横观各向同性水敏性地层斜井眼坍塌压力确定

在以泥页岩为目标储层的非常规油气藏水平井开发中, 钻遇井壁的稳定性问题突出, 一方面是由于在设计钻井液密度时简单采用各向同性介质模型和Mohr-Coulomb强度准则, 另一方面则是由于选用的钻井液性能欠佳, 难以保证建井全过程的井壁稳定。为此, 将地层视为横观各向同性介质, 采用Mogi-Coulomb强度准则判别岩石基体和层理面的稳定性, 同时考虑泥页岩水化作用对岩石力学强度参数的影响, 建立了水敏性地层井壁稳定分析模型, 分析了坍塌压力随井斜、方位、钻井液性能、钻井时间等因素的变化规律。研究结果表明:对于具有显著层理面结构的泥页岩地层, 采用横观各向同性模型比各向同性介质模型更能描述强度的各向异性;采用Mogi-Coulomb准则比Mohr-Coulomb强度准则得到的坍塌压力值与实际吻合程度更高, 在保证井壁稳定的前提下有利于降低钻井液密度, 实现钻井提速;提高钻井液封堵能力、减小泥页岩吸水扩散能力, 有利于延长坍塌周期。利用东濮、威远、焦石坝等工区多口页岩气水平井的实钻资料对本文方法进行了验证, 证实本文方法确定的钻井液密度窗口下值更能满足安全钻井需求。

Determination of Collapse Pressure for Deviated Wellbore in Transversely Isotropic Water-Sensitive Formation

Borehole stability in shale/mudstone strata is significant during the construction of horizontal well for unconventional hydrocarbon reservoirs,which is caused by two mainly reasons,one is the current borehole stability design model using simply isotropic media model and Mohr-Coulomb criterion,and the other is the inappropriate performance of drilling fluid.To solve this problem,a new borehole stability model for water-sensitive formation was established,which adopted transversely isotropic media model,Mogi-Coulomb criterion,and the effect of hydration to rock strength.The influence of deviation angle,azimuth,drilling fluid performance and drilling time on collapse pressure were analyzed.The results showed that transversely isotropic model was more suitable to describe strength anisotropy for laminated shale/mudstone; collapse pressure calculated by Mogi-Coulomb criterion is more consistent with drilling practice than Mohr-Coulomb,in this case,lower drilling fluid density can be used to keep borehole stability and promote rate of penetration;enhance sealing capacity and decline shale hydration diffusion capacity were helpful to extend collapse cycle.Several examples of shale gas horizontal wells drilled in Dongpu, Weiyuan and Jiaoshiba districts confirmed that the recommended method is highly reliable and effective.