Deviation Survey Planning

The data element Existence_Kind is use to differentiate between a planned survey and an actual one. In addition, there are data elements in the class Survey_Point_Detail that only apply to planned boreholes. For example, Build_Rate (radius of curvature at a point in a vertical plane) and Turn_Rate (radius of curvature at a point in a horizontal plane).

Well Planning

Regardless of which conventional survey method is used (single-shot, multishot, steering tool, surface readout gyro, MWD), the following three pieces of information are known at the end of a successful survey:

· Survey Measured Depth

· Borehole Inclination

· Borehole Azimuth (corrected to relevant North)

In order to ascertain the latest bottom-hole position, it is necessary to perform a survey calculation that includes the three inputs listed above. Only then can the latest bottom-hole coordinates be plotted on the directional well plot (TVD vs. Vertical Section on the vertical plot, N/S vs. E/W rectangular coordinates on horizontal plot). Projections to the target, etc., can then be done. A number of survey calculation methods have been used in directional drilling. Of these, only four have had widespread use:

· Tangential

· Average Angle

· Radius of Curvature

· Minimum Curvature

The Tangential Method is the oldest, least sophisticated, and most inaccurate method.

Average Angle and Radius of Curvature methods are commonly used in the field. Average

Angle method (in particular) lends itself easily to a hand-held calculator. Radius of

Curvature method is more widely used. However, official survey reports should not use

either of these methods, except when demanded by the customer.

Minimum Curvature method should be used for all office calculations and official survey

reports. Where possible, it should also be the chosen field calculation method. The Directional Driller is

advised to have a hand-held calculator at the well-site, which is programmed for both

Radius of Curvature and Minimum Curvature methods of survey calculation.