Abstract:

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.

Key words: gas drilling;gas and solid two-phase flow;transport of cuttings;erosion, drill pipe;CFD;numerical simulation

摘要：

针对气体钻井岩屑造成钻柱严重冲蚀的问题，基于气固两相流和冲蚀理论，建立了环空岩屑运移模型。在Eulerian坐标系下求解气体连续相流场，在Lagrangian坐标系下对离散相岩屑粒子进行了碰撞求解，采用Grant和Tabakoff公式求解岩屑粒子冲蚀速率，借助实验数据验证了仿真模型。利用该模型研究了气体钻井中钻杆居中以及不同钻杆偏心率、井径扩大率、注气量和机械钻速下环空岩屑粒子运移规律和钻杆冲蚀特性。研究表明，岩屑对钻杆接头及下坡面本体的冲蚀比钻杆本体及接头上坡面严重；钻杆偏心和井径扩大都使局部冲蚀速率峰值增大，且偏心越严重，井径扩大越大，局部冲蚀速率峰值增速越大；机械钻速增大，导致钻杆局部冲蚀速率峰值近乎线性增加；在工程常用注气量范围内，注气量对冲蚀速率峰值和冲蚀速率平均值的影响很小。根据冲蚀规律提出如下建议：减小钻杆斜坡坡度可减少钻杆接头及下坡面的局部冲蚀；采用低转速的空气锤钻井工艺可减缓因偏心引起的局部冲蚀；井眼扩大较大时，增大注气量，通过改变岩屑粒子轨迹，可减少岩屑对钻杆本体的多次冲蚀。

关键词: 气体钻井, 气-固两相流, 岩屑运移, 冲蚀；钻杆, 计算流体动力学, 数值模拟