Completion components integral to well design are selected for their required functionality.Thesecomponents range from simple profiles used for securing retrievable plugs,to valves of varying complexityand design used during the completion or production phases.Often,remedial work is required to removethese components if they are no longer working or needed when they are a hinderance to well access or itsproductivity.This paper presents two case histories of completion component milling operations that wereefficiently carried out by applying recent developments in combined tractor and mechanical applicationtechnologies.Electrohydraulic tractors were developed in the mid-1990s initially as means to convey electric line toolsin highly deviated sections of wells.Applications were soon developed to include rotational capabilityrun 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 amilling operation.Recent technology developments are providing an increased level of control,enablingmore complex component milling to be carried out efficiently and with greater degree of confidence.Suchcomponents,including flapper valves and nipple profiles,are made from a variety of steel alloys,shapesand dimensions.Efficient milling of these requires an optimal bit design,coupled with optimised millingparameters,for example,WOB,torque and RPM.The challenges of milling with limited available powerare discussed,new milling solutions are disclosed,and the importance of real time feedback of millingparameters 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 allrelevant milling parameters adjusted on the fly,delivering performance across all stages of the millingoperation.They will show the high level of instrumentation now available which ensures the millingoperation is conducted within prescribed and tested limits and allow performance parameters,designedand demonstrated in the lab,to be replicated one-to-one in the downhole environment.Improvements alsoinclude 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 otherissues in the well.The technology enables switching between the tractors driven and rolling rotational anchor functionalitywhilst providing continual rotation and back-reaming capability to minimise the possibility of a stucktool scenario.The case histories show that these developments have delivered unprecedented success inchallenging cased hole milling operations.
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