A study on the drill pipe erosion law in gas drilling

Erosion of drill strings by cuttings is very serious in gas drilling.Based on the gas and solid two-phase flow dynamics and erosion theory,the present paper established a transport model for annulus cuttings.A continuous gas phase flow field was solved under the Eulerian coordinate system,and the collision solution to discrete-phase cutting particles was obtained under the Lagrangian coordinates by applying the Grant and Tabakoff formulae to calculate the erosion rate of cutting particles and verifying the simulating model with the aid of experimental data.Then,this model was applied to study various problems encountered in gas drilling,such as the drill-pipe centralization,different eccentricity of drill pipes,enlargement of wellbore diameter,volume of gas injection,transport law of annulus cutting particles at the rate of penetration(ROP),and erosion features of drill pipes.The studies demonstrated that cuttings eroded the drill-pipe joint and its lower sloping surface more seriously than the drill-pipe body and the upper sloping surface of the joint.Both the drill-pipe eccentricity and the enlargement of wellbore diameter would increase the peak value of the local erosion rate,and the more the eccentricity and enlargement,the quicker the increase of the peak value of the local erosion rate.The increase of ROP led to an approximately linear increase in the peak value of the local erosion rate,and gas-injection volumes within the normal range of engineering influenced the peak and mean value of the erosion rate very little.The following recommendations were made according to the erosion law: the decrease in the sloping grade of drill pipes would reduce the local erosion of the drill-pipe joint and its lower sloping surface,the use of low-speed air-hammer drilling technics would reduce the local erosion caused by tool eccentricity,and the trajectory alteration of cutting particles could reduce frequent erosion of drill-pipe body as increasing volume of injection gas with more enlargement of wellbore.This study is of certain directive significance and applied value both in revealing the erosion law of drill pipes and in reducing the drill-pipe erosion.