LECTURES ON GEOCHEMICAL !N1 ERPRETATION OF ...

- 27 -DISSOLVED SlLICA- ENTHALPY OIAGRAMIn contrast to t emperature, enthalpy is a derived property that can beobtained from steam tables, provided that the temperature, pressure, andsalinity are known. It is useful to use enthalpy as a coordinate ratherthan temperature because the combined heat contents (enthalpies) of twowaters at different temperatures are conserved when those waters are mixed(neglecting small heat of dilution effects). but the combined temperaturesare not. Figure 3 shows the solubility of quartz in pure water at thevapor pressure of the solution, plotted as a function of the enthalpy ofthe solution. In contrast to the silica- temperature plot (Fig. 2) in whichthere are two values for dissolved silica at a given temperature (in liquidand steam), in the silica- enthalpy plot there is only one value for dissolvedsilica at a given enthalpy. The solubility of quartz in steam ispart of a bell- shaped, symetrical distribution of dissolved silica valuesthat first increase to a maximum value at an enthalpy of about 1595 JIg,and then decrease with a further increase in enthalpy. For practicalapplications, most reservoir fluids are sufficiently dilute so that enthalpiesof pure water can be used to construct cnthalpy- compos ition diagrams.BOO'" 600~'"E 400NoVi200.s:.•Critical point.. ~."• •o 500 1000 1500 2000 2500Entholpy. Jig3000FIGURE 3. Solubility of quartz in water at thevapor pressure of the solution as a functionof enthalpy.