Dynamics Of Whole-well Drill String Based On Mechanical Properties Of Hydraulic Oscillators

Horizontal extended distance is required as long as possible, so as to expose the layer substantially in the shale gas deep/ultra-deep horizontal well drilling operations. With the increase of horizontal section length, the excessive friction drag of drill string is prone to cause WOB transfer inefficient and bring about force holding effect which leading to dramatically decrease of ROP (Rate of Penetration). Among the solutions of releasing force holding effect and decreasing the friction drag, AGT (Axial-oscillation Generator Tool or Agitator) presents reliable apply effect and promising future.With the combination of AGT working principle, this paper discussed about abundant experimental research findings and theories of friction principles, as well as paper analyzed the reason of being hard to eliminate force holding effect and the relationship between friction and sliding velocity. It appears that the friction coefficient decrease with the increase of the relative slipping velocity of friction coupling in a certain scope. In addition, this paper cognized that Dieterich-Ruina friction law can describe the contact between the surfaces of drill steel and borehole rock, and modified the calculation method of friction coefficient in drilling.Based on the Elasticity and FEM method, considering the contact between drill string and borehole, this paper built the dynamics model of drill string in the practical borehole of horizontal well. When the contact happen between drill string and borehole, the Dieterich-Ruina friction law is taken into account in the friction coefficient calculation. According to this model, this paper analyzed the transient dynamic characteristic of whole well drill string with AGT by simulation method, and calculated the time response results of the total friction drag and wellhead axial load, as well as the axial friction and axial load of drill columns in different distances which close to the AGT in the drill string. Furthermore, this paper discussed analyzed contributory factors to the friction drag reducing effectiveness of AGT, including assembling position of AGT in drill string, maximum oscillating force and working frequency.The calculation and analysis of whole well drill string shows:The total estimated friction drag of dynamics method is approximately 25% lower than statics method (rigid string model); the total estimated friction drag decreases approximately 17% after utilizing the AGT; The significantly effective length is approximately 360m in the drill string up and down away from the AGT, and the farther from the AGT, the more insignificant of friction reducing effect, but the distance change has little effect on the friction reduction of the drill pipes nearby the bit that present a preferable friction reduction effect. Moreover, the assembling position of AGT closer from the bit, the larger of maximum oscillating force and the higher of the operational frequency, the total estimated friction drag decrease more. However, the followings calls for special attention:Firstly, it is significant for the friction reducing effect of the drill pipes around when the assembling position close to the bit; Secondly, when the maximum oscillating force is larger than 40 kN, the scale of friction reducing of the drill pipes around declines, thus the maximum oscillating force should be controlled under 40 kN; Thirdly, when the operational frequency is higher than 17 Hz, the scale of friction reducing of the drill pipes around declines; what's worse, when the assembling position closes to the bit, or the maximum oscillating force is grown up, or the operational frequency is increased, the axial load of BHA grows up.According to the analysis above, the recommended operation parameters of AGT (?172mm) can be given as:the AGT should be assembled away from the bit on the position range from 200m-360m; the oscillating force controlling parameters should be set as the operational pressure drop generated in 650 psi with cross pumping area in 11 in2,or the operational pressure drop generated in 550 psi with cross pumping area in 16 in2; the operational frequency should be controlled at the range from 16 Hz to 17 Hz.Research results in this paper could provide the theoretical support for ascertaining the proper operational parameters of AGT in the horizontal well drilling.