裸眼井筒的坍塌压力与漏失裂缝的起裂压力直接决定了钻井液的安全密度窗口。针对传统钻井漏失压力计算方法在坍塌非圆形井眼条件下的局限性,以及未考虑井眼尺寸效应的问题,采用位移不连续法和抗拉强度与断裂韧性双准则,建立数值模型,明确了圆形井眼以及非圆形“猫耳”坍塌井眼的漏失压力变化特征,同时研究了坍塌拉长井眼漏失裂缝的多点起裂位置以及偏移特征,并探讨了两者对漏失压力的影响规律。数值模拟结果表明:圆形井眼以及非圆形“猫耳”坍塌井眼的漏失压力均表现出尺寸效应,而纯“猫耳”形井眼的漏失压力与纯圆形井眼的漏失压力并没有显著的区别;对于小尺寸井眼而言,漏失压力是由裂缝断裂韧性所主导,受尺寸效应影响较明显;而对于大尺寸井眼而言,漏失裂缝的起裂是由抗拉强度所主导,受尺寸效应影响较弱;随着“猫耳”形井眼的拉长以及漏失起裂点处局部半径的增加,泥浆密度窗口会逐渐扩大;“猫耳”形井眼在长度上的拉伸会导致漏失裂缝偏离最大主应力方向,从而在井眼周围出现4个裂缝漏失点,形成X形漏失裂缝。该研究可为裸眼坍塌井筒所存在的漏失问题提供借鉴。
The collapse pressure of an open-hole wellbore and the initiation pressure of lost-circulation fractures directly determine the safe mud weight window during drilling.In light of the limitations of traditional lost-circulation pressure calculation methods under collapsed non-circular borehole conditions,as well as the neglect of borehole size effects,the displacement discontinuity method and a dual criterion(tensile strength and fracture toughness)were used to establish a numerical model,clarifying the characteristics of lost-circulation pressure variation for both circular and non-circular“cat ear”collapsed boreholes.The multiple initiation positions and deviation characteristics of lost-circulation fractures in elongated collapsed boreholes and their effects on lost-circulation pressure were also investigated.Numerical simulation results indicate that the lost-circulation pressure for both circular and non-circular“cat ear”collapsed boreholes exhibits a size effect.However,the lost-circulation pressure of a purely“cat ear”-shaped borehole does not differ significantly from that of a purely circular borehole.For small-sized boreholes,the lost-circulation pressure is governed by fracture toughness and is markedly influenced by size,whereas for large boreholes,fracture initiation is governed by tensile strength and the size effect is weaker.As the “cat ear” borehole elongates and the local radius at the initiation point increases,the mud weight window gradually widens.Axial elongation of the “cat ear” borehole causes lost-circulation fractures to deviate from the maximum principal stress direction,leading to four fracture initiation points around the borehole and the formation of an X-shaped lost-circulation fracture pattern.This study provides reference for addressing lost-circulation issues in collapsed open-hole wellbores during drilling.
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