MINERAL CHARACTERIZATION OF SCALE DEPOSITS IN INJECTION WELLS.pdf

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PROCEEDINGS, Thirty-First Workshop on Geothermal Reservoir EngineeringStanford University, Stanford, California, January 30-February 1, 2006SGP-TR-179

MINERAL CHARACTERIZATION OF SCALE DEPOSITS IN INJECTION WELLS; COSOAND SALTON SEA GEOTHERMAL FIELDS, CA

K.S. McLin1,2

, J.N. Moore2, J. Hulen

2, J.R. Bowman

1, B. Berard

3

1. University of Utah, Department of Geology and Geophysics135 South 1460 East Room 719, Salt Lake City, UT 84112-0111

2. Energy and Geoscience Institute, University of Utah423 Wakara Way Suite 300, Salt Lake City, UT 84108

3. CalEnergy Operating Corp7030 Gentry Rd, Calipatria, CA 92233

e-mail: [email protected]

ABSTRACT

The geochemical effects of injecting fluids intogeothermal reservoirs are poorly understood and maybe significantly underestimated. Decreasedperformance of injection wells after 5 to 7 years ofinjection has been documented in several geothermalfields. In this study, the effects of injecting flashedgeothermal fluids into the Coso and Salton Seageothermal fields, California are investigated bycomparing drill cuttings from the original injectionwells with samples from wells drilled on the samepads after injectivities in the original wells haddeclined. At Coso the fluids injected into 68-20 hadsilica contents up to 940 ppm and are grossly

supersaturated in silica with respect to quartz, thestable silica phase in the reservoir. X-ray diffractionand scanning electron microscope analyses of thereservoir rock penetrated by redrilled injection well68-20RD indicate that loss of injectivity in 68-20 wascaused by the deposition of silica as opal-Aaccompanied by trace amounts of calcite near thewell bore. As the scale deposits mature, the original

2 m spheres coalesce into larger spheres, up to 10

m in diameter and plate-like sheets. At the SaltonSea the fluids injected into Elmore IW3 RD arehypersaline and metal-rich but relatively silica poor.Scale deposits in the reservoir rocks near the

injection well consist of layered barite and fluorite,accompanied by minor anhydrite, copper arsenicsulfides, and traces of amorphous silica.

INTRODUCTION

Decreased performance of injection wells has beenobserved in several geothermal fields after only a fewyears of service, although the reason for thesechanges has not previously been established. In thisstudy, we present the result of petrologic

investigations of the mineral assemblages in thereservoir rocks surrounding injection wells at theCoso and Salton Sea geothermal fields, CA. Thesesamples were collected from injection wells drilledon the same pads as the original injection wells aftertheir injectivities had declined. Cuttings from Cosoinjection wells drilled from 1987 to 1993 on pad 68-20 and from Elmore IW3 RD at the Salton Sea wereexamined.

THE COSO GEOTHERMAL FIELD

The Coso geothermal field is developed in Mesozoicgranitic rocks of the Sierra Nevada Batholith on thewestern edge of the Basin and Range (Adams et al,2000). The heat driving the geothermal activity is

related to shallow intrusions that have given rise to38 rhyolitic domes during the last million years. Thereservoir host rocks range in composition fromdiorite to granite with varying degrees of alterationand veining (Kovac et al, 2005). Active and fossilfumaroles lie along a NE-SW trending belt thatextends through Devils Kitchen and Coso HotSprings. On the eastern margin of the field, knownas the East Flank, fossil sinter and travertine depositsare present (Adams et al, 2000). Geothermal powerproduction has been sustained at 240 MWe since1989. Between 1987 and 1993, six injection wellswere drilled on the 68-20 pad in the southern part ofthe field (Figure 2). Reservoir temperatures prior toinjection ranged from approximately 205-240oC. Thetemperatures of the injected fluids ranged from 110-120

oC. These injected fluids had silica contents

ranging from 174 to 965 ppm and were grosslysupersaturated in silica with respect to quartz, thestable silica phase in the reservoir. The compositionsof the injected fluids are given in Table 1.


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Figure 1. Simplified geologic map of the Cosogeothermal field showing the locations of the majorthermal features. The 68-20 injection pad is locatedin the southern part of the field.

68-20

68-20RD

68A-20

68A-20RD

68B-20

68B-20RD

67A-17

67B-17

67C-17

67-17RD

68 - 20

67 - 17

68-20

68-20RD

68A-20

68A-20RD

68B-20

68B-20RD

67A-17

67B-17

67C-17

67-17RD

68 - 20

67 - 17

Figure 2. Well trajectories for injection wells drilled

in pads 67-17 and 68-20. Locations of lostcirculation zones are shown as discs, and the amountof fluid lost is represented by the size of the disc.Axes in feet.

Table 1. Injected fluid chemistry from well 68-20,showing high, low, and average concentrations inmg/kg from 15 analyses. Brines were injected at 110-120

oC.

High Low Average

Na+

4,283 2,897 3,612

K+

941 362 614

Ca2+ 130 19 45

Mg2+

8.7 0 1.1

Fe 84.1 0.1 9.5

Al3+

10.4 0 1.1

SiO2 (aq) 965 174 657

B(OH)3 141.8 83 115.4

Li+

47 25 34

Sr2+

8.2 2.6 4.4

Astotal 26.24 2.85 9.54

Ba2+

116 0 9

HCO3-

229 77 167

Cl-

6,958 5,015 6,079

F-

5.7 1.6 2.5

SO42- 99 27 68TDS 12300 9233 11103

Lab pH 8.3 6.17 7.44

SEM and XRD Studies from 68-20RD Cuttings

Cuttings from Coso injection wells 68-20, theoriginal injection well, and 68-20RD, 68A-20, and68A-20RD have been examined at 3 m depthintervals. The rock type, the abundance of primaryand secondary minerals, and the abundance,mineralogy, and paragenesis of the veins was

documented at each interval. Thinly banded opalinesilica was observed in the cuttings from 68-20RD,but not in the original injection well 68-20 or in wells68A-20 and 68A-20RD. The banding and texturalrelationships suggest the silica represents fracturefillings and not alteration of preexisting minerals.The greatest density of silica precipitation was foundin cuttings from depths of 869-884 and 1710-1713 m.Samples of the precipitate from these two zones wereanalyzed using a scanning electron microscope andX-ray diffractometer. In this paper, SEM images andX-ray patterns of scale deposits are separated bydepth for descriptive purposes; however,morphologies and depth are not correlative.

.869-884 m depth

The silica deposits consist of opal-A spheres andplates. Figure 3 shows the morphologicalprogression associated with maturation of deposits.Textural relationships shown in Figure 3a indicate the

silica was deposited initially as spheres 1-2 m indiameter. As the deposit matures, the spheres

coalesce to form larger spheres up to 10 m indiameter (Fig. 3b). Further maturation is associated


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Figure 3. (a).-(d). SEM images of samples taken from 68-20RD at 869-884 m depth. Opal-A spheres 1-2 mm indiameter seen in (a). coalesce to form 10 mm spheres and sheets seen in (b)., (c)., and (d).. (e). X-ray diffraction

pattern of scale samples taken from the depth interval 869-884 m showing a broad opal-A peak centered at 22o

2-theta and quartz peaks at 21.5

oand 26.8

o2-theta

.

a. b.

c. d.

10 m 10 m

10 m 10 m

1000

0

100

200

300

400

500

600

700

800

900

35.015.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0

2-Theta

Counts

/Sec

e.


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Figure 4. (a).-(d). SEM images of samples taken from 68-20RD at 1710-1713 m depth. (a). Alternating silica layers

with varying density and visible porosity. (b)., (c). Silica spheres aligning to form strands (b). and sheets (c). (d).Tube structure covered with silica spheres. (e). X-ray diffraction pattern of scale samples taken from the depth

interval 1710-1713 m showing a broad opal-A peak centered at 22o

2-theta and quartz peaks at 21.5o

and 26.8o

2-theta.

20 m 10 m

20 m 5 m

a. b.

c. d.

1000

0

100

200

300

400

500

600

700

800

900

35.015.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0

2-Theta

Counts

/Sec

e.


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with the formation of plates and sheets. Infilling ofthe spaces between spheres provides a possibleexplanation for the dense, smooth surfaces seen inFigures 2b, 2c, and 2d. This maturation sequence issimilar to changes observed in young sinter depositsin New Zealand described by Rodgers et al (2004)and Lynne and Campbell (2004). Traces of calcite

locally coat the amorphous silica, suggesting itrepresents a later stage in the evolution of thedeposits.

The X-ray diffraction pattern of handpicked silica-rich samples from this depth (Fig. 3e) indicate thatthe deposit consists of opal-A with a broad peakcentered at 22

o2-theta. In addition, quartz peaks are

present in the X-ray diffraction patterns at 21.5o

and26.8

o2-theta, but quartz was not unambiguously

documented in the SEM images. It is possible thattraces of quartz were deposited by the injected fluidor through interactions with the amorphous silica.Alternatively, the quartz could represent fragments ofthe host reservoir rock that were incorporated into thedeposits of amorphous silica.

1710-1713 m depth

SEM images of silica deposits from a depth of 1710-1713 m show that they display generally similartextural and mineralogical relationships as those fromdepths of 869-884 m (Fig. 4). Both dense and porouslayers of silica are present. Textures shown inFigures 3b and 3c suggest that the denser layersdevelop as the silica spheres form strands and sheets.The formation of strands of small spheres suggests a

progression to a more stable silica form. The silicaplates in Figure 4c appear to be formed fromcoalesced opal-A spheres. An unusual tube likestructure coated with silica spheres is shown inFigure 4d. Similar features, interpreted as silicifiedbacteria, have also been observed in sinters fromgeothermal fields in New Zealand (Rodgers et al,2004). The X-ray pattern (Fig. 4e) of a sample from1710-1713 m indicates the silica consists of opal-Awith a broad peak centered at 22o 2-theta and quartzwith peaks at 21.5

oand 26.8

o2-theta.

THE SALTON SEA GEOTHERMAL FIELD

The Salton Sea geothermal system is developed inQuaternary deltaic sandstones and shales of theSalton Trough. The fluids injected into Elmore IW3RD-1 are hypersaline and metal-rich but containrelatively low concentrations of silica, which isremoved prior to injection. Elmore IW3 RD-1 wasdrilled in 1988 and used for injection until 1997. Dueto declining performance, the well was deepened in1997 from 2308 to 2405 m and renamed Elmore IW3RD-2. The temperature of injected fluid is 110

oC.

Injection chemistry is similar to Unit 6 ObsidianButte well fluids shown in Table 2, although the

silica is removed prior to injection. Also, theremoval of steam will increase the concentration ofchemical species in solution.

Table 2. Fluid chemistry from Unit 6 Obsidian ButteWell, showing high, low, and average concentrations

in mg/kg.

High Low AverageB(OH)3 350 300 320

Na+

50,000 40,000 45,000

Mg2+

45 30 40

Al3+

0.30 0.15 0.25

SiO2 (aq) 420 440 480

K+

16,000 11,000 13,000

Ca2+

29,000 22,000 25,000

Mn 1,200 750 1,000

Fe 1,500 700 1,200

Cu 6 3 4

Zn 400 250 325

As total 12 9 11

Rb+ 80 60 70Sr

2+525 350 450

Ba2+

220 140 180

Pb 100 60 80

NH4 450 300 375

Cl-

165,000 125,000 140,000

SO42-

150 50 100

Br-

100 80 90

I-

15 5 10

F-

25 15 20

HCO3-

100 40 70

TDS 250,000 220,000 235,000

pH 5.25 5.50 5.75

Studies from Elmore IW3 RD cuttings

A variety of scale deposits have been recognized inthe cuttings based on their mineralogy and textures.These deposits are observed at the top of Elmore IW3RD-2 and at a depth of approximately 2365 m. SEMand semiquantitative energy dispersive analysesdemonstrate that the scale deposits consist primarilyof variably colored layers of barite (BaSO4) andfluorite (CaF2) associated with minor anhydrite(CaSO4). Photomicrographs of these banded scaledeposits are shown in Figure 5. Minor amounts of

copper arsenic metal sulfides and traces of silica arealso present in the scale deposits (Fig. 6). Thepresence of gouge and microbreccia in the deeperzone suggests the scale was deposited in a fracturezone. Figure 7 shows that the anhydrite postdates theformation of barite.

DISCUSSION

Monomeric and polymeric deposition are twomechanisms of silica precipitation (Iler, 1979).Direct deposition of silica molecules onto solid


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Figure 5 (a)-(f). Photomicrographs of scale deposits from Elimore IW3 RD sampled from 2308-2405 m depth.Individual layers of barite and fluorite range from tens to hundreds of microns in thickness. (a)., (c)., (e). Images

taken under plane polarized light. (b)., (d)., (f). the same fragments under crossed nicols. (b). The gray mineral in

the left half of the large central fragment is barite. The right side of the fragment, which appears black undercrossed nicols is fluorite.

500 m

500 m

500 m

500 m

500 m

500 m

a. b

c. d

e. f.


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Figure 6. SEM image of a copper arsenic sulfidefrom Elimore IW3 RD sampled from 2308-2405 mdepth coated with amorphous silica.

Figure 7. SEM image of a barite layer in scale fromElimore IW3 RD sampled from 2308-2405 m depth.Anhydrite crystals are seen here coating barite. Bothminerals precipitated from the injected fluid.

surfaces is referred to as monomeric deposition. Theformation of a colloid in solution and its subsequentprecipitation is referred to as polymeric deposition.

Monomeric deposition tends to form a hard, densedeposit, while polymeric deposition forms a softer,porous silica scale. The textures of silica spheresobserved in deposits from Coso well 68-20RDindicate that polymeric deposition is the dominantprocess in precipitation of amorphous silica fromsolution. Smooth surface textures possibly representa second process of silica deposition. At higherdegrees of silica supersaturation, polymericdeposition is favored. As the silica precipitates, thedegree of supersaturation in the fluid decreases,favoring monomeric deposition. This is a possible

mechanism to explain the variety in textures andobserved porosity between layers. The interlayeringof barite and fluorite deposited in Salton Sea wellElimore IW3 RD, in contrast, is likely a reflection ofthe differing solubilities.

SUMMARY AND CONCLUSIONS

Examination of cuttings from redrilled injection wellsin both the Coso and Salton Sea geothermal fields hasyielded direct evidence for relating injectivity lossesto mineral precipitation. At Coso, deposits ofamorphous silica associated with traces of calcitewere found in the reservoir rocks adjacent to theoriginal injection well 68-20. This well hadexperienced a significant loss in injectivity within aperiod of 7 years. The silica deposits are layered,with individual layers ranging from tens to hundredsof micrometers. Apparent porosities vary from layerto layer with some displaying little visible pore space.Textural relationships indicate that the silica was

originally deposited as 1-2 m spheres of opal-A.The size and uniform diameter of the spheressuggests the silica layers formed as a colloidalprecipitate. As the deposits mature, spheres up to 10

m form. With further maturation, infilling of porespaces between spheres results in the formation ofsilica plates and sheets. These layers appear densewith little permeability. At the Salton Sea, scaledeposited in the reservoir rocks as a result ofinjection into Elmore IW3 RD consists mainly ofalternating layers of barite and fluorite. Minoranhydrite, copper arsenic sulfides, and amorphoussilica are also present.

REFERENCES

Adams, M.C., Moore, J.N., Bjornstad, S., andNorman, D.I. (2000) Geologic history of the Cosogeothermal system. Geothermal Resources CouncilTransactions, v. 24, p. 205-209.

Fornier, R.O., (1985) The behavior of silica inhydrothermal solutions. In: B.R. Berger and P.M.Bethke, eds., Geology and Geochemistry ofEpithermal Systems, Reviews in Economic Geology,v. 2, Society of Economic Geologists, p. 45-61.

Iler, R.K. (1979) The Chemistry of Silica-Solubility,

Polymerization, Colloid, and Surface Properties, andBiochemistry. John Wiley & Sons, Inc., New York.

Kovac, K.M., Moore, J.N., and Lutz, S.J. (2005)Geologic framework of the East Flank, Cosogeothermal field: Implications for EGSDevelopment. Proceedings, 30

thWorkshop on

Geothermal Reservoir Engineering.

20 m

Barite

Anhydrite

50 m

Silica


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Lynne, B.Y., Campbell, K.A. (2004) Morphologicand mineralogic transitions from opal-A to opal-CTin low-temperature siliceous sinter diagenesis, TaupoVolcanic zone, New Zealand. Journal ofSedimentary Research, v. 74, n. 4, p. 561-579.

Rodgers, K.A., Browne, P.R.L., Buddle, T.F., Cook,K.L., Greatrex, R.A., Hampton, W.A., Herdianita,N.R., Holland, G.R., Lynne, B.Y., Martin, R.,Newton, Z., Pastars, D., Sannazarro, K.L., Teece,C.I.A. (2004) Silica Phases in sinters and residuesfrom geothermal fields of New Zealand. EarthScience Reviews, v. 66, p. 1-61.

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MINERAL CHARACTERIZATION OF SCALE DEPOSITS IN INJECTION WELLS.pdf