Effect of the Spatial Distribution of Gravels on the Mechanical Properties of Conglomerates
Authors: Huang G. P. , Hu X. D. , Wu X. , Zhou F. J. , Gou X. Y. , Li M. H. 
Published: September 2022

ABSTRACT:
The spatial distribution of gravels is of great significance to the mechanical properties of the conglomerate. The effect of the spatial distribution of gravels on the mechanical properties of rocks is still lacking in knowledge and understanding. Five different distributions of gravels finite element models were first developed, and the Drucker-Prager constitutive model was adopted to study the effect of spatial distribution patterns of gravels on the mechanical properties of the conglomerate. Subsequently, the failure patterns, mechanical properties, and energy evolution were discussed. The result shows that the fracture initiation starts in the gravel distribution area, regardless of the spatial distribution modes of gravel. The failure in concentrated-location mode is dominated by complex fractures, while the failure of uniform-distributed mode is dominated by single fracture. The spatial distribution of gravels can directly affect the mechanical properties of conglomerates. Specifically, the compression strength of gravels with a concentrated distribution of gravels (19.3 MPa) is 68.93% of the strength of gravels with a uniform distribution (28.0 MPa). The total damage energies and the elastic energy under the uniform distribution are higher than that under the concentrated distribution, which causes the rock under the uniform distribution mode to have a higher compressive strength than that under other distribution modes. The results could provide some novel insights to understand the heterogeneous mechanical properties of conglomerate reservoirs.

1. INTRODUCTION
Conglomerate reservoirs are an important unconventional reservoir type that hosts many oil and gas resources worldwide (Krezsek et al., 2010; Kang et al., 2019; Mngadi et al., 2019; Chen et al., 2022). Understanding the mechanical properties of conglomerates is the key to the efficient development of conglomerate reservoirs. The diversities in the spatial distribution of the conglomerates make the conglomerates severely heterogeneous (Chen et al., 2021; Liu et al., 2018; Zhou et al., 2018; Duan et al., 2020). However, it is unclear about the effect of spatial distribution on the mechanical properties of the conglomerates. Currently, many scholars have carried out related research based on the heterogeneity of conglomerates (Jimenez et al., 2016; Lan et al., 2010; Renner et al., 2007; Abdelaziz et al., 2018; Shi et al., 2021) For instance, Sonmez et al., 2006, conducted the effect of gravel content on the compressive strength of conglomerate, and established the prediction model of gravel content and gravel strength on the uniaxial compressive strength of conglomerate (Sonmez et al., 2006). Li et al., 2013, conducted numerical simulations by RPFA2D to investigate the effects of closure pressure, gravel size, and gravel shape on the propagation of fractures. And they divided the propagation mode of fractures into five types (Li et al., 2013). Khan Larry et al., 2016, conducted laboratory experiments to investigate the effect of gravel content and contact pattern on conglomerates’ mechanical properties. The results showed that gravel content has an essential effect on the mechanical properties of conglomerates (Khanlari et al., 2016). Akram et al., 2018, conducted numerical simulations by PFC3D to investigate the effect of gravel particle size distribution, scale effect, and cementing material between matrix and gravel on the mechanical properties of uniaxial, triaxial, and Brazilian splitting of rocks (Akram et al., 2018). YE et al., 2019, conducted numerical simulations by PFC2D to investigate the effect of different particle ratios on the compressive and tensile strengths of rocks (Chen et al., 2021). Jianbo Wang et al., 2021, conducted uniaxial compression experiments and found that many micro-fractures were initiated around the gravel. The uniaxial compressive strength and elastic modulus of the gravel decreased with increasing gravel content (Wang et al., 2021). Luo et al., 2021, conducted uniaxial compression experiments based on the discrete element software PFC2D to investigate the effects of different cementation strength, gravel content, and gravel geometry on the mechanical properties of gravels and showed that the stress concentration around gravels is the microscopic mechanism of the difference in mechanical properties of gravels (Luo et al., 2021).