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Sedimentary rocks during burial maintain hydrostatic fluid pressure within their pore space if the fluids within the sediment can escape as the sediment compacts. If the fluid can not escape, compaction is retarded, and the fluid pressure rises (the sediment becomes overpressured), ultimately approaching the pressure exerted by the overlying rocks and contained fluids. The actual fluid pressure existing in a permeable formation can be determined by standard pressure-bomb measurements. The determination of the fluid pressure in shales, with their low permeability, previously has been difficult or impossible.
The fluid pressure within the pore space of shales can be determined by using data obtained from both acoustic and resistivity logs. The method involves establishing relations between the common logarithm of shale transit time or shale resistivity, and depth for hydrostatic-pressured formations. On a plot of transit time vs. depth, a linear relationship generally is observed, whereas on a plot of resistivity vs. depth, a non-linear trend exists. Divergence of observed transit times or resistivity values from those obtained from established normal compaction trends under hydrostatic pressure conditions is a measure of the pre-fluid pressure in the shale and thus in adjacent isolated permeable formations. This relation has been established empirically with actual pressure measurements i adjacent permeable formations. The use of these data and this method permits the interpretation of fluid pressure from acoustic and resistivity measurements with an accuracy of approximately 0.04 psi per ft., or about 400 psi at 10,000 ft. The standard deviation for the resistivity method is 0.022 psi per ft. and for the acoustic method 0.020 psi per ft.
Knowledge of the first occurrence of overpressures, and indeed of the precise pressure-depth relation in a geologic province, enables improvements in drilling techniques, casing programs, completion methods, and reservoir evaluations.
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