Pore Evolution of Lacustrine Organic-Rich Shales: Insights from Thermal Simulation Experiments

Abstract

The pore structure characteristics of shale reservoirs are an intense research topic in unconventional oil and gas exploration and development. To explore the evolution of the shale pore structure at different maturity stages, the immature organic-rich shale (Ro = 0.27%, TOC = 26.27%) of the Paleogene Huadian Formation is selected for a series of thermal simulation experiments in a closed system from the immature to overmature stages. The N2/CO2 adsorption experiment and the Frenkel–Halsey–Hill (FHH) model are used to characterize the pore development and fractal dimensions of the thermal simulation samples before and after Soxhlet extraction. The results show that most of the oil-generating stage occurs between 300 and 400 °C. With an increase in temperature, abundant liquid hydrocarbons will undergo secondary cracking to generate wet gas. The thermal simulation samples mainly develop wedge-shaped or slit-shaped pores. The pore size distribution mainly exists within the bimodal distributions 0.5–0.8 and 30–100 nm. The residue of liquid hydrocarbons can block some mesopores and macropores, which is shown by comparing the pore volume before and after Soxhlet extraction. Moreover, the pore structure of the samples is more complex than the pore surface during the hydrocarbon evolutionary process. The pore evolution of shale can be divided into three stages: the complex development stage of pores in the low-maturity stage, the massive development stage of pores in the middle maturity to high-maturity stage, and the uniform development stage of pores in the overmature stage. Additionally, during the entire thermal evolution of shale, residual oil and diagenesis are the main factors affecting the differential development of pores and the variation in the fractal dimension.

Study on Pore Structure and Fractal Characterization during Thermal Evolution of Oil Shale Experiments

“…This stage corresponds to the capillary condensation stage, indicating that the thermal simulation sample contains a certain amount of medium and large holes. 7 , 40 The N 2 adsorption capacity of thermal simulation samples ranged from 1.11 to 6.81 cm 3 /g (average 2.76 cm 3 /g).…”

“…The two most commonly used calculation methods are “ D = A + 3” and “ D = 3 A + 3”. At present, most mathematicians first use two calculation methods to calculate the fractal dimensions, 7 , 32 − 34 and then the fractal dimension satisfying the definition of fractal dimension of pore surface and pore structure is selected. 43 , 44 The relative pressure P / P 0 has two different linear segments in the intervals of 0–0.5 and 0.5–1, and the correlation coefficient ( R 2 ) of the two segments is greater than 0.969, indicating that there are obviously different FHH fractal characteristics in the two intervals ( Figure 7 ).…”

“…After the principle of complementary temperature and time in oil and gas generation was proposed, 2 , 3 many domestic and foreign scholars have performed relevant hydrocarbon generation simulation research. 4 − 7 According to the closure of the system, the thermal simulation system is divided into an open system, a closed system, and a semiopen system. 8 , 9 In recent years, scholars have simulated the evolution of pores during the maturation of organic-rich shale in different thermal simulation conditions 7 , 10 − 12 and have drawn some valuable experimental conclusions.…”

“… 4 − 7 According to the closure of the system, the thermal simulation system is divided into an open system, a closed system, and a semiopen system. 8 , 9 In recent years, scholars have simulated the evolution of pores during the maturation of organic-rich shale in different thermal simulation conditions 7 , 10 − 12 and have drawn some valuable experimental conclusions. Studies have shown that total organic carbon (TOC) content, kerogen type, clay minerals, and thermal maturity ( R o ) are the main factors controlling the pore structure of shale, among which thermal maturity is the most important factor.…”

“… 30 , 31 Many scholars appl iedfractal theory to the description and quantitative evaluation of pore structure and explored the influencing factors of fractal dimensions. 7 , 32 − 34 The fractal dimensions are usually between 2 and 3. The larger fractal dimensions represent more irregular surfaces and higher heterogeneity of pore structure..…”