Preliminary study of pore structure and methane sorption capacity of the Lower Cambrian shales from the north Gui-zhou Province

Abstract

The organic carbon content (TOC), mineral composition, specific surface area, porosity, pore volume and methane sorption capacity (MSC) of the Lower Cambrian Niu-ti-tang and Pa-lang shales from north Gui-zhou Province were investigated, aiming to characterize the pore structure and its controlling factors and the sorption capacity of these shales. The results indicate that these shales are characterized by low porosity and permeability, with a BET specific surface area ranging from 5.64 to 28.29 m2/g and micropore and mesopore volumes ranging from 0.02 to 0.54 cm3/100 g and 0.53–3.38 cm3/100 g, respectively. The positive correlations between porosity, specific surface area, micropore volume and TOC content suggest that organic matter is a primary factor in the pore structure development. However, the shales with TOC content of above 5% display a much low micropore volume and specific surface area. The Lower Cambrian shales display an excess MSC at 12 MPa (nex12MPa) and Langmuir MSC (nL) ranging from 0.30 cm3/g to 3.71 cm3/g and 0.41 cm3/g to 4.22 cm3/g, respectively. Both of the MSCs exhibit a positive correlation with TOC but no correlation among these shales with TOC content above 5% due to the lesser degree of organic pore development. In addition, the sorption capacity of these shales displays a fan-shaped positive correlation with the BET specific surface area and suggests that the higher BET specific surface area does not always result in high MSC in shales. Finally, the Pa-lang shales display a better pore property and higher MSC than that of the Niu-ti-tang shales and appear to be superior prospective shale gas reservoirs.