Other geochemical methods applied to Onikobe fluids

Geochemical methods established in other fields have been applied to Onikobe production water and steam analyses (Tables I and II). Some of these methods use triangular graphs, which provide a way to compare the relative (but not absolute) amounts of three components. Multiplying one or more of the concentrations used in constructing these graphs moves the points but does not change their relative positions, so a line of points, possibly showing mixing, remains a line.

5.1. The Na-K-Mg geothermometer diagram

There are a number of graphical methods that have been devised to test the state of equilibration, the homogeneity and the origin of geothermal fluids. The most widely used of these methods is the Na-K-Mg triangular diagram from Werner Giggenbach (1988). Figure 17 is a version of this graph modified for Onikobe data. This diagram is intended to combine the Na/K geothermometer, which equilibrates relatively slowly with an Mg-K geothermometer (invented by Giggenbach), which should equilibrate more rapidly. When the method works as intended, compositions of equilibrated waters should fall on the line, which represents full equilibrium. Because the Giggenbach Na/K geothermometer gives unrealistically high temperatures for Onikobe (Table III), the equilibration line in Figure 17 uses the more realistic Nieva Na/K geothermometer (Nieva and Nieva, 1987). On this graph the indicated Na/K temperatures cluster around 260ºC and the points group along the 260ºC line according to their pH suggesting that Mg-K equilibrium did not occur.

Figure 17. The combined Na/K and K-Mg geothermometer diagram using Nieva Na/K equation applied to Onikobe waters (see text for discussions).

5.2. The Cl-SO4-HCO3 and N2-He-Ar diagrams

Other widely used Giggenbach graphs compare the concentrations of Cl, SO4, and HCO3 with the object of indicating the extent of alteration of volcanic and near surface waters towards mature geothermal waters and compare gas components N2, He and Ar to indicate the volcanic or atmospheric origin of reservoir gases (Giggenbach, 1991). As useful as these methods are when

applied to high temperature springs and newly produced wells, they gave very little information when applied to Onikobe fluids. All points on the Cl-SO4-HCO3 graph fell at the Cl vertex. This lack of significant SO4 and HCO3 in Onikobe waters suggests that the acidity does not originate from SO2 and confirms the generally high acidity in most waters (acidity is incompatible with HCO3).

Similarly, all points on the N2-He-Ar graph fell between the compositions of air and gases in air saturated water (ASW). The points with air composition may result from air entering the sample bottle during steam collection, but could also enter the reservoir during injection. Similarly injection of water from the cooling tower could introduce some ASW gases. It is again notable that, as with the Cl-SO4-CO2 graph, there is no evidence of volcanic components in the Onikobe fluids.

5.3. The Cl-B-SO₄ diagram

In order to show the homogeneity of the Onikobe waters, Figure 18 was constructed showing relative amounts of Cl, B and SO4 in a graph introduced by Ellis (1970). In neutral, well-mixed geothermal waters both boron and chloride are conservative elements, neither reacting with rock minerals or separating during boiling or condensation processes, so that their ratio may be used to characterize waters with the same origin. However, the wide range of Cl/B ratios in Onikobe waters shows that, at low pH, these elements do not both act conservatively. In fact boron is volatile in low pH waters because it forms boric acid, which is soluble in steam. In Figure 18 neutral to alkaline waters (from wells 134 and 132) have low Cl/B ratios near 55:1 and the acid waters (from wells 130, 131 and 133) have high ratios near 90:1, with other wells intermediate in Cl/B. Surprisingly one of the well 135 waters with neutral pH also has a high Cl/B ratio.

Figure 18. The Cl-SO4-B triangular diagram applied to Onikobe production waters.

Note the clustering of points for wells and groups of wells.

5.4. A pH-Mg-Fe diagram

The solution of Mg and Fe by acid waters was described earlier as occurring first in the reservoir (where Mg minerals are found) and then in the well casing. Figure 19 shows that neutral waters (from wells 129, 134 and 135) have very high Mg/Fe ratios, slightly acid waters (from wells 127 and 128)

have lower Mg/Fe ratios (with Mg>Fe), and highly acid waters (from wells 130, 131 and 133) have low Mg/Fe ratios (with Fe>Mg). This graph suggests that for moderately low pH fluids reaction occurs first with rock minerals then for very low pH fluids continues in casings.

Figure 19. A pH-Mg-Fe triangular diagram applied to Onikobe production waters.

Note the progressive increase in the Fe/Mg ratio with decrease in pH.

5.5. The D’Amore gas grid diagram

In Figure 20, the gas geothermometer developed by Franco D’Amore (D’Amore and Truesdell, 1985) is applied to Onikobe gases. In this diagram the “s-shaped” curves represent reservoir temperatures from 125 to 350ºC and the other curves represent values of “Y” which approximate the ratio of reservoir steam to total fluid and range from 1 to –0.01 with negative values interpreted as the fraction of gas depletion resulting from boiling. As would be expected from the good agreement of measured, geothermometer and enthalpy-derived reservoir temperatures, there is little evidence of excess steam in the Onikobe reservoir. In fact only two samples (from well 127) showed positive y values and both of these indicated only about 0.1 % excess steam. The rest of the points on the diagram have negative y values, most indicating about 1% gas loss. The temperatures, ranging from 230 to 270ºC (with a few higher temperature outliers), approximate the measured and otherwise indicated values. Not all Onikobe samples could be represented on the diagram possibly because of their very low gas content.

The gas grid diagram shows that most production fluids are highly depleted in gas. This would be expected for waters that have repeatedly passed through steam separators and been reinjected into the reservoir. Thus the gas behavior confirms the indication from the increases in salinity that one of the reservoir fluids is recycled injectate.

Figure 20. The Dámore gas geothermometer ‘grid’ diagram applied to Onikobe production steam samples. Note that almost all points indicate negative values of ‘y’ indicating boiling of gas-depleted waters.