Calcium Carbonate Scaling in Geothermal Wells at Ohaaki

Abstract

Calcium carbonate scaling is a common occurrence in geothermal wells in Broadlands-Ohaaki. Broadlands- Ohaaki is a high temperature geothennalfield in the eastern margin of the Taupo Volcanic Zone (Tvz) in the North Island of New aaland. The field provides the steam requirements for the 11 6 W e Ohaaki power station.

Using the chemical anabsis of water ami gas dischargedfiom the wells, the chemical changes associated with iso-enthalpic boiling and subsequent formation of calcite scales is modeled using SOLvEQ, a computer program for computing aqueous-mineral-gas equilibria (Spycher and Reed, 1991), and CHILLER a reaction-path modding computer program (Spycher and Reed, 1990). The model assumes the deep fluid undergoes boilingfiom the initial reservoir temperature down to -150?C with every 5?C decrement. Saturatedgas and solid phases formed remain in contact with the solution throughout simulating a closed-ytem reaction process.

Maximum rates of calcite precipitation fiom modeling calculations are in the order of IF' to 1 Q3 grams per kilogram of geothermal fluid and occur less than 2O0Cfrom initial reservoir temperature. The scaling rates are consistent with iderred rates of calcite deposition in wellbores.