Study On Rock-breaking Rule In Deep Wells And Rock-breaking Fractal Mechanism

Exact Prediction and calculation of mechanical property and drillability of rock can provide the necessary base for the optimization of bits and designing parameters, thereby quickening penetration. Prediction and calculation of drillability of rock in the bottom of deep wells, however, is regarded as an important but unsolved problem in the drilling circle. This dissertation, based upon the status quo of deep wells drilling in Daqing, is to develop a method to evaluate rock drillability in the conditions of bottom hole of deep wells. In addition, the current method to assess the rock drillability needs coring, laboratory experiment and test. Because of the complex downhole lithology, frequent changes of core, it is impossible to conduct coring for the entire formation, and it is difficult to establish an continuous section map, which can result in many problems in bits selection and usage. Through the study on particle size distribution of cuttings during drilling, this dissertation establishes a fractal model of rock drillability, which is described by fractal characteristics, and establishes the evaluation indexes of rock drillability, which is described by fractal characteristics, therefore, provides the foundation for real-time determination of rock drillability and real-time optimization of bit types.Based on experiment, the dissertation establishes a correlation model for rock mechanics parameters, such as the value of rock drillability, hardness, strength and so on. The correlation analysis indicates that rock mechanics parameters, such as strength, hardness, are the integrated reflection of rock drillability. The model lays a foundation for calculating the value of drillability under the confining pressure.Aiming at the down hole conditions, the rock drillability problem under the confining pressure is studied through experiment and theoretical analysis. The dissertation establishes the quantitative relation between the parameters of rock mechanics and the confining pressure through the laboratory experiment of rock mechanics parameters under the confining pressure. The computational method for rock drillability under different confining pressures is developed through drillability analysis under the confining pressure. Taking the drilling of deep well in Daqing Oilfield as an example, the dissertation establishes the computational method of rock drillability in the condition of the different lithology and permeability, the results show that the difference of the drillability grade maybe get to 1-3 grade under the normal temperature and pressure and the downhole confining pressure, namely, the micro drilling time is 128 seconds under the normal pressure and temperature, but the micro drilling time is 256 seconds under the confining pressure. This indicates that downhole confining pressure significantly...