基于有限坍塌宽度的层理性泥页岩地层井壁稳定性评价

 钻井工程中一般允许实钻井眼存在有限宽度的崩落区域，如按传统井壁稳定理论模型进行设计会导致钻井液密度偏高、钻井成本增加、钻井效率降低、钻井液漏失甚至地层破裂等一系列问题。针对脆性层理泥页岩地层特性，根据测井资料和测试数据进行地质力学建模，利用基于有限坍塌宽度的定量风险分析方法合理地评估井壁稳定概率，优选钻井液密度窗口。以中国西部某油田泥页岩地层钻井实例进行分析，在基于有限崩落存在的情况下，通过定量分析方法在井壁稳定段及失稳段对钻井液密度窗口进行动态优化，可以提高机械钻速，降低钻井成本，评估的井壁稳定概率可达91%~98%，计算结果与实际情况基本吻合，说明该方法具有重要的工程价值和实际意义。

Stability evaluation for wellbore in layered shale formationbased on limited collapse width

Drilling engineering plays an indispensable role in the modern energy exploration and development. The wellbore stability directly determines the success or the failure of the drilling operation. The traditional wellbore stability model does not allow any degree of wellbore damage, but in the actual drilling operation, the stable wellbore generally allows a limited width of caving area. The design based on the traditional wellbore stability model will lead to a high density of drilling fluid, which not only will increase the drilling cost but also will reduce the drilling efficiency, more likely will cause the loss of drilling fluid or even the formation fracture. According to the characteristics of the brittle shale formation, a wellbore stability model is established. Based on the logging data and the test data, a geomechanical model is established. The quantitative risk analysis method based on the limited collapse width is used to reasonably evaluate the wellbore stability probability and optimize the drilling fluid density. Based on the case study of a certain oilfield in Qinghai Province, this paper uses this method to optimize the density window of drilling fluid in the stable and unstable sections of the wellbore, to some extent, to improve the ROP and reduce the drilling cost. The probability of the wellbore stability can reach 91-98% according to calculation and evaluation. The calculation results are basically consistent with the actual situation, which shows that this method is effective and the results are reliable and practical.