Improved Bearing Design in Workover Motor Boosts Operational Efficiency for Plug Milling in North America

Multi-stage stimulation in shale plays within North America land is still very predominant as of today and most operators continue with the already proven success of using frac plugs for zonal isolation. Post stimulation, intervention operation is carried out to remove the plugs via milling on coiled tubing or threaded tubing. Although dissolving or disintegrating plugs are gaining traction, vast majority is still of composite material and does require a proper bottomhole assembly (BHA) to ensure that the plugs are removed with maximum efficiency in one trip. Coiled tubing (CT) service providers for milling frac plugs are using CT with an outer diameter (OD) ranging from 2" up to 2 7/8", with 2 5/8" becoming the new normal compared to the previous normal of either a 2" or 2 3/8". Primary driver behind the use of a larger pipe is to ensure that CT can get to the target depth (TD) with the required weigh on bit (WOB) as well as to maximize the pump rates for milling in long lateral wells (>15,000 ft lateral length), thereby minimizing the need for short trips during the entire milling process. This increased flow rate and extended reach requirement in turn leads to the need for a more robust workover motor (WOM) that can handle the additional set down weights imparted on the BHA. Traditionally the WOM were derived from drilling services, where the process of hole making and stress acting on the motor is a lot different than the plug milling operations. Due to these differences, the reliability of the WOM can get compromised, which in turn will impact the operational efficiency. This paper will discuss the steps incorporated to prevent drive sub failures, which were mostly caused due to the combination of bending load and torque transmitted by unwarranted stalling. The overload would cause a loss in compression around the upper radial bearing, which eventually leads to drive sub failing. Improvement potential was identified by increasing material strength and optimizing contact areas. Extensive modelling was carried out to improve the design, and materials with improved strengths were identified. Further, improvements were also incorporated into the lower thrust rings, both inner and outer. Field trials were conducted and based on the modifications, it was observed that the WOM failures were eliminated, thereby completing the mill outs in a single trip. Improved design and material selection, helps in providing excellent service delivery, which in turn reduced the OpEx for customers carrying out plug milling in North America.