Like a driver steers the car, a captain steers the ship or the aeroplane, the directional driller steers the well to its final destination. Different methods and techniques are used to guide the well to the final target. All methods use some kinds of survey instruments. MWD tools are the most advanced of those available.
In directional drilling, well steering can be of two types:
Geometrical Steering involves using only survey and toolface data to adjust the position of the well bore.
Geological Steering involves use of formation related data's such as resistivity, neutron porosity and density or sonic measurements to orient the well bore accordingly.
We have discussed earlier that for guiding the well through the pre-planned well path all that's required to a directional driller is Inclination and Azimuth. These values are recorded at certain fixed intervals during the course of drilling and then the well path is adjusted.
While drilling with the SDMM (Steerable Down hole Mud Motor); there're two modes of drilling i.e Sliding & Rotary. In sliding mode, the directional driller make use of Toolface to deviate the well bore accordingly.
What is Toolface?
Let's explain this with an example.
Going from home to work, suppose you must travel straight 100 yards. take right turn and then travel 150 yards. The directions tells us which way to turn at specific points. For turning right, you must face right, and then move right.
The same principle applies to steering a well. Steering tools must be pointed in the direction that the well is to be drilled. The angle or direction at which the steering tool is pointed is called the Toolface.
There're two types of toolface;
Magnetic Toolface (MTF) given by magnetometers and
Gravity Toolface (GTF) given by accelerometers.
What are Magnetometers & Accelerometers?
Magnetometer is an instrument used for determining the strength and direction of the magnetic field in the vicinity of the instrument. Different types of tools uses different types of magnetometers.
Fluxgate magnetometer is the most commonly used one; and it's explained below:
When an electric current is applied around the bar of ferromagnetic material wrapped by a coil of wire, the bar becomes magnetized and generates its own magnetic field.
Now, a similar bar with coil of wire wrapped around it in the reverse direction is placed close to the previous one. The earlier one is named primary coil and the later one is named secondary coil.
When an electric current is passed through the coils the magnetic field produced in both cores will have same strength but opposite in direction and hence, cancelling out each other. But, when this arrangement is placed in an external field a measurable voltage in the secondary coil proportional to the strength of external field is induced. The measure of voltage induced by the external field will provide precise determination of the direction and magnitude of the local magnetic field relative to the magnetometer’s orientation in the borehole.
Generally, there are 3 magnetometers aligned orthogonally to read the magnetic field individually in the X, Y, Z direction and then the effective magnetic field is calculated.
Accelerometers are used to measure the earth’s local gravitational field. Each accelerometer consists of a magnetic mass (pendulum) suspended in an electromagnetic field. Gravity deflects the mass from its null position. Sufficient current is applied to the sensor to return the mass to the null position. This current is directly proportional to the gravitational force acting on the mass. The gravitational readings are used to calculate the hole inclination, toolface, and the vertical reference used to determine dip angle.
Generally, there are 3 accelerometers aligned orthogonally to read the gravity field individually in the X, Y, Z direction and then the effective gravity field is calculated.
The set of values such as Inclination, Azimuth, Magnetic & Gravity tool face, Total gravity field, Total magnetic field, Dip Angle/ Magnetic Inclination are as a result of the values from magnetometers and/ or accelerometers.
What does these two types of toolface means and why can't we use one toolface?
Magnetic toolface is the angle b/w the reference north and the high side of the tool.
Magnetic tool face is more accurate at the low angles as compared to gravity toolface. As we have discussed in previous section that, the magnetic readings get affected due to increased drillstring interference from the horizontal component as the angle increases. As per the company policies of directional drilling service providers, MTF values are obtained up to 5° or 8° or less. Above this angle there's change over to the GTF values.
Gravity toolface is the angle b/w the wellbore high side and the high side of tool.
Values of gravity toolface are more reliable at higher inclinations and they're free from any magnetic interference. Even inside casings or near to it or any other junk metals, accelerometers provides reliable inclination values.
Even though the same magnetometers and accelerometers are used for obtaining inclination and azimuth values but, while deviating the well in sliding mode, we rely on accurate toolfaces an hence MTF at low angles and GTF at high angles are used.
While programming MWD/LWD tools we keep them in prescribed position and assign it a value of 0° i.e the High Side. Consequently, taking this as a reference and moving clockwise the angles accordingly are 90° when it's extreme right, 180° when it's exactly opposite the high side i.e Down Side, 270° when it's extreme left and back to 360° or 0°.
As you could have guessed by now that, the direction towards the gravity is Down Side and the direction exactly opposite to it is called as High side. After programming M/LWD tool is aligned with the high side of the SDMM (0° is taken as the high side). This process of tool alignment while making up BHA is referred to as 'Scribing'.
When an electric current is applied around the bar of ferromagnetic material wrapped by a coil of wire, the bar becomes magnetized and generates its own magnetic field.
When an electric current is passed through the coils the magnetic field produced in both cores will have same strength but opposite in direction and hence, cancelling out each other. But, when this arrangement is placed in an external field a measurable voltage in the secondary coil proportional to the strength of external field is induced. The measure of voltage induced by the external field will provide precise determination of the direction and magnitude of the local magnetic field relative to the magnetometer’s orientation in the borehole.
Generally, there are 3 magnetometers aligned orthogonally to read the magnetic field individually in the X, Y, Z direction and then the effective magnetic field is calculated.
Accelerometers are used to measure the earth’s local gravitational field. Each accelerometer consists of a magnetic mass (pendulum) suspended in an electromagnetic field. Gravity deflects the mass from its null position. Sufficient current is applied to the sensor to return the mass to the null position. This current is directly proportional to the gravitational force acting on the mass. The gravitational readings are used to calculate the hole inclination, toolface, and the vertical reference used to determine dip angle.
Generally, there are 3 accelerometers aligned orthogonally to read the gravity field individually in the X, Y, Z direction and then the effective gravity field is calculated.
The set of values such as Inclination, Azimuth, Magnetic & Gravity tool face, Total gravity field, Total magnetic field, Dip Angle/ Magnetic Inclination are as a result of the values from magnetometers and/ or accelerometers.
What does these two types of toolface means and why can't we use one toolface?
Magnetic toolface is the angle b/w the reference north and the high side of the tool.
Magnetic tool face is more accurate at the low angles as compared to gravity toolface. As we have discussed in previous section that, the magnetic readings get affected due to increased drillstring interference from the horizontal component as the angle increases. As per the company policies of directional drilling service providers, MTF values are obtained up to 5° or 8° or less. Above this angle there's change over to the GTF values.
Gravity toolface is the angle b/w the wellbore high side and the high side of tool.
Values of gravity toolface are more reliable at higher inclinations and they're free from any magnetic interference. Even inside casings or near to it or any other junk metals, accelerometers provides reliable inclination values.
Even though the same magnetometers and accelerometers are used for obtaining inclination and azimuth values but, while deviating the well in sliding mode, we rely on accurate toolfaces an hence MTF at low angles and GTF at high angles are used.
As you could have guessed by now that, the direction towards the gravity is Down Side and the direction exactly opposite to it is called as High side. After programming M/LWD tool is aligned with the high side of the SDMM (0° is taken as the high side). This process of tool alignment while making up BHA is referred to as 'Scribing'.
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