Introduction 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 ? ” 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 is safe to assume that the casing strings are eccentric. This will require a different strategy.
A reduced OD stabilizer will be required below the mill to allow it to walk away from the ID of the casing being milled. This will prevent the mill from milling into the primary string casing. This is important because it is usually required that the primary string of casing still be testable after milling is complete. This is particularly important on wells where the hole angle is great enough that a large side load may be placed on the mill. A properly design mill with gage protection on the OD of the blades will also help protect from milling the primary string of casing. Cutting Removal
Removal of cuttings during milling has a great impact on the success of a milling job. If cuttings are not removed quickly and efficiently then downtime due to bird-nesting can become a problem. Bird-nesting occurs whenever a large number of cuttings accumulate in an area in the annulus. Surface equipment, flow line configuration, and milling fluids all become important for proper cutting removal. Surface Equipment A typical arrangement of flow equipment needed to remove cuttings on the surface is as follows. Flow is routed from the bell nipple through open flow ditches to two shale shakers with primary 24 to 50 mesh screens.
The fluid then exits the shale shaker through secondary 84 to 110 mesh screens. Large ditch magnets are strategically placed in the flow line and suction intakes. The magnets should be cleaned by rig personnel every half hour to remove settled swarf. Flow line Configuration To avoid flip-out of swarf in the bell nipple and BOP riser and the subsequent bird-nesting and pack-offs, it is necessary to reduce the size of the BOP and bell nipple riser from 30” to13 3/8”. This will allow a higher annular velocity to keep the BOP and BOP riser clear of swarf.
It is important that the flow lines from the riser to the shakers be as large and straight as possible with all valves removed. There should not be any obstructions in the flow lines to allow a bird-nest to form. Milling fluids The selection of the proper milling fluid is critical to a successful milling job. Polymer based fluids are generally preferable because of their increased lifting capacity. In order to obtain the maximum lifting capacity of the fluid it is preferable to keep the 6 rpm viscometer reading above 45 and the plastic viscosity as low as possible.