冲旋步进钻井提速方法的井底裂纹拓展特性

Propagation Characteristics of Bottomhole Cracks Created by the Percussive-Rotary Stepping Drilling ROP Improving Method

针对深部地层井底岩石硬度高、强度大和应力集中现象导致的岩石可钻性差、破岩效率低和使用寿命短的问题,提出旋冲钻井和差压步进破岩方法相结合的冲旋步进钻井提速新理念。为了研究冲击作用下差压步进钻头破碎地层岩石的裂纹拓展规律,采用有限-离散元(FDEM)方法建立了球齿冲击三维薄板模型,开展了球齿在不同冲击能量、冲击位置及岩石种类等条件下的冲击破岩过程模拟,获得了阶梯型井底岩石在冲击作用下的裂纹扩展规律。研究结果表明:由于阶梯面的存在,球齿在冲击作用下对阶梯型井底造成的预损伤区域比常规井底更大,冲击位置靠近阶梯面的裂纹扩展效果更好;双球齿冲击阶梯井底容易形成较大范围的岩石破碎区,降低了破碎阶梯井底地层岩石的难度;球齿冲击过程中对青砂岩造成的预损伤区域比花岗岩大,在冲击能量相同的情况下球齿冲击破碎花岗岩更为困难。利用冲旋步进钻井方法理论上可以进一步提高难钻地层钻井速度,但具体效果需要经现场试验及优化。所得结论可为深部难钻地层钻井速度的提升提供新的思路。

In order to solve the problems of poor rock drillability,low rock-breaking efficiency and short service life caused by high hardness,high strength and stress concentration of bottomhole rock in deep formations,a new concept of ROP improvement by percussive-rotary stepping drilling combining percussive-rotary drilling and differential pressure stepping rock-breaking method was proposed.In order to study the crack propagation law in formation rocks broken by differential pressure stepping bit under impact,an spherical button impact 3D sheet model was built using the finite-discrete element method(FDEM).The percussive rock-breaking process of the spherical button under conditions such as different impact energies,impact positions and rock types was simulated,obtaining the crack propagation law of stepped bottomhole rocks under impact.The research results show that due to the presence of stepped face,the predamage area caused by spherical button under impact in the stepped bottom hole is larger than that in the conventional bottom hole,and the crack propagation effect is better when the impact position is close to the stepped face.The double spherical button impact at stepped bottom hole is prone to forming a relatively extensive rock-breaking area,reducing the difficulty of crushing the formation rocks at stepped bottom hole.The predamage area caused by spherical button impact in blue sandstone is larger than that in granite,and under the same impact energy,the crushing of granite by spherical button impact is more difficult.The percussive-rotary stepping drilling method can theoretically further improve the drilling speed in bad ground,but the specific effect needs to be tested and optimized on site.The research conclusions provide new ideas for improving drilling speed in deep bad ground.