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→Vertical or straight wellbore
==Vertical or straight wellbore==
==Vertical or straight wellbore==
[[file:wellbore-trajectory_fig1.png|left|thumb|{{figure number|1}}Using a pendulum bottom hole assembly to drop angle.]]
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.
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 (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==