Method for determining sedimentary rock pore pressure caused by effective stress unloading

Abstract

An improved technique to more accurately calculate pore pressure of sedimentary rock due to subsurface fluid expansion where the technique is built upon a combination of known force balanced in situ loading limb mineralogical stress/strain relationships with locally variable unloading stress/strain relationships, including that in stress/strain space, the material properties governed loading limb is an upper limit for the many possible unloading limbs; also, a method for relating these different natural stress/strain paths and applying the correct path to calculate pore fluid pressure from in situ force balance is disclosed such that the method is preferably calibrated with in situ stress/strain data which allows for a lithologic sealing caprock to be identified and the locally prevailing in situ unloading limb stress/strain relationship to be estimated, where the forced balanced loading and unloading calibrations are applied to more accurately determine well casing depths using either wireline or real-time "measured while drilling" petrophysical data; and also solidity (1.0--Porosity) is the in situ parameter of choice which can be measured petrophysically in the borehole, where pore pressure is the fraction of the total external load which is borne by the fluids in the pore space of a sedimentary rock, and the solid framework of a granular sedimentary rock bears the force balance remainder of the external confining load as effective stress; so that loading and unloading power law linear stress/strain relationships are determined between effective stress and solidity for common sedimentary rocks.

Dependencies are inferred from the claim text.