Study On The Mechanism Of Flexible Drill Combination Anti - Skewing

Flexible BHA which is mentioned recently, has a good application effect in some oil blocks. A flexible joint which is installed in the BHA, can change the direction of bit deflection axis and loading states with connection of stabilizer. The deviation control and fast drilling mechanism research on flexible BHA is based on the application of the BHA in the formations tending to deflecting, and in this paper the study is carried out as follows:1.By using the finite element analysis method, vertical hole bottom rock model is established with anisotropic propertiesis. Different factors are discussed on the influence of stress field of hole bottom and the well deviation. Studies show that stress concentration positions of different joint dip angles are greatly influenced by the position of the joint plane intersecting the borehole; smaller liquid column pressure on the wellbore bottom results in tensile stress damage of rock more easily, producing deviation; if joint thickness is smaller, the number of stress concentration increases and rock is more easily damaged, causing deflection; horizontal stress magnitude and direction will change the direction of the borehole deviation and stress concentration is influenced by intersecting lines of joints plane and borehole; Under the condition of large wells diameter and WOB, the stress of wellbore bottom rock increases, which is more prone to damage.2.The anti deviation mechanism of flexible BHA is analyzed. The condition of deviation drop is that flexible joint is located in the center of the borehole axis and flexure direction of of drillstring is changed mainly by increaseing bit pressure, which makes the stabilizer near the bit close to the up wall, bit down to the low wall and makes bit cut the rock of the low wall more than the up wall. In addition, a static analysis model of flexible BHA is set up. The calculation example shows that the bit lateral force increases with the growing of the hole inclination angle and WOB.Comparing with the conventional pendulum BHA, there is better deviation control performance.3.Based on the interaction model of formation and bit, considering the effects of the anisotropy of formation and bit, this paper puts forward with "deviation trendence angle " to analyze the anti-deviation force of the anti-deviation drilling assembly; Calculation examples show that the deviation trendence angle of flexible drill string is larger than the pendulum assembly, whose deviation-drop tendency is more obvious and the diagonal performance plays better, for the same characteristics of formation and bit.4.The dynamics model of BHA was established with rotor dynamics and virtual work principle. The study of collar with a flexible joint and close to the drill explores the rule of the collar centroid whirling under the effects of bit pressure and well deviation. Dynamic simulation results show:The vortex in the drilling fluid is forward; As the well deviation angle is bigger, the collar centroid trending to the borehole is more obvious; While drilling, increasing the bit pressure appropriately is good to deviation control and inclination correction; It is WOB waving so frequently that affects the location and extent of drill cutting the borehole.