A Novel Methodology for Mercury Intrusion Porosimetry Analysis, Data Reduction, Blank Correction and Interpretation for Shales

摘要

A workflow for data reduction, blank correction and interpretation for Mercury Intrusion Porosimetry (MIP) on crushed shale samples is reviewed. Due to their nanoscale pore throats, shales require high pressures (<1,000 – 10,000psi) to intrude into air-filled pores. At these pressures; sample, Mercury, and system compression and Mercury thermal expansion are large and significantly impact the observed Mercury intrusion volumes during the experiment. System and Mercury compression and Mercury thermal expansion can be corrected for with blank runs on a mineral with known compressibility that occupies the same volume as a sample. Apparent Mercury intrusion volumes associated with sample matrix and pore compression are corrected using the MIP-derived bulk compressibility, an assumed matrix compressibility, and an assumed or independently measured porosity. Prior to intrusion during MIP, sample pores and pore throats experience increasing effective stresses that reduce their size. This suggests that the pore-diameter limit of MIP is greater than that predicted by the Washburn equation for un- stressed samples. Once filled with Mercury, a sample pore experiences zero net effective stress and undergoes expansion which is not measured during MIP. The pore expansion volume can be calculated using the measured sample bulk compressibility and measured Mercury intrusion volume. We argue here that the difference in stress state between the intrusion (stressed) and extrusion (unstressed) cycles during MIP is primarily responsible for observed hysteresis effects. Pressure equilibration time and ink-bottle effects are also examined here and are shown to increase the magnitude of volume and pressure discrepancies associated with hysteresis. Finally, crushing of sample material is required to increase surface area and effectively clean water and petroleum from shale samples. The analysis of crushed material requires large volumes of Mercury in the low-pressure portion of the experiment (<1,000 – 10,000psi) to fill in between and conform to sample particles. Conformance on crushed material occurs in excess of 1,000psi and the volume of Mercury associated with conformance increases with decreasing sample size.