Numerical Analysis of the Decrease in Injectivity of Wells in the Otake Geothermal Field, Japan

Abstract

Thefield tests of injecting geothermal hot water into two wells have been conducted for 638 t o 656 days in the Otake geothermal field. The permeability-thickness product decreased from 224 t o 0.79 darcy-m for Well and from 91 t o 5 darcy-m for Well R-2. This is probably caused by the decrease in permeability due to the deposition of silica i n cracks around the wells. A mathematical model, which describes the permeability decrease caused by the silica deposition in a porous reservoir, has been used t o analyze the f i e l d data. The rate constants of silica deposition used i n calculation were determined by the laboratory experiments on silica deposition in porous media. A reasonable match was obtained between the measured and the calculated water levels for Well The zone where the permeability decreased significantly was calculated to be in the vicinity o f the well, within 8 m in radius. Outside this area, the decrease i n permeability is negligible 1.