Directional Drilling Terminologies

AZIMUTH

There are three azimuth reference systems: True (Geographic North), Grid North and Magnetic North.

Geographic North: In geographic coordinates directions are referred to true north, or a true azimuth. Geographic north points to the North Pole; this direction is indicated by the polar star.

Grid North: Grid north is an arbitrary direction and is always in the direction of the positive ordinate axis of the specific grid used for a particular survey.

Magnetic North: Magnetic north can be measured by a simple magnetic compass. Magnetic azimuths are not constant due to the movement of the north and south magnetic poles and hence magnetic measurements may be in error due to local magnetic field variations. 

In oil wells, all surveys with ‘magnetic type’ tools are initially given an azimuth reading referenced to Magnetic North. However, the final calculated coordinates are always converted to either True North or Grid North.

Magnetic Declination: Magnetic north and true north do not coincide. The divergence between true north and magnetic north is different for most points on the earth’s surface, and in addition to this the magnetic north pole changes its position very slightly each year.

The angle in degrees between true and magnetic north is called the declination angle. The declination angle is negative if magnetic north lies to the west of true north and is positive if the magnetic north lies to the east of true north (refer figure below).

 

The azimuth of a wellbore at any point is defined as the direction of the wellbore on a horizontal plane measured clockwise form a north reference. Azimuths are usually expressed in angles from 0-360 , measured from zero north.

Note: West Declination is always Subtracted and East Declination is always Added. i.e., TRUE NORTH = MAGNETIC NORTH ± (DECLINATION)

Azimuth on horizontal plane, 20 degrees wrt True North

Azimuths can also be expressed in a quadrant system from 0-90 measured from north in the northern quadrants and from south in the southern quadrants.

The figure above shows azimuth reading of 135 equates to S45 E in quadrant readings.
Measured in: degree 

INCLINATION

The angle of the well bore defined by a tangent line at any point of wellbore and a vertical line is called the inclination. The vertical line is always parallel to the direction of earth's gravity. By industry standard, 0 degree inclination is vertical (downward pointing) and 90 degrees inclination is horizontal. An inclination (angle) greater than 90 degrees coincides with the term "drilling up".
Measured in: degree 

NOTE: AZIMUTH & INCLINATION ARE ALSO TERMED AS DIRECTION AND ANGLE RESPECTIVELY.

MEASERED DEPTH (MD) & TRUE VERTICAL DEPTH (TVD)

Measured Depth (MD): Measured depth (MD) is the distance measured along the well path from one reference point to the survey point.
Measured in: Feet (ft) or metre (m)

True Vertical Depth: The vertical distance from a point in the well (usually the current or final depth) to a point at the surface, usually the elevation of the rotary kelly bushing (RKB) is called the true vertical depth (TVD) at that point.
Measured in: Feet (ft) or metre (m)

It is to be noted that MD ≥ TVD in all cases.

VERTICAL SECTION (VS)

A projection of the borehole into a vertical plane parallel to the course bearing and scaled with vertical depth.

KICK OFF POINT (KOP), BUILD, HOLD & DROP

Kick off Point (KOP): The kick off point is defined as the point below the surface location from where the well is deflected from the vertical. The position of the kick off depends on several parameters including: geological considerations, geometry of well and proximity of other wells.

Build Up: It is the act of increasing the inclination of the drilled hole wrt vertical.

Build Section: That portion of the hole in which the inclination angle is increased; rate of buildup is usually expressed as the angular increase per 100 feet of measured depth.

Build Up Rate (BUR): It is the rate of change (degrees/100 feet or degrees/30 metre) of the increasing angle in the hole.

Drop off: It is the act of reducing the inclination of the drilled hole wrt vertical.

Drop Section: That portion of the hole in which the inclination angle is decreased; rate of drop off is usually expressed as the angular increase per 100 feet of measured depth.

Drop off Rate: The rate of change of the inclination in the part of the wellbore where the inclination angle is purposely returned toward vertical, usually expressed in degrees per feet or course length.

Hold: The act of maintaining the inclination and azimuth of the wellbore to remain constant as it is.

Tangent or Hold Section: The portion of hole in which the inclination and azimuth is maintained the same throughout the section.

In the figure below, KB means Kelly Bushing, RT means Rotary Table, DF means Derrick Floor, EOB is for End of Build (i.e., the point at which the Building ends and we either hold or drop the wellbore path).

RECTANGULAR COORDINATES


Rectangular coordinates of a target are usually given in feet/meters North/South and East/West of the local reference point. They can be easily derived by subtracting the grid coordinates of the surface location from those of the target.

The rectangular coordinates can be used to calculate the departure (horizontal displacement) between the surface location and the bottom hole target as follows:

Departure = [(Δ E/W)²+ (Δ N/S²)]^1/2

where: Δ denotes difference in coordinates between E/W or N/S 

 

POLAR COORDINATES

Polar coordinates can be derived from the rectangular coordinates. They are expressed as a distance (departure) and as a direction (either Quadrant or azimuth). 

Polar coordinates are derived from the rectangular coordinates as follows:

Azimuth = tan^-1 ((Δ E/W Coordinates)/( Δ N/S Coordinates))

Now let us try to solve a problem based on the above concept of Rectangular and Polar coordinates.

We have been provided the grid coordinates of the surface and target location. We need to find the Departure and Azimuth of the target from the surface location.  

Grid Coordinates: Target   6,334,400.00 N (m)      200,600.00 E (m)
Grid Coordinates: Surface 6,335,000.00 N (m)      200,400.00 E (m)
 

Now let us calculate the rectangular coordinates.

Δ N/S = N/S (target) - N/S (surface) = 6,334,400.00 - 6,335,000.00 = -600 m

Δ E/W = E/W (target) - E/W (surface) = 200,600.00 - 200,400.00 = 200 m

Now, Azimuth = tan^-1 ((Δ E/W Coordinates)/( Δ N/S Coordinates))

thus, Azm = tan^-1 (200/-600) = -18.4 degree

Also, Departure = [(Δ E/W)²+ (Δ N/S²)]^1/2

thus, Departure = [(200)²+ (-600)²)]^1/2 = 632.5 m

Hence in polar coordinates, the target is 632.5 m at an azimuth of 161.6 degree (S18.4W).These coordinates are plotted in figure below: