Responses of specific surface area and micro- and mesopore characteristics of shale and coal to heating at elevated hydrostatic and lithostatic pressures

Abstract

Samples of the low-maturity New Albany Shale (Middle and Upper Devonian to Lower Mississippian) and Mowry Shale (Late Cretaceous), both containing kerogen Type II, and samples of Wilcox Coal (Eocene), containing kerogen Type III, were heated to 60, 100, and 200 °C at hydrostatic ambient pressure, 100, or 300 MPa for 6 or 12 months in sealed glass and gold cells to investigate temperature and pressure effects on porosity and thermal maturity. In addition, lithostatic experiments were conducted in a hydraulic press at 100 MPa and 100 °C over a period of 6 months. Porosimetric characteristics of samples before and after experiments were investigated by using low-pressure gas adsorption and scanning electron microscope (SEM).An increase from ambient temperature to 200 °C caused increases in random vitrinite reflectance (Ro) for all samples, with Mowry Shale showing the largest increase from 0.57% to 0.65% and Wilcox Coal showing the smallest increase from 0.39% to 0.41%. For Mowry Shale and New Albany Shale, specific surface areas did not change in any notable way with an increase in temperature; specific surface area values for Mowry Shale ranged from 2.0 to 3.2 m2/g, and for New Albany Shale from 13.7 to 15.6 m2/g. Differences in Barrett-Joyner-Halenda (BJH) specific mesopore volumes and average mesopore size for the shales were also small to negligible. Considering the values of the original samples, we propose that these small differences are related to internal inhomogeneity of samples rather than to any temperature effect. Temperature-related changes in Wilcox Coal were more distinct. Specifically, there was a marked decrease in BET surface area, from 4.9 m2/g at 60 °C to 1.5 m2/g at 200 °C, and a decrease in both BJH mesopore volume and average mesopore size. The Wilcox Coal sample had large micropore surface areas (110–148 m2/g) compared to both shales, which had micropore surface areas below 10 m2/g. While Wilcox Coal showed a drop in micropore volume between 60 °C and 200 °C, no distinct or regular changes in micropore volume with temperature were documented for the other two samples.A sustained hydrostatic pressure increase from ambient to 300 MPa for 6 to 12 months resulted in insignificant changes in vitrinite reflectance values. Small differences in Brunauer-Emmett-Teller (BET) specific surface areas, micropore surface area, and volume may be related to internal sample heterogeneity rather than pressure treatment. Similar to the temperature effect, the Wilcox Coal sample experienced more pronounced changes compared to the shales. SEM observations on shales did not reveal porosity-related changes between the original and treated samples. No marked changes were documented for lithostatic pressure conditions at 100 MPa and 100 °C. We conclude that elevated isotropic hydrostatic or lithostatic pressure is unable to significantly affect the pore structure and pore-size distribution of shales, but it can make some modifications in the micropore and mesopore pore characteristics of low-rank coal.