Changes

==Overpressured reservoirs==

==Overpressured reservoirs==

===[[Drilling problems]] with overpressured reservoirs===

===[[Drilling problems]] with overpressured reservoirs===

As long as there is a silty or sandy bed within a few feet of the shales, the shales continue on a normal compaction trend. However, if no sandy beds are present, the water remains in the shale pores. As additional overburden is deposited, the shale then has to sustain all or part of the additional weight. This results in high pressure in the shale pore water. If there is a small, isolated sand body enclosed by the shale, whatever fluid it contains (water, oil, or gas), will share the same pressure. The fact that overpressures have been maintained for hundreds of millions of years over small vertical intervals indicates that the permeability of the enclosing shales can be virtually zero.

As long as there is a silty or sandy bed within a few feet of the shales, the shales continue on a normal compaction trend. However, if no sandy beds are present, the water remains in the shale pores. As additional overburden is deposited, the shale then has to sustain all or part of the additional weight. This results in high pressure in the shale pore water. If there is a small, isolated sand body enclosed by the shale, whatever fluid it contains (water, oil, or gas), will share the same pressure. The fact that overpressures have been maintained for hundreds of millions of years over small vertical intervals indicates that the permeability of the enclosing shales can be virtually zero.

The distribution of reservoirs and overpressuring is strongly controlled by the depositional environment (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_fig1.png|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. (From <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_fig2.png|thumb|{{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, 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, 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 (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]]).

 

[[file:pressure-detection_fig2.png|thumb|{{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. (After <ref name=pt03r8>Barker, C., Horsfeld, B., 1982, Mechanical versus thermal cause of abnormally high pore pressures in shales— discussion: AAPG Bulletin, v. 66, n. 1, p. 99–100.</ref>.)]]

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.

===Delayed indications===

===Delayed indications===

* ''Shale density''—Undercompacted shales, characteristic of overpressured zones, have a lower density (because of abnormally high [[porosity]]) than normal shales at a given depth. The density of shale cuttings can be measured by several methods. Also, the shape of drill cuttings from undercompacted shales may be different than those from normally compacted shales.

* ''Shale density''—Undercompacted shales, characteristic of overpressured zones, have a lower density (because of abnormally high [[porosity]]) than normal shales at a given depth. The density of shale cuttings can be measured by several methods. Also, the shape of drill cuttings from undercompacted shales may be different than those from normally compacted shales.

* ''Temperature''—There may be an increase in the temperature of the mud returns. Although it has been widely claimed that the geothermal gradient is higher in overpressured shales because of their abnormally high porosity and lower thermal conductivity, a doubling of shale porosity from 10 to 20% should cause a decrease in conductivity of only about 1% (with a correspondingly small increase in geothermal gradient). Thus, the increase in temperature is probably due to faster drilling and increased cavings in undercompacted shales.

* ''Temperature''—There may be an increase in the temperature of the mud returns. Although it has been widely claimed that the geothermal gradient is higher in overpressured shales because of their abnormally high porosity and lower thermal conductivity, a doubling of shale porosity from 10 to 20% should cause a decrease in conductivity of only about 1% (with a correspondingly small increase in geothermal gradient). Thus, the increase in temperature is probably due to faster drilling and increased cavings in undercompacted shales.

===Detection of overpressure with well logs===

===Detection of overpressure with well logs===

Undercompacted shales associated with overpressured zones have a much lower electrical resistivity than normally compacted shales (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 (Figure 4) (Hottman and Johnson,1965).

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}}.

 

[[file:pressure-detection_fig3.png|thumb|{{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_fig4.png|thumb|{{figure number|4}}Shale resistivity parameter, resistivity of normally pressured shale divided by observed resistivity of abnormally pressured shale, plotted against formation pressure gradient (FPG) and equivalent mud weight. (From <ref name=pt03r24>Hottman, C. E., Johnson, R. K., 1965, Estimation of formation pressures from log-derived shale properties: Journal of Petroleum Technology, v. 17, p. 717–723., 10., 2118/1110-PA</ref>; by permission of SPE.)]]

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>.