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{{publication
| image = development-geology-reference-manual.png
| series = Methods in Exploration
| title = Development Geology Reference Manual
| part = Wellsite methods
| chapter = Drilling problems
| frompg = 87
| topg = 87
| author = Phyllis Loose
| link = http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=612
| isbn = 0891816607
}}
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==
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.
==Key seats==
Key seats usually form as a result of dog legs. A ''key seat'' is formed when a channel or groove is cut in the side of the hole, parallel to the axis of the hole. The drill pipe dragging action through the sharp bend in a doglog creates the groove in the side of the wellbore. Key seats can be prevented by not creating doglegs.
==Hole instability==
Hole instability occurs when encountered formations flow, slough, or swell. The most unstable formations are shales and salt beds.
Instability can result from the following phenomena:
* Overburden pressure
* Earth movement forces
* Pore pressure
* 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.
Problems associated with hole instability include the following:
* Ineffective hole cleaning
* Stuck pipe
* Bridges and fill-up
* Wellbore enlargement
* Increased mud volume
* Increased cost
* Poor cement jobs
* Logging difficulties
Hole instability can usually be controlled by use of the proper [[drilling fluid]].
==Lost circulation==
Lost circulation is the complete or partial loss of drilling mud into a formation. The loss occurs when the total pressure exerted against the formation exceeds the total pressure of the formation. Formations succeptible to lost circulation have the following characteristics:
* Cavernous and open fissured
* Very coarse, permeable, and shallow, such as loose gravel
* Naturally or intrinsically fractured
* Easily fractured
* 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.
==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:
* Reduction in penetration rates
* Lost circulation
* The well being swabbed when drill pipe is pulled
* Stuck tools
This problem can be mitigated by using oil-based muds.
==See also==
* [[Mudlogging: Drill cuttings analysis]]
* [[Introduction to wellsite methods]]
* [[Wellbore trajectory]]
* [[Conventional coring]]
* [[Drilling fluid]]
* [[Land rigs]]
* [[Rig personnel]]
* [[Wellsite math]]
* [[Core handling]]
* [[Mudlogging: Equipment, services, and personnel]]
* [[Core alteration and preservation]]
* [[Wellsite safety]]
* [[Sidewall coring]]
* [[Show evaluation]]
* [[Rate of penetration]]
* [[Mudlogging: The mudlog]]
* [[Well planning]]
* [[Pressure detection]]
* [[Drill stem testing]]
* [[Measurement while drilling]]
* [[Offshore rigs]]
* [[Fishing]]
* [[Core orientation]]
* [[Mudlogging: Gas extraction and monitoring]]
==External links==
{{search}}
* [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=612 Find the book in the AAPG Store]
* [[pw:Drilling problems|Drilling problems]] on PetroWiki
[[Category:Wellsite methods]]
| image = development-geology-reference-manual.png
| series = Methods in Exploration
| title = Development Geology Reference Manual
| part = Wellsite methods
| chapter = Drilling problems
| frompg = 87
| topg = 87
| author = Phyllis Loose
| link = http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=612
| isbn = 0891816607
}}
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==
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.
==Key seats==
Key seats usually form as a result of dog legs. A ''key seat'' is formed when a channel or groove is cut in the side of the hole, parallel to the axis of the hole. The drill pipe dragging action through the sharp bend in a doglog creates the groove in the side of the wellbore. Key seats can be prevented by not creating doglegs.
==Hole instability==
Hole instability occurs when encountered formations flow, slough, or swell. The most unstable formations are shales and salt beds.
Instability can result from the following phenomena:
* Overburden pressure
* Earth movement forces
* Pore pressure
* 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.
Problems associated with hole instability include the following:
* Ineffective hole cleaning
* Stuck pipe
* Bridges and fill-up
* Wellbore enlargement
* Increased mud volume
* Increased cost
* Poor cement jobs
* Logging difficulties
Hole instability can usually be controlled by use of the proper [[drilling fluid]].
==Lost circulation==
Lost circulation is the complete or partial loss of drilling mud into a formation. The loss occurs when the total pressure exerted against the formation exceeds the total pressure of the formation. Formations succeptible to lost circulation have the following characteristics:
* Cavernous and open fissured
* Very coarse, permeable, and shallow, such as loose gravel
* Naturally or intrinsically fractured
* Easily fractured
* 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.
==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:
* Reduction in penetration rates
* Lost circulation
* The well being swabbed when drill pipe is pulled
* Stuck tools
This problem can be mitigated by using oil-based muds.
==See also==
* [[Mudlogging: Drill cuttings analysis]]
* [[Introduction to wellsite methods]]
* [[Wellbore trajectory]]
* [[Conventional coring]]
* [[Drilling fluid]]
* [[Land rigs]]
* [[Rig personnel]]
* [[Wellsite math]]
* [[Core handling]]
* [[Mudlogging: Equipment, services, and personnel]]
* [[Core alteration and preservation]]
* [[Wellsite safety]]
* [[Sidewall coring]]
* [[Show evaluation]]
* [[Rate of penetration]]
* [[Mudlogging: The mudlog]]
* [[Well planning]]
* [[Pressure detection]]
* [[Drill stem testing]]
* [[Measurement while drilling]]
* [[Offshore rigs]]
* [[Fishing]]
* [[Core orientation]]
* [[Mudlogging: Gas extraction and monitoring]]
==External links==
{{search}}
* [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=612 Find the book in the AAPG Store]
* [[pw:Drilling problems|Drilling problems]] on PetroWiki
[[Category:Wellsite methods]]