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

Abstract

Fractal dimension is an important parameter in the evaluation of tight reservoirs. For an outcrop section of the Nenjiang formation in the Songliao Basin, China, the pore structure and pore fractal characteristics of shale parasequences were investigated using fractal theory. In addition, factors causing pore structure changes were analyzed using the results of low-temperature nitrogen adsorption and scanning electron microscope (SEM) experiments. Conducive to gas migration and secondary pores development such as dissolution, results showed that nanoscale pores dominated by fracture-like morphology and consequent good internal connectivity were observed in each pore size section within the target layer. Each parasequence is characterized by a sequential upward decrease of average pore size and an upward increase of total pore volume, with an increasing number of pores from 2[Formula: see text]nm to 50[Formula: see text]nm. Pores are isolated from each other, with poor connectivity and relatively complex composition of brittle minerals and clay minerals. Main components of the brittle minerals, quartz and feldspar, occur in 20–50% and higher clay mineral content ranging from 50% to 70%. In the parasequence cycle, clay mineral gradually decreases while the brittle mineral content increases. Fractal dimension is negatively correlated with clay mineral content and positively correlated with brittle mineral (quartz and feldspar) content. The fractal dimension calculated by the imaging method and the FHH method shows an upward increasing tendency in each of the parasequence cycles. This is as a result of different phenomena, varied sediment hydrodynamic forces leading to particle size differences and increased brittle minerals resulting in microcracks, therefore, the fractal dimension of the large pores (imaging method) increases upward in the parasequence. Simultaneously, with increased content and accompanied dissolution of brittle minerals causing an increase of small pores from base to top of the parasequence, the fractal dimension of the small pores (FHH method) grows.

Coupling Effect of Temperature, Gas, and Viscoelastic Surfactant Fracturing Fluid on the Microstructure and its Fractal Characteristics of Deep Coal

“…For the quantitative analysis of the surface fracture morphology of coal samples, fractal characterization based on the box dimension of SEM images is considered a reliable technique. , The specific procedure is as follows: At first, the SEM images are binarized. Subsequently, the images are covered with boxes of the length δ.…”

A Fractal Approach for Predicting Unsaturated Hydraulic Conductivity of Deformable Clay

“…Since Mandelbrot and Wheeler [27] proposed the concept of fractal geometry, the fractal theory has been widely used in the analysis of physical geometry properties as a mathematical method [28][29][30][31]. The fractal theory relates the unsaturated hydraulic conductivity to characterization of soil structure [32,33], which provides a powerful tool to indirectly predict the hydraulic conductivity. Compared with the method of sectional calculation and integral calculation proposed by Fredlund et al [17] and Agus et al [18], the fractal model can simplify the complicated calculation procedure about relative hydraulic conductivity.…”

Adsorption of Methane, Nitrogen, and Carbon Dioxide in Atomic-Scale Fractal Nanopores by Monte Carlo Simulation I: Single-Component Adsorption

“…(1) Atomic-scale geometrically heterogeneous surfaces: statistical self-similarity, a typical geometric feature of surface heterogeneity that can hardly be described by Euclidean geometry, is a ubiquitous characteristic of most materials in nature . Thanks to the fractal theory advanced by Mandelbrot, , the surface fractal dimension functions as an applicable measure of surface geometric heterogeneity. − The fractal pore morphology has been examined by mercury intrusion porosimetry (MIP), − the fractal Frenkel–Halsey–Hill (FHH) adsorption isotherm equation based on nitrogen adsorption, − ,− nuclear magnetic resonance, ,, small-angle and ultrasmall-angle neutron scattering, ,− ...…”

“…These findings indicate marked surface geometric heterogeneity at the atomic scale. (2) Slitlike shape: the pore shape can be determined from the shape of the hysteresis loop of the nitrogen adsorption and desorption isotherms (see refs − , , , , − , − , − , − , − , , , − , − , , , , ) according to the classification by the International Union of Pure and Applied Chemistry (IUPAC) or by the field emission scanning electron microscope (see refs , , , − , , , , , , − , , , , , , , , , , , , , ) and MIP. ,,, The summary statistics reveal that slit pores constitute the largest...…”

“…(3) Nanoscale average diameter: IUPAC refreshed the pore classification by size, defining nanopores as the pores with diameters to an upper limit of about 100 nm . Usually, the average pore diameter ( d ) is calculated from the cylindrical pore model ( d = 4 v / a ) with the specific pore surface area ( a ) and the specific pore volume ( v ) obtained in carbon dioxide or nitrogen adsorption experiments (see refs − , − , − , , , − , − , , , − , , , , ). The distribution indicates that the overwhelming majority of the average diameters lies in the range of 0–50 nm (Figure c), which conforms to the definition of nanopores.…”