Modelling structured geothermal systems II: Application of linear stability analysis

This paper illustrates how information obtained from linearised mathematical models can be used to investigate the development in time of complex natural systems such as structured geothermal reservoirs. Solutions to the saturation equation resulting from a linear stability analysis are obtained in the form of unidirectional waves. From the direction of propagation of these waves, the upper layers of a vapour-dominated zone equilibrate earlier than the lower layers, and the lower layers of a liquid-dominated zone equilibrate earlier than the upper layers. Our results support the two-layer models suggested for Wairakei (New Zealand), The Geysers (California), and Larderello (Italy). They also explain why the inverse model with a liquid layer over a vapour-dominated zone proposed for the Kawah Kamojang system (Indonesia) is not valid. We suggest to replace this model by a single-phase liquid layer over a liquid-dominated convective zone. The latter two-layer structure 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.