Changes

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==

* 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'' (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_fig3.png|thumb|left|{{figure number|3}}Directional well design example: build and hold.]]

[[file:wellbore-trajectory_fig2.png|thumb|{{figure number|2}}Right-hand walk and lead angle.]]

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===

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 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).

[[file:wellbore-trajectory_fig3.png|thumb|{{figure number|3}}Directional well design example: build and hold.]]

[[file:wellbore-trajectory_fig4.png|thumb|{{figure number|4}}Using an open hole whipstock for sidetracking.]]

====Whipstock method====

====Whipstock method====

The oldest method of kicking off or deviating a wellbore uses an open hole ''whipstock'' (Figure 4), which is a casing joint with an upward-tapering wedge cut out of one side. Although this operation is time consuming, it is still occasionally used to sidetrack around fish or abandoned open holes. Whipstocks are also used to sidetrack out of casing by milling a “window” and deflecting the wellbore using a mud motor.

The oldest method of kicking off or deviating a wellbore uses an open hole ''whipstock'' ([[:file:wellbore-trajectory_fig4.png|Figure 4]]), which is a casing joint with an upward-tapering wedge cut out of one side. Although this operation is time consuming, it is still occasionally used to sidetrack around fish or abandoned open holes. Whipstocks are also used to sidetrack out of casing by milling a “window” and deflecting the wellbore using a mud motor.

[[file:wellbore-trajectory_fig4.png|thumb|{{figure number|4}}Using an open hole whipstock for sidetracking.]]

[[file:wellbore-trajectory_fig5.png|left|thumb|{{figure number|5}}Kicking off with a jet bit.]]

====Jet bit====

====Jet bit====

Another kicking-off method uses a ''jet bit''. The bit has one large nozzle that erodes a pocket from the hole bottom in the desired trajectory (Figure 5). Weight is applied to the bit while it is rotated into the pocket. This procedure is repeated until the desired trajectory is achieved. The disadvantage of jetting is that it is highly dependent on formation hardness. Some formations are too hard to be hydraulically eroded, and some soft formations erode too quickly, making it difficult to jet in the desired trajectory.

Another kicking-off method uses a ''jet bit''. The bit has one large nozzle that erodes a pocket from the hole bottom in the desired trajectory ([[:file:wellbore-trajectory_fig5.png|Figure 5]]). Weight is applied to the bit while it is rotated into the pocket. This procedure is repeated until the desired trajectory is achieved. The disadvantage of jetting is that it is highly dependent on formation hardness. Some formations are too hard to be hydraulically eroded, and some soft formations erode too quickly, making it difficult to jet in the desired trajectory.

[[file:wellbore-trajectory_fig5.png|thumb|{{figure number|5}}Kicking off with a jet bit.]]

[[file:wellbore-trajectory_fig6.png|thumb|{{figure number|6}}(a) Steerable bottom hole assembly, (b) Kicking off with a bent sub and straight mud motor.]]

====Bottom hole assemblages used to kick off wells====

====Bottom hole assemblages used to kick off wells====

* BHAs that use a downhole motor to provide bit power

* BHAs that use a downhole motor to provide bit power

There are two different downhole power sources: mud turbines and positive displacement mud motors. Both systems use the hydraulic energy of the mud to rotate the bit. Mud motors combined with a bend in the BHA are used to drill the well directionally. The bend is located in the motor housing (''bent housing motor'') (Figure 6a) or in a short sub (''bent sub'') directly behind the motor (Figure 6b). The purpose of the bend is to tilt the bit axis relative to the hole axis.

There are two different downhole power sources: mud turbines and positive displacement mud motors. Both systems use the hydraulic energy of the mud to rotate the bit. Mud motors combined with a bend in the BHA are used to drill the well directionally. The bend is located in the motor housing (''bent housing motor'') ([[:file:wellbore-trajectory_fig6.png|Figure 6a]]) or in a short sub (''bent sub'') directly behind the motor ([[:file:wellbore-trajectory_fig6.png|Figure 6b]]). The purpose of the bend is to tilt the bit axis relative to the hole axis.

 

To change course, drilling stops, the bend in the BHA is oriented to the new borehole trajectory and the bit drills ahead. This procedure is called ''sliding'' because the entire BHA above the motor is moving downhole without rotating. This system originally had several limitations:

To change course, drilling stops, the bend in the BHA is oriented to the new borehole trajectory and the bit drills ahead. This procedure is called ''sliding'' because the entire BHA above the motor is moving downhole without rotating. This system originally had several limitations:

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===

* ''Drill ahead''—After the well has been kicked off, the entire drill string is rotated to drill ahead while maintaining the trajectory. An undergauge stabilizer is often included on the motor to reduce the tendency to drop the angle. Further corrections in trajectory are made by orienting and sliding.

* ''Drill ahead''—After the well has been kicked off, the entire drill string is rotated to drill ahead while maintaining the trajectory. An undergauge stabilizer is often included on the motor to reduce the tendency to drop the angle. Further corrections in trajectory are made by orienting and sliding.

* ''Hold angle''—These require BHAs that are called ''packed'' because they contain many stabilizers. This tends to limit changes in wellbore trajectory (Figure 7a).

* ''Hold angle''—These require BHAs that are called ''packed'' because they contain many stabilizers. This tends to limit changes in wellbore trajectory ([[:file:wellbore-trajectory_fig7.png|Figure 7a]]).

* ''Drop angle''—The pendulum BHA, discussed earlier, is also used for dropping the angle (see Figure 1).

* ''Drop angle''—The pendulum BHA, discussed earlier, is also used for dropping the angle (see [[:file:wellbore-trajectory_fig1.png|Figure 1]]).

* ''Build angle''—This requires a near bit stabilizer to act as a fulcrum point. The bend in the drill collars above the near bit stabilizer causes the bit axis to tilt relative to the hole axis (Figure 7b).

* ''Build angle''—This requires a near bit stabilizer to act as a fulcrum point. The bend in the drill collars above the near bit stabilizer causes the bit axis to tilt relative to the hole axis ([[:file:wellbore-trajectory_fig7.png|Figure 7b]]).

[[file:wellbore-trajectory_fig7.png|thumb|{{figure number|7}}(a) Packed bottom hole assembly, (b) Fulcrum effect for build angle.]]

[[file:wellbore-trajectory_fig8.png|thumb|{{figure number|8}}Build rates for classification of horizontal wells.]]

==Horizontal wellbore==

==Horizontal wellbore==

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 workover services.

Horizontal wells are classified as ''long, medium'', or ''short radius'', depending on the build rate from vertical to horizontal (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.

 

===Long radius===

===Long radius===

Long radius design methods are used primarily for achieving extended reach from platforms and in appMcations where large horizontal displacement is desired. These wells are really just conventional directional wells with final hole inclinations of 90°. Build rates typically range from 2° to 6° per [[length::100 ft]]. More than [[depth::4000 ft]] of horizontal section can be drilled after reaching a 90 ° inclination.

Long radius design methods are used primarily for achieving extended reach from platforms and in applications where large horizontal displacement is desired. These wells are really just conventional directional wells with final hole inclinations of 90°. Build rates typically range from 2° to 6° per [[length::100 ft]]. More than [[depth::4000 ft]] of horizontal section can be drilled after reaching a 90 ° inclination.

BHAs used to provide wellbore trajectories for this type of well are similar to those used for conventional directional drilling. Steerable systems with bent housing motors are generally used for both the build and the horizontal sections. Slick BHAs with downhole motors are sometimes used to drill the horizontal portion of the well.

BHAs used to provide wellbore trajectories for this type of well are similar to those used for conventional directional drilling. Steerable systems with bent housing motors are generally used for both the build and the horizontal sections. Slick BHAs with downhole motors are sometimes used to drill the horizontal portion of the well.