Introduction Most high-temperature geothermal reservoirs are highly fractured systems. The fractures provide conduits through which fluid (and heat) can flow at sufficiently large rates to attract commercial interest. The rock matrix has a low flow capacity, but it stores most of the heat and fluid reserves. The fractures represent a very small fraction of the void volume, and probably contain less than 1% of total fluid and heat reserves in realistic cases. Sustained production from a fractured reservoir is only possible if the depletion of the fractures can be replenished by leakage from the matrix. The rate at which heat and fluid can be transferred from the matrix to the fracture s is therefore of crucial importance for an assessment of reservoir longevity and energy recovery. Yet most work in the area of geothermal reservoir evaluation and analysis has employed a "porous medium"-approximation, which amounts to assuming instantaneous (thermal and hydrologic) equilibration between fractures and matrix. Effects of matrix/fracture interaction have been investigated by Bodvarsson and Tsang ( 1981) for single-phase reservoirs, and by Moench and coworkers (1978, 1980) for boiling reservoirs. Moench's work addresses the question of pressure transients during drawdown and build-up tests in fractured reservoirs. The present paper focuses on a complementary aspect, namely, enthalpy transients. We use simple analytical expressions to analyze fluid and heat transfer between rock matrix and fractures. It is shown that heat conduction in a matrix with low permeability can substantially increase flowing enthalpy in the fractures. This affects fluid mobility and has important consequences for energy recovery and reservoir longevity. We present results of numerical simulations which illustrate these effects and show their dependence upon matrix permeability and fracture spacing.

Press the Back button in your browser, or search again.

Copyright 1981, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.