LECTURES ON GEOCHEMICAL !N1 ERPRETATION OF ...

- 37 -Consider a relatively dilute fluid in equilibriun with quartz in arese~oir at a temperature of 250·C (point R in Fig. 7). At that temperatureand at the vapor pressure of the solution, liquid has an enthalpy ofabout 1085 JIg and contains about 462 ms/kg dissolved silica. Conductivecooling during upflow (perhaps to a hot spring that has a relatively slowrate of discharse) without precipitation of a c~stal1ine silica phase,such as quartz or cristobalite. will result in the solution becoming justsaturated in respect to amorphous silica when the enthalpy decreases toabout 520 Jig at a pH of 7 or less (point C in Fig. 1 at a temperature ofabout 124°C). Further cooling with no increase in pH will result in thesolution becoming supe~satu~ated in ~espect to amorphous silica, and thatphase may p~ecipitate. Howeve~, the initial pa~tial p~essu~e of CO in 2the reservoi~fluid at 250·C is about 2.3S bars (obtained from an equationgiven by Arnorsson. 1985), so there could be a slight inc~easc in pH du~ingconductive cooling if CO is lost because of fluid decomp~ession. For a2situation in which a solution cools conductively f~om 250·C with an accompanyinginc~easein pH to 8 because of loss of CO 2, that solution wouldbecome just satu~ated in respect to amorphous silica at an enthalpy ofabout 480 Jig (about 11S·C). which is 60 Jig and 9·C lower than for thesituation in which there is no inc~ease in pH.Fo~commercial situations, cooling of the ascending fluid is likely tobe mainly by decomp~essional boiling (adiabatic) and the pH is likely toincrease du~ing upflow because of CO pa~titioning into the steam phase.2Fo~ adeabatic cooling of ~eservoi~ fluid R in Figure 7. the boiling solutionbecomes just satu~ated in ~espect to amorphous silica at an enthalpyof about 620 Jig at pH 7 (point E with a temperatu~e of 147·C), 51S J i g atpH 8 (point F with a temperature of I37·C). 520 Jig at pH 8.5 (point G witha tempe~ature of 124·C). and 425 Jig at pH 9 (point H with a temperatu~e of102°C). Assume that a wellhead steam separator is designed to operate at ap~essu~e of 3.3 ba~s (~esidual liquid enthalpy equal to 515 JIG after steamsepa~ation).If the pH of the flashed fluid does not exceed 1. the residualliquid in the separato~ (point F) would be supersatu~ated in respect to thesolubility of amorphous silica (point K) by about 70 mg/kg and silicascaling would likely result. On the othe~ hand, if the solution pHincreased to 8 as a result of CO sepa~ation into the steam phasc, the2solution would be just saturated in rcspect to amorphous silica at point F