Earth Science Frontiers ›› 2025, Vol. 32 ›› Issue (5): 361-376.DOI: 10.13745/j.esf.sf.2025.2.7

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Numerical simulation method on the impact of the difference of rock composition and structure on the development mechanism of fractured reservoirs: A case study from the granitoids in Jiyang Depression

HE Xiao1,2(), NIU Huapeng1,2,*(), ZHAO Xian1,2, ZHOU Haoyan1,2, LIN Weijun1,2, ZHANG Guanlong3, MENG Tao3, MU Xing3   

  1. 1. State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum (Beijing), Beijing 102249, China
    2. College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China
    3. Research Institute of Exploration and Development, Sinopec Shengli Oilfield Company, Dongying 257015, China
  • Received:2024-12-03 Revised:2025-02-27 Online:2025-09-25 Published:2025-10-14
  • Contact: NIU Huapeng

Abstract:

Fractured oil and gas reservoirs represent a significant source of growth for global oil and gas reserves. Their proven geological reserves constitute more than 30% of the global total. These reservoirs are widely distributed and possess substantial exploration potential. Rock composition and structure (mineral composition, grain size, texture) are fundamental factors controlling fracture development. While the particle discrete element method has been applied to study the mechanical properties of brittle minerals and microfracture mechanisms, the influence of variations in rock composition and structure on fracture development degree and its underlying mechanism have received relatively little attention. This study addresses the key scientific issue regarding the fracture mechanism in granitic bedrock considering variations in composition and structure. To address this issue, discrete element numerical models incorporating mineral content, grain size, orientation, macroscopic mechanical properties, and fracture development patterns were constructed to clarify the control mechanism of rock composition and structure on fracture development, thereby providing valuable insights for the exploration of fractured reservoirs. Using the fractured granitic bedrock reservoir in the Jiyang Depression as a case study, we quantitatively characterized the rock composition and structure through core observation, petrographic thin section analysis, and XRD. Based on these results, an initial discrete element numerical model for predicting rock composition and fractures was established. The microscopic parameters of the initial model were calibrated and verified using uniaxial compression tests and acoustic emission monitoring. Subsequently, a comprehensive quantitative model predicting the impact of rock composition and structure on mechanical properties and fracture development was developed. The main findings are as follows: (1) Quantification of intracrystalline fractures revealed that alkali feldspar makes the greatest contribution to reservoir fracture development, with its content showing a positive correlation with total microfracture density, followed by plagioclase. Quartz exhibits the lowest contribution, with its content negatively correlated. (2) As granite grain size increases from 2.0 mm to 5.0 mm, uniaxial compressive strength decreases. Consequently, smaller tectonic stresses are required to initiate microfractures, facilitating the development of fractured reservoirs. However, under sufficiently large tectonic stresses, microfracture density decreases. (3) Quantification of intercrystalline fractures showed that the inclination angle between mineral orientation and the tectonic stress direction is positively correlated with the proportion of intercrystalline microfractures. Compared to massive granite, gneissic granite exhibits lower compressive strength and enhanced microfracture connectivity, which favors the development of high-quality reservoirs. These findings provide an important theoretical basis for predicting fractured reservoirs.

Key words: fractured oil and gas reservoir, rock composition and structure, numerical simulation, quantitative prediction

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