Recent Developments in Carbonate Scale Inhibition and Reservoir Monitoring Hardware at the Dixie Valley, Nevada Geothermal Field

Abstract

Inhibiting calcium carbonate scaling in geothermal production wells requires delivering small amounts of a chemical below the jlash point. In geothermaljelds with relatively shallow flash points this can be accomplished by simply hanging 6.35 mm stainkss steel Capillary tubing in the wellbore. Infields with flash points below 600 m the tubingsftequently fail if not protected.

At the Dixie Valley geothermalfild the initial form ofprotection consisted of running capillary tubing strings inside a 1200 m long hangdown stri'ng. For 10 years this system proved generally reliable with some systems operating continuously for over I200 days. However, about one third of the time the downhole assembly become stuck and while pulling, the cap tubing parted. This resulted in well workovers to pull and rerun the hangdown strings.

In 1998 new inhibition systems were installed with capillaty tubings strapped to the outside of the hangdown string. Most wells now have two capillary tubings strapped to the hangdown string for scale inhibition. A research Vstem has five capillmy tubing strings providing for scale inhibition, continuous downhole pressure measurement, a thermocouple for continuous temperature measurement, and two opticalfibers for distributed temperature sensing measurements which allow for obtaining temperature profiles to the bottom of the hangdown stri'ng. The open hangdown strmg can accommodate logging with small diameter tools and repeated well kickoffs with Compressed air. These sysiems will cut operating costs of production wells by reducing the number of workovers needed to keep scale inhibition systems operating.