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→Overpressured reservoirs
* ''Lost circulation''—by raising the mud weight to control the formation pressure at the bit, the formation may rupture. The mud will then run out into a cavity of its own making.
* ''Lost circulation''—by raising the mud weight to control the formation pressure at the bit, the formation may rupture. The mud will then run out into a cavity of its own making.
When drilling in areas where overpressured zones are known to occur, it is necessary to be aware of both the pressure of the fluids in the pores and the pressure at which the formations will fracture. It is not enough simply to drill with heavier mud to prevent blowouts. If the mud is too heavy, the formation will rupture and lost circulation will result. It is usually impossible to determine these critical pressures in a new area in advance of drilling.
When drilling in areas where overpressured zones are known to occur, it is necessary to be aware of both the pressure of the fluids in the pores and the pressure at which the formations will [[fracture]]. It is not enough simply to drill with heavier mud to prevent blowouts. If the mud is too heavy, the formation will rupture and lost circulation will result. It is usually impossible to determine these critical pressures in a new area in advance of drilling.
===Geological cause of overpressured reservoirs===
===Geological cause of overpressured reservoirs===
The distribution of reservoirs and overpressuring is strongly controlled by the depositional environment ([[:file:pressure-detection_fig1.png|Figure 1]]). Overpressured reservoirs are commonly found where there are thick deposits of shaly sediments.
The distribution of reservoirs and overpressuring is strongly controlled by the depositional environment ([[:file:pressure-detection_fig1.png|Figure 1]]). Overpressured reservoirs are commonly found where there are thick deposits of shaly sediments.
[[file:pressure-detection_fig2.png|thumb|300px|{{figure number|2}}Common patterns of increasing pressure with depth. In the case illustrated by line A, the pressure increases normally to a certain depth, then increases abruptly to almost the weight of the overburden, which it then parallels. In the case of line B, the increase of pressure above normal follows the aquathermal gradient (constant water density) and then follows the fracture gradient.<ref name=pt03r8>Barker, C., Horsfeld, B., 1982, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0001/0050/0099.htm Mechanical versus thermal cause of abnormally high pore pressures in shales— discussion]: AAPG Bulletin, v. 66, n. 1, p. 99–100.</ref>]]
[[file:pressure-detection_fig2.png|thumb|300px|{{figure number|2}}Common patterns of increasing pressure with depth. In the case illustrated by line A, the pressure increases normally to a certain depth, then increases abruptly to almost the weight of the overburden, which it then parallels. In the case of line B, the increase of pressure above normal follows the aquathermal gradient (constant water density) and then follows the [[[fracture]] gradient.<ref name=pt03r8>Barker, C., Horsfeld, B., 1982, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0001/0050/0099.htm Mechanical versus thermal cause of abnormally high pore pressures in shales— discussion]: AAPG Bulletin, v. 66, n. 1, p. 99–100.</ref>]]
====Aquathermal effects====
====Aquathermal effects====
Aquathermal effects also cause overpressure. The temperature increases as sediment is buried, causing an increase in the volume of water. This in turn results in an increase in pressure if the sediment is sealed by an impermeable layer.<ref name=pt03r7>Barker, C., 1972, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0056/0010/2050/2068.htm Aquathermal pressuring—role of temperature in development of abnormal pressure zones]: AAPG Bulletin, v. 56, n. 10, p. 2068–2071.</ref>. For example, if a shale is totally sealed and there is no dilation to increase the pore volume, and if the geothermal gradient is [[temperature::25°C]] per [[depth::1000 m]], then the pressure increase is about 1.8 psi per ft. This is more than the increase in weight of the overburden. Consequently, this aquathermal pressuring will cause an increase of pressure up to the pressure at which the rocks fracture ([[:file:pressure-detection_fig2.png|Figure 2]]).
Aquathermal effects also cause overpressure. The temperature increases as sediment is buried, causing an increase in the volume of water. This in turn results in an increase in pressure if the sediment is sealed by an impermeable layer.<ref name=pt03r7>Barker, C., 1972, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0056/0010/2050/2068.htm Aquathermal pressuring—role of temperature in development of abnormal pressure zones]: AAPG Bulletin, v. 56, n. 10, p. 2068–2071.</ref>. For example, if a shale is totally sealed and there is no dilation to increase the pore volume, and if the geothermal gradient is [[temperature::25°C]] per [[depth::1000 m]], then the pressure increase is about 1.8 psi per ft. This is more than the increase in weight of the overburden. Consequently, this aquathermal pressuring will cause an increase of pressure up to the pressure at which the rocks [[fracture]] ([[:file:pressure-detection_fig2.png|Figure 2]]).
Pressure data from some U.S. Gulf coast wells suggest that the aquathermal effect is important.
Pressure data from some U.S. Gulf coast wells suggest that the aquathermal effect is important.