System for determination of downhole position
A measurement means for a downhole tool for determining the orientation of the axis of a downhole tool in relation to true north. It includes a gyro sensor capable of measuring angular rate, a rotation means for rotating the gyro sensor about a sensor axis, a measurement means to detect the angular rate at the gyro sensor as it is rotated to form a series of readings, and a processor capable of interpreting the signal amplitude from the series of readings and determining the points of greatest signal amplitude. The processor is also being capable of comparing the phase difference between the actual readings taken against what would be generated with the sensor pointing true north thus providing a downhole tool axis angular deviation from true north. The rotation means rotates the gyro sensor in discrete steps.
1. A measurement system, for a downhole tool, for determining the orientation of the axis of a downhole tool in relation to true north, comprising:
a gyro sensor capable of measuring angular rate,
a rotation means for rotating the gyro sensor about a sensor axis,
a pointing means for pointing the gyro sensor in a first direction towards the earth's centre and in a second direction at 90 degrees to the earth's centre,
a measurement means to detect the angular rate at the gyro sensor as it is rotated in each of the first and second directions to form a series of readings,
a processor capable of interpreting the signal amplitude from the series of readings and determining the points of greatest signal amplitude,
the processor also being capable of comparing the phase difference between the actual readings taken against stored data of what would be generated with the sensor pointing true north thus providing a downhole tool axis angular deviation from true north.
2. A measurement system according to claim 1 , wherein the rotation means rotates the gyro sensor in discrete steps.
3. A measurement system according to claim 1 , wherein the rotation means rotates the gyro about a sensor axis perpendicular to the angular rate being measured.
4. A measurement system according to claim 1 , wherein the rotation means comprises a tool orientation means for rotating the part of the downhole tool about the tool axis.
5. A measurement system according to claim 1 , wherein the rotation means is contained within the downhole tool and includes a drive motor on which the gyro sensor is mounted.
6. A measurement system according to claim 5 , wherein there is included a gimbal arrangement having two axes perpendicular to the sensor axis, which can orient the rotation means and gyro sensor to point the sensor axis towards the earth's axis of rotation.
7. A measurement system according to claim 6 , wherein there is included a triaxial accelerometer capable of detecting the earth's gravitational vector.
8. A measurement system according to claim 1 , wherein the gyro sensor is a solid state MEMS type gyro sensor.
9. A measurement system according to claim 1 , wherein there is a storage means for storing the measured readings or the determination of the orientation of the axis of a downhole tool in relation to true north.
10. A measurement system according to claim 1 , wherein there is a transmitter means for transmitting the measured readings or the determination of the orientation of the axis of a downhole tool in relation to true north to the surface.
11. A measurement system according to claim 1 , wherein the downhole tool includes a rotary drilling tool.
12. A measurement system according to claim 1 , wherein the downhole tool is deployed on coiled tubing.
13. A measurement system according to claim 1 , wherein the downhole tool is deployed on wireline.
14. A measurement method, for a downhole tool, for determining the orientation of the axis of a downhole tool in relation to true north, by rotating the gyro sensor about a sensor axis by means of gyro rotating means,
measuring the angular rate of a gyro sensor by means of a rotation sensor, comprising the following steps:
adjusting the alignment axes so that the sensor axis points to the earth's centre, then:
a) rotating the gyro sensor from an arbitrary starting point in discrete steps, and
b) at each step, stopping the rotor and measuring the rate of the gyro sensor,
adjusting the alignment axes so that the sensor axis are 90 degrees to the earth's centre and repeating steps a) and b) to form a series of readings,
using at least one processor, interpreting the signal amplitude from the series of readings and determining the points of greatest signal amplitude,
using the at least one processor, comparing the phase difference between the actual readings taken against what would be generated with the sensor pointing true north to provide a downhole tool axis angular deviation from true north.