Effective proppant placement in hydraulic fractures and fracture networks is crucial for optimizing hydrocarbon extraction in both conventional and unconventional petroleum reservoirs. It also plays a key role in enhancing the efficiency of enhanced geothermal systems (EGS) in geothermal reservoirs. This dissertation presents a comprehensive study on modeling proppant transport in hydraulic fractures and fracture networks. The research aims to develop advanced computational models to simulate and investigate proppant placement under various geological and operational conditions. The study integrates multiple physical processes, including fracture deformation, slurry flow, proppant transport, and heat transfer, within a unified simulation environment. Different numerical methods are employed to address the complexities of this multi-physics system. A three-dimensional displacement discontinuity method (3D DDM) is used to model rock and fracture deformation, while the finite volume method (FVM) is applied to simulate slurry flow, proppant transport, and heat transfer. Special attention is given to the impact of thermal effects, the influence of fracture intersections, and the role of the fracture closure process in shaping proppant transport and distribution. Simulation results illustrate how various parameters, such as reservoir temperature, proppant size, density, injection concentration, and pumping rate, affect the distribution of proppant in hydraulic fractures and fracture networks. Key findings reveal that the final proppant distribution and fracture conductivity are influenced by proppant and fluid properties, reservoir characteristics, and operational parameters. Optimizing proppant placement in hydraulic fractures or fracture networks requires a comprehensive consideration of these factors. The implications of this research extend to both the petroleum and geothermal industries, providing a robust tool for designing more effective hydraulic fracturing treatments. This dissertation contributes to the field by offering a deeper understanding of proppant transport dynamics and introducing a versatile modeling approach that can be adapted to various situations.