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Line 49: − − [[file:wellbore-trajectory_fig3.png|thumb|left|{{figure number|3}}Directional well design example: build and hold.]] − − [[file:wellbore-trajectory_fig4.png|thumb|{{figure number|4}}Using an open hole whipstock for sidetracking.]]
→Directional wellbore
==Directional wellbore==
==Directional wellbore==
<gallery mode=packed heights=200px widths=200px>
file:wellbore-trajectory_fig4.png|{{figure number|4}}Using an open hole whipstock for sidetracking
file:wellbore-trajectory_fig2.png|{{figure number|2}}Right-hand walk and lead angle.
file:wellbore-trajectory_fig3.png|{{figure number|3}}Directional well design example: build and hold.
</gallery>
''Directional drilling'' refers to any method employed to hit a predetermined subsurface target. One application of directional drilling is the development of offshore fields. Field development costs are reduced by directionally drilling many wells from one (or more) platforms. Other applications for directional drilling include the following:
''Directional drilling'' refers to any method employed to hit a predetermined subsurface target. One application of directional drilling is the development of offshore fields. Field development costs are reduced by directionally drilling many wells from one (or more) platforms. Other applications for directional drilling include the following:
* Drilling a relief well to kill a blowout
* Drilling a relief well to kill a blowout
* Increasing contact between the reservoir and the wellbore, (e.g., horizontal drilling)
* Increasing contact between the reservoir and the wellbore, (e.g., horizontal drilling)
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===