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Laboratory Measurement of Mudrock on Porosity, Pore and Pore Throat Size, and Permeability: Learnings From Comparison of Techniques


Porosity, pore or pore throat size distribution, and permeability are three key petrophysical properties in mudrocks and whose measurements remain challenging. Although many techniques can be applied for measurement of these parameters for mudrocks, results from different techniques are often not consistent, leading to a lack of standard methods or procedures for such measurements. In this study, porosity was measured using helium pycnometer, nitrogen adsorption, and mercury intrusion capillary pressure (MICP). The latter two techniques were also used for measurement of pore and pore throat size distribution, respectively. Permeability was measured using a modified gas-expansion method (MGE) for plug samples and the traditional Gas Research Institute (GRI) method for crushed samples with different plug/particle sizes. Comparison of helium pycnometer and nitrogen adsorption porosity measurements shows that the values are consistent for samples with insignificant amounts of pores larger than 200 nm, but that nitrogen adsorption measurements underestimate porosity for samples having substantial larger than 200 nm pores. Conformance and compression are two important sources of error for MICP analysis that lead to overestimation of porosity. A new conformance and compression correction method were developed. A previously published method was also applied for conformance and intrusion corrections. MICP porosity values after conformance and compression corrections are consistent with that from helium pycnometer. Apparently inconsistent pore size distribution from nitrogen adsorption and pore throat size distribution from MICP produce similar results in pore volume. Combination of these two size distributions reveals interesting findings on pore size composition of the pore network that cannot be seen in individual pore or pore throat size distribution. Permeability values from plug samples were found several orders of magnitude larger than that from crushed-rock samples. Permeability from both plug and crushed-rock samples shows a scaling relation to plug/particle size. This scaling behavior can be caused by inevitable sample damages and artifacts formed during sample preparation. The inability to catch the pressure decay from higher permeable layers in GRI method can also contributed to the scaling behavior of permeability.