井底压差对岩石破碎的影响机制

Mechanism for effect of down hole differential pressure on rock failure

综合地层应力、井底压力、孔隙压力以及牙齿吃入地层引起的应力,建立牙齿吃入深度的表达式,依据摩尔-库伦准则建立钻头牙齿吃入地层的岩石破碎模型。将综合地层和钻井参数因素的误差函数β与深度x的比值作为欠平衡钻井的应用参考参数,计算各种压差和β/x条件下的吃入深度,并分析其变化规律。结果表明:在压差的作用下,具有渗透率和孔隙度的待破碎薄地层内的孔隙压力并非原始地层孔隙压力,而是随时间和位置的变化而变化,其分布规律可以用一维不稳定渗流来表示和计算;牙齿吃入深度的表达式可以将压力、流体等因素对岩石破碎的作用清楚地表达出来;所建模型可以解释欠平衡钻井提高钻速的力学机制,以及定量描述随钻地层压力监测dc指数法的适用范围。

Based on the stresses in formation including drilling fluid pressure at down hole,pore pressure and stress caused by teeth invasion,the expression of teeth invasion depth was developed,and rock failure model for teeth invasion was established according to Morh-Coulomb principle.Using the ratio of error function and depth x as the reference parameters for underbalanced drilling application,invasion depths under all kinds of differential pressures and β/x were calculated,and the variation law was analyzed.The calculation results show that the real pore pressure of formation near down hole is not the original pore pressure because rock has permeability and porosity.The real pore pressure is related with time and location,and its distribution law can be expressed by one dimension unstable filtration theory.The expression of teeth invasion depth can clearly demonstrate respective effect of pressures and fluid on rock failure.This model can explain the mechanism of enhanced rate of penetration in underbalanced drilling,and it can depict the scope of application for dc index method correctly.