Rotary-Steerable System Concept Aims To Increase Efficiency, Reduce Costs

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The advent of the rotary-steerable system (RSS) introduced an efficient way to drill both current and future wells. However, current tools still fail to replace positive displacement motors (PDMs). High build-rate RSSs that maximize reservoir contact are more expensive than PDMs that deliver consistent and reliable build rates, whatever the formation, while RSS tools with a low cost of ownership may not deliver expected performances. This paper presents a concept that combines optimal functionalities and directional performances with cost‑effectiveness.

Lessons Learned in the Field

Hundreds of potential RSS configurations exist when considering engineering options such as steering principle, geometry, control mechanisms, rotating/nonrotating sections, and power source. In the past, many such systems have been run in two types of bent housing that equipped PDMs. Field experience has shown that an efficient RSS must incorporate features such as the following:

  • High build rates to maximize reservoir contact and deliver vertical, curve, and lateral sections in one run, thereby eliminating flat times
  • A slick, short nonrotating section to enhance reliability through a low-activation-frequency steering unit and consistent build rates without hole-cleaning issues
  • A steady, continuously active, and monitored step-by-step adjustable steering device associated with near-bit continuous inclination and azimuth measurement to drill vertical, curved, and lateral sections in one run with a smooth trajectory that facilitates operations during the whole life of the well

In addition, drillers seek timely near-rig maintenance to minimize both downtime and logistics, and all-mud-compatible low-drop-up pressure tools to drill with both the optimal mud formulation and maximum pressure. Failsafe downhole systems that ease the process of pulling out of hole in the event of failure are also beneficial.

However, these features are rarely available in a single tool. High-build-rate RSSs are more expensive than PDMs mainly because of maintenance cost and downtime. Most cannot deliver a smooth wellbore because of steering-control issues. Low total-cost-of-ownership rotary-steerable tools have limited functionality and thus inconsistent performance.

Addressing the Issue

To engineer an improved RSS, two complementary elements were incorporated:

  • A monobloc flexible drive shaft that maximizes system reliability by carrying both torque and weight on bit (WOB) through the RSS.
  • The short double-tilt unit bent housing, known currently as the point-and-push hybrid steering principle, that provides high build rates with a limited tilt angle.

Unlike most designs, the usual nonrotating main body is split into two sections (Fig. 1): first, an upper carrier integrating all sensors, actuators, electronics, and the antirotation unit, and, second, a lower steerable housing integrating the fully mechanical pressurization device. These modules are linked by a mono­bloc flexible drive shaft acting as both a multidirectional one-piece joint and a ­return spring.

Fig. 1—RSS architecture.


In this way, the build rate of existing flexible-drive-shaft RSSs can be multiplied by more than threefold. Because build rate is limited by casing-run constraints, the length of the tool itself as well as its main parts can be reduced dramatically. Also, reliability is maximized by lowering rotating bending stress for a given drive shaft characterized by its inertia and the Young’s modulus of its material.

Prototype Design

The 7¼-in.-outer-diameter (OD) prototype features a 2-m slick nonrotating section. The steady, internal, monitored, continuously activated steering unit is able to adjust step-by-step the build rate from 0 to 100% and from 0 to 360°. Its failsafe design ensures the return to neutral mode in case of failure to make pulling out of hole easier, an ability aided by the fact that the drive-shaft buckling load is much higher than the WOB.

The breakthrough architecture allows also measurement-while-drilling survey packages and gamma sensors to be placed as close as 6 ft from the bit connection. Both inclination and azimuth can be computed at the cutting face. An electromagnetic short hop can be added, as well as a toroidal resistivity at bit and a side-force-monitoring module.

In addition, all parts except the drive shaft are less than 1 m long. This key feature, associated with a low-part-count modular design, allows low-cost manufacturing and assembly.

A shorter tool means a lighter tool, which is, therefore, less sensitive to shocks and vibrations generated by the drill bit and the bottomhole assembly for longer life of the supports between the drive shaft and the upper carrier, as well as the lower steerable housing. Also, the concept allows the RSS to incorporate an embedded generator geared by the drive shaft for longer runs, lower operating costs, simplified logistics, lower risk, and environmental friendliness.

Simulation of Directional Potential

A numerical model of the prototype for an 8¾-in. hole size was built to evaluate two performance aspects:

  • The equilibrium curvature, which evaluates the steady-state behavior that must be reached rapidly after an activation.
  • The step-by-step response, which evaluates the transition response of the system before the steady state is reached.

Results indicate a linear response relative to the activation percentage of the tool, and a robustness relative to inclination, over-gauge, and bit steerability. Thus, expected build rates will be achieved with usual low-steerability bits with gauges from 2 to 4 in.

A laser has been installed at the bottom of the drive shaft of one prototype and a target set at almost 7 m to measure the build rate according to the three-point-geometry method. Laboratory results confirm the high-build-rate potential from the static simulations. The hybrid steering principle allows a high build rate even with medium active-gauge polycrystalline diamond bits for a calibrated borehole and smooth-trajectory wells.

Step-by-Step Responses

Results of transition simulations from a straight slant at 45° to a 100% activation show the importance of progressive activation to achieve expected build rate with a smooth trajectory, even with a basic proportional integral derivative (PID). High build rate can be achieved with a low steerability, resulting, for the same PID, in better hole quality.


On the basis of simulation and ­workshop-test results, this RSS has the potential to drill faster, and at lower cost, in various formations (abrasive, soft, or interbedded) and replace massively bent housing.

The 3.2-m-long 7¼-in. OD prototype has proved feasible and power generation from the drive shaft has been battery-free.

The next step is to integrate lessons learned from the simulation to the continuous steering modulator (such as the drop tendency function of inclination in order to eliminate the effect of gravity) and then field-test the prototype.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of the open-submission paper “Breakthrough Rotary-Steerable System To Reduce Well-Construction and Exploitation Costs,” by François Millet, SPE, Dynasteer, and Christophe Simon and Stéphane Menand, SPE, DrillScan. The paper was not presented at an SPE conference and has not been peer reviewed.

Rotary-Steerable System Concept Aims To Increase Efficiency, Reduce Costs

01 November 2019

Volume: 71 | Issue: 11

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