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Normal reservoir pressure is the pressure in the reservoir fluids necessary to sustain a column of water to the surface<ref name=pt03r19>Fertl, W. H., 1976, Abnormal formation pressures: New York, Elsevier Scientific Publishing Company, 382 p.</ref>. Normal pressures range between 0.43 and 0.50 psi/ft. Normal drilling muds weigh about 9 ppg (pounds per gallon) and exert a bottom hole pressure of approximately 0.47 psi/ft of depth.

Normal reservoir pressure is the pressure in the reservoir fluids necessary to sustain a column of water to the surface.<ref name=pt03r19>Fertl, W. H., 1976, Abnormal formation pressures: New York, Elsevier Scientific Publishing Company, 382 p.</ref> Normal pressures range between 0.43 and 0.50 psi/ft. Normal drilling muds weigh about 9 ppg (pounds per gallon) and exert a bottom hole pressure of approximately 0.47 psi/ft of depth.

By convention in the petroleum industry, ''overpressure'' refers to pressures higher than normal that require heavy drilling mud to keep formation fluids from entering the borehole. Pressures lower than normal are called ''subnormal''.

By convention in the petroleum industry, ''overpressure'' refers to pressures higher than normal that require heavy drilling mud to keep formation fluids from entering the borehole. Pressures lower than normal are called ''subnormal''.

====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, 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&deg;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, 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&deg;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.

A rising level of mud in the tanks indicates that more mud is coming out of the hole than is going in. This is called a “kick.” This happens because formation fluids are entering the hole and the well is threatening to blow out. The situation is extremely serious, and proper steps must be taken to get the gas, oil, or water out of the hole. The most common method is to close the blowout preventers and stop the pumps. After a few minutes, the pressure at the top of the drill pipe will equal the pressure in the formation minus the weight of the column of mud. This is the excess pressure that must be balanced by increasing the mud weight. The pumps are then started to circulate the extraneous fluid out of the hole. The drill pipe pressure is carefully controlled with the choke. If the equilibrium drill pipe pressure is exceeded, the well may lose circulation, and if it is too low, the well will blow out.

A rising level of mud in the tanks indicates that more mud is coming out of the hole than is going in. This is called a “kick.” This happens because formation fluids are entering the hole and the well is threatening to blow out. The situation is extremely serious, and proper steps must be taken to get the gas, oil, or water out of the hole. The most common method is to close the blowout preventers and stop the pumps. After a few minutes, the pressure at the top of the drill pipe will equal the pressure in the formation minus the weight of the column of mud. This is the excess pressure that must be balanced by increasing the mud weight. The pumps are then started to circulate the extraneous fluid out of the hole. The drill pipe pressure is carefully controlled with the choke. If the equilibrium drill pipe pressure is exceeded, the well may lose circulation, and if it is too low, the well will blow out.

[[file:pressure-detection_fig3.png|thumb|left|{{figure number|3}}Electric logs of two wells offshore Louisiana. Well A had normal pressure. Well B, 2000 ft away and across a growth fault, showed a sudden decrease in resistivity of shale (increase in conductivity) at about 11,100 ft. Shortly thereafter, the well showed indications of an impending blowout. (After Wallace, 1965.)]]

[[file:pressure-detection_fig3.png|thumb|left|{{figure number|3}}Electric logs of two wells offshore Louisiana. Well A had normal pressure. Well B, 2000 ft away and across a growth fault, showed a sudden decrease in resistivity of shale (increase in conductivity) at about 11,100 ft. Shortly thereafter, the well showed indications of an impending blowout. (After <ref name=Wallace_1965>Wallace, W. E., 1965, Abnormal subsurface pressures measured from conductivity or resistivity logs: The Log Analyst, v. 6, p. 26-38.</ref>.)]]

===Delayed indications===

===Delayed indications===

Undercompacted shales associated with overpressured zones have a much lower electrical resistivity than normally compacted shales ([[:file:pressure-detection_fig3.png|Figure 3]]). According to the Archie formula, doubling the porosity of a shale from 10 to 20% should cause its resistivity to drop to one-fourth. As a result, it is possible to determine accurately the degree of undercompaction of a shale from its resistivity and to estimate the pore pressure ([[:file:pressure-detection_fig4.png|Figure 4]]) [[(Hottman and Johnson,1965)]]{{Citation needed}}.

Undercompacted shales associated with overpressured zones have a much lower electrical resistivity than normally compacted shales ([[:file:pressure-detection_fig3.png|Figure 3]]). According to the Archie formula, doubling the porosity of a shale from 10 to 20% should cause its resistivity to drop to one-fourth. As a result, it is possible to determine accurately the degree of undercompaction of a shale from its resistivity and to estimate the pore pressure ([[:file:pressure-detection_fig4.png|Figure 4]]) [[(Hottman and Johnson,1965)]]{{Citation needed}}.

Because undercompacted shale has slow seismic velocity and low density, a high pressure zone can also be identified from sonic and density logs<ref name=pt03r33>Magara, K., 1978, Compaction and fluid migration: New York, Elsevier Scientific Publishing Company, 319 p.</ref>.

Because undercompacted shale has slow seismic velocity and low density, a high pressure zone can also be identified from sonic and density logs.<ref name=pt03r33>Magara, K., 1978, Compaction and fluid migration: New York, Elsevier Scientific Publishing Company, 319 p.</ref>

Note that as is always the case with well logs, there are pitfalls in interpretation, and the local geology and hole conditions must be taken into account.

Note that as is always the case with well logs, there are pitfalls in interpretation, and the local geology and hole conditions must be taken into account.

==Subnormally pressured reservoirs==

==Subnormally pressured reservoirs==

Subnormal reservoir pressures, that is, pressures less than 0.43 psi per ft of depth, are very common<ref name=pt03r12>Dickey, P. A., Cox, W. C., 1977, Oil and gas reservoirs with subnormal pressures: AAPG Bulletin, v. 61, n. 12, p. 2134–2142.</ref>. The cause of abnormally low reservoir pressures is not well understood. If a reservoir containing either gas or oil is isolated and then subjected to uplift and erosion, the removal of overburden causes an elastic rebound of the solids and an increase in volume of the pores. The elastic dilation of sandstones is about 7 × 10^–6 volumes per psi. Water expands only 3 × 10^–6 volumes per psi, so that the pressure of the pore water in the aquifer and the enclosing shales will drop, possibly sucking some of the water out of the aquifer. Most low pressure reservoirs are in areas where there has been uplift and erosion since the sediments forming the reservoir were deposited and lithified.

Subnormal reservoir pressures, that is, pressures less than 0.43 psi per ft of depth, are very common.<ref name=pt03r12>Dickey, P. A., Cox, W. C., 1977, Oil and gas reservoirs with subnormal pressures: AAPG Bulletin, v. 61, n. 12, p. 2134–2142.</ref> The cause of abnormally low reservoir pressures is not well understood. If a reservoir containing either gas or oil is isolated and then subjected to uplift and erosion, the removal of overburden causes an elastic rebound of the solids and an increase in volume of the pores. The elastic dilation of sandstones is about 7 × 10^–6 volumes per psi. Water expands only 3 × 10^–6 volumes per psi, so that the pressure of the pore water in the aquifer and the enclosing shales will drop, possibly sucking some of the water out of the aquifer. Most low pressure reservoirs are in areas where there has been uplift and erosion since the sediments forming the reservoir were deposited and lithified.

===Drilling problems with subnormally pressured reservoirs===

===Drilling problems with subnormally pressured reservoirs===