Qualification Of An Innovative Sealant To Ensure Hydraulic Isolation In TTRD Application In The Presence of a Downhole Pressure & Temperature Gauge Cable
Authors: Taoutaou S. , Bedel J. 
Published: March 2015

Abstract
In a mature field, where the strategy for accessing the remaining reserves is driven predominantly by cost reduction, through-tubing rotary drilling (TTRD) can provide the optimal solution. TTRD allows for low-cost sidetracking opportunities to near-wellbore targets by leaving the existing completion and Christmas tree in place. Additional time and cost savings are achieved once the well has been drilled as there is no need to run a new completion.
In a project in the North Sea, the casing design requires that the TTRD candidate wells have a dual-casing exit above the production packer to access the location of the proposed hydrocarbon target. This operation requires a cement plug to be circulated into the annulus between the 5.5 × 7-in. tubing and the 9 -in. casing. The purpose of this specific cement plug is to provide tubing stability during the window milling operation and annulus integrity for the lifecycle of the new TTRD well. It will then act as a production packer, and it will also serve as the first abandonment plug for the donor well. Finally, the top of cement of the annulus cement plug should also leave enough space below the 9 -in. casing to allow for the final abandonment.
The main complication with this cementing operation is caused by a downhole pressure and temperature gauge cable that runs from below the proposed kickoff point up to the tubing hanger. The risk is that this cable could potentially cause micro-annuli within the annulus cement plug and hence impair the integrity of the well. A further potential issue can arise if the gauge cable is sheared during the window milling operation, thus creating the potential for hydrocarbons to travel up through its core to the tubing hanger.
A cementing service company designed and qualified an innovative sealant that can be used to meet the above criteria. Large-scale laboratory testing was undertaken to meet the technical and operational acceptance criteria.

Introduction
The Schiehallion field in the North Sea was discovered in 1993 and sits in 300 m of water depth. Through-tubing rotary drilling (TTRD) has been identified as a low-cost solution to access to remaining reserves, and a front engineering design team looked into the feasibility (Johansen and MacLeod 2007). One of the challenges related to the implementation of the TTRD is cementing around a downhole pressure and temperature gauge cable which runs from below the proposed kickoff point up to the tubing hanger. The risk is that this cable could potentially cause micro-annuli within the annulus cement plug and hence impair the integrity of the well. A further potential issue can arise if the gauge cable is sheared during the window milling operation, thus creating the potential for hydrocarbons to travel up through its core to the tubing hanger.
To mitigate annulus pressure buildup and provide a complete seal in the annulus, an innovative sealant was qualified, and large-scale laboratory testing was undertaken to meet the technical and operational needs.
One of the three proposed TTRD candidate wells in Schiehallion has been identified as requiring a dual-casing exit above the production packer to access the location of the proposed target.
This operation requires a cement plug to be circulated into the annulus between the 5.5 × 7-in. tubing and the 9 -in. casing.