Fissure-Block Model for Transient Pressure Analysis in Geothermal Steam Reservoirs
At an earlier Stanford workshop, Moench (1978) presented a nonisothermal, radial flow, fissure-block, finite-difference model for geothermal steam reservoirs which was later used to simulate pressure buildup data for a steam well in Larderello, Italy (Moench and Neri, 1979). The model assumed the blocks to be impermeable but capable of conducting heat to the fissures which had been cooled by vaporization. In the present paper the model is revised to account for steam transport and vaporization within the blocks. This is a necessary consideration in order to accouht f o r the longevity of production wells i n The Geysers. The blocks, which may be i n i t i a l l y saturated w i t h l i q u i d water, are assumed t o have low i n t r i n s i c permeability and low p o r o s i t y relative to the fissures. Results computed with this finite-difference model are compared, for isothermal conditions, with the solutions of Boulton and Streltsova (1977). Under these conditions the model is similar to that of Kazemi (1969). When vaporization occurs in the blocks from a small amount of uniformly-distributed liquid water it is also possible to apply Boulton and Streltsova's solutions. This i s done by allowing for the apparent compressibility of the two-phase fluid mixture in the block. Comparison with Boulton and Streltsova's solutions under two-phase conditions is given in order to verify the finite-difference code. Numerical results are also presented showing pressure buildup following production with the blocks initially nearly saturated with liquid water. Effects of different thermal boundary conditions, block sizes and production times on pressure buildup curves are examined. After further refinements the model will be calibrated, using available pressure buildup data from representative wells in The Geysers, and used to provide a means for estimating pore pressure and temperature gradients within reservoir blocks. This information will be used to calculate changes in effective stress in the vicinity of a boiling front: a mechanism proposed to account for earthquake activity in the vicinity of The Geysers (Denlinger and Moench, 1979).
- Research Organization:
- USGS, Menlo Park, CA; USGS, Denver, CO
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AT03-80SF11459
- OSTI ID:
- 889123
- Report Number(s):
- SGP-TR-50-25
- Country of Publication:
- United States
- Language:
- English
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