Changes

[[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 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.

* 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

* Sidetracking around a “fish” or lost open hole

* [[Well types#Sidetrack wells|Sidetracking]] around a “fish” or lost open hole

* Sidetracking out of casing for recompletions or collapsed casing

* Sidetracking out of casing for recompletions or collapsed casing

* Drilling a relief well to kill a blowout

* Drilling a relief well to kill a blowout

* Faster payout

* Faster payout

Problems with horizontal wells include additional well costs and difficulties with formation evaluation, completion, and workover services.

Problems with horizontal wells include additional well costs and difficulties with formation evaluation, completion, and [[Workovers|workover]] services.

Horizontal wells are classified as ''long, medium'', or ''short radius'', depending on the build rate from vertical to horizontal ([[:file:wellbore-trajectory_fig8.png|Figure 8]]). As the build rate increases, the radius of curvature of the wellbore trajectory decreases. Long radius wells have smaller build rates and therefore reach a 90° inclination over a longer horizontal distance than short radius wells.

Horizontal wells are classified as ''long, medium'', or ''short radius'', depending on the build rate from vertical to horizontal ([[:file:wellbore-trajectory_fig8.png|Figure 8]]). As the build rate increases, the radius of curvature of the wellbore trajectory decreases. Long radius wells have smaller build rates and therefore reach a 90° inclination over a longer horizontal distance than short radius wells.