Repeated Fracturing Pressure And New Fracture Mechanics Mechanism

Refracturing is a key way for enhancing productivity and recovery for the maturing oil/gas field. It is also an effective means to integrated management and steady oil and control-water, especially for high water-cut oil field. For a long time, it is always a argument focus in the academic and project that whether the reorientation and new fracture can be achieved or not. Base on the stress changes nearby the wellbore ,in this paper, the mechanical mechanism of fracture creation for refracturing is systematically studied, the main achievements and understands is as follows:(1) Systematically study the characteristic of pre-refracturing stress field of well with vertical fracture and its main influencing factors. As the study shows, the induced stress resulted from primarily fracture, oil/gas production and some treatments such as water-injection will change pore pressure and further lead to the stress redistributing; injecting water into the formation for a long time depresses the reservoir temperature and also change the induced stress field. All of these factors change the distribution of in-situ-stress field of reservoir.(2) With the theory of elasticity, a model of induced stress field is established for primary fracture and the analytic solution is achieved by semi- retro-solution method.(3) Integrating percolating mechanics with rock mechanics and hypothesizing the reservoir is elastic-plastic, a mathematical model for percolation-solid phase balance is abstractly presented; as solving, adopting method of finite difference for percolating model, finite element method (FEM) for rock-change model while explicit alternation for fluid-solid coupling model and a stress field resulted from the change of pore pressure by oil/gas production is obtained. Quantitative analysis and simulation are also did for this induced stress field(4) With extended Hook's Law and plane elastic theory as well as the principle of energy conversation, a mathematical model for calculating thermoelastical stress is put forward. This stress is resulted from water injection nearby water injection well; based on the published empirical equations and by transforming and superpositing the thermal stress which caused by a cylinder in the center of infinite elastic formation, a calculation model of stress field for water injection well with fracture is developed. This stress field is resulted from the change of temperature. Quantitative analysis and simulation are also did for this induced stress field(5) As the simulation results shows, as the existence of the induced stress resulted from primarily fracturing and the variation of pore pressure etc.. the stress field in reservoir hasbeen changed with faster change for maximum horizontal main stress and slower change for minimum horizontal main stress, both of them are the functions of time and space. Therefore, when the induced stress differential is abundant enough to change the initial stress differential, there will be a stress-redirected zone near the wellbore and new fracture can be formed by refracturing(6) Based on the above researches on distribution of stress field and with rock mechanics, fracture mechanics and hydrofracturing mechanics, using the method of displacement discontinuity, initiating azimuth of fracture, fracture propagation and its extended track for refracturing are simulated, under the condition of complex stress that reservoir is of different stress values anywhereFor refractuing, the mechanism of new fracture's formation and its propagation rule are not only the keys and noduses, but also the crux impacting the refracturing optimal design. It is no doubt that the research in this paper indeed offers certain theory base to refracturing optimal design and a guidance to field treatment as well as enhance the technical and economical feasibility. It is of great significance for integrated management and for steady production and stimulation of maturing oil/gas reservoir.