煤层气钻井井壁稳定机理及钻井液密度窗口的确定

在煤层中钻井时,保持井壁的稳定具有重要意义。为此,利用有限元软件分析了近井壁裂纹尖端的受力状况,比较了尖端与井壁两处应力集中的情况,结果表明,裂纹尖端受力高于井壁的应力集中约一个数量级。从断裂力学的角度,研究了近井壁裂纹的扩展,分析了裂纹扩展在节理煤岩井壁失稳中所起的作用,讨论了裂纹表面闭合、孔隙压力及井壁渗透性等因素对井壁失稳的影响。利用经典断裂力学理论即应力强度因子理论和最大周向应力准则,对近井壁的裂纹进行了抽象建模,建立了煤岩井壁失稳的判据。根据失稳判据,提出拉剪破坏压力和压剪破坏压力的概念,两者分别为节理煤层钻井液密度窗口的上下临界值,两者之间为井壁稳定,两者之外便是井壁失稳。利用山西沁水盆地的岩心数据及现场钻井资料进行了实例计算,基于常规岩石力学方法求出的破裂压力和坍塌压力分别为19.1MPa、6.4MPa,而用所建立的方法得到的上下临界压力为18.1MPa、7.6MPa。由此证明了所建立的方法得到的密度窗口小于前者,与实际情况更加相符。

Mechanism of wellbore stability and determination of drilling fluid density window in coalbed methane drilling

It is important to keep the wellbore stable during coalbed methane drilling. Here in this paper the finite element analysis software is adopted to study the stress condition at the tips of the near wellbore cracks, which is compared with that at the wellbore. The result shows that the former stress is higher than the latter one by a magnitude order. From the perspective of the fracture mechanics, the development of the near wellbore cracks and its bad effects on wellbore instability in coal seam are both studied; and many other influencing factors on wellbore instability like surface closed crack, pore pressure, and permeability of a well wall, etc., are also discussed. Moreover, the classical fracture mechanics theories of stress intensity factor and the maximum circumferential stress principle are used in abstract modelling for near wellbore cracks, and then the criteria for wellbore instability in coal seam is developed, based on which the concepts of pull shear collapsing pressure and compression shear collapsing pressure are thus put forward. The two concepts are the upper and lower critical limits of the drilling fluid density window of the coal seam. If between these two values, the wellbore will be stable; if not, the wellbore will be unstable. Calculation example is made by using the core data and the field data taken from the Qinshui Basin of Shanxi. The fracture pressure and the collapse pressure obtained by the conventional rock mechanics method are 19.1 and 6.4 MPa respectively, while those obtained by the above mentioned method are 18.1 and 7.6 MPa respectively. This demonstrates that the density window obtained by this new method is smaller, showing more agreement with the actual situation.