Completion components integral to well design are selected for their required functionality. These components range from simple profiles used for securing retrievable plugs, to valves of varying complexity and design used during the completion or production phases. Often, remedial work is required to remove these components if they are no longer working or needed when they are a hinderance to well access or its productivity. This paper presents two case histories of completion component milling operations that were efficiently carried out by applying recent developments in combined tractor and mechanical application technologies.

Electrohydraulic tractors were developed in the mid-1990s initially as means to convey electric line tools in highly deviated sections of wells. Applications were soon developed to include rotational capability run in conjunction with the tractor, enabling milling of well debris or completion components. For this, the tractor is used not only for payload conveyance, but also to provide weight on bit (WOB) during a milling operation. Recent technology developments are providing an increased level of control, enabling more complex component milling to be carried out efficiently and with greater degree of confidence. Such components, including flapper valves and nipple profiles, are made from a variety of steel alloys, shapes and dimensions. Efficient milling of these requires an optimal bit design, coupled with optimised milling parameters, for example, WOB, torque and RPM. The challenges of milling with limited available power are discussed, new milling solutions are disclosed, and the importance of real time feedback of milling parameters to ensure success are illustrated.

This paper discusses new electronic and hydraulic developments applied to the tractor-milling platform. Case histories will demonstrate the hi-fidelity measurement, independent control and optimisation of all relevant milling parameters adjusted on the fly, delivering performance across all stages of the milling operation. They will show the high level of instrumentation now available which ensures the milling operation is conducted within prescribed and tested limits and allow performance parameters, designed and demonstrated in the lab, to be replicated one-to-one in the downhole environment. Improvements also include specific bit designs that have been developed though a rigorous testing program to minimise tool jamming and the metal debris created during the milling process, which could inadvertently cause other issues in the well.

The technology enables switching between the tractors driven and rolling rotational anchor functionality whilst providing continual rotation and back-reaming capability to minimise the possibility of a stuck tool scenario. The case histories show that these developments have delivered unprecedented success in challenging cased hole milling operations.

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