The Heat Output of the Waimangu, Waiotapu-Waikite and Reporoa Geothermal Systems (NZ): Do Chloride Fluxes Provide an Accurate Measure?

Abstract

Geothermal waters from the Waimangu, Waiotapu-Waikite and Reporoa geothermal systems find their way into three separate watersheds. The heat flow data from each of these drainage areas have been assessed making it possible to compare the heat outputs from two independent methods: direct heat measurements and the chloride flux method. For both the Waiotapu/Reporoa Valley drainage and the Waikite drainage a discrepancy exists between the two assessments, with the heat output observed at the surface (Waiotapu-540k110 M W ; Waikite-80 MW) nearly double of that calculated from the chloride flux (300 MW; 36 MW respectively). It appears that much of the throughput of chloride does not reach the surface within the area which was monitored and the basic assumption on which the method is based has been violated. For Waimangu the direct heat output is assessed as 51Ok60 MW. However the ratio of enthalpy to chloride concentration of the source fluid is not well determined. Depending on the ratio chosen the heat output could lie between 360 and 800 MW. Although the chloride flux is accurately known, the heat output cannot be measured accurately without well determined data on the source fluid at depth.

Soil CO2 emissions as a proxy for heat and mass flow assessment, Taupō Volcanic Zone, New Zealand

“…The historic heat flow values are all based on chloride measurements, which might be less conservative when it comes to fluid‐rock interaction [ Bibby et al ., ] and is also less mobile than CO 2 (CO 2 has a gas partitioning coefficient of 1185). For groundwater having reached saturation with respect to CO 2, the majority of the gas passes through to the surface.…”

Drone with thermal infrared camera provides high resolution georeferenced imagery of the Waikite geothermal area, New Zealand

“…obscured by vegetation). This value is for evaporation, conduction and radiation; it does not consider advective heat loss associated with ebullition, or advective heat loss associated with the Otamakokore stream flow (Bibby et al, 1995), which comprises nearly all surface out-flow from the survey area and was quantified previously (43 MW, Glover et al, 1992)(46MW;Healy, 1952). Assuming a similar heat flow today, the total heat loss for the Waikite survey area averaged ~86MW during the survey period.…”

A three dimensional numerical model of the Waiotapu, Waikite and Reporoa geothermal areas, New Zealand

“…Newson (1993) used the same approach in her modelling study to predict an equivalent heat loss (Q) of about 315 MW for the Waiotapu field. Using different ΔQ/ΔCl values, surface losses between about 200 and 300 MW were estimated by Bibby et al (1995) for Waiotapu. From recent measurements of Cl flux by Smith (2013) the surface heat loss for Waiotapu was calculated to be~295 MW.…”

“…It was not possible to obtain a good match for the RP1 well temperatures and the Reporoa surface heat output without adding a deep hot upflow beneath the Reporoa field. In selecting the location of possible hot upflow centres beneath the Reporoa field, results of resistivity surveys (Bibby et al, 1995) and the location of faults within the Reporoa Caldera (Nairn et al, 1994) were considered.…”