Effect of Pore Structure on Methane Sorption Capacity of Shales

Abstract

Abstract

50 marine shale samples of Lower Silurian Longmaxi Formation from 4 wells around the Sichuan Basin are chosen for examining total organic carbon(TOC), thermal maturity(Ro), XRD inflection, N2 and CO2 isothermal adsorption and CH4 high pressure adsorption (30°, 0∼15MPa, dry basis), in order to study the effect of pore structure on methane sorption capacity of shales. The relationships among methane sorption, pore structure and other internal factors(TOC, Ro, shale mineralogy) are investigated while the external factors(temperature, pressure, water) are invariable and the type of the kerogen is I₂∼II₁ type. Some conclusions can be deduced.(1)The internal influence factors can indicate the development of nanopores. High TOC and appropriate Ro(<3%) promote the development of micropores(<2nm), TOC is the main factor. The BET surface area and BJH pore volume for mesopores(2∼50nm) and macropores(>50nm) show a general increase with the increase of the brittle mineral content. The BET surface area and BJH pore volume increase first, then decrease with the increase of the clay mineral content, reaching the maximum value when the the content of the clay minerals is 45%∼50%. The relations of geochemical dates and mineral composition with the heterogeneous distributions of nanopores are obviously different for different formations in different areas.(2)The TOC, Ro and the content of clay minerals have a positive correlation with the methane sorption. The multi-controlling factors of the effect on methane sorption are obvious while the main controlling factor is the TOC content.(3)Methane is adsorbed in nanopores for various aperture in supercritical state, for the result of monolayer adsorption and micropore filling. The BET surface area and methane sorption content have the best correlation. The contribution of methane adsorbing on the surface of mesopores(40∼50nm) and macropores(>50nm) can't be negligible in high pressure(>5MPa). The ink-bottle shape pores(type H2) are better than wedge shape and V shape pores of flaky particles(type H3) for the methane sorption(IUPAC). The microporous slit pores have the best adsorption capacity of methane. The fractal dimension D for adsorption pores(<7.5nm) has a very good correlation of the methane sorption content.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016