Research On Load-transfer-law Of Multi-point Rocked Drill String

The number of complex trajectory wells such as directional wells,horizontal wells and extended reach wells is increasing day by day.In order to solve the technical problem of dragging pressure in sliding drilling of complex trajectory wells,vibration friction reduction technology has been widely concerned and developed rapidly.However,there are some problems such as limited scope of action of single vibrator and lack of theoretical basis for setting vibration parameters.To address the above problems and in order to further improve the drilling pressure transfer efficiency of sliding drilling,this paper proposes and carries out the mechanism study of multi-point vibration friction reduction.In this paper,the dynamic model of drill string and the finite difference solution scheme considering the multi-point bi-directional vibration,static-dynamic friction conversion and the interaction between drill bit and formation are established by combining theoretical analysis and numerical simulation.The law of multi-point vibration load transfer is preliminarily explored.The influence of the number of vibrators,the amplitude of vibration force,the frequency of vibration and the interval of vibrators on the dynamic characteristics of drill string,the distribution of friction,the transmission of drill pressure and the penetration rate are analyzed.The main control factors affecting the penetration rate of multi-point vibration sliding directional drilling are screened out,which provides the basis for the selection of the optimal parameters of multi-point vibration.The results show that:(1)compared with the single-point one-way vibration model,the drill string load transfer model considering the single-point two-way vibration of the vibrator established in this paper matches the field bottom hole pressure data better;(2)Compared with the single-point vibration,the multi-point vibration method proposed in this paper can achieve better friction reduction and pressure transfer efficiency.Taking the same bottom hole drilling pressure as the goal,the magnitude and frequency of vibration force for multi-point vibration are lower,which can avoid the problem of excessive local stress of drill string and improve the fatigue life of drill string;(3)The multi-point vibration method is more flexible than the singlepoint vibration method in drag reduction.According to the size of the friction of the pipe section where different vibrators are located,setting different parameters for different vibrators can effectively expand the drag reduction area and alleviate the problem of uneven distribution of friction of drill string;(4)The magnitude and frequency of the exciting force will affect the bottom hole pressure.When the working scope of the vibrator is within the frictional resistance zone,WOB increases approximately linearly with the increase of the amplitude of the exciting force.With the increase of vibration frequency,WOB increases first and then decreases,and there is an optimal frequency.At the same time,too high frequency will lead to the coupling phenomenon of low-frequency fluctuation and high-frequency vibration of bottom hole drilling pressure.The low-frequency fluctuation shows that the drill string is in the state of elastic drilling delivery.In practical application of multi-point vibration technology,selecting the optimal frequency can achieve higher transmission efficiency of drilling pressure,and limiting the vibrator to vibrate at a low frequency can achieve relatively stable WOB;(5)The number of vibrators and the interval of vibrators will also have an impact on the transfer of drilling pressure.The more the number of vibrators,the higher WOB and total drag reduction rate,it’s a non-linear relationship between the total drag reduction rate and the number of vibrators,but the results show that it’s a linear relationship between the loss of drag reduction efficiency of single vibrator and the number of vibrators.With the increase of interval between vibrators,WOB increases first and then decreases,and there is an optimal interval.According to the actual well conditions,the calculation results of this model can provide a reference for the selection of the optimal interval of vibrators.(6)The order of the influence of various controllable factors on penetration rate from large to small is: number of vibrators > amplitude > frequency >interval.By increasing the number of vibrators,the effect of low vibration force amplitude and frequency on load transfer can be effectively compensated.There are also some shortcomings in multi-point vibration.There may be some mutual interference phenomena such as vibration wave interference and overlap of drag reduction zone.The increase of the number of vibrators will also increase the cost and pressure loss.These problems need to be considered comprehensively in design.