Over 1982-83 therehavebeen substantial increases in the reservoir engineering contribution to the New Zealand geothermal programme. This has taken four forms : - more extensive and detailed analysis of pre- and post-production performance, primarily at Broadlands; - the extensive use of high-resolution pressure gauges to carry out interference tests at Rotorua, Ngawha and Broadlands; - the installation of an extensive monitoring system at Rotorua; - the commencement of detailed simulation of Broadlands. Past modelling of Broadlands used a lumped-parameter model. More detailed data analysis has produced clearlydefined histories of the two productive areas of the field, and more detailed estimates of the amounts of recharge deriving from deep and shallow sources. Some interference tests at Broadlands show results which fit closely to the line source solution or the line source solution with barrier. An extensive monitoring system is being installed to record changes in the Ohaaki rhyolite, a shallow very permeable aquifer extending far beyond field boundaries. Interference tests at Ngawha, using Paroscientific gauges mounted at wellhead or within the casing have shown responses over a distance of several kilometres, and transmissivities of several hundred darcy-metres. Similar tests at Rotorua over shorter distances have shown major structure or anisotropy within a very permeable aquifer, and anomalously large storativity. Monitoring of reservoir pressures by water level or transducer at Rotorua has shown these to respond tobarometric pressure, earth tides, fluctuations in domestic withdrawal and level of Lake Rotorua. Instrumentation of springs and geysers has just been completed. Detailed numerical modelling of Broadlands has been started, initially with a model of the natural state. Results are expected to be used in detailed design of the reinjection system. Analysis of detailed chemical and enthalpy transients at BR21 demonstrate that the well produces from a fractured medium.

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