Resistivity measurements were made on six synthetic bead cores and one synthetic sand core at varying conditions of pressure and temperature. Properties of the cores were "controlled" to provide a reasonably constant porosity in conjunction with varying pore sizes. The influence of overburden pressure was evaluated up to 10,000 psi, and the influence of temperature separately studied up to 320øF. In this study emphasis was placed on the effect of pore size distribution (surface area) on the formation resistivity factor, at varying pressures and temperatures. Use of the synthetic cores made it possible to exclude the other parameters. For the cores in question it was shown conclusively that relative formation resistivity factor may be correlated versus pressure (at constant temperature) with relative surface area as the third parameter. Relative formation resistivity factor can be defined as the ratio of the formation factor at pressure P to the formation factor at atmospheric pressure. This analysis indicates, therefore, that an equation of the form FP f (surface area) F will provide a suitable approach for predicting the effect of pressure on the formation resistivity factor. A definite trend in the rate of change of the relative formation resistivity factor was also noticed as an apparent function of the matrix character. This supports the contention that rock resistivity changes under pressure should be separately evaluated for specific group types such as sandstones, carbonates, and shales. A relative or pseudo surface was used rather than pore size distribution inasmuch as it is more readily identifiable with those factors which are hypothesized to play a role in the reported behavioral patterns. These are:
The increase in the length of the mean free path for current flow (increased tortuosity) as pores or constrictions close completely.
The increase in the constriction factor due to a more rapid relative decrease in the smaller pores.
