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A 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 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.
[[file:wellbore-trajectory_fig2.png|thumb|{{figure number|2}}Right-hand walk and lead angle.]]
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==
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.
[[file:wellbore-trajectory_fig4.png|thumb|{{figure number|4}}Using an open hole whipstock for sidetracking.]]
===“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).
====Whipstock method====
====Whipstock method====
Current technology has simplified directional drilling based on improvements in the following:
Current technology has simplified directional drilling based on improvements in the following:
[[file:wellbore-trajectory_fig7.png|left|thumb|{{figure number|7}}(a) Packed bottom hole assembly, (b) Fulcrum effect for build angle.]]
* Polycrystalline diamond compact (PDC) bits.
* Polycrystalline diamond compact (PDC) bits.
Steerable subs can be reset by changing the pump pressure. This changes the angle of the bent sub from straight to +1°. The main advantage of steerable systems is that, after achieving the desired wellbore deflection, it is possible to continue drilling without tripping. If needed, changes in wellbore trajectory can be made at any time in very gradual steps, which reduces the probability of severe dog legs. The main disadvantage of steerable systems is that they are more expensive than other deflection systems.
Steerable subs can be reset by changing the pump pressure. This changes the angle of the bent sub from straight to +1°. The main advantage of steerable systems is that, after achieving the desired wellbore deflection, it is possible to continue drilling without tripping. If needed, changes in wellbore trajectory can be made at any time in very gradual steps, which reduces the probability of severe dog legs. The main disadvantage of steerable systems is that they are more expensive than other deflection systems.
===Drilling a directional well===
===Drilling a directional well===