Method of Optimizing Motor and Bit Performance for Maximum ROP (2007)

Abstract Downhole motors are widely used to drill vertical, directional and horizontal wells in conjunction with PDC bits. When a bent housing Positive Displacement Motor (PDM) is oriented for slide drilling to manipulate a wells trajectory, the drill string is not rotating. Consequently, the rate of penetration (ROP) typically decreases. It is therefore important to optimize Bottom Hole Assembly (BHA) performance in conjunction with PDC drill bits. This paper demonstrates how Motor performance data coupled with an ROP model can predict the optimal weight on bit (WOB) required to achieve maximum possible ROP for a given section of hole to be drilled. This approach solves the ROP model to determine ideal WOB in respect of any restrictions that PDM performance equations apply on it. Bit wear is included in the ROP model and an analysis is performed to optimize a given interval of well bore. The optimization approach is illustrated with two examples for different formation types and one field case comparing the performance of two Motors with PDC bits. The optimum WOB, maximum average ROP and differential pressure values are the outputs from the analysis. This analytical approach can be used to determine the optimum PDM / PDC Bit combination to achieve maximum ROP through a wide range of operational conditions. Introduction Positive Displacement Motors (PDMs) have gained widespread use in vertical, directional and horizontal drilling applications. In directional and horizontal mode, bent housing PDM's are used to manipulate well trajectory, (inclination and azimuth), to intersect bottomhole targets. Slide drilling occurs when the bend in the PDM is orienting in a certain direction. During slide drilling the drill string is not rotating. In slide drilling mode, Bit rotation results entirely from the Motor from drilling fluid being pumped through the drill string. Drilling in slide mode can significantly reduce ROP and increase well costs. Therefore the overall performance of Bit and Motor combination can have an extremely significant impact on drilling costs. In a Positive Displacement Motor the power section converts hydraulic energy of mud flow into mechanical rotary power, the reverse action of Moineau pump principle[1]. Each positive displacement motor has a helical rotor assembled inside a helical stator. Also, the rotor has one less spiral or lobe than the stator that results in continuous seal line between the rotor and the stator. The length of helical pitch for the stator is bigger than the rotor which forms cavity spaces between them. These cavities move along the power section from the inlet to outlet by rotating the rotor. Mud flow fills the cavity connected to inlet and extends it by the pressure applied on the rotor body until the next cavity connects to the inlet. This process forces the rotor to rotate eccentrically inside the stator. The performance of the motor is controlled by the combination of the rotor/stator lobe configuration and the motor stage number. Increasing the number of rotor lobes gives higher output torque of the motor and lower rotational speed.