Geotechnics 2023, 3, 3–20. https://doi.org/10.3390/geotechnics3010002 www.mdpi.com/journal/geotechnics Article Digital Rock Mechanical Properties by Simulation of True Triaxial Test: Impact of Microscale Factors Wenjie Ma 1 , Yongfei Yang 1, *, Wendong Yang 2 , Changran Lv 1 , Jiangshan Yang 1 , Wenhui Song 1 , Hai Sun 1 , Lei Zhang 1 , Kai Zhang 1 and Jun Yao 1 1 School of Petroleum Engineering, China University of Petroleum, Qingdao 266580, China 2 College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, China * Correspondence: yangyongfei@upc.edu.cn Abstract: Complex fractures and pore structures in the rock strongly influence the mechanical prop‐ erties, and the process from compression to failure is complicated. Under the action of rock stress, pore structure deformation and fractures close or propagate, easily leading to deterioration in the rock me‐ chanical properties until rock failure. Thus, the effects of microscale factors are critical in mechanical properties such as rock strength, elastic modulus, and stress–strain state under the triaxial stress state. It is difficult for physical and mechanical experiments to obtain the qualitative rules of regular struc‐ tures, but numerical simulation can make up for this defect. In this work, the accuracy of the model was proven through a comparison with previous experimental results. The true triaxial numerical simulation experiments were conducted on representative rocks and natural pore structures. These simulated results revealed that the pore and throat parameters will change abruptly when the particle model volumetric strain is between 0.0108 and 0.0157. When the fracture angle is between 45° and 75°, the fracture has a great influence on the peak stress. The angle between the natural fracture and the fracturing direction should be less than 45° as much as possible. Clay affects the rock strength by in‐ fluencing the force chains formed by the rock skeleton. Fracturing is easier when the structural clay content is higher than 25%. It is easier to fracture in a direction parallel to the laminated clay when the clay content is below 27%. This work indicates the effects of rock particles, fractures, and clay on the mechanical parameters, providing key fundamental data for further quantifying the fracturing pat‐ terns. Keywords: digital rock; mechanical properties; stress–strain state; microscale structures; numerical simulation 1. Introduction Deep underground rocks store a large number of oil and gas resources, and their phys‐ ical properties are diverse and heterogeneous [1,2]. There are complex fractures and pore structures in the rocks, and the process from compression to the failure of rocks is compli‐ cated [3–5]. Under the action of rock stress, pore structures and rock skeleton deformation, fractures close or propagate and connect with other fractures [6,7]. Thus, the effects of mi‐ croscale factors are critical not only in the mechanical properties, but also in engineering operations such as fracturing and logging [8]. The rock physics experiment is the most basic rock physics research method, and the conventional triaxial test and true triaxial test are simple to operate [9]. According to the engineering design of construction and the environmental conditions, the true triaxial test can change the stresses of three axes to simulate the real stress environment accurately. However, the traditional rock physics experiment has the disadvantages of long experiment period, high cost, poor repeatability, and large error under low porosity and permeability [10]. Citation: Ma, W.; Yang, Y.; Yang, W.; Lv, C.; Yang, J.; Song, W.; Sun, H.; Zhang, L.; Zhang, K.; Yao, J. Digital Rock Mechanical Properties by Simulation of True Triaxial Test: Impact of Microscale Factors. Geotechnics 2023, 3, 3–20. https://doi.org/10.3390/ geotechnics3010002 Academic Editors: Md Rajibul Karim, Md Mizanur Rahman, Khoi Nguyen and Asif Iqbal Received: 7 December 2022 Revised: 11 January 2023 Accepted: 17 January 2023 Published: 26 January 2023 Copyright: © 2023 by the authors. Li‐ censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con‐ ditions of the Creative Commons At‐ tribution (CC BY) license (https://cre‐ ativecommons.org/licenses/by/4.0/).