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Drilling problems include any difficulties encountered while drilling a well. The most common drilling problems are the creation of doglegs and key seats, hole instability, lost circulation, and excessive bottom hole temperatures.

Drilling problems include any difficulties encountered while [[drilling a well]]. The most common drilling problems are the creation of doglegs and key seats, hole instability, lost circulation, and excessive bottom hole temperatures.

==Doglegs==

==Doglegs==

A ''dogleg'' is usually defined as any deviation greater than 3° per [[length::100 ft]], and it occurs when a sharp change of direction is taken in the wellbore. Typically, a dogleg is caused by a change in the [[dip]] of the formation or by a change in the weight applied to the bit. Severe doglegs can result in stuck casing, drill pipe failure, and inability to run casing to total depth. If casing is successfully run through the dogleg, excessive wear on production equipment can occur. The use of properly placed stabilizers, large diameter drill collars, and the proper weight applied to the bit will minimize the formation of doglegs.

A ''dogleg'' is usually defined as any deviation greater than 3° per [[length::100 ft]], and it occurs when a sharp change of direction is taken in the wellbore. Typically, a dogleg is caused by a change in the [[dip]] of the formation or by a change in the weight applied to the bit. Severe doglegs can result in stuck casing, drill pipe failure, and inability to run [http://www.glossary.oilfield.slb.com/en/Terms/c/casing.aspx casing] to total depth. If casing is successfully run through the dogleg, excessive wear on production equipment can occur. The use of properly placed stabilizers, large diameter drill collars, and the proper weight applied to the bit will minimize the formation of doglegs. (See [[Wellbore trajectory]].)

==Key seats==

==Key seats==

==Hole instability==

==Hole instability==

Hole instability occurs when encountered formations flow, slough, or swell. The most unstable formations are shales and salt beds.

Hole instability occurs when encountered formations flow, slough, or swell. The most unstable formations are [[shale]]s and [[Evaporite|salt beds]].

Instability can result from the following phenomena:

Instability can result from the following phenomena:

* Overburden pressure

* Overburden pressure

* Earth movement forces

* Earth movement forces

* Pore pressure

* [http://www.glossary.oilfield.slb.com/en/Terms/p/pore_pressure.aspx Pore pressure]

* Water absorption, swelling, or dispersion

* Water absorption, swelling, or dispersion

Instability occurs when the relief of overburden pressure exceeds the yield strength of the drilled formation, resulting in flow of the formation (plastic flow). Abnormally high pore pressures can cause blowouts when present in highly [[permeability]] formations. If the pressure differential between the wall of the hole and the fluid in the hole is large, the formation may slough off. Structural stresses can also cause hole instability.

Instability occurs when the relief of overburden pressure exceeds the [http://dictionary.reference.com/browse/yield+strength yield strength] of the drilled formation, resulting in flow of the formation ([http://dictionary.reference.com/browse/plastic+flow plastic flow]). Abnormally high pore pressures can cause [[Wikipedia:Blowout (well drilling)|blowouts]] when present in highly [[permeability|permeable]] formations. If the pressure differential between the wall of the hole and the fluid in the hole is large, the formation may slough off. Structural stresses can also cause hole instability.

Problems associated with hole instability include the following:

Problems associated with hole instability include the following:

* Ineffective hole cleaning

* Ineffective hole cleaning

* Stuck pipe

* Stuck pipe

* Bridges and fill-up

* [http://www.glossary.oilfield.slb.com/en/Terms/b/bridge.aspx Bridges] and fill-up

* Wellbore enlargement

* Wellbore enlargement

* Increased mud volume

* Increased mud volume

* Cavernous and open fissured

* Cavernous and open fissured

* Very coarse, permeable, and shallow, such as loose gravel

* Very coarse, permeable, and shallow, such as loose gravel

* Naturally or intrinsically fractured

* Naturally or intrinsically [[fracture]]d

* Easily fractured

* Easily fractured

* Underpressured or depleted

* [[Underpressure causes|Underpressured]] or depleted

Lost circulation results in increased mud expense and may cause subsurface blowouts. The proper drilling fluids and use of lost circulation material will minimize lost circulation.

Lost circulation results in increased mud expense and may cause subsurface [[Wikipedia:Blowout (well drilling)|blowouts]]. The proper drilling fluids and use of lost circulation material will minimize lost circulation.

==Bottom hole temperatures==

==Bottom hole temperatures==

Extremely high bottom hole temperatures can occur in deep wellbores or in areas of abnormally high geothermal gradient. These excessively high bottom hole temperatures (greater than [[temperature::250°C]]) can cause drilling problems because of the accelerated thickening of water-based drilling fluids. The increase in viscosity and density of the drilling can may result in the following:

Extremely high bottom hole temperatures can occur in deep wellbores or in areas of abnormally high [[geothermal gradient]]. These excessively high bottom hole temperatures (greater than [[temperature::250°C]]) can cause drilling problems because of the accelerated thickening of [[Drilling_fluid#Water-based_muds|water-based drilling fluids]]. The increase in [[viscosity]] and density of the drilling can may result in the following:

* Reduction in penetration rates

* Reduction in penetration rates

* Lost circulation

* Lost circulation

* The well being swabbed when drill pipe is pulled

* The well being [http://www.glossary.oilfield.slb.com/en/Terms/s/swab.aspx swabbed] when drill pipe is pulled

* Stuck tools

* Stuck tools

This problem can be mitigated by using oil-based muds.

This problem can be mitigated by using [[Drilling_fluid#Oil-based_muds|oil-based muds]].

==See also==

==See also==

* [[Introduction to wellsite methods]]

* [[Introduction to wellsite methods]]

* [[Wellsite safety]]

* [[Wellsite safety]]

* [[Well planning]]

* [[Well planning]]

[[Category:Engineering]]

[[Category:Engineering]]

[[Category:Engineering – Development and Operations]]

[[Category:Engineering – Development and Operations]]