井壁失稳风险的可靠度理论评价方法

国内外学者针对井壁稳定的问题已经开展了较为深入的研究,建立了多种经验模型、解析模型和数值模拟方法,但针对输入参数的不确定性及其对井壁稳定分析结果影响的研究则较少、认识尚不明确。为此,以井壁稳定力学解析模型为基础,结合可靠度理论下的一次二阶矩方法,建立了基于可靠度理论的井壁失稳风险评价方法,研究了不同钻井液当量密度下的井壁稳定可靠概率,并考察了参数不确定程度对井壁稳定分析结果的影响规律。研究结果表明:①井壁稳定输入参数的分布规律基本满足正态分布,变异系数越高,则样本数据的不确定性越强,对井壁稳定分析结果的影响将更加显著;②随着钻井液当量密度的增加,井壁垮塌的概率逐渐降低,但井壁被压漏的概率也逐渐增加,在井壁坍塌和破裂可靠概率曲线交点以下能够找到一个合适的安全窗口;③各地质因素不确定性对井眼稳定的影响顺序为:地应力孔隙压力岩石强度。结论认为,在井壁稳定分析中准确确定地应力的大小、降低其不确定程度的影响,可以提高井壁稳定评价的准确性。

A risk assessment method of wellbore instability based on the reliability theory

Domestic and foreign scholars have carried out thorough studies on wellbore stability and have established a lot of empirical models, analytical models and numerical simulation methods. However, the uncertainty of input parameters and its effect on analysis result of wellbore stability are less researched and no clear understanding is obtained. In this paper, a risk assessment method of wellbore instability based on reliability theory was established on the basis of mechanical analytics model of wellbore stability, combined with First Order Second Moment method from reliability theory. By virtue of this method, the reliability probability of wellbore stability at different equivalent densities of drilling fluid was analyzed, and the influence of the uncertainty degree of parameters on the analysis result of wellbore stability was investigated. And the following research results were obtained. First, the distribution law of the input parameters of wellbore stability fits for normal distribution. As the variation coefficient increases, the uncertainty of the sampled data increases and its influence on the analysis results of wellbore stability get more significant. Second, as the equivalent density of drilling fluid increases, the probability of wellbore collapse decreases gradually and the probability of wellbore leakage increases gradually. An appropriate safety window can be confirmed at the intersection between reliability curves of wellbore collapse and fracture. Third, the influence of the uncertainty of different geological factors on the wellbore stability is ordered as: in-situ stress > pore pressure > rock strength. In conclusion, when wellbore stability is analyzed, it is necessary to determine the in situ stress accurately to reduce the influence of its uncertainty degree, so as to improve the accuracy of wellbore stability evaluation.