ZH凹陷硬脆性裂缝油页岩地层坍塌压力研究

Study on the collapse pressure of hard and brittle fractured oil shale in ZH sag

硬脆性裂缝油页岩地层钻进易发生井壁坍塌,严重制约页岩油气有效开发。考虑化学势差、水力压差和裂缝渗流对近井带地层压力的影响,建立页岩井周应力分布模型,基于页岩地层裂缝面产状,构建了任一裂缝面产状下剪应力与正应力计算模型,并以裂缝面上剪应力大于摩擦力为含裂缝面页岩地层的破坏准则,形成了含裂缝面页岩地层的流-固-化-热井壁稳定多场耦合模型。ZH凹陷硬脆性裂缝油页岩地层井壁稳定性分析结果表明,该区块沿最小水平地应力方位钻进,地层坍塌压力最低值为1.173 g/cm^(3),地层裂缝存在使得该方位坍塌压力最低值升高0.05 g/cm^(3);水平井眼10 d时间坍塌压力升至1.38 g/cm^(3);渗流作用使得地层坍塌压力升高0.04 g/cm^(3),其中低温低活度钻井液可以有效降低井壁坍塌压力,利于ZH凹陷油页岩地层井壁稳定。

When drilling through the hard and brittle fractured oil shale,borehole collapse is likely to happen,which seriously restricts the efficient development of shale oil and gas.In this paper,the shale circumferential stress distribution model was established by considering the influences of chemical potential difference,hydraulic pressure difference and fracture seepage on the formation pressure near the wellbore.Then,the model for calculating the shear stress and normal stress under the occurrence of any fracture surface was established based on the occurrence of fracture surface in shale formation.Finally,the hydraulic-mechanical-chemicalthermal multi-field coupling model of stable wellbore in the fracture surface containing shale was formed based on the failure criterion of fracture surface containing shale that the shear stress on the fracture surface is larger than the friction force.The analysis on the wellbore stability in the hard and brittle fractured oil shale of ZH sag shows that when the drilling is performed in the direction of the minimum horizontal geostress in this block,the minimum formation collapse pressure is 1.173 g/cm^(3),and the existence of formation fractures leads to the increase of the minimum collapse pressure in this direction by 0.05 g/cm^(3).The 10 d collapse pressure of horizontal hole rises to 1.38 g/cm^(3).What’s more,the seepage leads to the increase of the formation collapse pressure by 0.04 g/cm^(3),and low-temperature and low-activity drilling fluid can reduce the borehole collapse pressure effectively,which is favorable for the wellbore stability in the oil shale of ZH sag.