Changes

* 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===

* Polycrystalline diamond compact (PDC) bits.

* Polycrystalline diamond compact (PDC) bits.

* Improved motor design.

* Improved motor design.

* Reliable [[measurement while drilling]] (MWD) tools and systems. This system allows the directional driller to monitor the azimuth and inclination of the borehole continously near the bit and make changes as required (see [[Measurement while drilling]]).

* Reliable [[measurement while drilling]] (MWD) tools and systems. This system allows the directional driller to monitor the [[azimuth]] and inclination of the borehole continously near the bit and make changes as required (see [[Measurement while drilling]]).

* Downhole adjustable bent subs, called ''steerable subs''.

* Downhole adjustable bent subs, called ''steerable subs''.