Stress distribution model of hard and brittle mud shale based on hydration damage

Abstract

Hard and brittle shale developed in the middle and deep layers of the Bohai Sea cause severe challenges in drilling. During the drilling process, drilling fluid easily penetrates the rock along the microcracks, which leads to the deterioration of the mechanics and strength parameters of the rock. This consequently changes the stress distribution in the borehole wall. In addition, the fluid seepage hydrates the rock, which affects the relationship between the change in water content at any position and at any time in the rock mass is solved. The hydration equivalent of the elastic modulus is regarded as damage evolution, and a damage tensor considering hydration is introduced. A constitutive relationship of hard and brittle mud shale considering the effect of hydration damage is established, and the stress distribution around the well of the hard and brittle mudstone under non-uniform stress is solved by a semi-numerical and semi-analytical method. In addition, the influence law of hydration damage on the stress distribution around the well is analyzed. Taking an 8-1/2-inch section of an exploratory well in the Bozhong 19-6 gas field as an example, the stress distribution and collapse pressure analysis of the well bore revealed that after the section of the well was drilled for 100 h, the formation stiffness caused by hydration damage significantly decreased and borehole instability occurred inside the borehole wall. The risk of collapse was greater in the direction of the smallest in situ stress. The effect of hydration expansion on the stress distribution was limited, showing only a slight effect on the tangential stress near the well wall. The collapse of hard and brittle mud shale is mainly related to deterioration of the rock stiffness performance and strength parameters caused by hydration damage. The section incurred a short period of collapse, in which the maximum diameter expansion rate was equivalent to 5.9%, and the density of drilling fluid used on site was relatively low. Therefore, the density of the drilling fluid should be appropriately increased to enhance its sealing properties. The model reveals the hydration collapse mechanism of hard and brittle shale to a certain extent, which provides a theoretical basis for the safe and efficient drilling in similar formations.