Abstract
In Underbalance Coiled Tubing Drilling (UBCTD), well pre-setting can be defined as the required drilling or work-over activity to set the last casing string at the top of a productive layer. This cost can be avoided by utilizing a thru-tubing casing exits (whipstock). This paper will address the challenges and enablers for direct wells’ intervention without pre-setting.
To close the gap between sending existing wells for pre-setting and direct well intervention with UBCTD, an extensive case-by-case analysis on UBCTD candidates is carried out. In essence, the approach was selected on the basis of utilizing existing technologies to their full potential or re-designing the tools to be able to fulfill the requirements. Since some candidates don't tolerate failures and allow for only one attempt, tools were always subjected to the highest level of quality checks and deployed in the best possible conditions.
Setting thru-tubing whipstocks in a typical well will require a low to medium inclination, rigid cement behind the casing and enough room below end of tubing. One of the main challenges in UBCTD is weight transfer for milling a window through the metal, especially in high inclination. Applying excessive weight on the window could agitate and dislocate the whipstock. Consequently, this could result in having stuck incidents while passing through the window during tripping. Thus, for such an application, the whipstock has been empowered with stronger slips for robust casing engagement.
In another challenge for cases with limited room below end of tubing, the window milling BHA was redesigned to be shorter. Also, close monitoring on the milling parameters was employed to ensure smoothness of the window and to avoid losing it and, indeed, losing the well. Such practice becomes standard to observe the milling parameters and benchmark them to guarantee the best window possible. This approach allows entry on wells with poor or absent cement behind the casing.
Utilizing the current wells’ assets as they are, and avoiding re-entry or mechanical pre-setting can save tremendous cost. Salvaging wells can be tricky with multiple choices and this would be always be a great opportunity with the tools and process upgraded. Thru-tubing whipstocks can now be handled in different fashions based on case-by-case applications.
