Impact of Degassing Time and Temperature on the Estimation of Pore Attributes in Shale

Abstract

The influence of degassing time and temperature on low-pressure gas adsorption (LPGA) behavior of shales was examined in this study. Two organic-rich shales of contrasting maturity, reactivity and organic matter type, were crushed to <1 mm and <212 μm grain-sizes and degassed at 110, 200, and 300 °C for 3 and 12 h, respectively. Our results indicate that degassing duration has a minimal influence on pore-character interpretations from LPGA experiments, while the degassing temperature shows a strong influence on the pore attributes. For both shales, reliable porosity estimates were obtained when the samples were degassed at 110 °C. When the degassing temperature was increased to 200 and further to 300 °C, distinct changes in adsorption isotherms and other pore structural features were observed. For the mesoporous low-mature shale (collected from a lignite mine) when the degassing temperature was kept at 200 °C, a macroporous character was induced with a manifold increase in pore diameter. Results from thermogravimetry and Rock-Eval indicate abundance of reactive kerogen, which undergoes alteration when degassed at higher temperatures. When the degassing temperature was kept at 300 °C, the organic matter underwent further alteration and showed an isotherm similar to the shales degassed at 110 °C. Similarly, for the oil-window mature shale sample, a transition towards macroporous structure was observed when the sample was degassed at 200 and 300 °C, compared to a mesoporous structure observed when degassed at 110 °C. The results from fractal dimensions also support the above inferences, indicating the presence of simpler structures at higher degassing temperatures. Reduction in pore volume (110–200 °C) and its further rise (200–300 °C) are also evident in the micropore domain, more distinctly in the oil window mature shale. Our results strongly indicate that degassing temperature should be kept at around 110 °C for reliable shale pore character estimation.

Perspectives of Gas Adsorption and Storage in Kerogens and Shales

“…Low-temperature subcritical physisorption studies are used routinely to characterize the porous structures of a wide range of materials. In the case of shales and kerogens, it is essential to ensure that representative sampling is undertaken for an appropriate particle size range and that degassing conditions are precisely controlled. − Most of the adsorption research for both sub- and supercritical adsorption has used samples, which have been degassed under a high vacuum at 383 K, so that all adsorbed water vapor and CO 2 were removed. ,,, This temperature was used to ensure the removal of adsorbed water without significant modification of the shale structure and gives a baseline for adsorption measurements for a dry sample.…”

“…32−35 Most of the adsorption research for both sub-and supercritical adsorption has used samples, which have been degassed under a high vacuum at 383 K, so that all adsorbed water vapor and CO 2 were removed. 12,13,34,36 This temperature was used to ensure the removal of adsorbed water without significant modification of the shale structure and gives a baseline for adsorption measurements for a dry sample.…”

Impact of Supercritical CO2 on Shale Reservoirs and Its Implication for CO2 Sequestration

“…Organic-hosted porosity is one of the most critical elements in successful shale plays. Different techniques viz., imaging (e.g., broad ion beam-scanning electron microscopy, and focused ion beam-scanning electron microscopy), gas adsorption (low pressure and high pressure gas adsorption techniques), mercury intrusion porosimetry, neutron scattering (e.g., ultrasmall/small angle neutron scattering), etc., are used to assess shale porosity. ,,,− In addition to several factors, such as the presence of surface functional groups, the mesopores and micropores, and the specific surface area, etc., thermal maturity levels have been identified to strongly control the development of organic porosity and, hence, the gas adsorption capacity of shales and carbonaceous matter. − Loucks et al observed organic nanopore development to be strongly dependent on the thermal maturity levels of the samples and inferred that this is essentially caused by the conversion of kerogen to hydrocarbons and the formation of pores within the organic matter in the process.…”

Water Distribution in Marine Shales: Based on Two-Dimensional Nuclear Magnetic Resonance and Low-Temperature Nitrogen Adsorption

“…Different pretreatment methods have a significant influence on the measured results. Different pretreatment methods have a significant influence on the measured results . Low-temperature nitrogen adsorption experimental process was carried out in accordance with GB/T 21650.2-2008 (determination of pore size distribution and porosity of solid materials by the mercury intrusion method and gas adsorption method) in this study.…”

“…Different pretreatment methods have a significant influence on the measured results. 53 Low-temperature nitrogen adsorption experimental process was carried out in accordance with GB/T 21650.2-2008 (determination of pore size distribution and porosity of solid materials by the mercury intrusion method and gas adsorption method) in this study. Before the test, the samples were dried (120 °C, 24 h) and then degassed ( Note: I is illite; S is smectite; I/S is illite/smectite interlayer; %S is interlayer ratio; C is chlorite.…”