力-化耦合作用下水化坍塌周期研究

在钻井过程中,钻井液与泥页岩相互作用所引起的水化应变,改变了井周应力分布,减小了泥页岩岩石强度,使得岩石力学参数也出现了改变,最后的结果导致井壁失稳,发生井壁坍塌。作者基于前人针对水化作用对于岩石力学参数影响的研究,以弹性力学和岩石力学等理论为基础,通过室内实验得出吸水扩散系数,同时考虑水化作用产生的弱化影响和附加水化作用所产生的应力,建立了力-化耦合作用下坍塌压力预测模型,绘制了水化坍塌周期图版。研究结果表明,含水量与吸水处距离成负相关关系,与吸水时间呈正相关关系,同时水化作用也会对岩石力学参数造成影响。南海目标储层在使用密度为1.12 g/cm^(3)的水基聚合物钻井液的条件下坍塌周期在10 d左右;当钻井液密度为1.13 g/cm^(3)时,坍塌周期为21 d。最终从水化坍塌周期图版可以看出,在钻开泥页岩地层后,将泥浆密度逐渐提升至1.14~1.15 g/cm^(3)之间,有益于长时间维持井壁稳定,防止井壁坍塌。该研究成果丰富了泥页岩井壁稳定研究内容,为安全钻井周期的设计提供了理论依据。

In the process of drilling,the hydration strain caused by the interaction between drilling fluid and shale changes the stress distribution around the well,reduces the shale rock strength,and changes the rock mechanical parameters,which leads to wellbore instability and wellbore collapse.Based on the previous studies on the influence of hydration on rock mechanical parameters,as well as the theories of elasticity and rock mechanics,the author obtains the water absorption diffusion coefficient through indoor experiments.Meanwhile,considering the weakening effect of hydration and the stress caused by additional hydration,the prediction model of collapse pressure under mechanical coupling is established,and the period chart of hydration collapse is drawn.The results show that there is a negative correlation between water content and the distance of water absorption,and a positive correlation between water content and water absorption time.At the same time,hydration will also affect the rock mechanical parameters.The collapse period of the target reservoir in the South China Sea is about 10 days under the condition of using water-based polymer drilling fluid with density of 1.12 g/cm^(3),and 21 days when the density of drilling fluid is 1.13 g/cm^(3).Finally,the hydration collapse cycle chart shows that after drilling the shale formation,the mud density is gradually increased to 1.14~1.15 g/cm^(3),which is beneficial to maintain wellbore stability and prevent wellbore collapse for a long time.The research results enrich the research content of shale wellbore stability and provide a theoretical basis for the design of safe drilling cycle.