Slim Hole Thru-Tubing Casing Exit on Coiled Tubing: A Case Study Discussing the State-Of-The-Art in Casing Exits for A Coiled Tubing Drilling Project in Israel

Abstract The type of operation described in this paper has often been rejected by service companies because it is perceived to be unfeasible. This case study aims to highlight an example where detailed engineering mitigated the risks associated with the project and resulted in the execution of a previously unfeasible sidetrack, requested by an Israeli oil company which wanted to access a new and untapped section of a reservoir. This operation required passing through a 3.877" ID production tubing, featuring a landing nipple with a 3.562" ID, and drilling a new sidetrack from an existing deviated wellbore. Coiled Tubing Drilling (CTD) was the only option available for this project, given the slim-hole nature of this thru-tubing re-entry application. In addition, underbalanced drilling was required to boost production from a fractured formation. CTD is the only technique which can drill in true underbalanced conditions because it has continuous pipe and a closed-loop circulation system. Consequently, the reservoir never needs to be put in an over-balanced regime. The successful outcome of the operation validated the technology and the procedures being used and demonstrated how coiled tubing is the only option available for casing exit and milling operations on super-slim (below API recommended drift diameters) thru-tubing applications. A 3-1/2" OD whipstock attached to the bottom of a 3-1/8" OD Directional CTD Bottom Hole Assembly (BHA) had to pass through the production liner (3.958" ID) and the restriction given by the completion nipple (3.562" ID) to perform the casing exit and access the reservoir. The very small clearance posed a possible risk of getting stuck in hole. To deliver the project, a standard coiled tubing unit was converted to run e-coil. A camera run was carried out on the e-coil to inspect the tubing string and then a specially designed drift was run in hole to confirm the ability to pass the restriction, before running in hole with costly equipment. With the drift able to pass through the restriction, the casing exit operation was able to follow. The whipstock was set at the correct depth and oriented in the right direction using the real-time data from the CTD BHA. The depth of the whipstock was correlated using Gamma Ray (GR) and Casing Collar Locator (CCL) measurements. The orientation was measured using the gravity toolface and then the CTD Orienter was used to turn the toolface of the whipstock to the right setting orientation. 3.54" string mills and window mills were attached to the bottom of the CTD BHA to mill and clean out the window. The CTD BHA was run on e-coil and provided real-time feedback on the downhole conditions to the directional driller and the casing exit specialist.