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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
Volume:
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Title: Pressure
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Abstract:
Abnormal formation pressure
requires a seal; without a seal pressures would equalize to normal hydrostatic. Abnormal pressures originate from several interrelated processes, but temperature change appears to be the principal cause.
Both epeirogenic movements with associated erosion and deposition and long-term changes in climate can alter the temperature of a sealed formation at depth. Abnormal pressure
resulting from temperature change caused by change of overburden thickness must be corrected for inherited
pressure
and change of hydrostatic
pressure
related to elevation difference. Osmosis, precipitation, or solution by trapped pore fluid and carbonization effects are minor in comparison with temperature effects.
Overburden stress cannot cause abnormally high pressure
at present drilling depths. The loss of porosity with depth in all sedimentary rocks appears to be a chemical process rather than mechanical compression.
Pressure
differentials between wells may indicate ambiguously either no fluid flow (wherein the
pressure
difference is maintained by a seal) or flow (wherein the
pressure
drop is from fluid friction in the permeable medium). A relatively small amount of flow across a seal can equalize pressures, retarding further flow. The fluid expelled as a result of loss of porosity during geologic time also flows at a low rate.
The geologist concerned with pressure
problems must be aware of (1) the many variables involved in subsurface pressures, (2) the low number and ambiguity of
pressure
measurements, (3) the need to establish what constitutes "normal"
pressure
to determine abnormal
pressure
, and (4) the possibility of uniqueness in any field situation.
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