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[[file:wellbore-trajectory_fig1.png|300px|thumb|{{figure number|1}}Using a pendulum bottom hole assembly to drop angle.]]

[[file:wellbore-trajectory_fig1.png|300px|thumb|{{figure number|1}}Using a pendulum bottom hole assembly to drop angle.]]

A [[Well types#Conventional wells|vertical hole]] is called a “straight” hole. However, some minor deviation from vertical often occurs naturally. This is related to formation properties, such as dip angle and hardness, and to other factors, such as the BHA, the bit design, and the weight on the bit.

A [[Well types#Conventional wells|vertical hole]] is called a “straight” hole. However, some minor deviation from vertical often occurs naturally. This is related to formation properties, such as [[dip]] angle and hardness, and to other factors, such as the BHA, the bit design, and the weight on the bit.

Two types of BHAs are commonly used to drill a vertical hole: slick and pendulum. A ''slick BHA'' consists of a drill bit, drill collars, heavy-weight drill pipe, and regular drill pipe. The name ''slick'' is related to the absence of stabilizers. Slick BHAs have limited application due to their high potential for becoming differentially stuck. Square or spiral collars can be used in conjunction with slick BHAs when differential sticking is known to occur. In addition, slick BHAs can be run when there is a risk of losing the BHA due to hole problems.

Two types of BHAs are commonly used to drill a vertical hole: slick and pendulum. A ''slick BHA'' consists of a drill bit, drill collars, heavy-weight drill pipe, and regular drill pipe. The name ''slick'' is related to the absence of stabilizers. Slick BHAs have limited application due to their high potential for becoming differentially stuck. Square or spiral collars can be used in conjunction with slick BHAs when differential sticking is known to occur. In addition, slick BHAs can be run when there is a risk of losing the BHA due to hole problems.

A ''pendulum BHA'' is probably the most often used assembly for drilling a vertical hole. A pendulum BHA is similar to a slick BHA, but contains one or more stabilizers ([[:file:wellbore-trajectory_fig1.png|Figure 1]]). The closest stabilizer to the bit acts as a pendulum point. Gravity tends to force the bit to the “low side” of the hole, decreasing hole angle. Pendulum BHAs are run at a high RPM rate and a low weight-on-bit (WOB) rate in areas where deviation needs to be minimized.

A ''pendulum BHA'' is probably the most often used assembly for drilling a vertical hole. A pendulum BHA is similar to a slick BHA, but contains one or more stabilizers ([[:file:wellbore-trajectory_fig1.png|Figure 1]]). The closest stabilizer to the bit acts as a pendulum point. [[Gravity]] tends to force the bit to the “low side” of the hole, decreasing hole angle. Pendulum BHAs are run at a high RPM rate and a low weight-on-bit (WOB) rate in areas where deviation needs to be minimized.

==Directional wellbore==

==Directional wellbore==

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''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 rig|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:

* Building a surface location away from the bottom hole location to avoid cultural or topographic restrictions

* Building a surface location away from the bottom hole location to avoid cultural or topographic restrictions

* 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''.

[[Category:Wellsite methods]]

[[Category:Wellsite methods]]