                                             1




                                                                   ua
                                                       Ma
and started operation in




                                                                      F
                                                         lpa
                                                                                 13




                                                                          au
                                                            ís


                                                                             lt
                                                            Fa
November 1995.
                                 2174000




                                                                 ult
                                                                                                                    6
                                                                                                                         Ma
                                                                                                                           zta
                                                                                                                              loy
                                 2173000                                                                  39                     aF
                                                                                                                        12            au
                                                                                                                                        lt   24

 H-7    (deeper   reservoir),   2172000
                                                                                        2

drilled in 1983 to a depth of                   Well
                                                                                                                                       18
                                                                                                                                                  CFE
2340 m, started operation in     2171000
                                                Studied well                                      Xalapaxco Crater

1984,     up   to   now     in                  Reinjection well


production.
                                                CFE
                                 2170000                                                                                14

                                                    661000 662000 663000 664000 665000 666000
                       Results for well H-1, 1D
 1986-1994: Input of hotter
                                         -5
recharge    in   well   H-1,
                                                             H-1
temperature changed from                                     H-1D
260ºC in 1986 to >300ºC in
1994.
                                        -10

 1995-2000: Recharge from                         Vapor gain (Y>0)


peripheral fluids rich in gas in                   1.0
                                               0.50
well H-1D. Peripheral fluids                       0.25
                                                                                                 2
                                                                                                                                    5
                                                         0.10                                                92
are constituted by steam                                                                              3                   4




                                   FT
                                                                                                 1
                                        -15                                                 0                                                    350
                                                                                                 93            94
                                               0.05                              99                   89
phase returns and injected air.                                         86        88
                                                                                       97
                                                             0.01                                         96                             325
 2000-2005: Input of hotter                                   0.005                                                                       T oC

recharge,          temperature                                                                                       95
                                        -20                                       0)
                                                                               (Y=
                                                                                                                              300
changed from 280ºC to                               0.001
                                                                         uid
                                                                      liq
                                                                gle
>325ºC in 2005. This occurred                                Sin
                                                         0                                           -0.05
due to natural increase in                                     -0.001                   -0.01
                                                                                                               275



permeability and an increase            -25
                                                         Vapor loss (Y<0)

in seismicity in the zone                     -8                                            -6                                      -4                 -2
                                                                                                               HSH2
(Lermo et al, 2002)
                             Results for wells H-1, H-1D
                250
                                                              Total
Mass flowrate



                200                                           Liquid
                                                              Steam
  (ton/hr)




                150

                100

                 50

                  0

                      1980   1985   1990   1995   2000     2005    2010


      Results regarding the input of hotter recharge qualitatively
      agree with production data in wells H-1, 1D. Peaks in produced
      fluids are seen in 1988 and 1990-92 in well H-1. 2001 - 2005
      important increase in fluid production in well H-1D
                      Results for well H-1, 1D
Water produced by well H-
1D from 1999 to 2005 tends                                                              Na/1000
toward the equilibrium line
but      relatively    lower
reservoir temperature is
                                                                                        H-1, 1D
estimated           (<300°C)                                               120


compared        with      the                                        140


temperature estimated by                                       160                         0.01% Mg

gas data                TCCG
                                                         180                Equilibrium (Giggenbach, 1988)
                                                                                 200           160
                                                                                                      120
                             Na/K                  200
                                                              83                  Equilibrium CCG
                                          220                      240          (Nieva and Nieva, 1987)
 A trend toward less
                                                                            200
                                                                                          160                  80
                                     240                 95
                                                                                                       120
 equilibrated waters
                                               5               94
                                   260                                    89 93
                                                                     87       88
                                               2     3
                                                                0
 is    seen    during
                              280                                                  98
                                         300
                             300                   280                     96             86

 1995-1998, due to
                          320                                        97     99
                                         320
                         340                                                                    82

 the       reinjection                               0.05% Mg
 returns                                                                                             1% Mg
                                                                                                             3.5% Mg

              K/100                                                                                                    Mg
                               Results for well H-1D
                   200
                                                        Total discharge
Chloride (mg/kg)

                   150                                  Separated water

                   100

                    50

                     0

                     1980   1985   1990   1995   2000     2005       2010

In well H-1D chloride in separated water and in total discharge
decreased during 1996-1999 because of the steam reinjection
returns which chloride depleted. Since 1999 chloride increased.
                      Results for well H-7
 1985-1995: Input of hotter       -5

recharge,      temperature
changed from 275ºC in 1986              1995-2000
                                                                                                         H-7

to >335ºC in 1995.
                                  -10        Vapor gain (Y>0)
 1995-2000: Recharge from                                1.0
peripheral fluids rich in gas.           0.50                                             86
                                                                                                          0
                                                                                                               2 4
                                                                                                                   2000-2005

Peripheral      fluids     are                                                             85
                                                                                                             99 97 3                5
                                         0.25
constituted by steam phase
                                                                                                       87                 95
                                             0.10                                                         89 1
                                                                                                                         96
returns and injected air.    FT   -15        0.05                                                  1985-1995
                                                                                                             88 93
                                                                                                                   94
                                                                                                                      92
                                                                                                                                        350




 2000-2005: Input of hotter                                    0.01

recharge,       temperature
                                                                                                                              325
                                                                 0.005
                                                                                                                         T oC
changed from 290ºC to                          0.001                      u id
                                                                               (Y=
                                                                                     0)
                                                                     iq
350ºC     in   2005.     This     -20                            l
                                                             gle
                                                        S in
                                                                                                                   300
                                                    0
occurred due to natural                             -0.001
increase in permeability and                                                               -0.05
an increase in seismicity in                                          -0.01
                                                                                                   275



the zone (Lermo et al, 2002)      -25
                                                Vapor loss (Y<0)

                                        -7                                    -6                          -5             -4
                                                                                                     HSH2
                Results for well H-7

Water produced from 1998                                                  Na/1000
to 2006 (but 2002-2003)
tends toward equilibrium
but     lower    reservoir                                                    H-7
temperature is estimated                                        120


(<300°C) compared with                                    140



the temperature estimated                           160                           0.01% Mg


by gas data             TCCG
                                              180                     Equilibrium (Giggenbach, 1988)

                                                                                  160
                                  Na/K 200                                                   120
                                                                       200
A trend toward less
                                                                            Equilibrium CCG
                                        220               240           (Nieva and Nieva, 1987)
                                                                        200        160                   80
equilibrated waters
                                    240             260
                                              85                                                   120
                                  260                      87
                                                  92 4 93
is seen during 1995-         280
                                             280
                                                95      88
                                                           2  0 99
                            300                     65
1998, due to the         320
                        340             320
                                           300 94     96
                                                        89 97 1
                                                                  3

reinjection returns
                                                                 98
                                   340                                                            86
                                                0.05% Mg

                                                                                            1% Mg
                                                                                                       3.5% Mg

                K/100                                                                                            Mg
                    Results for well H-7
Data for 1995-2000 are                                  Cl
sulfate   or    bicarbonated
“peripheral waters” due to
the reinjection returns




                                                         Ma
                                                            ture
                                                                 wa
                                                                   ter
                                                        H-7
Data for 2001-2004 and




                                                                      s
                                               rs
2005-2006       tend      to




                                             te
                                          wa
“chloride   waters”,    this
                                                              6




                                         nic
                                                          3
agrees with deep hotter

                                     lca
                                                       94 2 4 91 89
                                                            99      88


                                  Vo
recharge. Data for 2006                                     92 1
                                                          84 93
                                                                    5
                                                                       0

tend to “mature waters”
                                                  95         96
                                                               98




                                                                                     Pe
                                                                           85
                                                                                87




                                                                                        ri
                                                           97
                                                                                     86




                                                                                          ph
                                                                                            era
                                                                                                l
                                                                                               wa
                                                                                                    ter
                                                                                                       s
                        Steam-heated waters
                  SO4                                                                                      HCO3
                                 Results for well H-7

                    80
                                                                   Total
    Mass flowrate




                    60
                                                                   Liquid
                                                                   Steam
       (ton/hr)




                    40

                    20

                     0

                         1980   1985   1990   1995   2000   2005       2010

Due to steam reinjection returns an increase in steam production (for a
constant orifice diameter) occurred from 1995 to 2000, the steam mass flow
rate changed from ~ 42 to 52 tons/hr and subsequently decreased (because
of a reduction in the orifice plate diameter).
                      Conclusions
 Gas data have been useful to understand important characteristics
of Los Humeros reservoir. For pre-exploitation conditions evidence of
both, mixing and convection processes were found.


 FT-HSH2 method was used to study the changes in gas
composition of wells from the Los Humeros geothermal field and the
occurrence of intermittent recharge processes was identified


 Two wells were given as example H-1 (upper reservoir) and H-7
(deep reservoir). According to the trends, the input of deeper hotter
fluids has occurred during specific periods of time in both wells.


 For wells H-1, 1D the overall trends found for 1986-1995 and from
2000 to 2005 data indicated the entry of hotter recharge and
specially in 2005 very high temperature was estimated (333°C) for
the reservoir fluid.
                        Conclusions
 In well H-7 the 1985-1995 and 2000-2005 trends indicated the
inflow of hotter fluids. The temperature estimated for 2005 was
350°C. The water classification tends to “chloride waters”. The
increases in temperature were due to the H2S increase in the
discharges.
 Production data and the chemistry of the water supported the
recharge hypothesis in both wells.

 Other evidences in the field suggested increase in reservoir
permeability and were attributed to higher seismicity in the zone.

 In both studied wells a decrease in temperature and increase in
reservoir steam fraction were observed to occur from 1995-2000
data. This tendency was interpreted as a result of the presence of
steam phase reinjection returns in well discharges, (reinjection in the
field started in 1995).

 Reservoir temperatures could be overestimated, however the FT-
HSH2 tendencies with time are useful to qualitatively assess the
reservoir characteristics as a result of exploitation.

				
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