临界井径计算分析与工程应用

针对钻井过程中松散砂岩、易水化泥页岩等地层易发生井下井径扩大、缩径甚至卡钻等复杂问题，提出了临界井径的新概念，采用了理论分析与现场模拟等方法，研究了其形成原理、影响因素。建立了临界井径反映井壁受钻井液冲刷的相关规律。为了定量描述井壁受冲刷的程度与井眼变化态势，定义了井壁冲刷系数，并用其来反映在特定钻井液流变参数与钻井液排量联合作用的情况下钻井液对井壁冲刷能力的大小。针对不同地层井壁对冲刷能力的不同需求，提出了基于井壁冲刷系数的钻井液流变参数与排量优选的理论及工程应用方法，并在钻井设计与优化施工中得到了广泛地应用。结果表明，基于临界井径的钻井液流变参数与施工排量优化理论，为钻井液流变参数与施工排量的设计与实施优化提供了一种可行的技术方法。

Calculation,analysis and engineering application of critical well diameter

In the process of drilling in unconsolidated sandstone and hydratable shale, downhole hole enlargement and shrinkage, even pipe sticking and other complex situations tend to occur easily. To solve these problems, a new concept of critical well diameter was put forward, and its formation principles and influential factors were investigated by means of theoretical analysis and field simulation. The related laws of critical well diameter to reflect the scour of drilling fluid to the wall were established. In order to quantitatively describe the scour degree of borehole wall and the change situation of borehole, wall scour coefficient was defined and used to reflect the scour capacity of drilling fluid to the wall under the joint effect of the rheological parameter and displacement of the specific drilling fluid. To satisfy the requirements on the scour capacity from the walls in different strata, the theory and engineering application method on rheological parameter and displacement optimization of drilling fluid based on wall scour coefficient were proposed and extensively applied to drilling design and construction optimization. It is indicated that the rheological parameter and construction displacement optimization theory of drilling fluid based on critical well diameter provides a kind of feasible technology for designing and optimizing the rheological parameter and construction displacement of drilling fluid.