Removing debris from the wellbore has been an industry goal for years. But now eliminating the opportunity for debris to build up or become stuck inside the wellhead is taking on a higher priority because drilling debris is not just a production and HSE risk; it is a financial risk. Drilling debris significantly impairs drilling, delays production and is a major cost to operations.
Drilling managers are seeking to keep the wellhead clean and are searching for protection against remnants of drilling fluid and other debris such as metal shavings or cement plug. The problem begins with debris being deposited at the same time the hanger is landed. As cement returns through the riser, debris or shards can be deposited in the profile of the wellhead and on its load shoulders. Debris also can scratch the elastomer seal elements passing through it, causing well control issues. Packoffs, wear bushings and BOP testers also are subject to the effects of debris.
Periodic removal of debris is needed and often is not completed to speed up drilling operations. As cuttings are circulated back to the surface, they meet with increasing hole diameters as they move from inside the casing into the wellhead. This slows the velocity of the cuttings, causing them to settle in exactly the place that causes the most issues.
Reality of risk, cost of status quo
Whether drilling a complex well on land or offshore, remnants of drill fluid or other debris can interfere with the proper installation and performance of wellhead systems, leading to costly remedial operations. In fact, based on operator feedback, it is estimated that installation issues due to debris occur a significant amount of the time and could lead to several millions of dollars in nonproductive time (NPT). NPT costs could be double for new wells with new crews or new rigs.
For example, an operator received a call from the rig informing him that the Christmas tree could not fully land on the tubing hanger/compact housing. After taking measurements, the team ascertained that the tubing hanger was locked in the housing but tilted. The connector was modified, installed and tested 48 hours later. While the operator praised the service company for its fast response and great support, the installation failure cost the operator two days of NPT. This could have been avoided if not for debris sitting on top of the seal assembly.
Debris, misalignment impact
These wellhead installation failures can be traced to casing hangers not being landed correctly, resulting in the inability to set the packoff or causing it to leak. Leaking pack-offs and tubing hangers are mainly caused by hangers not being centralized or not being able to fully land due to debris on the landing shoulders. Wells with a shallow deviation can add further risk to align-not always certain.
With the use of outward biased lock rings for the tubing hanger, debris often accumulates and solidifies behind the ring for years, preventing the ring's collapse to free the hanger. It is sometimes necessary to machine onsite to retrieve a completion.