Near-bit inclination, bit RPM and stick-slip severity in % from a customer well in Texas where excessive stick-slip hindered the ROP. (Images courtesy of Pathfinder Energy Services)

The introduction of a commercial RSS in the 1990s revolutionized directional drilling. Since then, RSS technology has become more reliable and is now a mature, standard application. Due to the inherent complexity of RSS technology, however, system reliability is still lower than that of MWD and LWD tools.

Now, new technology is changing drilling dynamics. PathFinder Energy Services has developed an 81?2-in.-hole-size PathMaker RSS, based on the earlier, proven 121?2-in.-hole-size PathMaker, for drilling more challenging extended-reach and horizontal wells. The RSS consists of a non-rotating housing with three active and independent pads that maintain constant contact with the well bore. Directional steering is achieved by positioning the pads to offset the tool center from the centerline of the hole. The system drills a continuous curve and does not rely upon on/off biasing. The constant contact of the blades with the borehole provides near-bit caliper measurements as well as extra stability to the lower part of the bottomhole assembly (BHA), reducing undesirable drill bit vibrations and instability.

Despite the extra stability gained at the RSS, it is inevitable that the tool is exposed to high vibrations due to torsional vibration (stick-slip) induced from the bit-formation interaction, lateral vibrations caused by the mass imbalance of drill collars and other BHA components, and axial vibrations resulting from the high-low pattern of a rock bit in hard formation. Moreover, some of these vibrations do not travel to the surface and can only be detected downhole.

Real-time stick-slip and vibration detection (RSVD), designed to detect near-bit vibration at the RSS, has been incorporated into the newly developed 81?2-in.-hole-size RSS. Timely remedies to harmful vibrations can increase drilling efficiency and protect MWD/LWD tools and the RSS from damage caused by vibration. With the RSVD system, operators can be aware of harmful downhole conditions and can adjust surface parameters to optimize drilling performance.

Integrated near-bit sensors

Integrating near-bit sensors with the RSS allows consistent measurement of borehole caliper as well as the three principal types of downhole vibration: torsional, lateral, and axial. Near-bit sensors also have facilitated comparative analysis of drill bit and BHA performance among different BHAs and wells.

The sensor system is now a standard, integrated component of all PathFinder RSSs and provides real-time near-bit borehole caliper and vibration severities on all commercial runs. Using these integrated sensors can be extremely advantageous:
• The sensors always measure the near-bit data at the same position in a BHA (either point-the-bit or push-the-bit RSS);
• Real-time near-bit data is available for all of the RSS runs to improve drilling efficiency;
• The use of integrated sensors does not add extra complexity to the RSS, which means the systems deliver superior reliability;
• The integrated sensors are more economical and affordable than specialized sensor subs;
• The use of the sensors is transparent to the RSS operator and requires no extra sensor/battery setup; and
• Unlike specialized vibration subs, the sensors do not alter the BHA length.

Well Results

Since 2005, the RSVD system integrated into the 81?2-in.-hole-size RSS has been extensively tested in various wells in North America, including vertical and horizontal wells both onshore and offshore. Throughout the commercial pilot runs, the real-time drilling screen was available to a multidisciplinary drilling optimization team consisting of engineers specializing in directional drilling, RSS optimization, research, and design. Remote satellite communication and Internet technologies enabled the team to monitor downhole drilling conditions around the clock. This team provided critical feedback to the on-site directional drillers and MWD engineers for minimizing vibration-related failures and for improving drilling efficiencies.

The RSVD system has been used to improved rate of penetration (ROP) in a number of wells. In one case, the vertical section of an S-shape well was drilled with the fully automated 81?2-in.-hole-size RSS in Texas. The borehole inclination was originally 1.06?. The RSS quickly dropped the inclination back to less than 0.1? and maintained the inclination to an average value of 0.05? for approximately 1,000 ft (305 m). The average inclination of the near-vertical borehole was based on the MWD static surveys. No time was wasted downlink programming the RSS in this vertical drilling application because the RSS was set up to continuously seek low-side. Several rig repairs were carried out during three days, adding non-productive time. The overall penetration rate in the vertical section was 14.1 ft/hr (4.2 m/hr).

RSVD revealed during drilling operations that excessive stick-slip hindered the ROP on this well. Figure 1 shows the near-bit inclination, bit RPM, and stick-slip severity computed at the RSS. The downhole computer applies a low-pass filter to raw RPM readings and records a mean RPM in memory. It was noticed that the filtered RPM was still noisy due to stick-slip.

The RSS assembly was pulled because of a low ROP and a suspected undergauge bit. In fact, the bit and near-bit stabilizer were both undergauge. The following run was a conventional steerable motor with a similar PDC bit. This assembly averaged 23 ft/hr (7 m/hr), indicating that stick-slip was causing the low ROP. In this application, the use of a motor-assist RSS assembly would have reduced stick-slip at the bit and ultimately led to improved ROP. The sensor system helped plan and design optimized rotary-steerable BHA for the subsequent wells in the same formation and well plans.

In a well in New Mexico, the 81?2-in.-hole-size RSS was used with a small under-powered rig. The RSS drilled 3,260 ft (994 m) at an ROP of 39.7 ft/hr (12.1 m/hr), accumulating 82.2 drilling hours and 164.2 circulating hours. Weight on bit was maintained at 15,000 to 20,000 lb. Well data, shown in Figure 2, shows that no major vibrations were observed in the first half of the run. In the second half, stick-slip became problematic, with stick-slip severity between 100% and 200%. Very little lateral or axial vibration was observed in the real-time data. The RSS operators were able to manage the stick-slip problem and completed 1,700 ft (518 m) of the problematic footage without downhole failure. These wells are typically drilled at an average ROP of 15 ft/hr (4.6 m/hr) with a conventional steerable motor and rock bit (for steerability).
The use of RSS and PDC bit, with proper vibration management, provided a 164% increase in ROP.

Reaping rewards from RSVD

The stick-slip and vibration detection system integrated into the RSS is now a standard component because of its effectiveness in detecting downhole dynamic conditions in real time. Because the RSVD is always available to directional drillers and MWD engineers, the sensor system leads to optimized drilling parameters in most RSS runs, even in complex 3-D wells, because it allows the vibration information to be fully used.

Well results indicate that the RSVD feature helps minimize the detrimental effect of downhole vibrations and prolong the optimum performance of the RSS and other BHA components. The effective use of the sensor system has resulted in increased RSS reliability and performance.