Technique Models Effect of Local Doglegs on Running of Completions and Casing in Hole
Authors: Carpenter Chris 
Published: January 2022

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202431, “RSS and Motor Directional Analysis: Modeling the Effect of Local Doglegs on Running in Hole of Completions and Casing,” by Benjamin Nobbs, SPE, Stéphane Menand, SPE, and Ngoc-Ha Dao, SPE, DrillScan, prepared for the 2020 SPE Asia Pacific Oil and Gas Conference and Exhibition, originally scheduled to be held in Perth, Australia, 20–22 October. The paper has not been peer reviewed.
Rotary steerable systems (RSS) and steerable motors pose unique challenges when modeling bottomhole-assembly (BHA) directional behavior. The complete paper presents a methodology that aims to allow the anticipation of problems such as mechanically stuck pipe or lockup situations when running in hole (RIH) casing or completion strings. Correctly evaluating the risk of BHA, casing, or completion strings being stuck or locked up when RIH can ultimately provide a template for ultimate reduction of nonproductive time.

Local Doglegs and Downhole Equipment
A pure rotary BHA has no ability to change its directional behavior during the run. Its directional capabilities are defined by stabilizer position and outer diameter, and, although no ability exists to control directional behavior directly, this type of BHA may be sensitive to weight on bit (WOB), mud weight, formation changes [unconfined compressive strength (UCS)], and inclination. With proper design, it is possible to design a rotary BHA that is insensitive to these parameters.
A motor and turbine can operate with or without a bent housing. A bent housing imposes a small angle offset from the axis of the drillstring, with the result that two distinct modes of directional behavior occur, sliding and rotating. Because of its increased rotary speed, rotary mode is prioritized over sliding mode (ideally, 30% sliding and 70% rotating). Because a combination of these distinct drilling modes exists, the actual tortuosity may be quite different when compared with standard survey spacing (Fig. 1).
A slight overgauge will be present when operating in rotating mode because of the bent sub. Much like a rotary BHA, a motor may be sensitive to operational parameters. The type of polycrystalline diamond compact bit recommended is one with a long passive gauge that is required to drill in the direction of bit tilt.
An RSS can be modelled in two distinctive modes: point-the-bit and push-the-bit. A point-the-bit system operates in much the same way as a downhole motor by varying the tilt at bit. A push-the-bit system operates by applying a side force at pads that pushes against the wellbore. Both types of RSS allow continuous rotation of the drillstring and reduce the risk of differential sticking and, ultimately, stuck pipe. A point-the-bit system may operate with or without proportional steering.
A push-the-bit RSS system can operate either by varying the magnitude of force or having a constant force that is varied for a set period in the desired tool-face orientation (TFO). Because of the method of activation (using pads that hit against the side of the wellbore), the local doglegs that can be induced are slightly higher.