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

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