The Decrease of Capacity in Re-Injection Wells in the Takigami Field, Japan

Abstract

The Takigami Geothermal power plant (Unit: 25MW, Location : Fig.1) has been running since Nov.1st, 1996. The annual capacity factor in Takigami has been over 96% in the past two years. The capacity of production wells is almost stable, and the decrease of production rate is negligible. This result almost coincides with the expectation before operating. But in the re-injection wells (total injection rate 1,100 t/h), the decrease of the capacity is remarkable, and the rate of decrease in injection capacity has exceeded our expectations. Specifically, the rate of decrease in injection capacity was about 25%/year (270 t/h/year) in the first year after start-up, and 18% (195 t/h/year) in the second year. Recently, the rate of decrease has slightly improved, but the decrease is still continuing. At first, we expected injection rates to decline about 5%/year, because the closed system of re-injection (at a re-injection temperature of about 130 degrees C) was adopted in Takigami, so the unsaturated silica content would be relatively low, and silica scale problems would not occur as a result. In addition, the permeability in the re-injection area was very high, so the pressure interference among re-injection wells was almost negligible. According to the results of several investigations, it is inferred that the decrease of the capacity of the re-injection wells has been caused by silica scale clinging to and blocking the fissures of the reservoir, and not by pressure interference among re-injection wells. It is necessary to search for methods of inhibiting silica scaling to achieve stable operation at Takigami.