Research On Crack Closure Mechanism Based On Rock Meso-mechanics

For the drilling and production in fractured formations, a series of engineering problems often arise due to the changes of the fracture aperture such as plugging failure and reservoir stress sensitivity. The key to resolve those problems is the law of fracture closure behavior. The macro deformation of fracture derived from the deformation of asperity. Predecessors established many fracture closure model basing some hypothesis of mechanical behavior of asperity. However, those existing models of fracture closure have simplified the asperity a lot, and currently there is no yet experimental studies on the mechanical properties of rock asperity. it is not clear that how the rock asperity deforms. Therefore, to carry out the study of the mechanical properties of rock asperity is helpful to understand the law of frature closure from mesoscopic. It's also beneficial to reduce the drilling costs in fractured formation, and improve the development efficiency of fractured reservoirs.This thesis is aimed at the tight sandstone in Sichuan Xujiahe group. Based on the rock meso-mechanics, CT scan, experiments test and numerical simulation are carried out to study the mesoscopic heterogeneous structure and mechanical properties of rock asperity. The main research contents of this thesis is followed as below:(1) The mesoscopic heterogeneous structure of rock asperity is reconstructed by CT scan. Combining with the rock thin analysis, the mesoscopic heterogeneous model has been established, which establishes the foundation of numerical simulation later.(2) A series of experiments test to asperity have been conducted. In monotonic loading experiments, the full normal load-deformation curve of asperity is obtained. The contrast between experimental data and Hertz contact theory has been conducted. In cyclic loading experiments, the damage accumulation phenomenon has been observed.(3) The influence of mesoscopic heterogeneous structure of rock asperity has been conducted combining the mesoscopic model and finite element simulation method. The Simulation results shows that the asperity has size effect.Through the above research, the actual appearance and internal structure of the rock asperity have been captured. These research results provide a theoretical basis not only to reveal the fracture dynamic changes Spatial Variation under the drilling conditions, the flow characteristics, the law of wellbore-formation coupled flow, but also for the managed pressure drilling, while drilling plugging, while drilling reservoir evaluation, and reservoir protection.