The sensors used in steering tools and MWD/LWD tools are solid-state electronic devices known as magnetometers and accelerometers which respond to the earth's magnetic field and gravitational field respectively

Since the magnetometers may be affected by the steel collars and drill pipe, the probe must be seated within non-magnetic collars. The probe slots into the muleshoe key, which is aligned with the scribe line of the bent sub.

The probe therefore measures the direction in which the scribe line of bent sub is pointing

The orientation of the bent sub can be measured relative to Magnetic North (magnetic toolface) or with respect to the High Side of the hole (gravity toolface)

If we place a plumb bob at the centre of any section of wellbore, the plumb bob orients itself in the direction of "g", vertically downwards. The direction opposite to the orientation of plumb bob is the high side of the wellbore.

At low inclinations (0-5°) magnetic toolface is used, since the High Side is not well-defined at that stage. Once the angle increases, however, and the hole direction becomes established, the gravity toolface is used (i.e. toolface is reported as a number of degrees to the left or right of High Side). 

The High Side of the hole can be defined by the accelerometers. The High Side is directly opposite to the gravity vector, which is the sum of the three gravitational components measured by the accelerometers.
Now, before taking this discussion ahead, you should know what scribe line and  the tool face alignment and orientation  means.

Look at the figure below. It shows an adjustable bent sub which gives us an option to adjust the amount of bent we wish to provide to the motor. It generally ranges between 0 to 3 degrees.

Now to provide a desired bent, we hold the upper and lower ABH (Adjustable Bent Housing) using the tong and by using chain tong on orientation sleeve we coincide the upper and lower angular marking to the desired value.
Suppose we desire to provide 2.89 degree initial bent. So after coinciding the two angular marking, what the bent sub looks is like in the figure below

The line passing through the two coincident angular markings give us bent sub tool face, i.e. the orientation of our bent sub. See figure below 

Now have a look at this figure:
This is how the accelerometer and the magnetometer is arranged in the MWD unit aligned  in same axis (z axis).

Typically, three magnetometers and three accelerometers are used to measure the three components of the gravity vector and the Earth magnetic field vector in the sensor frame.

The voltage outputs from the accelerometers are denoted by Gx, Gy and Gz, corresponding to the three orthogonal axes.

Similarly the magnetometer outputs are Hx, Hy and Hz.

z axis points down the axis of the tool and the y axis is defined as being in line with the toolface.

Now in order to accomplish the directional drilling task, the operator needs to know the orientation of the bent sub. The relationship between the directional sensor and the bent sub is fixed for each bottom hole assembly. From the directional sensor measurement, the directional sensor tool face is known. If the angular difference between the directional sensor reference point and the bent sub is measured on the surface, then the operator can use this measurement and the directional sensor tool face reading to determine the orientation of the bent sub, namely, the bent sub tool face. Such angular difference is sometimes called tool face offset.

In the prior art, the angular difference is determined by the use of a scribe line on the exterior of the instrument housing.

Now our next job is orient our MWD/LWD tool in the direction of bent.

Below is a typical BHA arrangement for a 8 1/2" Hole Section :

After we are done with adjusting the bent sub to a desired angle, we make up the string stabilizer, the float sub, the UBHO sub and 1 (one in number) NMDC (also called Monel). To know more about Stabilizer, Float Sub and UBHO sub refer my blog post : Stabilizer, Float Sub , UBHO Sub. 

Now out job is to lower the MWD tool assembly into the NMDC. 

The MWD tool is run inside the NMDC (Monel).

It gets seated in the mule shoe sub (UBHO Sub) which is at the bottom of NMDC.

The tool face of MWD usually doesnot remain aligned with the toolface of downhole motor.


Magnetic Tool Face
It is the direction, in the horizontal plane, the bent sub scribe line is pointing with regard to the north reference (Grid, Mag, or True). 

Magnetic orientation is used when the inclination of the well bore is less than 5°. When the inclination is below this amount, the survey instrument cannot accurately determine the highside of the instrument for orientation purposes. The toolface will be presented in azimuth or quadrant form, referenced to magnetic north. The magnetic toolface reading is whatever magnetic direction the toolface is pointed. 

Gravity Tool Face
It is the angular distance that a bent sub scribe line is turned, about the tool axis, relative to the high side of the hole. 

If the inclination of the well bore is above 5° to 8°, then the gravity toolface can be used. 
The toolface will be referenced to the highside of the survey instrument, no matter what the hole direction of the survey instrument is at the time. 
The toolface will be presented in a number of degrees either right or left of the highside. 

GTF orientation is represented by figure below :

برچسب‌ها: Tool Face, Magnetic Tool Face, Gravity Tool Face, Tool Face Orientation
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