High-Temperature-Induced Pore System Evolution of Immature Shale with Different Total Organic Carbon Contents

Abstract

The pyrolysis process of source rock, especially organic-rich immature shale, is required for oil and gas extraction, during which the evolution of the pore structure system in the immature shale determines the heat conduction and fluid flow under the heating treatment. Although some sound achievements have been made regarding the pyrolysis of immature shale, the effect of the total organic carbon (TOC) content on the pore structure evolution of immature shale remains unclear. With respect to this issue, in this work, a series of N 2 adsorption/desorption and nuclear magnetic resonance (NMR) experiments were conducted, and fractal dimension theory was also introduced to analyze the pore structure evolution of immature shale subjected to heating treatment in a quantitative manner. The results indicate that the adsorption branch of the nitrogen adsorption–desorption isotherm can be divided into three stages. The pore structure of different TOC immature shales does not change significantly, and they are all slit-shaped. In addition, immature shale with a higher organic content has a higher hydrocarbon expulsion strength and a higher pore volume growth rate, which indicate that the pyrolysis of organic matter greatly affects the pore structure of immature shale during heating. This phenomenon shows that the pyrolysis of organic matter greatly influences the pore structure of immature shale during the heating process. The pores of immature shale in the study area have significant fractal characteristics, the fractal dimension is between 2.397 and 2.636, the pore space of the sample is extremely small, the pore structure is extremely complex, and the heterogeneity is strong.

Evolution of the Anisotropic Thermophysical Performance for Low-Maturity Oil Shales at an Elevated Temperature and Its Implications for Restoring Oil Development

“…During the heating processing, the heating rate was 5 °C/min to avoid the possible thermal-shock-induced damage of the sample, and each measuring temperature point remained 2 h for the sufficient thermal behavior of low-maturity oil shale, before the measurement operation. The reliability of this heating operation is supported by previous works. , …”

“…Therein, the high TOC content ensures a greater AC K value and indicates a stronger anisotropy of thermal conductivity of the orthogonal directions relative to shale bedding, while a lower TOC content triggers an inferior AC K value, a minor anisotropy of thermal conductivity. This is probably because of the different pore space evolution at an elevated temperature for samples with different TOC contents . Besides, the anisotropic phenomenon of K values also indicates that the heat can move faster through the bedding-parallel direction but slower through the bedding-perpendicular direction under all temperature conditions.…”

“…The inputs of density are ∼2.18, ∼2.11, and ∼2.14 g/cm 3 for the samples with high, middle, and low TOC contents, respectively; these inputs are measured at 25 °C. However, the heating operation enables the dehydration of clay minerals and the pyrolysis of organic matter, making the oil shale sample slightly lightened (usually less than ∼8%). ,, That is to say, the heating process tends to downgrade the density of oil shales at an elevated temperature, where, however, the real-time density in an elevated temperature environment can hardly be measured. Besides, the sample density may also be downgraded by the sample volume increase as a result of thermal expansion at high temperatures.…”