Drilling a well involves using a drill bit inserted into the ground on a drill string. Also included on the drill string may be various tools for, performing tasks associated with drilling the wellbore. For example, when drilling a well, a drill operator often wishes to deviate a wellbore or control its direction to a given point within a producing formation. This operation is known as directional drilling. One example of this is for a water injection well in an oil field that is generally positioned at the edges of the field and at a low point in that field (or formation).
One type of drilling tool for drilling a deviated wellbore is a rotary steerable tool (RST) that controls the direction of a well bore. The RST tool uses an actuator, to manipulate the relative position of an inner sleeve with respect to an outer housing to orient the drill string in the desired drilling direction. The RST tool further includes a “brake” to lock the position of the inner sleeve relative to the outer housing once the desired relative position is obtained. A processor instructs the actuator to move the position of the direction of application of the force on the mandrel. The processor may also be used for determining when the direction of the force applied by the direction controller should be moved. The actuator in the outer housing may move the inner sleeve using a drive train with a very high gear ratio, for example 10,000:1. To determine the relative orientation of the inner sleeve to the outer housing, the RST tool uses the rotation of the motor and a known initial orientation of the inner sleeve to the outer housing to determine a “motor” reference position. As the motor turns, it energizes reference poles. The RST tool monitors and processes the energization of the reference poles, or “clicks”, to resolve the magnitude and direction the motor has turned. The RST tool uses the motor travel information, in addition to the known gear ratio between the inner sleeve and the actuator, to determine the position of the inner sleeve relative to the outer housing at any given time.
One issue that may occur is the ability of the RST tool to process the “clicks” of the motor reference poles. If an excessive external force is applied to the outer housing, the brake is designed to slip, which results in the motor and its drive train turning in that direction. Because the gearing ratio back to the motor may be over 10,000 to 1, the speed at which the end of the motor is spinning may create “clicks” faster than the processor may be able to process. Thus, the processor may miscount the number of “clicks”, resulting in the calculated versus actual position on the inner sleeve relative to the outer housing being out of sync.
Other types of downhole tools may also be included on the drill string. Additionally, other types of downhole tools may be comprised of a mandrel, an inner sleeve, and an outer housing. Still further, other downhole tools may include the use of a magnet on the inner sleeve as a “home position” and a magnetic sensor on the outer housing that detects the magnetic field of the magnet as it rotates relative to the sensor. However, such systems may only determine one position of the inner sleeve relative to the outer housing. Any positions other than the “home position” may not be detected. Additionally, a problem might arise if the magnetic sensor does not detect the magnet and the magnet never rotates past the sensor.