In several shale plays in North America (for example, the Eagle Ford), a common trajectory for 8.5-in or 8.75-in section consists of vertical, curve, and lateral sections. When running conventional positive displacement motors (PDMs), these sections are drilled in several trips. A new motor was developed to drill the vertical, curve, and lateral in a single run, eliminating trips and reducing drilling times. This new motor has a short bit-to-bend distance with higher dogleg severity (DLS) capability at lower bend angle settings. New high torque transmission and driveshaft were designed to pair with specially developed high torque power sections to achieve improved performance and tool life. When developing the new motor, a question arose about stabilizer placement because it greatly impacts wellbore quality, directional tendency, and toolface control. Traditional methods such as laboratory testing and static analysis were not able to provide sufficient insight; therefore time-based drilling dynamics analysis was used to design the optimal BHA for this new motor.

The time-based drilling dynamics simulation considers the effect of cutting structures and the transient dynamics of the drillstring. The cutting element and rock interaction was considered, enabling 3D bottom hole geometry to be calculated based on rock removal. Extended length of drilling can be mimicked and the wellbore surface quality and directional tendency can be evaluated. An optimal BHA was selected for this application based on simulation results.

The new short bit-to-bend motor and the proposed BHA design were tested in several basins in US land. In South Texas, the first three runs all reached total depth (TD) with high rate of penetration, showing excellent reliability and significantly improved performance. The only concern was that the toolface angle was not easily controllable when it kicked off near the Austin Chalk Formation. A simulation scenario was designed to mimic the motor drilling in such a transitional formation. Results revealed that sudden toolface angle change occurs when there is a sudden change in rock type and strength. The BHA was optimized accordingly based on the learnings from the study. The improved BHA was tested in the five following field runs and showed a step change in toolface control. Based on the successful field test runs, this short bit-to-bend motor was commercialized in March 2014.

Time-based drilling dynamic simulation provided a novel and unique solution to directional drilling using PDMs, delivering insights not achievable using traditional methods. The BHA design using such technique is one of the keys to successful motor development.

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