Structural correlation indicates that the main conduit of the tritium and the NDS from 1R8D to the production wells is the Sambaloran Fault. Figure 73 shows the schematic of the likely path of the tritium-bearing fluid. By first order of approximation, it is logical to think that wells 2R3D and 2R4D would yield the tracer first since these are situated nearest the injector well. It should then follow that the tracer recovery would diminish from well 2R3D towards well 202.

Figure 74 shows the cross-section and the structures intersected by wells where tritium and NDS were detected. As shown, the Sambaloran Fault is the only structure intersected by the wells which showed breakthroughs. Thus, it is the likely conduit of the tritiated brine.

Considering the proximity of the injector well, 1R8D, with the nearest monitor production well, 2R3D/2R4D, at ~140 m (well bottom separation), a much higher recovery of tracer was expected than was actually measured.

FIG 74. Cross-section of the wells showing the structures intersected. Highlighted in dark green are the intersects of the Sambaloran Fault. (CFL — Central Fault Line; MEF—Mahiao East Fault; Ura — Urangon Fault; SSU — South Sambaloran Unit).

Similarly, the tracer recoveries in wells 214 and 202 are considered low since there were already chemical and thermal indications as early as 2002 that when 1R8D was commissioned for brine injection (from South Sambaloran production wells in an adjacent sector), brine returns were then observed in wells 214 and 202, amongst others.

Although wells 202, 214 and 2R3D have brine returns based on geochemical monitoring and tracer breakthrough, the low tracer recovery suggest that these wells are still predominantly fed by the upper steam zone. This implies that most of the tritium is still in the deeper portion of the reservoir, along with the liquid zone. This is corroborated by flow measurements showing medium to high enthalpy discharges at the well head (W214 ~2500-2700 kJ/kg enthalpy, W202 ~2000-2200 kJ/kg, 2R3D ~1800-1900 kJ/kg).

Tritium, on the other, has a fractionation factor of 1. Thus, equal concentrations go to the water and vapour phases. The low recovery seen in wells 2R3D, 214 and 202 may suggest that the 10 Ci injection is insufficient.

The two pulses seen in wells 2R3D and 214 imply that the first pulse passed through the structural conduit, the Sambaloran Fault, from 1R8D and the second pulse which occurred at a later period could mean that the tracer travelled with the brine to the deeper part of the reservoir and later flashed and again passed through another conduit to appear in these wells (Fig. 75).

It has been reported that the northern Tongonan wells, 101 and 105D, showed declines in their gas concentrations and geothermometers, and this was attributed to the injection returns from well 1R8D. The discharges from these wells remained dry despite the prognosticated brine returns simply because of their shallow production zones, which are tapping the degassed steam from the Tongonan injection sink.

On the basis of the above-mentioned scenario, the absence of tritium in wells 105D, 101 and 109D could indicate two things. Firstly, there may not have been enough tritium injected into well 1R8D to effect a breakthrough in these northern Tongonan wells. Secondly, the current preferential flow of the tracer and the brine returns is to the east through the Sambaloran Fault, as indicated by the

early tracer breakthroughs in the wells along the structure. Thus, breakthrough of the tracer to the northern Tongonan wells could be observed much later. However, as shown by chemical changes in these wells, with the use of well 1R8D, the connection between these wells cannot be discounted.

Using minimal tracer recovery experimental data, no decline in temperature was observed in TRCOOL simulation (Fig. 76). Thus, no historical match was processed for the wells, in terms of cooling or thermal decline. The ~10°C decline in the fluid temperature (based on TQuartz) of well 202 did not match the cooling prediction based on tritium and NDS recoveries.

These observations and processed data could, therefore, imply one, or a combination, of the following: (i) tritium injected may have been insufficient to be detected in the monitor wells, however short their distances are; (ii) because of the nature of NDS tracer, monitoring in a highly two phase environment will give minimal recovery; (iii) the chemical breakthroughs observed in previous years could mean that well 1R8D is not the sole source of brine/cold waters capable of effecting such changes in thermal and chemical parameters.