In their ongoing push to simultaneously improve operational efficiencies and reduce drilling costs, drillers have to carefully weigh their options when it comes to selecting the right rotary steerable system (RSS). The optimal drilling system for a given formation will help the driller achieve a high average ROP while minimizing tortuosity and deviations in the well path.
Some degree of deviation is necessary for course correction purposes—to redirect the drillbit to ensure the wellbore reaches target depth and location, and maximizes contact with productive zones in the reservoir. However, too much tortuosity, which is being defined for these purposes as the unwanted undulations around the well plan, raises the cost of the drilling operation. It also can diminish hole quality, which increases the potential for problems while running the completion and can negatively impact production in both the short and long term.
Different mechanisms, similar drawbacks
Two of the most commonly used steering mechanisms are push-the-bit and point-the-bit systems. In a typical push-the-bit system, a force is applied against the wellbore side wall through multiple pads mounted on the body of the RSS. One pad is activated at a time by diverting some mud flow through a controller valve, and the action of the pad pushing against the wellbore directs the bit in the desired direction. Point-the-bit systems instead rely on internal deflections running through the RSS to tilt the bit into the desired well path.
Both systems work on the principle of on/off steering cycles. The pads or tilting mechanisms are deactivated, or off, until a predetermined inclination threshold is crossed. At this point, the systems reactivate to build angle and correct the inclination to remain within the desired value. It is quite common to overshoot the target when passing the correct inclination angle until the switch-off threshold is achieved. Consequently, the pads and tilting mechanisms go into an ongoing cycle of opening and closing to meet the directional plan, and the wellbore will continuously oscillate around the planned well path until target depth is reached.
This on/off steering principle, and the resulting tendency to overshoot the target, makes the drilling process longer and less efficient. These systems also tend to create more tortuous well paths that compromise ultimate recovery.
Continuous proportional steering
A third RSS mechanism known as continuous proportional steering operates by a different principle. Instead of an on/off cycle, the system delivers directional control by applying pressure through three independent pads mounted on a slow rotating sleeve. Internal hydraulics power the steering pads, with an electronic processor controlling the continuous distribution of pressure to the pads. Each pad receives the exact proportional force magnitude required to keep the bit moving in the desired direction.
Steering control is not affected by drilling dynamics, like bit pressures, flow rates and drilling fluid properties, as it is in many competing systems. As a result, the technique ensures more precise and consistent directional control.
Continuous proportional steering is the underlying technology in the AutoTrak RSS, developed by Baker Hughes, a GE company (BHGE). The system has been successfully deployed in horizontal wellbores around the world, reliably delivering smoother, in-gauge holes and precise well placement as well as improved overall drilling performance. BHGE system’s operational flexibility enables operators to match the drillbit design to the formation challenges, thus improving efficiencies on the rig and in the hole.
Verifying field performance
An operator in the Middle East has deployed all three RSS technologies in its wells, but the operator was uncertain as to how effective each mechanism was at delivering a high-quality hole that would optimize subsequent completions and production operations. Therefore, the operator partnered with BHGE to conduct a joint study aimed at quantifying the tortuosity and deviations created by the three RSS tools.
The study quantitatively compared available RSS technologies by running high-resolution wireline surveys in more than 700 wells in the same field. Although different RSS technologies were used in these wells, other conditions such as geologies, trajectories and bottomhole assemblies (BHAs) were similar. The study effectively isolated the RSS as the primary variable, enabling a like-for-like comparison between steering technologies. All survey data were gathered from areas of mature directional drilling operations, ensuring that the early learning curves for RSS BHA design and operation were already complete.
The high-accuracy, high-density wireline surveys took measurements at 3-in. intervals throughout the horizontal sections of the wells. This provided highly accurate insights into hole quality that could not be observed with MWD surveys. The longer survey intervals required for MWD systems—typically anywhere from 9 m to 29 m (30 ft to 95 ft) to collect one datapoint— simply would not offer an accurate representation of wellbore hole quality concerning tortuosity. While MWD results show that push-the-bit and pointthe- bit systems do not deliver the same hole quality as continuous proportional steering, the wireline data revealed pronounced differences and high local doglegs invisible in MWD datapoints.
The novelty of the wireline survey approach lies in its ability to precisely evaluate wellbore tortuosity using a far greater number of datapoints. Four points are recorded over a distance of .30 m (1 ft), which provides a much finer level of analysis compared to standard MWD results.
The results of the study confirmed this. When examining the high-resolution wireline data, it became clear that both the push-the-bit and point-the-bit RSS tools significantly increased wellbore tortuosity. The underlying on/off steering mechanics used by these systems were incapable of drilling a horizontal wellbore without a significant level of tortuosity. Increased tortuosity during drilling has immediate cost and efficiency consequences, requiring reaming, backreaming and cleanup cycles. As these consequences are multiplied across wells, tortuous wellbores can have a significant cumulative effect on field development, production and profitability.
A comparison of well tortuosity using high-resolution wireline surveys from three offset wells clearly shows that the continuous proportional steering method delivers substantially superior hole quality and minimal tortuosity compared to push-the-bit and point-the-bit techniques. (Source: BHGE)
Conversely, the study demonstrated that continuous proportional steering reduced tortuosity by four to six times when compared to push-the-bit and point-the-bit methods, and it consistently delivered better hole quality in horizontal drilling. Continuous proportional steering left a clear path that improved drilling efficiency with a greater ROP, reduced the likelihood of stuck pipe or lost BHAs, improved control to stay in the pay zone and reduced time to target depth. It also reduced costs and improved efficiencies while running casing and installing completions. A less tortuous wellbore increases the well’s production potential by improving reservoir contact and makes for easier workovers and abandonment operations as the well matures.
The operator has used this research study as a useful reference to analyze the performance and efficiency of RSS mechanisms across drilling and workover operations. Other operators are encouraged to deploy similar high-density wireline surveys in their fields and see for themselves how continuous proportional steering systems, such as AutoTrak RSS, help improve the efficiency and accuracy of directional drilling compared to traditional RSS technologies.
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