Stress Sensitivity of Mercury-Injection Measurements

Many petrophysical properties of tight rocks, such as permeability and electrical resistivity, are very stress sensitive. However, most mercury-injection measurements are made using an instrument that does not apply a confining pressure to the samples. Here we further explore the implications of the use and analysis of data from mercury-injection porosimetry or mercury-injection capillary pressure measurements (MICP). Two particular aspects will be discussed. First, the effective stress acting on samples analyzed using standard MICP instruments is described. Second, results are presented from a new mercury-injection porosimeter that is capable of injecting mercury at up to 60,000 psi into 1- or 1.5-in. core plugs while keeping a constant net stress up to 15,000 psi. This new instrument allows monitoring of the electrical conductivity across the core during the test so that an accurate threshold pressure can be determined.

Although no external confining pressure is applied (unconfined) when using the standard MICP instrument, this doesn't mean that the measurements can be considered as unstressed. Instead, the sample is under isostatic compression by the mercury until it enters the pore space of the sample. As an approximation, the stress that the mercury places on the sample is equal to its threshold pressure. Thus, the permeability calculated from standard MICP data is equivalent to that measured at its threshold pressure. Not all the samples have the same stress dependency, thus comparing measured permeabilities at a single stress with values calculated from standard MICP data, corresponding at different threshold pressures, can lead to erroneous correlations. Therefore, the estimation of permeabilities from standard MICP data can be flawed and uncertain unless the stress effect is included.

Results obtained from the new mercury-injection system porosimeter under net stress, are radically different from those obtained from standard MICP instruments, such as the Autopore IV. In particular, the measurements at reservoir conditions produce threshold pressures that are three times higher and pore-throat sizes that are one-third of those measured by the standard MICP instrument. The results clearly indicate that calculating capillary-height functions, sealing capacity etc. from the standard instrument can lead to large errors that can have significant impact on subsurface characterization.