钻井过程中井筒裂缝动态扩展规律研究

Research on dynamic propagation law of wellbore fractures in drilling process

为了搞清井筒裂缝的动态扩展规律,针对渤海湾渤中区域东营组硬脆性泥岩地层微裂缝漏失问题,利用有限元数值模拟法,建立了裂缝动态扩展与井筒压力、地层岩石力学参数的有限元模型,分析了单条裂缝与成组裂缝的延伸长度、宽度、沿裂缝应力分布随压差及初始裂缝长度的变化规律。结果表明,随着井筒压差的增加,裂缝尖端位置处的等效应力约呈0.78 MPa/MPa的趋势增加,井壁裂缝宽度约呈0.01 mm/MPa的趋势增加;随着初始裂缝长度的增加,裂缝尖端处的有效应力约呈0.06 MPa/mm的趋势增加,井壁裂缝宽度约呈0.003 mm/mm的趋势增加,裂缝初始延伸所需的井筒压差逐渐降低。与单条裂缝相比,成组裂缝宽度及延伸长度变化幅度更大。以渤中区域某深井3000 m处漏失情况为例开展模拟,得到漏失处井筒正压差3.9 MPa,裂缝宽度增加80μm,堵漏时综合考虑井筒压差产生的裂缝宽度变化与地层原有裂缝宽度的尺寸,对堵漏材料颗粒进行设计。该成果对工程实际具有一定指导意义。

In order to find out the dynamic propagation law of wellbore fracture,aiming at the leakage of micro fracture in Dongying Formation of hard and brittle mudstone in Bozhong area of Bohai Bay,a finite element model of fracture dynamic propagation,wellbore pressure and formation rock mechanical parameters is established by using the finite element numerical simulation method.And the extension length,width,distribution along fracture stress with pressure difference and group fracture along with the change rule of initial crack length are analyzed.The results show that with the increase of wellbore pressure difference,the equivalent stress at the crack tip increases about 0.78 MPa/MPa,while the fracture width about 0.01 mm/MPa.With the increase of the initial fracture length,the effective stress at the fracture tip increases about 0.06 MPa/mm,while the fracture width of the shaft wall about 0.003 mm/mm,and the wellbore pressure difference required for the initial fracture extension is gradually reduced.Compared with single fracture,the width and extension length of group fracture vary more.Taking the leakage situation of a deep well 3000 m in Bozhong area as an example,the positive pressure difference of the wellbore at the leakage position is 3.9 MPa,and the fracture width is increased by 80μm.When plugging,the change of fracture width caused by the pressure difference of the wellbore and the size of the original fracture width should be considered comprehensively to design the plugging material particles.This achievement has a certain guiding significance for engineering practice.