| Abstract |
Characterizing the Hot Dry Rock (HDR) reservoir is a very important issue for the practical use of the HDR geothermal power. As one of the reservoir characterization methods, we have been conducting a geochemical study at the Hijiori and Ogachi HDR test sites, to develop a geochemical method for estimating the reservoir surface area. As part of the development of a geochemical estimation method for the reservoir surface area: (1) We estimated dissolution values (dissolved weights of Na and K) in the production water due to water/rock interactions. (2) We conducted preliminary laboratory test to measure the apparent rate constant of the reservoir rock dissolution and to assess the relationship between the dissolution value and the grain size of the rock sample. From these two studies, the surface area of the Ogachi HDR reservoir was preliminarily estimated. Estimated contents of sodium and potassium contributed by the dissolution of reservoir rock into the production water from the OGC-2 well during each circulation are summarized as follows; The estimated sodium contents were 225 ppm, 218 ppm, 210 ppm and 200 ppm, and the estimated potassium contents were 18.3 ppm, 19.2 ppm, 21.0 ppm and 16.0 ppm, in 1993, 1994, 1995 and 1997, respectively. The following results were obtained from the laboratory interaction experiments: (1) Under the limited experiment condition but like the Ogachi reservoir, the apparent sodium dissolution rate constant of the reservoir rock sample for different grain sizes was obtained. The following relationship between the apparent rate constant for Na and the rock/water ratio (A/M ratio: A represents the surface area of the reservoir and M represents the water volume in the reservoir) was derived; K(ppm/min)=0.001368(A/M)+0.00728 (2) Considering this rate constant for Na and the estimated Na content in the production water during each circulation period at Ogachi, the rock/water (A/M) ratio in the Ogachi reservoir was estimated. Using the A/M ratio and values obtained from tracer tests, the surface area of the Ogachi reservoir in each year was estimated as 6463x103 m2 in 1994, 6656x103 m2 in 1995 and 4174x103 m2 in 1997. |