Use of Ice Plugs in Geothermal Anti-Scalant Tubing Systems

Abstract

Anti-scalant tubing is commonly used in geothermal wells to allow chemical injection of anti-scalant fluids from the surface, delaying scale formation downhole. This tubing is intended to be continuously exposed to severe well conditions creating a need for monitoring and inspecting the injection string periodically. One of the methods used to inspect the tubing is called tubing lift. This consists of retrieving part of the anti-scalant tubing string from the well. This procedure moves the localised stress point on the tubing string and also allows inspection of part of the anti-scalant tubing at the surface.
Over time, this excess tubing collected at the surface becomes problematic, taking up space at the wellhead, and needs to be removed. A common solution to this problem is to cut and re-join the tubing with the slack removed. Unfortunately, this process has the inherent risk that when the tubing is cut, for a short period, the well remains open to the atmosphere. To avoid this exposure, the alternative is to retrieve the entire tubing string followed by unspooling & spooling, cutting and re-running the string in the well. However, this is a timely and costly endeavour for such a small change in the anti-scalant tubing string.
To improve the cut and re-join process, the idea of a temporary well barrier was explored, and ice plugs were found to be a suitable solution to well containment. Ice plugs have been used in process industries in a variety of applications, but there currently exists no documented evidence of their use in geothermal anti-scalant tubing maintenance.
This paper discusses the use of ice plugs in geothermal anti-scalant tubing operations, and how they have provided a simple and inexpensive solution in contrast to a full tubing retrieval while still addressing the well control risk of the cut and re-join method.