Numerical simulation of lost-circulation fracture initiation in collapsed wellbores

doi:10.3969/j.issn.1006-6535.2026.02.018

Abstract

Abstract: 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.

Key words: lost-circulation pressure, lost-circulation fracture initiation length, dual criterion, borehole collapse, multi-fracture borehole

CLC Number:

TE348

LI Shuai, XU Jiangwen, WANG Mingxing, GONG Jie, CHEN Xi, FU Haifeng, YAN Xingming, QIAN Fengxue. Numerical simulation of lost-circulation fracture initiation in collapsed wellbores[J]. Special Oil & Gas Reservoirs, 2026, 33(2): 159-165.

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