LECTURES ON GEOCHEMICAL !N1 ERPRETATION OF ...

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- 22 -vapor pressure of pure water (coexisting liquid and steam). there is nomixing of hotter and colder waters during upflow, and there is eitherconduetive cooling of the ascending water or adiabatic cooling with steamseparation at 100°C. Morey et al. (1962) noted that on a plot of logdissolved silica versus the reciprocal of absolute temperature, the datafor quartz fall approximately aLong a straight line over the temperaturer ange 20 0 to 250°C. Also, in the temperature range 20 0 to 250°C solubilitiesof chalcedony. n- cristobalite, ~-cristobalite. and amorphous silicalie along nearly straight lines on a similar plot (Fig. 1). In addition,a plot of log dissolved silica (after steam separation at lOOGe) versusthe reciprocal of absolute temperature also is approximately a straightline in the range 90° to 250°C....:'""E '"oC/)100010010/ /C/ ////«t 50 100 200 300 ·c3.5 3.0 2.5 2.0 1.510 3 fT. 'KFIGURE 1. Solubilities of varioussilica phases in water at the vaporpressure of the solution.A ~ amorphous s i lica, B = opal- CT(incorrectly identified as~- cristobalite by Fournier, 1973),C : ~- cristobalite, D = chalcedony.and E : quartz (from Fournier. 1973).
- 23 -Chalcedony is a very fine- grained variety of quartz composed of aggregatesof very tiny crystals. The individual quartz grains are so smallthat they have C'clatively la~e surface ener-gies compared to "normal"quartz. and this results in an increase in solubility. Chalcedony isunstable in contact with water at temperatures above about 120- 1BO°Cbecause the smallest- sized crystals completely dissolve and larger- sizedcrystals grow large enough that surface ene~y is no longer a factor.Temperature, time, fluid composition, and prior history (recrystallizationof amo~houssilica versus direct precipitation of quartz). all affect thesize attained by quartz crystals. Thus, in some places (particularly 100&lived systems) well - c~ystalline qua~tz may cont~ol dissolved silica attemperatures less than lOO·C. and in othe~s chalcedony (very fine- grainedquartz) may control dissolved silica at temperatures as high as l80·C(particularly younger systems).Equations Applicable in the Temperature Range 20-250·CEquations for the above straight lines (Useful only for reservoirtemperatures less than 250"C) are as follows. with the silica concentration.S. in mg/kg:Quartz- no steam loss t·c =1309 - 273.155.19 - log S(1)Quartz- maximum steam loss t"C • 1522 - 273.15at lOO·C 5.75 - log SChalcedony tOC 1032 - 273.154.69 - log SCl- Cristobalite t .. c • 1000 - 273.154.78 - log SP-Cristobalite t"C = 781 - 273.154.51 - log SAmorphous silica t·c '" 731 - 273.154.52 - log S(2)(3)(4)(5 )(6 )
Page 1 and 2: ,. ,O~KUSTOFNUNNATIONAL ENER GY AUT
Page 3 and 4: PREFACESince the foundation of the
Page 5 and 6: P~eTHE SOLUBILITY OF SILICA IV HYDR
Page 7: - 1 -CARBONATE TRANSPORT AND DEPOSI
Page 10 and 11: - 4ca~bonic acid (Table 3). Equatio
Page 12 and 13: - 6 -Values of y a~ewater and the r
Page 14 and 15: - 8 -The s implest equation ~ep~ese
Page 16 and 17: - 10 -c0"0"'0>,'"0>>-.- .D"0"'Q)Q'"
Page 18 and 19: - 12 -The effect of partitioning of
Page 20 and 21: - 14 -FIGURE 5. The computed activi
Page 22 and 23: - 16 -REFEREYCESA~o~sson.S .• 191
Page 24 and 25: TABLE 2.Coefficients for use with e
Page 26 and 27: - 20 -TABLE 4. Values of ionie cnar
Page 30 and 31: - 24 -Figure 2 shows the solubility
Page 32 and 33: - 26 -TABLE 2. Temperatures, enthal
Page 34 and 35: - 28 -800600~ci>E 400NoVi200o 500 1
Page 36 and 37: - 30 -800~ 600"- 0>E 400NoVi200o 50
Page 38 and 39: - 32 -has an enthalpy L , and the r
Page 40 and 41: - 34 -tempe~atu~e in kelvins. Molal
Page 42 and 43: - 36 -SILICA SCALING POT ENTIAL AS
Page 44 and 45: - 38 -and no silica scaling will oc
Page 46 and 47: - 41 -CATION GEOTHERKOKETRYRobert O
Page 48 and 49: - 43 -fo~ dilute solutions). Thus,
Page 50 and 51: - 45 -3.002.50~ 2.00Q-;"1.50z~g-' 1
Page 52 and 53: - 47 -whe~e ~ is 1/3 fo~ wate~s equ
Page 54 and 55: - 49 -Kg concent~ations in geotherm
Page 56 and 57: - 51 -Na/l00070IMMATURE WATERSK/l00
Page 58 and 59: - 53 -CONCLUSIONSIn evaluating cati
Page 60 and 61: 55CONVECTION AS A MECHANISM FOR TRA
Page 62 and 63: 57and mainly wi thin interconnectin
Page 64 and 65: 59During drilling, the mud weight r
Page 66 and 67: 61TABLE 2. Calculation of specific
Page 68 and 69: 63REFERENCESG~iffiths.R.W., 1981. L
Page 70 and 71: 66Temperature __, t 1/
Page 72 and 73: 68crystallizing about 0.1 km 3 of r
Page 74 and 75: 70usually to the side along a ~ift
Page 76 and 77: 72Although the above calculations a

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