2. Straight Hole ?
"There's no such thing as a straight hole." All wells, whether by accident or by design, exhibit changes in hole angle and direction.
Directional drilling refers to all activities that are required to design and drill a wellbore to reach a target, or a number of targets, located at some horizontal distance from the top of the hole.

3. Straight Hole ?
Plan view of 4 vertical wells drilled to 6000 ft

4. Straight Hole ?
Crooked hole tendencies were considered early on as a serious drawback to the widespread use of rotary rigs. But the benefits of rotary drilling more outweighed this perceived limitation, and so the industry began developing ways to keep wells on course. Thus was born the science, the art of deviation control.
Once the industry began devoting serious effort to limiting wellbore deviation, it was natural to start working toward the next step of actually guiding the drill bit to a specific downhole target: a practice known as controlled directional drilling.

5. Deviated Hole
Although the exact causes of wellbore deviation are unknown, we can list the following as contributing factors:
Drill string: buckling
Formation type: lithology, anisotropy, dip
Bottomhole assembly size and configuration: stabilizer types, size, positioning; drill collars; reamers and other tools
Drilling parameters: weight on bit, hydraulics
Hole angle
Annular clearance
Bit type, design features

6. Deviated Hole
We can break down the mechanical actions of drilling a rock into three categories: percussive action, drag-rotary action and combined percussive/drag-rotary action.

7. Deviated Hole
The deviating forces that the drill string imparts to the bit relate directly to the string's configuration, the hole geometry, and the weight on the bit.
A drill string is a flexible, elastic member, unable to resist lateral loads and subject to buckling under axial loads.
The shape of this buckling depends on how much weight is applied at the bit. Once buckling occurs, the bit is no longer vertical, and hole deviation results.

8. Deviated Hole
Wellbore deviation results from forces acting at the bit. We can separate a study of these forces into two parts:
Bit/rock interaction — the study of bit behavior in various rocks under the action of applied bit loads
Drill string mechanics — the analysis of drill string behavior under the action of imposed forces

9. Deviated Hole
Factors that are of practical importance in affecting drill string behavior include:
Drill collar stiffness and unit weight
Hole inclination
Clearance between the drill string and the hole
Weight-on-bit
Formation characteristics

10. Deviated Hole

11. Deviated Hole
The key to control the deviation is to control borehole deviation, minimizing abrupt changes in hole angle so as to allow for running downhole tools, setting casing. There are three basic techniques for controlling wellbore deviation:
Use a pendulum assembly: consisting of the bit, drill collars and strategically positioned stabilizers, to decrease hole angle.
Use packed hole assemblies: consisting of reamers, short collars and stabilizers, or square drill collars, to "lock in" the bit and maintain a constant hole angle.
Use directional drilling tools: downhole motors, bent housings etc.

12. Deviated Hole
Pendulum Assembly

13. Deviated Hole
Strictly speaking, a pendulum is that part of any bottomhole assembly between the bit and the lowest point of contact between the drill collars and the borehole wall. In practice, the term pendulum assembly refers to a drill collar arrangement in which a stabilizer centers the drill collars in the wellbore at some point above the bit.

14. Deviated Hole
Placing the stabilizer too high causes the drill collars to contact the hole wall between the bit and the stabilizer, near the point where they would have contacted if no stabilizer was used. In other words, the stabilizer has no effect on deviation.

15. Deviated Hole A: Welded blade B: Integral blade C: Sleeve
D: non-rotating sleeve

16. Deviated Hole

17. Deviated Hole Packed Hole Assembly
The packed hole assembly employs large-diameter drill collars and multiple stabilizers to provide added stiffness to the bottomhole assembly and thus reduce deviation tendencies. Obviously, designing such an assembly requires a good knowledge of local drilling characteristics.
Problems that can accompany the use of large-diameter tools in packed- hole assemblies include increased torque, a reduction of the available weight-on-bit, accelerated hole erosion, and small annular clearances, which could present problems in case fishing becomes necessary.

18. Deviated Hole Directional Drilling Tools
It is normal to think of directional drilling in the context of reaching out some horizontal distance from a well's surface coordinates. But the same tools and procedures used for drilling extended-reach wells may be employed in "crooked hole country" to control wellbore deviation.
Some common tools including:
Mud motors
Rotary Steerable Systems
Deflection tools

19. Directional Drilling
The earliest and still the most widespread application of controlled directional drilling is offshore development, where cost, logistical and environmental considerations make it essential to be able to drill multiple wells from a single surface location.

20. Directional Drilling

21. Directional Drilling
Reaching inaccessible locations: Reservoirs often underlie mountainous terrain, urban developments or other surface obstacles where building a rig location and drilling vertically to target would be impractical.
Offsetting wells from the target location to take advantage of natural wellbore deviation tendencies: Certain formations have well-defined drilling characteristics, such as the well-known tendency for bits to "walk updip." If we are aware of these characteristics, we can minimize drilling costs by offsetting the surface location and letting the bit drift to target.

22. Directional Drilling
Salt domes: Directional drilling is a means of reaching reservoirs that underlie overhanging salt domes. By "drilling around" the salt, problems such as hole washouts, mud losses and corrosion may be avoided.
Redrills or re-entries: Redrilling or re-entering from an existing wellbore is often an economical way of exploring for new producing horizons, or putting new life into old fields. Although redrills and re-entries are nothing new in the oilfield, their importance has grown over the years due to improvements in slim hole drilling and coiled tubing capabilities.

23. Directional Drilling
Sidetracking or straightening: Directional drilling methods can be applied to remedial work. For example, it may be more practical to sidetrack around a stuck tool or other wellbore obstruction than to perform an expensive and possibly unsuccessful fishing job. Directional methods can also be used to straighten a crooked hole.
Relief well drilling: Directional drilling is a valuable tertiary well control tool. When a blowout occurs and surface control methods are impractical, an alternative is to drill a relief well and pump kill fluids directly or indirectly into the blowout well's annulus.
Drilling under riverbeds: Drilling beneath riverbeds for pipeline installation is another common application of directional drilling methods.

24. Directional Drilling
Horizontal drilling is a special application of directional drilling, which involves directing a well laterally through a productive interval.

25. Directional Drilling
In certain types of reservoirs (e.g., thin formations with good vertical permeability and tendencies toward drawdown-related problems such as water or gas coning, micro- perm zones), horizontal wells are a proven means of
Increasing formation exposure
Improving well deliverability
Eliminating water and gas coning
Reducing overall development costs

26. Directional Drilling
Multilateral drilling involves drilling multiple wellbores from a single vertical or horizontal "trunk," thus making it possible to intersect multiple geologic units from a single surface wellbore. As is the case with single horizontal wells, their applicability is based on geological and reservoir characteristics.