№7 July - August 2012Table of contents Issue Archive
№7 July - August 2012Table of contents Issue Archive
№ 6 (June 2012)
As reservoirs are depleted, the necessity to drill directional wells to penetrate and access reserves is gaining momentum. Directionally drilled wells are increasing in complexity due to intricate wellbore trajectories, tighter zones, and challenging drilling mechanics.
By Stephen Kelly, Weatherford International Ltd.
Application of rotary-steerable systems (RSS) in these wells improves penetration rates, borehole quality and turtuosity. Additionally, RSS tools will reduce high torque and drag and stick slip when compared to drilling directionally with mud motors. The decision to use RSS technology will result in longer sections drilled and wellbore evenness so that casing is optimally run.
Directional wells drilled using high-performance mud motors can initially be cost-effective. However, various issues that can make the well difficult to complete may result in significant non-productive time and cost. Common problems when drilling directionally with mud motors occur due to uneven hole gauges and micro-doglegs, which can make casing difficult to run. The risk of stuck pipe is ever-present while sliding with a mud motor in long horizontal sections. An RSS can avoid these costly concerns due to its point-the-bit process as opposed to pushing the bit of a mud motor.
It is estimated that 23% of all directional drilling worldwide is completed with RSS, accounting for $3.5 billion of an estimated $15 billion market. This number is expected to continue to increase as directional work becomes more demanding.
Proper tool selection is vitally important to operators, balancing engineering considerations against cost. While an RSS can perform the duties of a high-performance mud motor, in most cases the cost justification is warranted only after thorough well planning and engineering considerations are taken into account.
Rotary-steerable technology does not employ sliding to directionally control a well. An RSS will constantly rotate while controlling the direction with point-the-bit in the desired path. Rotation of the whole drill string can help to avoid stuck pipe and helical buckling providing proper weight transfer to the bit to optimize the rate of penetration; saving time and money. Sliding and rotating with a motor can have drastic differences in ROP, particularly in horizontal sections.
When comparing sliding of an RSS with that of a mud motor there can be some key disadvantages when employing a motor. Sliding with a mud motor in long horizontal sections is difficult with a mud motor as controlling the motor direction becomes increasingly challenging with the length of the section. When sliding with a mud motor, the drill string is not rotating leaving the drilling fluid in a steady state where cuttings are not easily removed and may pack off around the drill string causing it to stick to the borehole wall. By pushing the bit with a sliding motor, drag is increased; constant rotation with RSS avoids this issue.
As wellbore length increases it can become tough to apply the correct downhole rpm to program an RSS to drill in the required direction. A mud motor will be difficult to control in long lateral sections where the low-side of the drill-string will rest on the borehole where it can become wound-up causing problems in holding an accurate tool-face. This will cause both problems in maintaining the desired direction and costly rig time to adjust the mud motor. RSS technology can be communicated with both drill-string rotation and pressure fluctuations from specialized surface equipment that will program it to drill along the intended well-path with precision directional control.
In conjunction with RSS technology, logging-while-drilling (LWD) tools can be used to provide excellent azimuthal borehole images and offer superior logging data due to the nature of an in-gauge wellbore delivered by the RSS. Improved logging data is beneficial to geoscientists where an RSS is used to geo-steer in tight bed boundaries. For example, azimuthal density and resistivity borehole in geo-steering applications cannot be optimally provided when drilling with a mud motor, due to the lack of rotation while sliding. The continual rotation of an RSS will deliver a high quality image in a gauged hole enabling the wellbore path to stay in the heart of the pay-zone.
An RSS will provide operators with improved rates of penetration, enhanced hole cleaning, smoother wellbores, precision well placement, and high quality logging data while delivering a higher degree of wellbore quality and reducing overall costs.
Accurate Control in Various Environments with a Rotary Steerable System
An RSS was used successfully to control dogleg and drill a 1.26 extended-reach index, followed by a difficult three-dimensional drop-and-turn section through a depleted Brent formation reservoir in the North Sea.
The first run drilled from 10,728 to 18,760 feet (3,370 to 5,718 meters) was completed on track with a minimal dogleg of 1.5 / 100 feet (30 meters) while minimizing torque. The run was finished in order to change the drilling bit. The second run involving a difficult 3D well path performed precisely with controlled doglegs of 3.5 / 100 feet (30 meters) over a length of 3, 965 feet (1,209 meters). The RSS significantly outperformed high-performance mud motors. The operator has since adopted drilling with RSS and LWD technology as a recommended best practice.
The same technology was used to drill a well in the Olmos formation in South Texas resulting in a world record at the time for total footage run (completion) of 9,421 feet (2,872 meters). The RSS assembly was used to drill from 5,750 to 15,308 feet (1,753 to 4,666 meters) completing a vertical well path, a building curve and lateral section. The well was drilled six days ahead of schedule resulting in a savings of approximately $650,000. The lengthy completion string was run with no reported issues.
The evolution of RSS has lead way to motorized rotary-steerable systems. This technology places a mud motor on the RSS in order to achieve additional RPM at the bit while using the precise directional control of the RSS, eliminating the need for sliding.
In an onshore well in Austria the motorized RSS was run to drill from a depth of 3,766 to 6,339 feet (1,148 to 1,932 meters). The ROP increased from 16 to 49 feet/hour (5 to 15 meters/hour) when compared to offset wells using conventional RSS. Using this technology, the operator drilled 433 feet (132 meters) further than previously achievable while all directional objectives and full geo-steering control were successful.
While the previous case studies prove the effectiveness of RSS, high-performance mud motors can offer significant savings, provided they are used under the right conditions. RSS technology is the best choice in narrow bed boundaries where exceptional directional control is required.
High-performance mud motors Offer Cost Savings Under the Right Conditions
While RSS technology offers specific advantages over high-performance mud motors, there are scenarios where the latter is an excellent option. When evaluating an RSS vs. a high-performance mud motor, it is important to accurately estimate the cost savings that an RSS will have on overall operations as well as costly damaged equipment and lost in hole costs.
For example, if an RSS is lost in hole during drilling operations, the replacement cost of that equipment is likely to exceed the million dollar mark. Conversely, a high-performance mud motor would be in the vicinity of $200,000. If borehole instability, which may cause a lost BHA is of major concern, then a mud motor may be the better choice.
High-performance mud motors are compatible with all bits; RSS are not. When a particular bit is selected based on the formation to be drilled, it may not be possible to use that bit with an RSS. This can decrease the ROP if an RSS is used as bit selection may not be optimal.
RSS rely on the surface rotation of the drilling rig to transfer the required RPM to the RSS downhole. Smaller rigs are unable to provide the speed required for top performance of an RSS, thus negating their advantages. A motorized RSS can be used to supplement the required RPM but will add substantial costs to operations. The high-speed of an RSS can cause casing wear and damaged to the drill-string.
If an RSS is not applicable for any reason, a high-performance mud motor can deliver similar performance in large target zones where precision directional control is not a primary objective. High-performance mud motors feature advanced rubber elastomers that deliver increased torque, power, and efficiency. High-performance mud motors also have the ability to achieve higher build-rates over RSS systems.
Although rate of penetration can be on par with RSS systems, high-performance mud motors will most often result in poor hole cleaning, uneven hole gauge, and varying doglegs. Ultimately, the quality of the wellbore will suffer decreasing the quality of logging data, complicating casing running and completing the well.
High Performance Mud Motor Applications
Accurately engineered well design, and BHA considerations in combination with a high-performance mud motor can help to achieve exceptional results. The selection of a high-performance mud motor can in some instances out-perform RSS in the right situation.
A high-performance mud was used in offshore Qatar in order to reduce vibration and minimize sliding. 3,390 feet (1,033 meters) was drilled in a build section from an inclination of 33 to 55 with an average ROP of 127 feet/hour (38,7 meters/hour). The motor surpassed the expected ROP and set a new record for this section. An engineered BHA design minimalized sliding requirements to only 4% of the total section while maintaining an on-target well path.
Knowing when to use an RSS vs. a high-performance mud motor can be a performance and cost saving measure. Meticulous engineering and cost modeling must be performed when the selection is being made. Various factors such as bit selection, reservoir formation, casing design, well bore temperatures and pressures, rig specifications, and other aspects should be considered to determine the technology to be deployed.
An RSS is the staple choice when drilling in complex zones with tight targets. A high-performance mud motor has the ability to achieve similar results to an RSS when the target range is large and sliding is minimized.
Revolution™ rotary steerable system is a trademark of Weatherford.