Changes

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

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

 

[[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>.)]]

pressure-detection_fig3.png|{{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.<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>

pressure-detection_fig4.png|{{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.<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

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 equation|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 equation|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}}.