PDC钻头布齿参数与地层适应性的试验研究
邹德永1, 徐城凯1, 易杨1, 陈修平2, 于金平1
1.中国石油大学(华东)石油工程学院
2.中国石油塔里木油田公司油气工程研究院
通讯作者:徐城凯,1993年生,硕士研究生;研究方向为油气井岩石力学与工程。E-mail:xchkai@163.com

作者简介:邹德永,1962年生,教授,博士生导师;主要从事油气井工程方面的教学与科研工作。地址:(266580)山东省青岛市黄岛区长江西路66号。电话:18561409382。ORCID: 0000-0002-5405-080X。E-mail:zouyan@upc.edu.cn

摘要

根据地层性质合理选择或设计PDC钻头可以显著提高钻井效率。为指导PDC钻头合理选型及个性化设计,选取21只试验钻头,在6种不同可钻性级别的岩石上进行钻进试验,研究了在钻进不同可钻性级别的岩石中切削齿尺寸、后倾角、布齿密度的合理取值问题。结果表明:①当岩石可钻性级别小于Ⅲ时,破岩效率随切削齿尺寸的增大而增大;②当岩石可钻性级别大于Ⅲ时,16 mm切削齿的破岩效率最高;③切削齿尺寸对破岩效率产生不同影响规律的岩石可钻性级值分界点为3.11;④在6种不同可钻性级别的岩石中,随切削齿后倾角增大,破岩效率均呈线性规律降低,但随着岩石可钻性级别的增大,后倾角对破岩效率的影响程度降低;⑤在不同可钻性级别的岩石中,随着布齿密度增大,破岩效率均呈幂函数规律降低。该研究成果为按岩石可钻性级别合理选择PDC钻头提供了依据。

关键词: PDC钻头; 台架试验; 破岩效率; 切削齿尺寸; 切削齿后倾角; 布齿密度; 岩石可钻性; 地层适应性
An experimental study on PDC bits' cutter parameters and formation adaptability
Zou Deyong1, Xu Chengkai1, Yi Yang1, Chen Xiuping2, Yu Jinping1
1. College of Petroleum Engineering, China University of Petroleum <East China>, Qingdao, Shandong 266580, China;
2. Oil and Gas Engineering Research Institute, PetroChina Tarim Oilfield Company, Korla, Xinjiang 841000, China
Abstract

If a PDC bit is selected or designed rationally according to the formation properties, its drilling efficiency can be improved significantly. For providing reference for a rational selection and personalized design of PDC bits, 21 test bits were selected to perform the drilling test on six rock samples in different types and with different drillability grades. Then, the optimal values of cutter sizes, back rakes and cutter densities for different rock drillability grades were analyzed. It is shown that when the drillability grade is less than Ⅲ, the drilling efficiency increases with the increase of the cutter sizes. Otherwise, the rock breaking efficiency is the highest when the cutter size is 16 mm; that the critical point of rock drillability grade at which the effect law of cutter size on the rock breaking efficiency changes is 3.11; that as for the rocks with different drillability grades, the rock breaking efficiency decreases linearly with the increase of back rakes. However, the higher the rock drillability grade is, the less the effect of back rake on the rock breaking efficiency is; and that with the increase of cutter density, the rock breaking efficiency is reduced by the power function. These research results provide a basis for the rational selection of PDC bits according to rock drillability grades.

Keyword: PDC bit; Bench test; Rock breaking efficiency; Cutter size; Back rake; Cutter density; Rock drillability; Formation adaptability

根据地层性质合理选择或设计PDC钻头对于提高钻井效率具有重要意义。地层性质不同, 与之相适应的钻头结构参数, 尤其是对PDC钻头破岩效率有重要影响的布齿参数也就不同[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]。Sinor等[21]、邹德永和蔡环[22, 23]通过单齿切削试验, 对PDC钻头布齿参数与地层的适应性进行了研究。然而, PDC钻头破岩过程是多齿联合作用过程, 单齿切削试验结果往往具有一定的片面性。如Akbari等[24]发现13 mm切削齿破岩效率高于16 mm切削齿; Hough Jr[25]得出后倾角越大, 钻头破岩效率越高的结论。但实际情况却并非如此。因此, 笔者通过全尺寸PDC钻头的室内台架试验, 对切削齿尺寸、后倾角、布齿密度等布齿参数与地层的适应性进行了系统的研究, 分析了布齿参数在不同可钻性级别的地层中的合理取值问题, 以期为PDC钻头选型和个性化设计优化设计提供理论依据。

1 试验钻头设计
1.1 钻头剖面形状及水力结构设计

设计加工了21只PDC钻头, 钻头直径均为114.3 mm。为了避免冠部剖面形状及水力结构设计的不同对试验结果产生影响, 所有试验钻头都采用相同的冠部剖面形状和水力结构, 其参数如表1所示。

表1 实验钻头结构参数表
1.2 钻头布齿参数设计

试验钻头均设计了4个布齿刀翼, 以等磨损原则布置切削齿, 并形成完整的井底覆盖。图1所示为切削齿尺寸为16 mm的钻头设计。各试验钻头切削齿尺寸、后倾角及布齿数量设计如表2所示。

图1 钻头布齿设计图及实体图

表2 试验钻头布齿参数表
2 室内台架试验
2.1 试验装置

本试验所用装置为中国石油大学(华东)岩石破碎实验室XY-2B型钻机, 如图2所示。

图2 XY-2B型钻机图

该钻机由转速控制系统、钻压控制系统、位移传感器以及数据采集系统等部分组成, 并用清水代替钻井液作为循环介质。

2.2 试验方法及参数

本次试验共选用6种可钻性级值不同的岩石, 每种岩石力学参数及对应钻压转速如表3所示。

表3 试验参数表

每种钻头在固定的钻压和转速下, 在每一种岩石中钻孔3次, 每次钻孔深度为350 mm, 记录每次钻孔时间。按下式计算破岩效率:

式中vr表示破岩效率, m/(h· kN); h表示单孔钻进深度, 本试验中固定为350 mm; ti表示单次钻进时间, s; W表示钻压, kN。

3 试验结果分析
3.1 切削齿尺寸对破岩效率影响规律

不同岩石可钻性下破岩效率与切削齿尺寸关系曲线如图3-a所示; 不同切削齿尺寸下破岩效率与岩石可钻性级值关系曲线如图3-b所示。

图3 切削齿尺寸和岩石可钻性级值对破岩效率影响图

由图3-a可知, 钻进可钻性Ⅲ 级下岩石(红色砂岩)时, 随切削齿尺寸增大, 破岩效率呈增大趋势; 钻进可钻性Ⅲ 级以上岩石时, 破岩效率随切削齿尺寸变化规律一致, 均呈先增大后减小的趋势, Ø 16 mm切削齿的破岩效率最高。也就是说, 对可钻性Ⅲ 级以上的地层, Ø 16 mm切削齿最合适。

由图3-b可知, Ø 16 mm切削齿破岩效率曲线与Ø 19 mm切削齿破岩效率曲线存在唯一交点, 则此交点对应横坐标即为切削齿尺寸对破岩效率产生不同影响规律的岩石可钻性级值分界点Kd0, 约为3.11。

上述规律可用比钻压(接触应力)来解释。表4列出了不同尺寸的切削齿在6种岩石中钻进时的比钻压。可以看出, 在红色砂岩中, Ø 19 mm切削齿钻头钻进比钻压最大; 在其余5种岩样中, Ø 16 mm切削齿钻头钻进比钻压最大。比钻压越大, 说明攻击能力越强, 故破岩效率最高。

表4 不同尺寸切削齿钻进比钻压计算值表
3.2 切削齿后倾角对破岩效率影响规律

Ø 13 mm、Ø 16 mm、Ø 19 mm切削齿钻头破岩效率与切削齿后倾角关系曲线如图4-a~4-c所示, Ø 16 mm切削齿钻头破岩效率随可钻性级值变化曲线如图4-d所示。

图4 切削齿后倾角与破岩效率关系曲线图

由图4-a~4-c可知, 在3种切削齿尺寸条件下, 6种岩石上PDC钻头破岩效率均随着切削齿后倾角角度增加近似线性降低; 由图4-d可知, 随岩石可钻性级值增加, 后倾角对破岩效率影响程度明显降低。

PDC钻头切削齿后倾角减小, 则切削齿与岩石的接触面积减小, 钻压不变时, 切削齿与岩石接触部位应力增加, 整个PDC 钻头破岩效率提高。但是切削齿后倾角并非越小越好, 试验与现场实际均证明, 后倾角小, 则PDC钻头受冲击力较大, 钻头寿命短。因此在钻进硬地层时, 因后倾角对破岩效率影响较小, 且钻头受冲击力大导致钻头寿命较短, 此时钻头寿命为首要考虑因素, 建议采用大于20° 的后倾角以保证钻头寿命; 对软地层, 后倾角对破岩效率影响较大, 且钻头受冲击力小, 钻头寿命长, 此时破岩效率为首要考虑因素, 建议采用小于20° 的后倾角以提高破岩效率。

3.3 布齿密度对破岩效率影响规律

不同岩石可钻性条件下破岩效率与切削齿数量关系曲线如图5-a所示; 不同切削齿数量条件下破岩效率与岩石可钻性级值关系曲线如图5-b所示。

图5 钻头破岩效率与布齿密度关系曲线图

由图5-a可得到:在钻进6种不同可钻性级值岩石时, PDC钻头破岩效率均随布齿密度的增加而近似呈幂函数形式降低。由图5-b可知, 随岩石可钻性级值增加, 布齿密度对破岩效率影响程度明显降低。

相同钻压下, 布齿密度越小, 单齿承受载荷越大, 切削齿吃入岩石能力越强, PDC钻头破岩效率越高。但是试验与现场实际均表明:布齿密度低使得单齿载荷过大会导致崩齿、碎齿等现象, 严重影响PDC钻头使用寿命。因此在钻进硬地层时, 因布齿密度对破岩效率影响较小, 且钻头易发生崩齿、碎齿现象, 此时钻头寿命为首要考虑因素, 建议采用高密度布齿以保证钻头寿命; 对软地层, 布齿密度对破岩效率影响较大, 且崩齿、碎齿现象不严重, 此时破岩效率为首要考虑因素, 建议采用中低密度布齿以提高破岩效率。

4 结论及建议

1)钻进可钻性级值小于3.11的岩石时, PDC钻头破岩效率随切削齿尺寸的增大而提高; 钻进可钻性级值大于3.11的岩石时, 16 mm的切削齿钻头破岩效率最高。

2)随切削齿后倾角增大, PDC钻头破岩效率近似线性降低; 随岩石可钻性增大, 后倾角对破岩效率的影响程度降低。综合考虑钻头破岩效率及抗冲击性能, 对软地层, 建议采用小于20° 的后倾角以提高破岩效率; 对硬地层, 建议采用大于20° 的后倾角以提高抗冲击性能。

3)随布齿密度增加, PDC钻头破岩效率近似呈幂函数形式降低; 随岩石可钻性增大, 布齿密度对破岩效率的影响程度明显降低。综合考虑钻头破岩效率及使用寿命, 对软地层, 建议采用低密度布齿; 对硬地层, 建议采用高密度布齿。

The authors have declared that no competing interests exist.

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