A new analysis demonstrates a rough correlation between microseismicity with focal depths ~ 2 km and injection in one well located in the southeastern portion of the Geysers geothermal field (in and near Unit 13), confirming what Stark (1990) has shown for the area to the northwest. The abrupt onset of micro seismicity near well McKinley-5 early in 1980 follows the initiation of injection there by about three months. Until 1986, injection and numbers of earthquakes within 3000 feet of this well appear to be roughly correlated, with injection increases or decreases leading the rise or fall of microearthquake numbers. For the years after 1986, there seems to be no correlation between earthquake frequency and injection; this may be due to the overall decline of injection volumes in the well. A similar analysis was conducted for well Thorne-7, but no correlation is evident. However, after the start-up of injection in 1984, microseismicity near this well did become more continuous from month to month.

In the design of geothermal installations, the possibility of scale precipitation caused by silica over saturation has often been taken as a design constraint upon steam separation pressure. At installations where over-saturation is allowed to develop, the resulting scale formation is a nuisance and in some cases affects the performance of injection wells. A review of the thermodynamics, reaction mechanisms, and kinetics of silica deposition, compared with actual experience at various locations, shows that it is possible to predict low-level scaling with sufficient confidence for production and injection system design. With foresight it thus becomes possible to suppress or manage the silica scale. A useful suppression technology is fluid pH reduction, achieved by mixing with noncondensible gases and/or steam condensate. Results from several geothermal systems will be presented. Further improvements of predictive technique will benefit from more uniformity in data collection, designing experiments, reporting of results, and reporting measurements of scaling in actual production systems.