Development of a Protective Shroud for Downhole Injection Tubing

Abstract

Downhole injection of antiscalant fluid is used to limit run down in two phase well output due to calcite scaling. The use of this process in the Kawerau geothermal field had been failing due to breakage of the injection tubing at the wellhead. A shroud was constructed to protect tubing from forces generated as the flow turns at the wellhead tee. This shroud failed after a short time in service. Analysis was carried out which suggested the failure was due to fatigue caused by two phase slug flow in the well bore and wellhead piping. A new shroud was designed to withstand the fatigue loading. Field measurements of wellhead vibrations where taken to determine the frequency spectrum of the wellhead and compared to the fundamental frequency of the failed shroud. The new shroud design incorporated features to raise its fundamental frequency above the wellhead vibration frequencies. The new shroud design has been installed on several wells in different geothermal fields, one of which has been in operation since April 1998. The operational history of the new shroud demonstrates the success of the design.