AN INTERPRETATION OF SMECTITE PRECIPITATION IN PRODUCTION WELLS CAUSED BY THE MIXING OF DIFFERENT GEOTHERMAL FLUIDS

Abstract

A trouble by smectite-amorphous silica precipitation happened at two production wells in the Onikobe Geothermal Power Station, northern part of
Japan. It is considered that the reason of the down-hole scaling is caused by the mixing of two kinds of fluids in a production well, based on the facts as follows.

During the drilling, there were two lost circulation zones at shallow and deep. The precipitation occurred above the shallow feed zone.
The pH of production fluid had changed radically from neutral to acid while the scaling trouble had occurred. According to the observation of cuttings
and the analysis of alteration zoning of the vicinity wells, acidic altered minerals distribute at the shallow feed zone, whereas neutral alteration at the deeper. In addition to the facts above, our chemical modeling
calculation, using SOLVEQ and CHILLER (Spycher and Reed, 1990), indicates that the mixing of the acid and the neutral fluids with some definite ratios would make a smectite supersaturated condition.