Study on classification, grading, characterization and regulation for wellbore instability of broken formation in Tarim Basin

There are multiple broken formations developed in the Tarim Basin. However, there are currently no better rock mass quality assessment methods and parameters for evaluating the plugging performance of drilling fluids for broken formation. Firstly, this paper proposes a method to calculate the damage index of the wellbore surrounding rock by using logging data, key rock mechanics parameters and equation coupling damage model to characterize the degree of rock mass fragmentation and verifies it with experiments. Then, the stress level of formation is synthesized by strength stress ratio, horizontal maximum principal stress and rock triaxial compressive strength. At the same time, considering the disturbance of rock water sensitivity and drilling fluid, the rock mass quality grading coefficient of multi-parameter-controlled wellbore surrounding rock is established. Moreover, the plugging coefficients are determined by multiple types of experiments to characterize the plugging performance of the drilling fluid, providing direction for the optimization of the drilling fluid. The study shows that this article establishes a rock mass quality grading coefficient calculation method based on logging data, introducing parameters such as water sensitivity and drilling fluid disturbance and referring to the basic rock mass quality index coefficient and correlation function method. It can quickly and comprehensively evaluate the quality of the wellbore surrounding rock mass of broken formation and predict the risk of wellbore collapse. This study fills the gap of wellbore surrounding rock mass quality evaluation for broken formation. The grading indicators provide theoretical support for the targeted preferential plugging performance of the drilling fluid.