Modelling structured geothermal systems II: Application of linear stability analysis

This paper illustrates how information obtained from linearised mathematica l models can be used to investigate the development in time of complex natu ral systems such as structured geothermal reservoirs. Solutions to the satu ration equation resulting from a linear stability analysis are obtained in the form of unidirectional waves. From the direction of propagation of thes e waves, the upper layers of a vapour-dominated zone equilibrate earlier th an the lower layers, and the lower layers of a liquid-dominated zone equili brate earlier than the upper layers. Our results support the two-layer mode ls suggested for Wairakei (New Zealand), The Geysers (California), and Lard erello (Italy). They also explain why the inverse model with a liquid layer over a vapour-dominated zone proposed for the Kawah Kamojang system (Indon esia) is not valid. We suggest to replace this model by a single-phase liqu id layer over a liquid-dominated convective zone. The latter two-layer stru cture as well as the three-layer structure with a liquid layer between the upper vapour and lower liquid-dominated zones are proven to be possible in a homogeneous reservoir. Computer simulation results are presented. (C) 199 9 Elsevier Science Ltd. All rights reserved.