In directional drilling, location of the well is specified by 'Inclination' (or Angle) and 'Azimuth' (or Direction) and these measurements are taken w.r.t a fixed reference system, so that the course of well bore can be exactly calculated and recorded.
These reference systems include:
- Depth references
- Inclination references
- Azimuth references
1. Depth References
During the course of directional well, there are two depths-
Measured Depth (MD) - It's the depth measured along the actual well bore from the surface reference point to the survey point. This depth can be measured from pipe tally, wireline depth counter or mud logger's depth counter etc.
True Vertical Depth (TVD) - It's the vertical depth measured from the surface reference point to the survey point. This depth is always calculated from the deviation survey.
In most drilling operations, RKB (Rotary Kelly Bushing) is used as a reference point to measured depth.
For floating drilling rigs, a mean rotary table elevation has to be used.
In offshore, MSL (Mean Sea Level) is sometimes used. Variations in actual sea level from MSL can be read from tide tables or can be measured.
In most drilling operations, RKB (Rotary Kelly Bushing) is used as a reference point to measured depth.
For floating drilling rigs, a mean rotary table elevation has to be used.
In offshore, MSL (Mean Sea Level) is sometimes used. Variations in actual sea level from MSL can be read from tide tables or can be measured.
2. Inclination references
Inclination of the well bore is the angle between the vertical and the well bore axis at a particular point. The vertical reference is the direction of local gravity vector.
3. Azimuth references
In directional surveying there are three azimuth reference systems, namely;
Why are there three types of North?
All the magnetic instruments measures direction w.r.t magnetic north but, unfortunately magnetic axis of the earth changes along with time; so they're referenced to True North or Grid North in order to obtain a stable reference (since earth's rotational axis remains same).
In absence of magnetic interference, magnetometers points to magnetic north.
In absence of magnetic interference, magnetometers points to magnetic north.
True/ Geographic North (TN) is the direction of geographic north pole which lies on the Earth's rotational axis.
Magnetic North (MN) is the direction towards which the north seeking arrow of the compass points; it lies to the north of magnetic axis of the earth.
Magnetic Declination is the difference in degrees between TN and MN at a given location.
Magnetic declination values are updated by magnetic monitoring stations placed throughout the world.
Azimuth readings taken w.r.t MN is called as Raw Azimuth.
East Declination : MN lies to the east of TN & is +ve.
Corrected Azimuth = Raw Azimuth + East declination
West Declination: MN lies to the west of TN & is -ve.
Corrected Azimuth = Raw Azimuth + (-West Declination)
Examples:
Q 1. If the measured azimuth is 100°; magnetic declination is -20°. Calculate corrected azimuth?
Sol'n: -ve sign indicates west declination.
Corrected Azimuth = Raw Azm + Mag Dec
= 100 + (-20) = 80°
Q 2. For the above, calculate corrected azimuth if magnetic declination is 10°.
Sol'n: +ve sign indicates east declination.
Corrected Azimuth = Raw Azm + Mag Dec
= 100 + 10 = 10°
Grid North (GN) is the north direction represented on the grid maps.
During drilling operations we're working on a curved surface (i.e Earth's surface), but while calculating horizontal co-ordinates we assume we're working on a flat surface. Obviously, it's not that easy to project earth's curved surface on a flat map and hence, different type of projection systems are used.
Among them UTM (Universal Transverse Mercator) is widely used and accepted; we'll be using the same.
East Grid Correction (when GN lies to the east of TN) remains +ve.
West Grid Correction (when GN lies to the west of TN) remains -ve.
The result of Grid Correction is called as Grid Convergence.
Grid convergence will be zero if magnetic declination is zero and hole location lies along central meridian.
Total Correction = Magnetic Declination - Grid Correction
Corrected Azimuth = Raw Azimuth + Total Correction
Corrected Azimuth = Raw Azimuth + {Magnetic Declination - Grid Correction}
Examples:
Q 3. If the measured azimuth is 100°; magnetic declination is -20°; grid correction is -15°. Calculate corrected azimuth?
Sol'n: Corrected azimuth = Raw Azm + {Mag Dec - Grid correction}
= 100 + {-20-(-15)} = 100 + {-5} = 95
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