There have been significant attempts to optimize the various bottom-hole assemblies (BHAs) used in rotary steerable (RS) applications because the optimal RS assembly can provide operators with significant cost savings by providing less vibration, high BHA stability, high-quality borehole, and thus predictable steerability.
This paper presents the development of BHA analysis models using simplified mathematical equations. RS BHA optimization process is demonstrated through the extensive field test program. The predictions of BHA response and directional drilling performance of the RSS have been examined with the numerous field test data taken in a controlled and non-commercial environment, allowing single step changes in both the drill bit features and RS configurations.
The testing is unique since the specific rotary-steerable system (RSS) works in field-configurable point-the-bit and push-the-bit modes. Between two distinct RSS operation modes, consistency in stiffness, weight, force applying capability, and control system lead to a direct comparison of different BHA models. Confidential test sites were selected for evaluating different RS BHA configurations in both push-the-bit and point-the-bit modes. The systematic BHA testing was conducted in two different test facilities in North America.
A unique sensor system, integrated into the specific RSS, provided real-time measurement of near-bit borehole caliper and near-bit stick-slip and vibration 1,2. This feature allowed real-time evaluation of bit and BHA stability and borehole quality while the maximum build-up test was performed. After each test run, memory data was retrieved and used for more detailed assessment of bit and BHA performance.
BHA configuration tests were systematically conducted in a controlled environment so that the relationship between BHA analysis models and actual BHA behaviors could be identified. As a result, the systematic testing and verification lead to the optimal RS BHA design in both push-the-bit and point-the-bit configurations for stability, steerability and borehole quality.
Introduction
The introduction of the commercial RSS in the 1990s revolutionized directional drilling 3,4,5. Since then, the rotary-steerable system (RSS) technology has remarkably advanced. Today, RSS has become a mature technology, and it is used in a wide variety of directional drilling applications. As such, the driving principles and mechanisms used in commercial RSS have diversified.
There are two major schools of RSS design: push-the-bit and point-the-bit configurations. A push-the-bit RSS applies a side force to a bit with multiple expandable steering pads right above the bit, whereas a point-the-bit RSS generates a deflection angle between the low-end tool axis and the borehole axis. There are advantages to both systems, depending upon the application. The following section provides a short discussion of push- and point-the-bit RSS.
