超深井侧钻段泥岩井壁失稳分析

为了解决西部超深硬脆性泥岩地层侧钻过程中井壁易垮塌的难题,从矿物特征及钻井液作用下地层强度的变化规律出发,确定了井壁失稳原因,考虑了层理面产状、井眼轨迹及地应力的综合影响,根据桑塔木组井壁围岩强度破坏条件建立了造斜井段井壁失稳地质力学模型.利用提出的井壁稳定力学模型分析可得,TKX-CH井侧钻段泥岩造斜初期坍塌压力当量密度为1.22 kg/L,实钻采用钻井液密度1.12 kg/L,井下掉块严重;井斜角达到58°时,将钻井液密度降至1.11 kg/L,井下掉块即得到抑制.现场试验表明,相同井斜角和侧钻方位角条件下,随着差应力比值的增大以及泥岩裸露在钻井液中时间的增长,维持井壁稳定的钻井液密度增大.研究认为,钻井设计时应根据地应力状态优选合理的造斜方位,以有效规避地层井壁围岩坍塌失稳风险高的井段,降低安全钻进风险. 

Wellbore Instability Mechanism of Deep Sidetracking Wells in Mudstone Strata

To solve the wellbore instability problem of a deep sidetracking well in hard-brittle mudstone strata of west China,this paper indicated the mechanism of wellbore instability considering the effects of both minerals and drilling mud on rock compressive strength.Based on failure condition of wellbore,a model of well instability was built considering the contributions of bedding plane occurrence,in-situ stress,and well trajectory,etc.The calculations of this model for the sidetracking mudstone segment in Well TKX-CH showed that the collapse pressure is 1.11 kg/L.Serious rock-collapse happened in practice when drilling mud was 1.12 kg/L.This situation improved as the drilling mud density decreased to 1.1 kg/L,when the deviation angle reached to 58°.When the sidetracking orientation and deviation angle are constant,the density of drilling mud to keep well stable increases as the ratio of stress difference increases and as exposure time of mudstone inside drilling mud increases.Therefore,in order to decrease the risk of borehole instability,in-situ stress,bedding planes and exposure time of mudstone should be considered systematically in drilling design.