Changes

To hit a subsurface target, control must be exercised on both the angle of hole inclination from vertical (the ''drift'' or ''angle'') and the azimuth angle (the ''direction''). Wellbores have a tendency to move from left to right as the hole is drilled. This phenomenon, known as “walking to the right,” is presumably due to right-hand rotation of the bit and drill string and is affected by inclination angle, rotary speed, weight on the bit, formation dip and strike, and bit design. Most directional wells are oriented to the left of the direction of the target azimuth by an amount known as the ''lead angle'' ([[:file:wellbore-trajectory_fig2.png|Figure 2]]). By compensating for right-hand walk in this fashion, the wellbore is allowed to move naturally to the right, forming an arc into the target.

To hit a subsurface target, control must be exercised on both the angle of hole inclination from vertical (the ''drift'' or ''angle'') and the azimuth angle (the ''direction''). Wellbores have a tendency to move from left to right as the hole is drilled. This phenomenon, known as “walking to the right,” is presumably due to right-hand rotation of the bit and drill string and is affected by inclination angle, rotary speed, weight on the bit, formation dip and strike, and bit design. Most directional wells are oriented to the left of the direction of the target azimuth by an amount known as the ''lead angle'' ([[:file:wellbore-trajectory_fig2.png|Figure 2]]). By compensating for right-hand walk in this fashion, the wellbore is allowed to move naturally to the right, forming an arc into the target.

===“kicking off” a directional well===

===“Kicking off” a directional well===

In directional wells, the wellbore will be deviated at a preselected depth known as the ''kick-off point''. An example of a directional well plan called “build and hold” is shown in [[:file:wellbore-trajectory_fig3.png|Figure 3]]. A borehole inclination of at least 15° is desirable since it is harder to maintain directional control in holes with shallower deviation angles (Adams, 1985). However, wells with higher deviation angles can present operational problems (such as running wireline logs to total depth).

In directional wells, the wellbore will be deviated at a preselected depth known as the ''kick-off point''. An example of a directional well plan called “build and hold” is shown in [[:file:wellbore-trajectory_fig3.png|Figure 3]]. A borehole inclination of at least 15° is desirable since it is harder to maintain directional control in holes with shallower deviation angles (Adams, 1985). However, wells with higher deviation angles can present operational problems (such as running wireline logs to total depth).