Investigation of organic-shale nanopores in the Lower Cambrian Niutitang Formation using low temperature N2 and CO2 adsorption: Multifractality and classification

Abstract

Nanopores in organic shale reservoirs are of great significance for shale gas accumulation and carbon dioxide geological sequestration. To explore the multifractality and classification of nanopores in the Lower Cambrian Niutitang Formation shales, nine shale samples were selected from the well TX1 in North Guizhou, South China. Then, we conducted TOC analysis, XRD analysis and SEM to investigate shale material composition and nanopore morphology and low temperature N 2 and CO 2 adsorption experiments were used to quantitatively characterize the nanopore structure. The multifractal theory was applied to study the spatial distribution of nanopores. Results showed that nanopores in Niutitang Formation shales are mainly organic pores with pore sizes less than 50 nm, followed by intraparticle dissolution pores; interparticle pores are rarely observed. According to the results of the N 2 adsorption experiments, our samples can be divided into three types. Type I samples have the highest average pore size, whereas Type III samples have the highest pore volume and surface area. Results of the CO 2 adsorption experiments indicated a trimodal distribution of measured pores. Our study revealed the multifractality of nanopores, and the complexity of pores measured via N 2 adsorption is stronger than that measured via CO 2 adsorption. Results of multifractal analysis via N 2 adsorption demonstrated that Type I samples have the strongest pore agglomeration and Type III samples have the most uniform pore size distribution. For nanopores measured via CO 2 adsorption, subtle differences exist in multifractal characteristics of the various samples. An increase in the pore volume measured via N 2 adsorption corresponds to a decrease in pore aggregation and heterogeneity and an increase in pore connectivity. For nanopores detected via CO 2 adsorption, no obvious correlations were observed between the pore structure and multifractal parameters. We propose a new nanopore-classification scheme based on pore heterogeneity and connectivity. Nanopores of the selected samples can be classified as micropores (<10 nm), mesopores (10–25 nm), and macropores (>25 nm). Fractal properties of nanopores obtained from SEM images showed that mesopores have weaker heterogeneity and better connectivity than micropores and macropores, which also indicates the reasonability of our pore classification. Multifractal theory was applied to analyze the full pore size distribution obtained by CO 2 and N 2 adsorption experiments, then nanopores were divided into micropores, mesopores and macropores according to differences in heterogeneity and connectivity. • Multifractality of nanopores detected via low temperature N 2 and CO 2 adsorption. • The complexity of the nanopore systems measured via N 2 adsorption is stronger than that measured via CO 2 adsorption. • For nanopores detected via N 2 adsorption, obvious correlations exist between pore volume and multifractal parameters. • Nanopores of Niutitang shales can be divided into micropores (< 10 nm), mesopores (10 – 25 nm), and macropores (> 25 nm).