Nanopore Structure of Lower Cambrian Niutitang Shale in South of China: Insights from Gas Adsorption, MICP and FE-SEM Analyses

Abstract

Various and multidisciplinary techniques, both qualitative and quantitative, have been developed to characterize pore structure in black organic-rich shales for understanding the mechanism of shale gas accumulation. Low pressure gas (CO2 and N2) adsorption experiments and Mercury Intrusion Capillary Porosimetry (MICP) were conducted to quantitatively determine pore size distribution (PSD), pore volume and surface area in the Lower Cambrian Niutitang Formation in TM1 and TX1 wells, which were drilled in the Cen’gong block, South of China. CO2 adsorption isotherms are both ascribed to type I, indicative of microporous materials. Surface areas and pore volumes of micropore vary from 8.23 to 37.39m2/g and from 3.06 to 10.89×10-3cm3/g, with an average of 20.02m2/g and 6.11×10-3cm3/g, respectively. N2 adsorption showed reversed S-shaped isotherms, which are classified into type H2 and H3 hysteresis loops, indicating inkbottle pores and slit-or plate-like pore shape. BET surface areas and total pore volumes are in the range of 4.5~21.91m2/g and 12.18×10-3~28.81×10-3cm3/g with an average of 14.76m2/g and 21.50×10-3cm3/g, respectively. Subsequently MICP can provide quantitative and reproducible pore information from 3nm to 36μm, the measured-porosity vary from 0.52 to 2.14% and the measured-porosity of TM1 well are smaller than TX1 well. And finally high resolution FIB-SEM analysis is applied to investigate the nano-scale pore structure of Niutitang shale and suggesting that organic matter pores (OMP) are more popular in samples with TOC content higher than 2.0% with a diameter less 50nm, but sample TM-4 with the highest TOC of 10.01% has little OMP, while sample TX-3 with the TOC of 7.51% has amounts of OMP. The abundances of OMP in TM1 well are smaller than TX1 well in Cen’gong block. Pore volume, and surface area from CO2 adsorption increases with respect to TOC, indicating that TOC content is mainly associated with micropores, but BET surface area, micropore and mesopore volume obtained from N2 adsorption increases with respect to TOC before TOC contents are less than 7.51%, above a TOC value of 7.51%, the surface areas and micropore volumes are diminished in the higher TOC content, whereas macropore volume decreases with respect to TOC content. All results show that pore structure characteristics of TX1 well are better than TM1 well and the Niutitang shale demonstrates the greatest potential for shale gas production in South of China.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018