Abstract

This paper describes how extensive testing has allowed the determination of a link between cutting structure geometry, gauge pad configuration, and achievable hole curvature or steerability. For a specific point-the-bit system, the tests have shown that steerability of the bit is not substantially dependent upon the cutting structure geometry; rather, the nature and extent of gauge pad features are crucial to the directional response of the system. Gauge pad bearing efficiency must be high while simultaneously allowing the cutting structure to tilt as directed by the rotary-steerable system (RSS).

The several case histories presented, from operations in the North Sea and offshore Indonesia, verify the results of laboratory tests regarding the relationship between design features of fixed cutter bits and steerability of a point-the-bit RSS. Furthermore, these case histories demonstrate that a point-the-bit RSS, together with an optimized bit design, can lead to improved drilling efficiency in providing good directional response, high rates of penetration (ROP), excellent-quality wellbore smoothness, improved logging-while-drilling (LWD) log response, and ease of completion.

Introduction

Whether an RSS is a point-the-bit or a push-the-bit system, the choice of bit for it is critical to optimize performance because each system operates differently. To match a bit to an RSS, all of the key characteristics of a bit design must be considered; but probably the most important criteria are stability and steerability. Durability and aggressivity, specific to the properties of the lithology, must be considered as well. Compromise is needed in this selection and is of particular importance in view of the focus on bit stability that more sophisticated real-time downhole measurement systems have generated. An understanding of contributors to stable drilling is paramount for improvement of drilling performance.

RSS Features

Recently an 8 1/4-in. collar sized RSS (the RS 825) has been developed and added to a family of rotary-steerable tools. These systems use point-the-bit drilling technology, incorporating a near-bit stabilizer to orient the drill bit axis with that of the intended hole trajectory. The operating principle of these tools consists of a non-rotating sleeve through which a rotating drive shaft passes. Through the actuation of a hydraulic system, the drive shaft can be deflected away from the centerline of the wellbore.¹

The Revolution^® RSS contains a high-accuracy rotations-per-minute (RPM) measuring device. The key component of the measuring device is a toothed timing device, containing 24 teeth, that rotates with respect to a stationary sensor. The sensor, in turn, measures the time interval between teeth passing the sensor. These data are used as part of the control mechanism of the the RSS and allow the accurate measuring of the differential RPM between the rotating shaft of the tool and the non-rotating outer sleeve. As long as the non-rotating sleeve is stationary, the differential RPM measurement is equivalent to the downhole drillstring RPM, providing the ability to monitor stick-slip events downhole and control the RSS accordingly. The measured downhole RPM and a calculated stick-slip function are stored in the engineering memory of the RSS and transmitted in real time to the surface, along with other rotary-steerable and LWD measurements, via the measurement-while-drilling (MWD) mud-pulse telemetry system. The engineering memory within the RSS can be configured to record these measurements at varying sample rates, depending on requirements, and the RPM and stick-slip data communicated to surface are used by the field engineer in optimizing drilling parameters.

Testing

As part of the plan for the integrity testing program for the new RS 825 system, controlled test holes were to be drilled through large concrete blocks to evaluate the directional response of the system. Earlier tests² and field experience with other sizes of RSS had highlighted the requirement to evaluate the response of different styles of polycrystalline diamond compact (PDC) bit on directional performance; therefore, the decision was made to conduct a series of laboratory tests to investigate the effect of bit geometry on RSS stability and steerability while simultaneously development testing the RS 825.