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Sealing efficiency of shales
Author(s) -
Luo Xiaorong,
Vasseur Guy
Publication year - 1997
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/j.1365-3121.1997.tb00005.x
Subject(s) - overpressure , geology , hydraulic conductivity , oil shale , compaction , geotechnical engineering , sedimentary rock , deformation (meteorology) , dissipation , petrology , soil science , geochemistry , thermodynamics , paleontology , oceanography , physics , soil water
The sealing efficiency of shale layers is studied through the hydraulic parameters which are required for sustaining overpressure during geologically significant periods. Assuming a 1D sedimentary complex composed of a sealing layer overlying a permeable one, we give approximate solutions for the dissipation of a given initial overpressure. The time constant for relaxation involves the thickness, hydraulic conductivity and specific storage of the seal and also those of the permeable layer. The values of the various hydraulic parameters are discussed. It is argued first that the specific storage corresponding to plastic deformation during burial compaction is larger than the one which would correspond to elastic deformation. When taking into account (i) plausible values for specific storage of the upper shale layer and (ii) the storage effect of the lower permeable layer, it is found that a shale layer of several tens of m with an hydraulic conductivity of the order of 10 ‐15 ms ‐1 maintains overpressure for 1 Myr. For such hydraulic parameter values, in the absence of on‐going pressuring forces, initial overpressures would decay with a time constant (corresponding to a decrease by a factor e ∼, 2.7) of 1 Myr. This is interpreted as supporting a dynamic origin for observed abnormal pressures.