LBNL-42057

The Importance of Natural Fluid Recharge to theSustainability of the Cerro Prkto Resource

A.H. Truesdell,l M.J. Lippmann,z H. Guti&rez P.,3 and J. de Le6n V.3

lconsu]t~t

Menlo Park, California

2E~h sciencesDivisionErnest Orlando Lawrence Berkeley National Laboratory

University of CaliforniaBerkeley, California 94720

3comisi~n Federal & El~&~ci&d, ~exicali, BCN, Mexico

June 1998

This work was supported in part by the Assistant Secretary for Energy Efficiency and Renewable Energy,Office of Geothermal Technologies, of the U.S. Department of Energy under Contract No. DE-AC03-76SFOO098.

DISCLAIMER

This report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal Iiabiiity or responsibility forthe accuracy, comp~eteness, or usefuhess of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.

DISCLAIMER

Portions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.

Truesdell et al.

THE IMPORTANCE OF NATURAL FLUID RECHARGE TO THE SUSTAINABILITYOF THE CERRO PRIETO RESOURCE

A.H. Truesdell’, M.J. Lippmann2, H. Guti&rez P.3 and J. de Leon V.’

‘Consultant, Menlo Park, CA2E.0. Lawrence Berkeley National Laboratory, Berkeley, CA3Comision Federal de Electricidad, Mexicali, BCN, Mexico

ABSTRACT

The Cerro Prieto field in northern Mexico has beenunder commercial exploitation for 25 years. At presentits three power plants are generating around 600 MWe(the total installed capacity is 620 MWe). Almost 1800million tons of fluids have been produced between 1973and 1997 with only about 140 million tons injectedback into the reservoir (injection started in 1989).

In spite of the large net fluid (and heat) extraction ftomthe system, wells continue to supply steam to the powerplants. This is largely due to the natural recharge of thereservoir. The inflow of cooler waters does not occureverywhere. Groundwater recharge along the northboundary of the present wellfield seems to be minor, butthe lateral influx through the western and southern edgesof the field, as welI as vertical through two normal faults,has been documented. The amount of natural fluidrecharge at Cerro Prieto is estimated based on changes inchloride in the produced fluids.

Because of the pressure support provided by naturalrecharge, not only has the life of the field been extended,but also that of individual wells. The behavior of wellsin areas a&cted by this fluid inflow contrasts withthat of wells located where recharge is only minor, ornon existent. Other wells are influenced by the injectionof waste geothermal waters.

INTRODUCTION

The importance of fluid recharge in liquid-dominatedgeothermal systems is well established. Without activecirculation (i.e., recharge and discharge) of hot fluids,these systems would not exist or would be short lived(i.e., Donaldson et al., 1983; Lippmann and Truesdell,1990). The case of vapor-dominated systems is quitedifferent; since they are driven by heat conduction, massrecharge is lacking or tends to be negligible.

Cerro Prieto is a large, hot (320-350”C) field (totalcapacity at least 780-800 .MWe; Hiriart-Le Bert andGuti&-rez-Negrin, 1996) hosted in sedimentary andmetasedimentary rocks, but otherwise it is a typicalwater-dominated system. In a number of papers the

evidence for groundwater recharge at Cerro Prieto wasdiscussed. We now review the most relevant results andconclusions reached by these earlier studies.

The reservoir pressure drawdown caused by theexploitation of the field (Bermejo et al., 1979; Pelayo etal., 1989), which started in 1973, resulted in an almostimmediate inflow of cooler, more dilute groundwatersinto the western, shallower Alpha reservoir. This wasevident from changes in the chloride and silica contentand isotopic characteristics of the produced fluids (i.e.,Truesdell et al., 1979a, b; Grant et al., 1984; Stallard etal., 1987). The paths followed by the groundwater werestudied by Truesdell and Lippmarm (1986), who showedthat the waters invaded the Alpha reservoir horizontallyffom the west and vertically down the normal, east-dipping Fault L.

Later, as the development of the field proceeded towardsthe east, studies tended to focus on the much larger Betareservoir. Initially groundwater recharge to this deeperaquifer was believed to be lateral, along the edges of thesystem, except perhaps to the north where there is animpermeable boundary (Truesdell and Lippmann, 1990;Lippmann et al., 1991; Truesdell et al., 1995). Morerecent chloride and isotope data, however, clearlyindicate that the major recharge to the Beta reservoir isby groundwater flowing down the normal, SE-dippingFault H (Truesdell et al., 1997; Truesdell andLippmann, 1998). These more recent papers also discussthe efkts of man-made recharge (i.e. iqjection) on thechemistry of the produced fluids.

As indicated by Grant et al. (1984) geothermal fluids inthe Alpha reservoir mixed with cooler waters even &forecommercial-scale fluid production began. This mixingwas not uniform as shown by the chloride content offluids produced by different wells. In response toreservoir drawdown caused by exploitation, only localboiling occurs near most Alpha wells (i.e., no extendedsteam zone is formed). This is due to the lateral andvertical groundwater recharge that provides significantpressure support (essentially the reservoir responds ashaving a constant-pressure boundary).

The behavior of Beta wells is quite tierent and variesover the system. Until about 1985, before power plants

1

Truesdell et al.

CP-11 and 111came on line more than doubling theinstalled capacity in the field, production fi-om thisdeeper reservoir was relatively small. However becausetemperatures are higher and fluid recharge is morerestricted than in the alpha reservoir, an extended boilingzone began to form very early. After 1986, some wellsshowed adiabatic steam condensation, others producedfluid enriched in chlorides due to boiling (Truesdell andLippmann, 1998). Other wells in the Beta reservoirpresent clear indications of the efi%ctsof horizontal andvertical groundwater recharge. Still others show mixingwith the waters being injected along the western edges ofthe system (Guti6rrez Puente and Ribo Muiioz, 1994).

OBSERVATIONS

Contours of reservoir chloride across the area of thereservoir and plots of the changes with time of reservoirchloride (Cl) for individual wells provide indications ofthe time and amount of groundwater (or injectate)reaching each well. For the Alpha reservoir the earliestmaps show a chloride maximum in the middle of thefield with lower chloride on all sides except thenorthwest (Figure 1), and later maps show a strongminimum in the center (Figure 2). A typical centralAlpha well, M-35, initially (1974) had reservoir Cl near10,000 ppm which decreased slowly to 8500 ppm by1979, when Cl sharply dropped to nearly level out at4500 ppm (Figure 3). In 1992 injected brine reached thiswell and Cl increased rapidly. This behavior isinterpreted as showing production from reservoir waterwith a C1 gradient followed by the arrival of agroundwater front and an injectate fi-ont. Wells at thenorthern part of the Alpha reservoir decrease slowly in CIfrom their initial values until they are strongly affiectedby injected brine. Southern wells behave similarly, butdo not show injectate.

The response of the Beta reservoir is much morecomplex as seen in a contour map of reservoir chloridefor 1997 (Figure 4). Much of the chloride behavior inBeta wells has been described by Truesdell andLippmann (1998), here only chloride changes in wellslocated near reservoir boundaries and points of fluidinflow will be described. The histories of reservoirchloride concentration of these wells are shown inFigures 5-7 and their locations in Figure 4. These wellsare typical of their areas, but others show the samechanges.

Wells near the margin of the reservoir are consideredlikely to show evidence of groundwater recharge. In thewest, the Beta reservoir rock (sandstone with minorshale) probably interfiigers with coarser sediments(Halfinan et al., 1984) which could provide an avenue fwentry of groundwater. Less information is available aboutother boundaries. Chloride changes for well E-2 (Figure5) on the west indeed show groundwater inflow with a

gradual decrease from 13,000 to 10,000 ppm, followedby a sharp drop to 4000 ppm and in 1993 a rapidincrease. This is interpreted in the same way as thebehavior of well M-35 in the Alpha reservoir, althoughthe E-2 pattern is not as clear.

The southern boundary is evidently also leaky togroundwater as illustrated by well T-395 (Figure 5).Chloride in this well has decreased linearly fi-om 10,000to 7000 ppm. This suggests that there is a gradient inchloride to the south. To the north of the field thesystem appears closed to groundwater entry as shown byessentially constant chloride in well M-155 (Figure 6).This was suggested earlier because boiling appearedrapidly in the north when the CP-111powerplant went online (Truesdell et al., 1997). Well T-394 in the east alsoshows constant chloride (Figure 6), but the reservoirboundary is probably t% to the east as indicated bydrilling.

Other parts of the Beta reservoir have shown rapidchanges in chloride due to the entry of waters &omoutside. At the northwest part of the intersection of faultH with the top of the upthrown block (Figure 4), lowerchloride waters appeared soon after the start of large-scaleproduction. The area with lower chloride has spread 3.5km east- west and probably along the entire length of theintersection in the drilled field. A typical well at thecenter of this area is E-41 (Figure 7) which shows asteady decrease in reservoir chloride ffom 11,000 ppm toabout 4000 ppm. A similar decrease was shown by wellM-193 (Truesdell and Lippmann, 1998). This wasinterpreted as showing flow of groundwater down fault Hinto the upper part of the reservoir. It was estimated thatup to 1996 about 100 million tons had flowed into thereservoir in this area.

A strong high-chloride anomaly appears in the south-central part of the field centered on well E-55 (Figure 7).This represents return from brine injected into well E-6increasing from 0.5 xl Octons in 1990, to 4.3 x 1Octonsin 1991 and about 5-6 x 106 tons per year thereafter(unpublished CFE data). The shape of the Cl versustime curve indicates the arrival of a strong front.

METHODOLOGY

Observations of groundwater inflow into the Cerro Prietoreservoirs have been mostly qualitative (see Introduction)or limited to parts of the field. In order to quanti@ thecontribution of groundwater inflow to the totalproduction of the field a mass balance approach wasused. Although ~oundwaters adjacent to the exploitedreservoirs have not been sampled in situ, somewellfluids consist nearly entirely of groundwater that hasdisplaced the original thermal water (Figure 3).

This groundwater has been altered in temperature and

2

Truesdell et al.

reactive constituents, but certain unreactive componentsremain unchanged. In particular these are chloride ionand the hydrogen and oxygen isotopes of water whichallow the calculation of the flaction of groundwater in thetotal produced fluid. Isotopes have great advantages inthis calculation because they are less at%xtedby boilingand condensation processes (Stallard et al., 1987; Vermaet al., 1996; Truesdell et al., 1997), but the isotopedatabase is sparse, particularly in early years. For thisreason it was decided to use chloride ion which theComisi6n Federal de Electricidad (CFE ), the operator ofthe field, has analyzed once or twice a year for each wellfrom the start of production. A chloride balance on thereservoir fluid can be written as

Cl presentreservoirwater= X ● Cl groundwater+ (l–X) ● Cl originalreservoirwater (1)

where X is the mass fiction of groundwater. Thisequation assumes that mixing is the only processaffecting chloride. Although mostly true for the shallowerAlpha reservoir, it does not apply to large parts of theBeta reservoir in the CP-IH area (in the northeastern partof the field) which have undergone strong boiling andphase segregation during production (Truesdell andLippmann, 1998).

Another complication with the application of Eq. 1 isthat concentrations are referred to the liquid phase in thereservoir rather than to the total discharge. This isnecessary for wells that produce excess enthalpy fluids ascommonly found at CP-111.Therefore additional chlorideand enthalpy balances were used to calculate Cl inreservoir Iiquid for both all-liquid and excess steamreservoir fluids. These equations are

Cl reservoirwater= Cl separatorwater : :;~~r (2)

H totalfluid– H separatorwaterY separator=

H separator stem – H separator water(3)

and

H total fluid – H reservoirwaterY reservoir=

H reservoirstearrr– H reservoirwater(4)

where Y is the steam (mass) ii-action and H is enthalpy.Production enthalpy (H total fluid) is measured monthlyfor each well by CFE, reservoir enthalpy values werebased on Na-K-Ca geothermometer temperatures, andthermodynamic data for pure water were used (Keenan etal., 1969). The reservoir quantities used in Eqs. 1 to 4apply strictly only to fluid at the inlet to the well, but“reservoir” is used here due to common practice,

In order to calculate the mass fraction of groundwater

(X), using Eq. 1 the chloride concentrations ingroundwater and original reservoir water must be knownor estimated. Ciroundwater chloride was estimated fromthe chloride histories of wells like M-35 (Figure 3) inwhich original reservoir water has been completelyreplaced by groundwater, or nearly. so. Wells used fwthis estimation include M-26, M-3 1, M-35 and M-90producing from the Alpha reservoir, and M-193, E-4, E-27 and E-41 producing tlom the Beta reservoir. Theconcentrations of chloride in groundwater indicated ti-omthese wellfluids range from near 3500 to 4000 ppm. Forthe computations, a value of 4000 ppm was assumed fbrboth reservoirs.

Obtaining concentrations of chloride in original reservoirwater is somewhat more dlfflcult. Chloride gradients inthe earliest Alpha fluids suggest that the edges of thereservoir were affected by groundwater inflow and mixingbefore the start of production. The data for 1973 (the fmtyear of production) show four wells in the center of thedrilled area with 10,000 + 500 ppm Cl. Away born thisarea the chloride drops off to about 8500 ppm (Figure 1).For the Beta reservoir, 1986 data (before the start ofintense boiling in the CP-111area) show an elongatedarea in the east-central part of the field with five wellswith reservoir Cl above. 12,000 ppm, while the rest ofthe wells show reservoir Cl of 11,000 *1000 ppm(Truesdell and Lippmann, 1998). Based on these datathe original geothermal fluid is assumed to have areservoir chloride of 10,000 ppm in the Alpha reservoirand 11,000 ppm in the Beta reservoir.

RESULTS

Applying Eqs. 1 to 4 to waters produced from the Alphareservoir is straightforward. Chloride concentrations of10,000 ppm and 4000 ppm were used as the originalcompositions of the reservoir water and the enteringgroundwater, respectively. Figure 8 shows the evolutionof Cl in the reservoir, with a line for each wellconnecting the yearly average concentrations. The spreadin Cl concentrations at the start of production apparentlyresults fi-om mixing with groundwater which startedbefore the plant went on line. The slope of the change inchloride is similar fm most wells, and some wells nearthe center of the reservoir that started with higher C 1,cross over to the trend of the low-Cl wells (Figure 8).This resulted from the entry of groundwaters (flowingdown fault L) into the Alpha reservoir near its center(Figure 2). Many wells show upturns atler 1991 due toincreasing mixture with brine from the evaporating pondinjected into the reservoir along the western edge of thefield; injection began in 1989, becoming important afkr1991 (Table 1). The results of these calculations arepresented in Table 1 which shows, for each year, thetotal production from the Alpha reservoir and the partoriginating Ilom groundwater. Note that after 1983, morethan half of the total Alpha production was tlom

3

Truesdell et al.

groundwater recharging the system.

As inti~catedearlier, the Beta reservoir evolution is morecomplex. The extreme boiling in the northwestern CP-111part of the reservoir caused wellfluids to fluctuate inchloride as dilution with adiabatic condensate occurredin wells with the largest amounts of excess steam,followed in the same wells by high-chloride watersresidual to the earlier boiling. These processes have beendescribed at length elsewhere (Truesdell and Lippmann,1998). As a result of these processes (and the largenumber of wells producing Iiom the Beta reservoir), thediagram of changes in reservoir chloride for single wellsis confhsing and relatively uninformative (Figure 9).Only at the NE end of the intersection of fault H with thetop of the upthrown Beta reservoir (in the CP-111mea)did groundwater en~ and strong boiling coincide. Inany case boiling in this area did not last long because itwas quenched by the increase in reservoir pressure causedby the entry of groundwater.

At fwst we considered removing wells from thecomputation if they had unusually low or high reservoirchloride. However the groundwater fractions calculatedwith and without these wells diifered little, probablybecause they are few. The concentration changes wereshort lived and partly compensatory. We therefore leflthem in the calculatio~ but for the calculation of thefi-action of groundwater we did remove wells stronglyafkcted by injection of brine from the pond, for thereason that there was no compensating dilution process.Although only three wells are strongly affected byinjectate, the inclusion of these wells lowered the averagegroundwater fi-actionby about 10%.

The fkaction of groundwater was calculated for each Betawell on a yearly basis (usually there are only one or twochloride analyses in a year) using original reservoir waterand groundwater chloride concentrations of 11,000 and4000 ppm, respectively. The results of these calculationssummed for each year are given in Table 1. The part oftotal production due to groundwater is much smallerthan in the Alpha reservoir. Initially the results fluctuatefrom a thirtieth to a quarter of the total, but after 1985the relative amount is stable at between a fourth and afifth of the total production. Note the amounts injectedshown in the last column are about the same orsomewhat less than the groundwater recharge into bothreservoirs.

DISCUSSION

Mixture with groundwater was emphasized by Grtint etal. (1984) as the dominant reservoir process in both thenatural and exploitation states for the Cerro Prieto Alphareservoir. These authors tier from the natural stateenthalpy-chloride gradient that mixing with cooler wateracted as the dominant natural-state cooling mechanism,

and that exploitation was dominainflowing groundwater withbehavior is quite different ffom WNew Zealand in which boiling i:mechanism.

Chloride-time plots for central(e.g., Figure 3) show two regiachloride lasting a few years andthe arrival of a chemical fronlTruesdell and Lippmann, 198groundwaters entered tiom th(reservoir producing the gentle cinitial and early exploitationreservoir pressures declined susalinity groundwaters from abovlinto the reservoir producing thechemistry.

On the basis of the distributicsubsurface, we estimated that theat temperatures at and above 25((CFE unpublished data). Assureof 0.15 and an average fluid deflestimated initial Qwe-exploitatiobrine in the reservoir at these hig5.6 x 109 metric tons. This atimes the mass that has been pnsince 1973 (Table 1). In otherheat stored in the rock, the resejnatural groundwater recharge, angeothermal liquids, the Cerro Prisustain production for a longestimation of the commercial lifionly be obtained using numeric;which will also permit conexploitation and injection scenari

The advantages of “leaky” bounand exploitation of geothelemphasized by Lippmann andVerma et al. (1996). The latter alinflow of cooler groundwatersexploiting heat stored in fluids arsystem, because reservoir presslheat is swept to producing wellsdrilling wells for injection. Conin Table 1, it seems likely thproduction of Cerro Prieto has bethe inflow of groundwater. Theconstitutes about one quarter oftwhich the level of liquid saturathe bottom of most wells. Thumost wells would eventually prcsteam and would decrease in flovIII zone of intense boiling (see diTruesdell et al., 1997).

4

Leakv reservoir boundaries hel~ sustain m-oduction andexte{d the commercial lifetime if a geoth&md field, butsince recharge is mainly fkom cooler groundwater, thetemperature of the reservoir rocks and fluids mustprogressively decrease. Thus, power plant design shouldallow for the use of lower separation pressures as theexploitation of a geothermal field proceeds. .

ACKNOWLEDGMENTS

The work was partially supported by the AssistantSecretary for Energy Efficiency and Renewable Energy,Office of Geothermal Technologies of the USDepartment of Energy under contract No. DE-AC03-76SFOO098.

REFERENCES

Bermejo M., F.J., Navarro O., F.X., Castillo B., F.,Esquer, C.A. and Cortez A., C. (1979). Pressurevariations at the Cerro Prieto reservoir duringproduction. Proc. 2nd. Symp. Cerro PrietoGeothermal Field, Mexicali, BCN, Mexico, Oct. 17-19, 1979, pp. 473-493.

Donaldson, I.G., Grant, M.A., and Bixley, P.F. (1983).Nonstatic reservoirs: The natural state of thegeothermal reservoir. Journal of Petro[eumTechnology, 35, 184-194.

Grant, M.A., Truesdell, A.H. and Maii6n, A. (1984)Production induced boiling and coId water entry inthe Cerro Prieto geothermal reservoir indicated bychemical and physical measurements. Geothermics,13, 117-140.

Guti6rrez Puente, H. and Rib6 Muiioz, M.O. (1994).Reinfection experience in the Cerro Prietogeothermal field. Geotheml Resources CouncilTrans., 18,261-267.

Halfman, S.E., Lippmann, M.J., Zelwer, R. andHoward, J.H. (1984). Geological interpretation ofgeothermal fluid movement in Cerro Prieto field,Baja California, Mexico. American Association ofPetroleum Geologists Bull., 68, 18-30.

Hiriart-Le Bert, G. and Guti&rez-Negrin, L.C.A. (1996).Geothermal-electric development program inMexico. Geothermal Resources Council Trans., 20,581-586.

Keenan, J.H., Keyes, F.G., Hill, P.G. and Moore, J.G.(1969). Steam tables - Thermodynamic properties of

Truesdell et al.

water including vapor, liquid and solid phases.Wiley, New York, 162 p.

Lippmann, M.J. and Truesdell, A.H. (1990). Beneficialefkcts of groundwater entry into liquid-dominatedgeothermal systems. Geothemnal Resources CouncilTrans., 14,721-727.

Lippmann, M.J., Truesdell, A., HaMnan-Dooley, S.E.and Matlon M., A. (1991). A review of thehydrogeologic-geochemical model for Cerro Prieto.Geothermics, 20,39-52.

Pelayo L., A., Hemimdez G., M.L. and Ocampo D.,J.D.D. (1989). Analisis de la recarga de agua Ma enel campo geot&mico de Cerro Prieto. Proc. CFE-DOE Symp. in Geothermal Energy, DOE CONF8904129, pp. 271-276.

StalIard, M.L., Winnet, T.L., Truesdell, A.H., Coplen,T.B., Kendall, C., white, L.D., Janik, C.J. andThompson, J.M. (1987). Patterns of change in waterisotopes from the Cerro Prieto geothermal field1977-1986. Geothermal Resources Council Trans.,11, 203-210.

Truesdell, A.H. and Lippmann, M.J. (1986). The lack ofimmediate effkcts from the 1979-80 Imperial andVictoria earthquakes on the exploited Cerro Prietogeothermal reservoir. Geothermal Resources CouncilTrans., 18,405-411.

Truesdell, A.H. and Lippmann, M.J. (1990). Interactionof cold-water aquifers with exploited reservoirs of theCerro Prieto geothermal system. GeothemudResources Council Trans., 14,1.735-741.

Truesdell, A.H. and Lippmann, M.J. (1998). Effects ofpressure drawdown and recove~ on the Cerro PrietoBeta reservoir in the CP-IH area. Proc. 23rdWorkshop on Geothermal Reservoir Engineering,Stanford, CA, pp. 90-98.

Truesdell, A. H., Lippmann, M.J., and GutierrezPuente, H. (1997). Evolution of the Cerro Prietoreservoirs under exploitation. Geothermal ResourcesCouncil Trans., 21,263-270.

Truesdell, A.H., Lippmann, M.J., Quijano, J.L. andD’Amore, F. (1995). Chemical and physicalindicators of reservoir processes in exploited high-temperature, liquid-dominated geothermal fields.Proc. World Geothermal Congress, Florence, Italy,18-31 May 1995, pp. 1933-1938.

5

Truesdell et al.

Truesdell, A.H., Maii6n, A., Jim6nez, M.E., S5nchez,A. and Fausto, J.J. (1979a). Geochemical evidenceof drawdown in the Cerro Prieto geothermal field.Geothermics, 8,257-265.

Truesdell, A., Rye, R.O., Pearson, F.J., Jr., Olson,E.R., Nehring, N.L., Whelan, J.F., Huebner, M.A.and Coplen, T. (1979b). Preliminary isotopic

studies of fluids from the Cerro Prieto geothermalfield. Geolhermics, 8,223-229.

Verrnri, M., Quijano, L., Guti6rrez, H., Iglesias, E. andTruesdell, A.H. (1996). Isotopic changes in thefluids of the Cerro Prieto lli reservoir. Proc. 2 lthGeothermal Reservoir Workshop, Stanford, CA, pp.93-99.

Table 1. Groundwater (GW) contribution to production fi-omitheCerro Prieto Alpha and Beta reservoirs, compared tototal injection (quantities given in millions of metric tons)

t Year I Alpha Reservoir I Beta Reservoir I Injection ITotal GW Total GW Total

1973 23.35 I 4.351974 22.79 I 3.24 I

‘3.90 I I I I1975 21.421976 26.181977 25.791978 -c OL

1979 29.58 I 7.98 I 7.2(J I 2.19 I1980 9:?1 I -7‘7< Q701 1701 I

6.538.73

Lu. ou I 7.40 1.02 0.29---- --- —-. .-

1981 L-r.u7 I 7..

1982 22.35 I 8.5/ I ,LI..J1 11009 -* n? 0 09 -- OA

AJ. L 1 1 1 .1-J I u./7 1 1.!7 I9A ~n * 36 15.00 I 0.56 I

n-/l -19’71 n 21

170J ,LL. ul O.O.J LL.64 1.851984 19.33 8.74 20.41 2.281985 16.74 8.54 24.93 3.101986 15.81 8.64 90.38 14.591987 15.64 9.02 90.69 18.98

I21.06 I1988 13.34 8.46 84.04

1989 12.85 8.17 90.38 18.41 1.481990 19.41 12.54 99.29 18.90 4.501991 18.15 10.82 100.00 23.70 10.03

6

Truesdell et al.

M:l 1,. 1

3586203

w

35860m{

35a5m

h

/.2 ,,!k$idL

665030 6652QU

Figure 1. Map of reservoir chloride concentrationsfor the Alpha reservoir in 1973 (in ppm).

t

● ●

75 80 85 90 95

Year

35a6YYl

33atm3~

35awxl

35843CNI.

35$WXU-

M-)14 t I

-’l

Fault L ~

PM-J30

\l&

645em 665603

Figure 2. Map of reservoir chloride concentrations(ppm) for the-Alpha reservoir in 1983 with the positionof fault L (Halfinan et al., 1984) shown at reservoir level.

Figure 3. Reservoir chloride concentration history forAlpha reservoir well M-35.

7

Truesdell et al.

36875al

3587m0

35863XI

w36m0

3586500

359w00

3584500

6&X 66&a 66&Kl 6471X0 66icd3 668nm Me& 66$im

Figure 4. Map of reservoir chloride concentrations for the Beta reservoir in 1997 (in ppm). The location of fault H (Halfimtnet al., 1984) at the reservoir level is shaded; producing wells are shown by crosses “wi~ fiical wells (in Figures 5-7) named.

16C03 ,

● E-2

●☛

● ☛●

●✎

●

● .”*.,

● **

● ●● *

9

2coo~80 85 95 10I

Y~ar

llCOJ

EQQ lm

6033

, , , , ,

● ● “ T-395●

●

● :0 .**● 9

● ●*● %“b ● ● ●,.OO

.* ●

.0●*

86 88 90 92 94 96 98 1131

Year

Figure 5. Reservoir chloride changes(in ppm) for wells near the western (E-2) and southern (T-395) margins of the Betareservoir showing groundwater inflow.

8

g M-155Qa’ 1~ ●

●

~ ● “0’ ●*●A* ● :;. ‘“* ●*

20

●* ●; ● “

*●*

g 8cal-●

g ●

g

84 86 88 90 94 96 98

Yearw

130XI

EQQ, 120W

llm

lomx

9aYl

8030

Truesdell et al., , , , , , ,

T-3948

●

●☛

● ☛☛

● ● ☛ ● .” ●*”””**● *● *●***.

●●*

●

, , ,868890929496 98i

Year

Figure 6. Reservoir chloride changes in (ppm) for wells near the northern (M- 155) and eastern (T-394) margins of the Betareservoir showing no inflow of groundwater.

iEQQ 18QY3

J E-55●

●✎☛☛ ●

o~ 1lowo, I

88 m 92 94 96 98 88 w 92 94 96 98

Year Year

Figure 7. Reservoir chloride changes (in ppm) for wells that show entry of waters from outside the reservoir. Well E-41 is atthe center of the area affected by groundwater flowing down fault H into the Beta reservoir. Part of well E-55 production isflom brine injected into well E-6.

Iwm i 4-

Figure 8. Changes of reservoirchlorideconcentrationsforwellsof the Alpha reservoir to 1997 (in ppm).

I&xc!t

0 k78%3 B2M86 Wew%w

Y;ar

Figure 9. Changes of reservoir chloride concentrations for welkof the Beta reservoir on line in 1986 (in ppm). Inclusion of allwells would render the plot unreadable without addingsignificant information.

9