Abstract: The concealment and complexity of geological environment will lead to great uncertainties in geomechanical parameters and rock mechanics parameters. Ignoring the influence of uncertainty in wellbore stability analysis may lead to unreasonable drilling fluid density design, which will directly affect the wellbore stability during drilling. Domestic and foreign researchers have carried out in-depth studies on the wellbore stability under the condition of determined parameters, rather than on the influence of uncertain parameters on the wellbore stability, especially for the evaluation of the instability risk of arbitrary deviated well. Therefore, a risk assessment method of wellbore instability based on reliability theory is established using the poroelastic model and breakout width model. Further, using Monte-Carlo stochastic method, this paper simulates the wellbore instability risks of vertical, deviated and horizontal wells in CW gas field in Sichuan Basin, and systematically analyzes the influence of the mean value and variance of the parameters on wellbore instability. The results show that:(1) with the increase of deviation angle, the equivalent density of collapse pressure increases significantly, the equivalent density of fracture pressure decreases significantly, and the instability law of different wells is basically consistent with that of conventional model; (2) when considering the influence of parameter uncertainty, the equivalent density of collapse pressure increases, the equivalent density of fracture pressure decreases, and the safety density window of drilling fluid gradually narrows, indicating that the influence of parameter uncertainty cannot be ignored; (3) the collapse width has a significant impact on wellbore collapse; the larger the collapse width, the higher the reliability of wellbore collapse; on the premise of no wellbore collapse accident, the controllable occurrence of wellbore collapse is conducive to improve the drilling success rate; (4) the sensitivity analysis of the mean value and variation coefficient of parameters shows that the most significant factor affecting wellbore stability is in-situ stress, followed by pore pressure and rock strength; with the increase of variation coefficient, the equivalent density of collapse pressure gradually increases, the equivalent density of fracture pressure gradually decreases, the safety density window of drilling fluid gradually narrows, and the higher the risk of wellbore instability is.

Key words: reliability theory, risk assessment, wellbore stability, collapse pressure, fracture pressure, Monte-Carlo method

摘要： 由于地质环境的隐蔽性和复杂性，地质力学参数和岩石力学参数具有很大的不确定性，井壁稳定分析中忽略不确定的影响可能导致钻井液密度设计不合理，进而直接影响钻井井壁稳定。国内外学者对确定参数条件下的井壁稳定开展了深入的研究，但对不确定参数条件下井壁稳定影响的研究并不深入，尤其是对于任意斜井失稳风险的评价。为此，在井壁稳定孔弹性力学模型和崩落宽度模型基础上，建立了基于可靠度理论的井壁失稳风险评估方法，采用Monte-Carlo随机方法模拟了四川盆地CW气田直井、斜井和水平井的井壁失稳风险，并系统分析了参数均值和方差对井壁失稳的影响规律。研究结果表明：①随着井斜角增加，井壁坍塌当量密度显著增加，井壁破裂当量密度显著降低，不同井型失稳风险与常规井壁稳定规律基本一致。②考虑参数不确定影响后，坍塌当量密度增加、破裂当量密度降低，安全密度窗口逐渐变窄，说明参数不确定影响不可忽略。③崩落宽度对井壁坍塌影响显著，崩落宽度越大则井壁坍塌可靠度越高，在不发生井壁失稳事故的前提下，允许适当的井壁崩落有助于提高成功钻井概率。④4参数均值和变异系数敏感性分析结果表明，影响井壁稳定最为显著的因素为地应力，其次为孔隙压力和岩石强度；随着变异系数的增加，坍塌当量密度逐渐增加、破裂当量密度逐渐降低，安全密度窗口逐渐变窄，井壁失稳的风险越高。

关键词: 可靠度理论, 风险评估, 井壁稳定, 坍塌压力, 破裂压力, Monte-Carlo方法