裂缝性地层钻井液漏失模型及漏失规律

Drilling fluid loss model and loss dynamic behavior in fractured formations

针对裂缝性地层钻井作业时的钻井液漏失问题,采用幂律模式钻井液,将地层中裂缝看作任意倾角、可变形、裂缝面粗糙且存在滤失的二维单条裂缝模型,引入裂缝力学开度和裂缝迂曲度参数表征裂缝面粗糙度对钻井液漏失规律的影响,推导了钻井液漏失模型,并基于该模型分析了钻井液漏失规律。研究结果表明,幂律模式钻井液的剪切稀释性会造成漏失初始阶段漏失速率的升高;钻井液漏失速率随裂缝迂曲度变大而减小,随着裂缝开度增加裂缝迂曲度对钻井液漏失速率的影响降低;初始裂缝开度、裂缝倾角、裂缝面积、裂缝长度越大,钻井液漏失速率越大;矩形裂缝的钻井液漏失速率低于正方形裂缝;裂缝面滤失综合系数越大,钻井液漏失速率越高;井眼与裂缝相交于裂缝中心位置时钻井液漏失速率最高;随着井底压差增大,钻井液漏失速率明显升高;裂缝法向刚度越高,钻井液漏失速率越小。

In view of the lost circulation in fractured formations, a two-dimensional transient model for describing a power-law drilling fluid loss in an arbitrarily-oriented, compressible, permeable, rough-walled fracture was introduced. In this model, the mechanical fracture aperture and fracture tortuosity were considered to investigate the effect of fracture roughness on fluid loss dynamics. The governing equation of power-law fluid loss model was given and solved to analyze the fluid loss dynamics in fractured formations. The results show that the shear thinning behavior of power-law drilling fluid can result in high fluid loss rate at the initial stage of loss event; the fluid loss rate decreases as the fracture tortuosity increases, meanwhile, the effect of fracture tortuosity on fluid loss rate will decrease as the fracture aperture becomes larger; the larger the initial fracture aperture, fracture dip, fracture dimensions or fracture length, the higher the fluid loss rate will be; the fluid loss rate of rectangular fractures is much lower than that of square fractures; the higher the total leak-off coefficient, the higher the fluid loss rate will be; the fluid loss rate is the highest when the wellbore intersects the fracture at the center location; the fluid loss rate increases sharply as the differential pressure increases; the larger the normal stiffness of the fracture, the lower the mud loss rate will be.