Peninsular Malaysia has a number of fields where reserves are not economical to be further exploited using conventional drilling methods. With the increase in oil prices over the last few decades, an operator decided to embark on a new drilling technique of using a coiled-tubing drilling (CTD) unit to successfully access a bypassed reservoir.

One of the candidates chosen for the project was Well D in which the completion includes 3 1/2-in. tubing in 7-in. production casing. The challenge in this well was to place the whipstock through the cased tubing before drilling operations could be performed because there was no through-tubing (3 1/2-in.) whipstock design for 7-in. casing available on the market. The placement of the whipstock was crucial due to the existence of a shale zone uphole through which it was to be drilled that would create a borehole stability issue. The other reasons taken into considerations were to avoid dual-casing exit and the potential associated complications as well as to provide a uniform wellbore to create adequate annular velocity for cuttings transportation to the surface. The team proposed an atypical solution to assist whipstock setting by plugging the entire 7-in. production casing column until the end of tubing with cement.Once the cement was hardened and tested, a directional drilling assembly with a 2.8-in. speed mill was used to mill a pilot hole through the cement plug, boring along the high side of the casing. Next, a caliper log was run on slickline to confirm hole diameter, and a whipstock was then run into the pilot hole and set with it facing toward the casing. These operations were successfully performed by a rig-less CTD package.The milling assembly was run smoothly and the sidetrack window was effectively cut prior to drilling the target sand.

The successful case study presented summarizes in detail the technique used to mill such a wellbore, the challenges presented, and considerations madein designing the job, which proved to becorrect and accurate on the first attempt.