Dual Permeability Modeling of Flow in a Fractured Geothermal Reservoir

Abstract

A three dimensional fracture system synthesis and flow simulation has been developed to correlate drawdown characteristics measured in a geothermal well and to provide the basis for an analysis of tracer tests. A new dual permeability approach was developed which incorporates simulations at two levels to better represent a discrete fracture system within computer 1 i m i t at ions. The first incorporates a discret e simulation of the largest fractures in the system plus distributed or representative element simulation of the smaller fractures. The second determines the representative element properties by disc r e t e simulation of the smaller fractures. The fractur e system was synthesized from acoustic t e l e view e r data on the orientation and separation of three distinct frac t u r e sets, together with additional data from the literature. Lognormal and exponential distributions of fracture spacing and radius were studied with the exponential distribution providing more reasonable results. Hydraulic apertures were estimated as a function of distance from the model boundary to a constant head boundary. Mean values of 6.7, 101 and 46 vm were chosen as the most representative values forth e three fracture sets. Recommendations are given for the addition a l fractur e characterization needed to reduce the uncertainties in the model.