Casing Milling

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Sidetracking has been proven to be a cost saving solution over drilling new horizontal wells to increase production from existing wells. Most problems that might be encountered when removing casing can be identified prior to beginning a lining removal job. By identifying these potential problems, the proper equipment can be used effectively to keep the job running smoothly. Technical advances in mill design such as the development of a cutting structure that effectively manages the cuttings size and is more wear resistant has increased the efficiency of the entire operation.


A cost effective means to enhance production and provide for reservoir stimulation from existing platforms is to sidetrack wells from existing slots to reach new bottom hole locations. In the process of recovering these slots to facilitate the sidetrack, one or more casing strings must e removed from the existing well. Casing removal is accomplished by a combination of process such as cutting, pulling and milling. In typical wells 13 3/8” and 9 5/8” casing is cut and pulled from the well. Liners from 5 ½” through 9 5.8” may be milled to facilitate their removal.

Liners and casing cemented to the surface require removal by milling long intervals which have historically been slow and at best, time consuming process. Numerous operational problems exist including difficulty in cuttings removal and potentially stuck string from “bird-nesting” of cuttings. With new technology available, these problems can be eliminated or minimized to make liner removal a low risk economical choice for slot recovery.

This paper describes the process for recovering these platform slots and provides a review of new technology that enhances the economics of the process. Case studies will show how proper selection of the BHA, mud and mud handling system, mill design, and milling parameters make slot recovery and liner removal fast and economical. Many of the problems associated with past practices are eliminated.

Milling Considerations

Prior to embarking on a prolonged milling job, there are a number of issues that should be considered. These considerations can mean the difference between a project being completed on time and on budget or project running into multiple problems and costing much more than anticipated. With proper preplanning the probability of performing a good effective job will be greatly increased.

Casing milling jobs are now being planned and completed at hole angles above 50 & 60 degrees and through long tangent sections. In the pat without the assistance of technology, these types of jobs would have a high degree of uncertainty of being completed. The result was that a section would be cut or a Whipstock would be set to facilitate a sidetrack far above the desired target depth for the completed milling job. It is important to consider the following before beginning a job: (1) Casing eccentricity, (2) Cutting removal, and (3) Pilot mill design.

Casing Eccentricity.
An important aspect of planning a milling job is to determine if there is any eccentricity between the casing strings. This is an important consideration for determining the proper stabilization below the pilot mill. If the inner casing to be milled is held in place with cement and with casing centralizers installed then there is a good chance that the casing strings will be concentric with one another. Therefore, the stabilizer OD of the pilot assembly should be the drift ID of the inner casing string being milled. This will ensure that the mil tracks true to the centerline of the inner casing. It will also ensure that the collars are milled completely. If the collars are not completely milled, problems can be encountered at a later time when the skinned collar is circulated to the surface. Usually this results in a plugged flow line. Should the casing not have centralizers installed and the hole angle is 30 degrees or more, it...
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