here are three azimuth reference systems: True (Geographic North), GridNorth and Magnetic North.

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

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

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

In oil wells, all surveys with magnetic type tools are initially given anazimuth reading referenced to Magnetic North. However, the finalcalculated coordinates are always converted to either True North or GridNorth.

Magnetic Declination: Magnetic north and true north do not coincide. Thedivergence between true north and magnetic north is different for mostpoints on the earths surface, and in addition to this the magnetic northpole changes its position very slightly each year.The angle in degrees between true and magnetic north is called thedeclination angle. The declination angle is negative if magnetic north lies tothe west of true north and is positive if the magnetic north lies to the eastof true north (refer figure below).

The azimuth of a wellbore at any point is defined as the direction of thewellbore on a horizontal plane measured clockwise form a north reference.Azimuths are usually expressed in angles from 0-360 , measured from zeronorth.Note: West Declination is always Subtracted and East Declination is alwaysAdded. 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 measuredfrom north in the northern quadrants and from south in the southernquadrants.The figure above shows azimuth reading of 135 equates to S45 E inquadrant readings.Measured in: degree

INCLINATION

The angle of the well bore defined by a tangent line at any point of wellboreand a vertical line is called the inclination. The vertical line is always parallelto the direction of earth's gravity. By industry standard, 0 degree inclinationis vertical (downward pointing) and 90 degrees inclination is horizontal. Aninclination (angle) greater than 90 degrees coincides with the term "drilling

up".Measured in: degree

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

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

Measured Depth (MD): Measured depth (MD) is the distance measuredalong 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 (usuallythe current or final depth) to a point at the surface, usually the elevationof the rotary kelly bushing (RKB) is called the true vertical depth (TVD) atthat 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 coursebearing 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 thesurface location from where the well is deflected from the vertical. Theposition of the kick off depends on several parameters including: geologicalconsiderations, geometry of well and proximity of other wells.

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

Build Section: That portion of the hole in which the inclination angle isincreased; rate of buildup is usually expressed as the angular increase per100 feet of measured depth.

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

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

Drop Section: That portion of the hole in which the inclination angle isdecreased; rate of drop off is usually expressed as the angular increase per100 feet of measured depth.

Drop off Rate: The rate of change of the inclination in the part of thewellbore 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 toremain constant as it is.

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

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

RECTANGULAR COORDINATES

Rectangular coordinates of a target are usually given in feet/metersNorth/South and East/West of the local reference point. They can be easilyderived by subtracting the grid coordinates of the surface location fromthose of the target.The rectangular coordinates can be used to calculate the departure(horizontal displacement) between the surface location and the bottom holetarget as follows:Departure = [( E/W)2+ ( N/S2)]1/2where: denotes difference in coordinates between E/W or N/S

POLAR COORDINATES

Polar coordinates can be derived from the rectangular coordinates. Theyare expressed as a distance (departure) and as a direction (either Quadrantor azimuth).Polar coordinates are derived from the rectangular coordinates as follows:Azimuth = tan-1 (( E/W Coordinates)/( N/S Coordinates))

Now let us try to solve a problem based on the above concept ofRectangular and Polar coordinates.We have been provided the grid coordinates of the surface and targetlocation. We need to find theDeparture and Azimuth of the target from thesurface 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. N/S = N/S (target) - N/S (surface) = 6,334,400.00 - 6,335,000.00 = -600 m E/W = E/W (target) - E/W (surface) = 200,600.00 - 200,400.00 = 200m

Now, Azimuth = tan-1 (( E/W Coordinates)/( N/S Coordinates))thus, Azm = tan-1 (200/-600) = -18.4 degree

Also, Departure = [( E/W)2+ ( N/S2)]1/2thus, Departure = [(200)2+ (-600)2)]1/2 = 632.5 m

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