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[[File:M91Ch2FG5.JPG|thumb|300px|{{figure number|2}}Various types of drilling operations, offshore and onshore. Drill ship Jack Ryan courtesy of BP (www.bp.com). Jackup rig courtesy of Maersk Oil and Gas (www.media.maersk.com). Semisubmersible rig and land rig in the Sahara Desert, Libya, courtesy of Woodside Energy Ltd. (www.woodside.com.au).]]

[[File:M91Ch2FG5.JPG|thumb|300px|{{figure number|2}}Various types of drilling operations, offshore and onshore. Drill ship Jack Ryan courtesy of BP (www.bp.com). Jackup rig courtesy of Maersk Oil and Gas (www.media.maersk.com). Semisubmersible rig and land rig in the Sahara Desert, Libya, courtesy of Woodside Energy Ltd. (www.woodside.com.au).]]

The scale and cost of a drilling operation differs between wells onshore and those offshore. An onshore well is drilled with a relatively cheap land rig ([[:file:M91Ch2FG5.JPG|Figure 2]]); offshore, the operation is several times more expensive.

The scale and cost of a drilling operation differs between wells onshore and those offshore. An onshore well is drilled with a relatively cheap [[land rig]] ([[:file:M91Ch2FG5.JPG|Figure 2]]); offshore, the operation is several times more expensive.

In shallow water, typically about 6–45 m (20–150 ft) deep, drilling is conducted by a jackup rig. A jackup is a rig that has three or more legs that sit on the sea floor. In moderately deep water (more than 45 m [150 ft] deep), a floating or semisubmersible rig is used. The semisubmersible rig is kept in place by several anchors.<ref name=Reed_1992>Reed, B., 1992, [[Offshore rigs]], ''in'' D. Morton-Thompson and A. M. Woods, eds., Development geology reference manual: AAPG Methods in Exploration Series 10, p. 65-66.</ref>

In shallow water, typically about 6–45 m (20–150 ft) deep, drilling is conducted by a jackup rig. A jackup is a rig that has three or more legs that sit on the sea floor. In moderately deep water (more than 45 m [150 ft] deep), a floating or semisubmersible rig is used. The semisubmersible rig is kept in place by several anchors.<ref name=Reed_1992>Reed, B., 1992, [[Offshore rigs]], ''in'' D. Morton-Thompson and A. M. Woods, eds., Development geology reference manual: AAPG Methods in Exploration Series 10, p. 65-66.</ref>

In deep water, a drill ship is the preferred option. Deep water is defined as water depths between 500 and 2000 m (1640 and 6562 ft).<ref name=Weimerandslatt_2004>Weimer, P., and R. M. Slatt, 2004, Petroleum systems of deep-water settings: Society of Exploration Geophysicists/European Association of Geoscientists and Engineers Distinguished Instructor Series 7, 465 p.</ref> The drill ship is maintained in place by dynamic positioning. Computers constantly calculate the position of the drill ship using global positioning system technology or in response to signals from transducers on the sea bed. Signals are sent to propellers and lateral thrusters on the sides of the vessel. These readjust the location of the ship to keep it stable against the forces of wind and water currents.

In deep water, a drill ship is the preferred option. Deep water is defined as water depths between 500 and 2000 m (1640 and 6562 ft).<ref name=Weimerandslatt_2004>Weimer, P., and R. M. Slatt, 2004, Petroleum systems of deep-water settings: Society of Exploration Geophysicists/European Association of Geoscientists and Engineers Distinguished Instructor Series 7, 465 p.</ref> The drill ship is maintained in place by dynamic positioning. Computers constantly calculate the position of the drill ship using global positioning system technology or in response to signals from transducers on the sea bed. Signals are sent to propellers and [[lateral]] thrusters on the sides of the vessel. These readjust the location of the ship to keep it stable against the forces of wind and water currents.

[[File:M91Ch2FG6.JPG|thumb|300px|{{figure number|3}}A well is not drilled all the way through. Metal casing strings are run to isolate specific sections of the hole before drilling further.]]

[[File:M91Ch2FG6.JPG|thumb|300px|{{figure number|3}}A well is not drilled all the way through. Metal casing strings are run to isolate specific sections of the hole before drilling further.]]

Every now and again, the hole will collapse in on itself. This will happen where the earth stresses exceed the rock strength. Salt sections or shale sections at shallow depths containing water-sensitive clays are prone to this. Water-sensitive clays can expand by reacting with drilling fluids, particularly low salinity muds. This can cause the borehole wall to founder and bury the drill bit irretrievably. A decision may then be made to branch off from what hole is left, and this is called sidetracking.

Every now and again, the hole will collapse in on itself. This will happen where the earth stresses exceed the rock strength. Salt sections or shale sections at shallow depths containing water-sensitive clays are prone to this. Water-sensitive clays can expand by reacting with drilling fluids, particularly low salinity muds. This can cause the borehole wall to founder and bury the drill bit irretrievably. A decision may then be made to branch off from what hole is left, and this is called sidetracking.

Another problem that can occur is lost circulation, whereby the drilling mud is lost in large quantities into a fracture or a highly permeable interval. Adding fibrous material to the mud will solve the problem. This clogs up the lost circulation zone and prevents any further losses.

Another problem that can occur is lost circulation, whereby the drilling mud is lost in large quantities into a [[fracture]] or a highly permeable interval. Adding fibrous material to the mud will solve the problem. This clogs up the lost circulation zone and prevents any further losses.

==Well operations after drilling has ceased==

==Well operations after drilling has ceased==