Research And Application Of Stress-induced Stress Field In Dagang Tight Oil Reservoir

Dagang Oilfield tight reservoir has the characteristics of large reserves, poor physical properties, low porosity and permeability. The oil yield has little effect though conventional fracturing, which greatly restricts the further development of Dagang Oilfield tight reservoir. So PetroChina opens a special topic to focus on research of tight oil reservoir complex crack formation mechanism. In current, the study of induced stress field is not systematic in fracturing. So, it's important to have further study of the mechanism and influence of induced stress field in fracturing is of great significance to generate complex fractures in tight oil reservoir.This paper uses Dagang Oilfield tight oil reservoir as the research object. Based on the experimental Analysis of tight oil reservoir rock mechanics and ground stress characteristics, the paper applies the displacement discontinuity method to establish induced stress field mathematical model of the openhole and perforating well and the model can be programmed to solve. Meanwhile, we use the software to simulate and analyze the impact factors and the laws of horizontal wells and vertical wells induced stress field and guidance tight reservoir fracturing optimization. Through this study's research, the following conclusions and understanding can be obtained.(1) The Young's modulus of Dagang Oilfield Konger and Shahejie group tight oil reservoir is low, average of 16138.6MPa, and the Poisson's ratio is medium to high, average of 0.27. The maximum horizontal principal stress of Konger is 53.3MPa, and the minimum is 44.4MPa. The maximum horizontal principal stress of Shahejie is 70.2MPa, and the minimum is 63.6MPa. The horizontal stress difference coefficient of two reservoirs is small, but difference value is great. So the level of induced stress still need be reduced to increase the degree of complex fractures.(2) The mathematical model of the fracture induced stress field is established by the displacement discontinuity theory, and combines with fluid pressure in the wellbore, temperature difference, the fracturing fluid loss, ground stress and perforation stress etc. Through superposition principle, the mathematical models of induced stress field for openhole and perforating wells are respectively established.(3) The paper applies VB to prepare Dagang Oilfield tight oil induced stress field software, and combines with fracture breakdown pressure model of the fracturing process. The analysis shows that the breakdown pressure of considering fracture induced stress is closer to actual breakdown pressure value, so indirectly verifying the accuracy of this thesis induced stress field model.(4) Induced stress of hydraulic fracture of vertical wells has a great impact on the in-situ stress field. The longer the initial fracture length, the greater the initial fracture net pressure, the smaller the in-situ induced stress different coefficient, the greater the influence of the induced stress is. On the basis, the secondary sand fracturing technology in vertical wells is proposed, and the application has achieved good results in Dagang oilfield.(5) The effect of induced stress field of staged fracturing in horizontal wells is analyzed. The results show that the induced stress field of cluster perforation is more conducive reducing ground stress, and the use of horizontal wells induced stress field of crack spacing reduces the optimal stress difference is about 50m; Meanwhile, the greater fracture net pressure, the longer the fracture length, the more helpful reduce stress difference, there is conducive to complex fractures in the formation.This study provides theoretical support and methodological tool for fracturing optimization design in tight oil reservoirs, and facilitates effectively tight oil development of Dagang Oilfield.