A Novel Experimental Technique for Investigating Pore Pressure Effect on Mechanical and Acoustic Properties in Sands and Shales

ABSTRACT: Pore pressure causes changes in reservoir properties primarily through effective stress. As fluid pressure drops, effective confining pressure increases resulting porosity and permeability reductions with both elastic (recoverable) and anelastic (non-recoverable) components. If the pore pressure reduction is large enough, the accompanying increase in effective stress may cause a permanent loss of strength, and may result in plastic deformation. The magnitudes of these changes as well as deformation behavior of unconsolidated shaly sands and shales have been experimentally investigated in this study. Pore pressure cycling measurements were conducted to obtain static and dynamic moduli under true in situ as well as abnormal and depleted reservoir pressure conditions. The measurement technique discussed here provides a continuous measurement of the state of the rock in addition to contributing calibration of log and seismic velocities and effective stress. This is essential for accurate pore pressure prediction from methods that use log or seismic data as input. Data from a single core measurement may be sufficient to determine the variations in the mechanical and acoustic properties of the rock in lifetime of the reservoir.