Abstract

A conceptual model is presented to describe the flow mechanisms and estimate the reserves of vapor-dominated, fractured geothermal reservoirs. The model accounts for two permeability systems; (a) high permeability associated with the flow conduits such as large fractures or fault planes, and (b) low permeability associated with the randomly-fractured rock matrix. A characteristic rock transmissibility per unit bulk volume is defined for use as a correlation parameter. Initial mass in place is represented by the product of porosity and initial water saturation.

The effects of various geological and reservoir parameters on mass recovery efficiency were determined by simulating the conceptual model. Simulation results indicate that the dip and spacing of flow conduits, porosity, permeability, initial water saturation and reservoir thickness are the most effective parameters which influence recovery. Two correlation charts are presented relating mass recovery efficiency and decline in reservoir pressure to these parameters.

The procedures for using the correlations to estimate geothermal reserves and for checking the system's thermodynamic equilibrium are outlined. An illustrated example is included.