# Research on Mechanical Properties and Failure Mode of Conglomerate Based on Discrete Element Method
## 1 Introduction  
With the march of China's oil industry and the progress of exploration and development technology, the development of unconventional oil and gas resources has become the focus of global oil and gas research, and has become the next key research object of oil and gas exploration and reservoir development in China [1]. Unconventional oil and gas resources include tight sandstone gas, shale gas, tight oil, coalbed methane, etc. Tight oil and gas resources feature a short investment cycle, and tight oil and gas are stored in sandstone or conglomerate reservoirs. In 2018, the world's largest glutenite oilfield was discovered in Xinjiang, with confirmed reserves of about 520 million tons. [2] The conglomerate reservoir generally is characterized by low porosity and low permeability, which results in an insufficient self-production capacity. Therefore, it is necessary to master its mechanical properties and improve reservoir permeability by hydraulic fracturing. Conglomerate reservoirs in China are mainly distributed in northwestern Junggar Basin, Karamay Oilfield, Bohai Bay Basin, Fute Oilfield of Erlian Basin and Dongxin Areaof Shengli Oilfield [3; 4]. Conglomerate reservoirs are mainly developed in fault zones with complex sedimentary environments and tectonic settings, which determine their strong heterogeneity and complex mechanical properties. Researchers at home and abroad have studied the influence of gravel on the mechanical properties of a conglomerate through physical experiments and numerical models and proved the strong heterogeneity of the conglomerate. The existence of gravel has a great impact on the mechanical strength of the cementation surface, and the cementation mode can be divided into argillaceous, calcareous cementation or semi cementation [5]. [PERSON] [6] and [PERSON] [7] studied conglomerates with different cementation strengths in Mahu, Xinjiang, and found that cementation strength has a significant influence on mechanical properties and fracture propagation of conglomerates. [PERSON] [8] and [PERSON] [9] found that the conglomerate shows strong heterogeneity and stress concentration due to the existence of gravel, which leads to the linear decrease in compressive and tensile strength of the conglomerate with the increase in gravel content. In order to study the influence of gravel content on conglomerates, researchers used natural and artificial cast rock samples to study the influence of gravel sorting and different particle sizes on the mechanical properties and deformation parameters of conglomerates [10; 11; 12; 13]. Physical experiments are interfered by external factors and have limitations such as non-repeatability, so numerical simulation and other means have been widely used. Through RFPA, ABAQUS and other finite element software, it is found that cracks in gravel mainly show four propagation modes [14; 15; 16]. The finite element method has some limitations, such as the mesh cannot update the data in real time and it is difficult to divide the mesh at the crack tip. However, there are great similarities in cementing modes between particles and 'gravel and matrix' in the discrete element method, which is more suitable for analyzing discontinuous media materials such as a conglomerate. Therefore, researchers began to use the discrete element method to analyze discontinuous media materials such as the conglomerate [17]. [PERSON] [18] used the discrete element method to divide the conglomerate into a composite medium composed of three parts: gravel, matrix and cementation surface. The critical energy release rate was used to study the mechanical properties and fracture propagation law of gravel with different particle sizes, contents and fracture toughness. [PERSON] [19] and [PERSON] [20] used PFC2D to construct broken particle clusters with different particle sizes to study the mechanical properties evolution and heterogeneity effect of the conglomerate under uniaxial compression.