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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''.

==Overpressured reservoirs==

==Overpressured reservoirs==

[[file:pressure-detection_fig1.png|300px|thumb|{{figure number|1}}Schematic diagram showing the location of abnormal pressures in southern Louisiana. The continental and deltaic facies contains sandy beds. The nerltic (nearshore) facles contains a few silty and sandy beds that connect laterally to the deltaic facies. The outer shelf facies contains almost no sandy beds, and the pore fluids cannot escape. The growth faults are seals that stop the lateral flow of pore water toward the neritic facies.<ref name=pt03r13>Dickey, P. A., Shriram, C. R., Paine, W. R., 1968, Abnormal pressures in deep wells of southwestern Louisiana: Science, May 10, v. 160, p. 609–615., 10., 1126/science., 160., 3828., 609</ref>]]

[[file:pressure-detection_fig1.png|300px|thumb|{{figure number|1}}Schematic diagram showing the location of abnormal pressures in southern Louisiana. The continental and deltaic facies contains sandy beds. The nerltic (nearshore) facles contains a few silty and sandy beds that connect laterally to the deltaic facies. The outer shelf facies contains almost no sandy beds, and the pore fluids cannot escape. The growth faults are seals that stop the lateral flow of pore water toward the neritic facies.<ref name=pt03r13>Dickey, P. A., C. R. Shriram, and W. R. Paine, 1968, Abnormal pressures in deep wells of southwestern Louisiana: Science, May 10, v. 160, p. 609–615., 10., 1126/science., 160., 3828., 609</ref>]]

===Drilling problems with overpressured reservoirs===

===Drilling problems with 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., and B. Horsfeld, 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&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, [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&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.

====Tectonic phenomena====

====Tectonic phenomena====

Tectonic phenomena also produce overpressures. In the Gulf of Alaska, fluid pore pressures up to 0.85 psi per ft were found due to horizontal compressive stress in the rocks. In Western Alberta, large thicknesses of Paleozoic carbonates have been thrust over soft Cretaceous shales, resulting in overpressuring of the lenticular oil-bearing sandstones that extend under the overthrust (such as Leafland and Pembina).

Tectonic phenomena also produce overpressures. In the Gulf of Alaska, fluid pore pressures up to 0.85 psi per ft were found due to horizontal compressive stress in the rocks. In Western Alberta, large thicknesses of Paleozoic carbonates have been thrust over soft Cretaceous shales, resulting in overpressuring of the lenticular oil-bearing sandstones that extend under the [[overthrust]] (such as Leafland and Pembina).

====Thermal cracking of organic matter====

====Thermal cracking of organic matter====

Much less attention has been paid to subnormally pressured reservoirs than to overpressured reservoirs. This is probably because there are fewer spectacular [[drilling problems]] associated with subnormal pressures and underpressures. However, problems exist that can be serious.

Much less attention has been paid to subnormally pressured reservoirs than to overpressured reservoirs. This is probably because there are fewer spectacular [[drilling problems]] associated with subnormal pressures and underpressures. However, problems exist that can be serious.

If the reservoir pressure is much lower than the pressure in the [[drilling fluid]], severe formation damage can occur. The drilling mud filtrate penetrates the reservoir, causing swelling and migration of clays, which may plug the pore throats. Even a little water in the hole can kill a low pressure producing gas well. The water is drawn into the pores by capillarity and ruins the [[relative permeability]] to gas. In the case of low pressure gas sandstone reservoirs, it is desirable to set casing at the top of the reservoir interval and drill with gas, salt water, or oil-based mud to minimize formation damage.

If the reservoir pressure is much lower than the pressure in the [[drilling fluid]], severe formation damage can occur. The drilling mud filtrate penetrates the reservoir, causing swelling and [[hydrocarbon migration|migration]] of clays, which may plug the pore throats. Even a little water in the hole can kill a low pressure producing gas well. The water is drawn into the pores by capillarity and ruins the [[relative permeability]] to gas. In the case of low pressure gas sandstone reservoirs, it is desirable to set casing at the top of the reservoir interval and drill with gas, salt water, or oil-based mud to minimize formation damage.

Also, if the gas reservoir has a low pressure, there may be no indication of gas on the mudlog. The logs of many abandoned [[dry hole]]s should be reexamined to look for bypassed gas zones.

Also, if the gas reservoir has a low pressure, there may be no indication of gas on the mudlog. The logs of many abandoned [[dry hole]]s should be reexamined to look for bypassed gas zones.

[[Category:Wellsite methods]]

[[Category:Wellsite methods]]