During the production of reservoir fluids, the pore fluid pressure decreases as the overburden pressure remains constant Consequently, pore compression occurs as the disparity between external and internal pressure increases. The technique developed in this study has a different approach to the conventional method resulting in a procedure which is both highly efficient and cost effective. This is achieved by neglecting the cr, which is typically an order of ten less than the cb, thus permitting the almost routine determination of cp. A microcomputer controlled porosimeter has been utilized to carry out compressibility studies to obtain rapid reliable results in a highly cost effective manner. The compressibility characteristics of twelve samples, 2 inches in length and 1.5 inch in diameter, can be determined to a maximum frame stress of 10,000 psi in a matter of hours. The applied technique assumes that: (a) pore compressibility behavior depends only on the effective frame stress based on the theory of poroelasticity; (b) irreducible brine and rock matrix expansion due to reduction in pore pressure are negligible; (c) hydrostatic pore volume compressibility can be converted to uniaxial strain condition based on constant Poisson's ratio for all loading conditions.

Demonstrative studies were applied to core samples from limestone and sandstone reservoirs. A total of 714 carbonate plugs were tested at five different frame stresses between 500 and 3,500 psi. A further 32 sandstone samples were tested, primarily at four different pressures ranging from 500 psi to 3,500 psi. Of these six samples were tested at pressures from 4,000 psi to 9,500 psi at 500 psi increments. An exponential relationship in the form of:
cp=mP
Eq. (1)

was found to give excellent correlation with an R2> 97% for all cases. The slope of the exponential line allows one to determine the pore volume compressibility, porosity at stress, and relative porosity change of each core plug at any pressure corresponding to the relevant depth.

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