Pore Structure and Fractal Characteristics of Niutitang Shale from China

Abstract

A suite of shale samples from the Lower Cambrian Niutitang Formation in northwestern Hunan Province, China, were investigated to better understand the pore structure and fractal characteristics of marine shale. Organic geochemistry, mineralogy by X-ray diffraction, porosity, permeability, mercury intrusion and nitrogen adsorption and methane adsorption experiments were conducted for each sample. Fractal dimension D was obtained from the nitrogen adsorption data using the fractal Frenkel-Halsey-Hill (FHH) model. The relationships between total organic carbon (TOC) content, mineral compositions, pore structure parameters and fractal dimension are discussed, along with the contributions of fractal dimension to shale gas reservoir evaluation. Analysis of the results showed that Niutitang shale samples featured high TOC content (2.51% on average), high thermal maturity (3.0% on average), low permeability and complex pore structures, which are highly fractal. TOC content and mineral compositions are two major factors affecting pore structure but they have different impacts on the fractal dimension. Shale samples with higher TOC content had a larger specific surface area (SSA), pore volume (PV) and fractal dimension, which enhanced the heterogeneity of the pore structure. Quartz content had a relatively weak influence on shale pore structure, whereas SSA, PV and fractal dimension decreased with increasing clay mineral content. Shale with a higher clay content weakened pore structure heterogeneity. The permeability and Langmuir volume of methane adsorption were affected by fractal dimension. Shale samples with higher fractal dimension had higher adsorption capacity but lower permeability, which is favorable for shale gas adsorption but adverse to shale gas seepage and diffusion.

Experimental Investigation of Evolution of Pore Structure in Longmaxi Marine Shale Using an Anhydrous Pyrolysis Technique

“…There was also a reduction in shale porosity in the oil window as indicated by many previous studies [30,31], and the primary reason was pore blocking by liquid hydrocarbon [32]. Then, there was a rapid increase in pore content after the oil window due to OM cracking, which formed a large number of pores, and the OM pores were mostly micropores and fine mesopores [5,21]. Therefore, the increasing micropore SSA and PV below 600 °C may be attributed to further OM cracking.…”

“…Then six artificial shale samples (pyrolysis products) and the initial sample were tested using gas physisorption (N 2 and CO 2 ) and mercury intrusion to characterize the pore structure. The methods used in this study were the same as in our previous studies [5,21,22] which described in detail the experimental apparatus and process, and they are not discussed further in this paper.…”

“…In general, the fractal dimension values range between 2 and 3, and the larger the fractal dimension value, the more complex the pore structure [39,40]. In this study, fractal dimensions of micropores and mesopores were calculated using a Frenkel-Halsey-Hill (FHH) model based on the N2 adsorption isotherm [5], and the fractal dimensions of macropores were calculated using the model developed by Pfeifer and Avnir [41], which is based on mercury intrusion data [17,42]. The formulas for calculating the fractal dimensions of micropores, mesopores (equation 1), and macropores (equation 1) can be described as follows:…”

“…The marine shale from south China is characterized by large thickness, high total organic carbon (TOC) content, high brittle minerals content, and high thermal maturity [3,4]. High thermal maturity is a major feature of marine shale in south China (vitrinite reflectance (Ro) values typically range between 2.0 and 4.5%), which is much higher than shale from North America (Ro values are commonly 1-2.5%) [5]. Many studies have argued that high thermal maturity is a major risk for further exploration of marine shale gas in the Yangtze Platform of south China [6].…”

Pore Structure Characterization for a Continental Lacustrine Shale Parasequence Based on Fractal Theory

“…Among them, the FHH method has a wide application range, and convenient calculation. [39][40][41][42][43] When Avnir and Jaroniec 44 studied the adsorption of gas molecules on porous solids with uneven surfaces, the fractal dimension was introduced into the adsorption potential theory, and the gas adsorption model on the fractal surface (>2 nm) in the capillary condensation region was established, namely the FHH equation…”