Rotary Steerable System proves effective on HDD project.
The right project
Both InterCon and Schlumberger wanted to use the PowerDrive on a relative short crossing in a rock formation to critique the tool's performance. Each company knew that differences between oilfield conventions and the HDD industry would crop up and need to be dealt with. Despite the obstacles, InterCon's directional drilling management team made up of Bruce Gabrielse and Steve Allen were looking forward to using the tool to complete challenging HDD crossings.
Panhandle Energy's Cass County, MO, 1,300-foot crossing of two small creeks and a county road proved to be the perfect choice. InterCon called Schlumberger's HDD representative, InterSyn Technologies to mobilize tools and support personnel from Schlumberger's Oklahoma City US Land PowerDrive Service Center Center and incorporated Mi HDD's expertise in fluid engineering and products for their support.
Rotating makes the difference
Conventionally, a downhole motor with a bend to control toolface has been used to drill rock formations on HDD crossings. In order to steer, the motor has to slide with the drillstring not rotating. The torque generated by the bit and the drag generated by the drillstring result in toolface fluctuations and reduced weight on the bit.
On the other hand, rotary steerable systems are able to make changes in inclination and azimuth while the drillstring rotates continuously. They can produce a cleaner, straighter borehole while reducing drag, improving the transfer of weight to the bit and increasing the rate of penetration. The unique feature of Schlumberger's Rotary Steerable System--PowerDrive--is that all external parts of the tool rotate, and no stationary parts are touching the borehole. Continuous rotation combined with the absence of annular bottlenecks significantly reduces the chance of cuttings buildup, packing off and getting stuck.
To date, the practical upper limit of HDD drills has been about 6,000 to 7,000 feet. PowerDrive Rotary steerable tools have been used in the oil and gas industry to drill horizontal displacements in excess of 30,000 feet.
One tool--many options
The PowerDrive tool utilizes mud flow in both the control and bias sections to enable the tool to steer or drill straight. First, mud flows through turbines in the control section of the tool producing the electrical power for the guidance electronics and control valve positioning. The control valve directs a small portion of the mud flow in the bias section to one of three steering pads located just behind the bit. These pads extend and push against the formation in the direction opposite the intended path to produce side cutting force and steer the drill string. The simple design, utilizing only mud power, with no downhole hydraulic pumps or electrical motors, can inherently improve reliability.
The steering settings of the tool can easily be reset without tripping out of the hole by varying the mud flow through the tool. It can also be set in one of two modes. When in steering mode, the pads extend according to the programmed toolface and more or less frequently according to the amount of steering force the driller wants. Toolface and steering force can be adjusted to direct the tool along the intended path and counteract walking tendencies in the formation.
The tool can also be programmed for "inclination hold." Using its own inclinometers located 7 feet behind the bit; the tool adjusts steering pad movement automatically to hold the drill path at the current inclination. This feature is very useful for drilling tangents in formations that have strong build or drop tendencies. The target inclination can be adjusted in 1/2 degree increments and left or right steer can also be applied and adjusted while in inclination hold mode.
Both InterCon and Schlumberger agreed that this crossing was going to require a lot of communication. Discussing all options for each situation allowed everyone involved to learn and develop confidence in the tool's capabilities for directional crossings. InterCon's veteran drilling supervisor Don Masters made sure everyone involved knew that production would be secondary to learning from each other and ensuring that InterCon delivered a quality crossing to their customer.
Schlumberger drilling engineers provided a hydraulic study to InterCon indicating that the mud system would need to provide 1,200 psi at a 450 gpm flow rate to complete the 9 7/8-inch pilot bore while maintaining the 6 3/4-inch tool's steering force. This was asking the American Augers DD-330 rig for all of its pump's capacity, but InterCon's Allen was confident the rig could deliver. Approximately 200 feet into the drill, it was noted that the tool was not responding to commands.
It was decided to pull out of the hole and review the tool's on-board diagnostics to determine exactly what mud flows and communications the tool was receiving. Because the tool uses mud flow for its power and communication, the flow meters and pressure gauges on the rig need to be accurate.
The review indicated that the rig's flow meters were not indicating the true drill-stem flow and the tool was not recognizing the downlinks to change the steering settings due to this discrepancy.
Armed with accurate information on the mud flow, Schlumberger's PowerDrive engineer came up with the flows needed to properly communicate to the tool. The PowerDrive went back into the borehole and there were no more problems downlinking to the tool for the remainder of the job.
When needed, the tool delivered 1.5 degree builds per joint (4.8 degree/100 foot DogLeg Severity) using only 80 percent of its steering force. This short crossing had little to no tangent on the bottom, so a moderate build was held from the end of the entry tangent to the start of the exit tangent. The tool was drilling on line and building predictably at the planned 1 degree per joint to the point the survey tool read 103 degree inclination which indicated 104 degrees at the bit. The formation also exhibited a slight tendency to the right. At this point, a downlink was sent to the tool, placing it in inclination hold with 15 percent left steer. The final seven joints were drilled with the borehole holding a steady 104 degrees and exiting less than a foot from the planned exit point.
Mi HDD ran the Drilplex HDD system throughout the pilot hole with a special 2 percent lost circulation material additive called MIXII to combat the fractured rock zones without plugging the internal PowerDrive flow paths, thus ensuring power and communication stayed intact. While maintaining constant returns and providing excellent penetration rates with optimum cutting transport the system proved a perfect match for the PowerDrive and geology.
InterCon's surveyor, Pat Unwin, felt that in addition to being able to steer while rotating, "the inclination hold feature makes PowerDrive a very useful tool in the HDD industry."
InterCon's Masters concurs. "We're excited to explore how this tool works and learn how we can apply it on future jobs. Being able to steer while rotating is going to open up a whole new range of projects where HDD was previously considered impossible or where the risk and expense of two intersecting bores was required."
FOR MORE INFORMATION:
Rotary steering tool: InterSyn, (262) 352-4639, or circle #185
Utility contractor: InterCon, (608) 227-7473, or circle #186
Bryan Wichmann, InterSyn Technology, Houston, TX