The rotary steering system described here operates on the priciple of the application of a sideforce in a similar way to the non-rotating systems described before.
However, in these systems it is also possible to rotate the drillstring even when drilling directionally. It is therefore possible to rotate the string at all times during the drilling operation. This is desirable for many reasons but mostly because it has been found that it is much easier to transport drilled cuttings from the wellbore when the drillstring is rotating.When the drillstring is not rotating there is a tendency for the cuttings to settle around the drillstring and it may become stuck.
There are a number of tools which have been developed in order to allow the string to be rotated whilst drilling in the oriented mode but only one of these devices will be described below. Other systems (developed and offered by other service companies) can be found on the internet.
The main elements of the rotary steerable steering system that is described here (the AutoTrak¨ RCLS system) are the: Downhole System and the Surface System
The downhole system consists of:
- The Non-Rotating Steerable Stabiliser;
- The electronics probe and
- The Reservoir navigation or MWD Tool.
Non-Rotating Steerable Stabilizer
The Steering Unit contained within a non-rotating sleeve controls the direction of the bit. A drive shaft rotates the bit through the non-rotating sleeve. The sleeve is decoupled from the drive shaft and is therefore not affected by drill string rotation.
This sleeve contains three hydraulically operated ribs, the near bit inclinometer and control electronics. Pistons – operated by high pressure hydraulic fluid – exert controlled forces separately to each of the three steering ribs. The system applies a different, controlled hydraulic force to each steering rib and the resulting force vector directs the tool along the desired trajectory at a programmed dogleg severity.
This force vector is adjusted by a combination of down hole electronic control and commands pulsed hydraulically from the surface
|Non Rotating Steerable Stabilizer|
The micro-processing system inside the Auto Trak RCLS calculates how much pressure has to be applied to each piston to obtain the desired toolface orientation.
In determining the magnitude of the force applied to the steering ribs, the system also takes into account the dogleg limits for the current hole selection.
In field tests, the sleeve has been seen to rotate at approximately one revolution every W hour, depending on both the formation type and ROP. To compensate, the system continuously monitors the relative position of the sleeve. Using these data, AutoTrak RCLS automatically adjusts the force on each steering rib to provide a steady side force at the bit in the desired direction.
|End section of Non-Rotating Steerable Stabilizer|
The Electronics Probe controls the interface between all tool components and manages the exchange of data to and from the surface. This section also contains directional and tool vibration sensors. Azimuth measurements from the tri-axial magnetometer monitor and control the steering unit in conjunction with the near bit inclinometer, providing early readings of tool inclination changes. The vibration sensor helps ensure that AutoTrak RCLS is operated within specifications and at maximum efficiency.
Reservoir Navigation Tool / MWD
The Reservoir Navigation Tool (RNT) sub – with Multiple Propagation Resistivity (MPR) and Dual Azimuthal Gamma Ray (GR) sensors – enables real-time geosteering within the reservoir. Using two frequencies and dual transmitters, the RNT provides four (4) compensated resistivity measurements for accurate determination of Rt under a variety of conditions. The system provides deep-reading 400 kHz measurements and high vertical resolution 2 MHz readings. While drilling horizontally, the 400 kHz readings can detect contrasting bed boundaries and fluid contacts up to 18 feet (5.5 m) from the tool. In a horizontal application, this enables drillers to anticipate boundaries more than 250 ft (75 m) ahead of the bit. These two frequency readings and Dual Azimuthal Gamma Ray measurement enable AutoTrak operators to downlink course corrections to keep the well in the zone of interest.
AutoTrak’s Surface System has two main elements:
- Surface Computer System
- By-Pass Actuator
Surface Computer System
The Surface Computer System encodes the downlink signals for transmission to the tool and decodes the MWD signals received from downhole. It also provides standard directional and LWD outputs. This system includes the central processor and an MWD decoding unit. Downlink communication with the AutoTrak RCLS tool is controlled either by the computer or manually from the keypad. The downhole system is programmed by using the negative pulse telemetry created in the surface By-Pass Actuator.
The By-Pass Actuator (BPA) valve unit transmits commands to the downhole tool through negative mud pulse telemetry. Each valve unit is fully certified by Det Norske Veritas. The by-pass actuator is connected to the standpipe and can divert some of the mud flow to create a series of negative pulses in the drill pipe. The tool senses and decodes these as downlink instructions. A complete downlink
command can take between 2 and 8.5 minutes depending upon the complexity of the downlink. After the AutoTrak RCLS downhole tool receives the downlink information, it sends a confirmation message back to the surface, then reconfigures itself for the task required. Automated operation of downlink can be performed as drilling proceeds, allowing control of AutoTrak RCLS without interrupting the progress of the well.