Numerical Simulation Of The Effect On Accuracy Of Stress Measurement In The Progress Of Drilling

Initial stress plays an important role in many fields. The most direct and effective way to get initial stress is stress measurement, but the aiguille and drill pipe make strong mechanical vibration and high impact loads in the process of drilling, and it changes the stress in hole-wall,and then effects accuracy of stress measurement. In this paper, the process of stress measurement in borehole is simulated by using finite element method, this paper studies the effects of rock property, stress border and fractures on stress changes in hole-wall and summarizes the sensitive factors of accuracy of stress measurement.Process of stress measurement in borehole is simulated. The results indicate that, the displacement in y direction is large, the displacement in x direction is one magnitude smaller than that in y direction, so the rock has small damage by the process of drilling. The maximum value of peak stress relates to two factors, one factor is the drilling speed, and another is the distance from the hole, stress transmission speed is 2cm/s when the drilling speed is 0.5cm/s.The calculated results show that, rock property, edge stress and fractures have a variable influence on stress changes in hole-wall before and after drilling. Poisson ratio and density of rock has little influence, so they are not the sensitive factor. The sensitive factors include elastic modulus of rock, yield strength, edge stress and fractures. Water-bearing state in rock impacts elastic modulus and yield strength, and then impacts stress changes, so the different water-bearing state in rock has significant impact to accuracy of stress measurement.The results indicate that, fractures property, edge stress, the distance of fractures from the borehole and compound fractures have a variable influence on stress changes. Poisson ratio of fractures has little influence on stress changes, it is not the sensitive factor. Elastic modulus of fractures, the distance of fractures from the borehole, edge stress and compound fractures are the sensitive factors. Above studying results provide reference for stress measuring in borehole.Considering the relationship between stress measurement and geological structure, the law of stress distribution is analyzed in the vicinity area and the end of fractures. Other factors are also studied, including elastic modulus of fractures and rock, the angle between maximum horizontal principal stress and fractures direction, edge stress and fractures patterns. The results indicate that, the sensitive factors include fractures scale, elastic modulus of fractures and rock, the angle of maximum horizontal principal stress and fractures direction, edge stress and fractures patterns.