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

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-chemical-thermal 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/cm³, and the existence of formation fractures leads to the increase of the minimum collapse pressure in this direction by 0.05 g/cm³. The 10 d collapse pressure of horizontal hole rises to 1.38 g/cm³. What’s more, the seepage leads to the increase of the formation collapse pressure by 0.04 g/cm³, 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.