Influence of rotary-steerable-system design on borehole spiraling

The paper investigates the influence of the design of push-the-bit rotary-steerable systems (RSSs) on the tendency to drill spiraled boreholes by analyzing the directional stability of the bit trajectory. In this model, differences in RSS designs are accounted for conceptually by assigning a lateral stiffness to the RSS pads. This simple device, which introduces a dependence of the force on the pads upon the deflection of the bottomhole assembly (BHA) relative to the borehole axis, enables exploration of the influence of the actuation mechanism, with the RSS behaving at the ends of the spectrum either as a soft or as a stiff node of the drilling structure. According to this analysis, low pad stiffness has little consequence on the general behavior of the system. However, as the pad stiffness increases, any perturbation in the borehole geometry sensed by the pads alters the drilling direction of the bit and triggers, under certain conditions, self-excited oscillations in the borehole geometry. By increasing the transverse rigidity of the BHA in the vicinity of the bit, stiff RSS pads thus enhance the propensity of a drilling structure to drill spiraled holes and generate, if the system is directionally unstable, borehole oscillations with pitch that corresponds to the distance between the bit and the pads. In contrast, a directionally unstable BHA equipped with RSS characterized by a low stiffness produces spiraled holes with a wavelength corresponding to the distance between the bit and the first stabilizer.