A typical method of creating casing exit windows for multilateral completions includes using tool strings comprised of mills for traversing whipstocks and creating elliptically shaped windows. This paper describes an innovative casing exit milling bottomhole assembly (BHA) and the method by which its design and predicted performance were optimized using inhouse-created drillstring dynamics finite element analysis (FEA) software.

Relatively longer casing exit windows are required to be generated for subsequent passage of contemporary drilling equipment. Sophisticated simulation technology is being used in designing the milling BHAs to create these longer exits. Paramount to generating a long window is maintaining sufficient restraining forces on the bottom-most mill to keep the mill tracking the full ramp length. Preferably, the BHA is designed so that the bottom-most mill can traverse against the entire ramp, and then deviate away from the ramp to avoid undesirable re-entry into the exited wellbore. The challenge is to optimize the milling BHA's endured contact forces so that its fatigue life is increased.

The aforementioned analytical tool helped in designing the milling BHA so that its operational bending stresses were reduced and its fatigue life was increased. Purposefully designed shapes and placements of the BHA's bladed cutting sections provided a favorable distribution of restraining forces, thus enabling an optimal-length casing exit window.

As directional drilling assemblies continue to be made longer and stiffer, there is an increased significance of using simulation tools to design milling BHAs. These newly designed BHAs will continue to be enhanced to maximize window length for improved ingress and egress of rotary steerable drilling BHAs. Extensive analysis, qualification testing, and field trials were performed on these systems. Case histories describing operational results will be shared and discussed during the paper presentation.

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