It is vital to design an optimum drilling fluid density for retaining the stability of wellbore wall in drilling engineering. Under estimated drilling fluid density will induce shear failure of wellbore wall. Conversely, wellbore wall will occur tensile failure. Drilling engineers have been looking for an optimum drilling fluid density for a long time. A new method to compute the optimum drilling fluid density was established in this study. Wellbore wall occurs displacement in the process of forming wellbore. The displacement of wellbore wall causes wellbore collapse or fracture. By designing a drilling fluid density to prevent the displacement of wellbore wall from happening, the collapse or fracture of wellbore can be decreased or avoided. Surrounding rocks of wellbore are subject to four forces, overburden pressure, maximum horizontal principal stress, minimum horizontal principal stress and hydrostatic wellbore fluid column, respectively. Mathematical expressions of radial displacement of surrounding rocks of wellbore and drilling fluid pressure were derived through elasticity theory. The latter expression can be used to compute the optimum drilling fluid density. Basic parameters, Young's modulus, Poisson's ratio, overburden pressure, maximum horizontal principal stress, minimum horizontal principal stress, in the expression can be obtained by utilizing logging information. In this work, the new method were used to compute the optimum drilling fluid density of three wells extracted randomly in Tarim Oilfield. Results show that the drilling fluid density values obtained through this method and actual drilling fluid density values in well site have a good fit.

设计最佳的钻井液密度以保持钻井工程中井眼壁的稳定性至关重要。在估计的钻井液密度之下，会引起井筒壁的剪切破坏。相反，井眼壁会发生拉伸破坏。长期以来，钻井工程师一直在寻找最佳的钻井液密度。本研究建立了一种计算最佳钻井液密度的新方法。井眼壁在井眼形成过程中发生位移。井眼壁的位移导致井眼坍塌或破裂。通过设计钻井液密度以防止发生井眼壁的位移，可以减少或避免井眼的坍塌或破裂。井筒的围岩要承受四个力 最大水平主应力，最小水平主应力和静水井筒流体柱。通过弹性理论推导了井筒围岩径向位移和钻井液压力的数学表达式。后一个表达式可用于计算最佳钻井液密度。通过利用测井信息可以获得表达式中的基本参数，杨氏模量，泊松比，上覆压力，最大水平主应力，最小水平主应力。在这项工作中，该新方法被用来计算塔里木油田随机抽取的三口井的最佳钻井液密度。