Collapse pressure prediction and borehole trajectory optimization for coal measure strata

Coal cleats are developed in a stagger manner, so conventional models for predicting borehole collapse pressure cannot accurately guide drilling operations or even may cause borehole wall instability incidents. The physical and chemical properties and microstructure of coal were analyzed and then mechanical instability was determined as the main type of borehole wall instability. A model for calculating the collapse pressure of coal in horizontal wells was established based on the theories of elastic mechanics, rock mechanics and single weakness plane failure criteria. With this model, borehole trajectories in coal intervals of the DB-X01 and DB-X04 wells and the Honghe Oilfield were optimized and the best inclination and azimuth for drilling were obtained. The influences of cleat occurrence, geostress mechanism and borehole trajectory on collapse pressure were analyzed using the J₂kz coal in Well DB-X01 as an example, which provided an idea and reference for the optimization of borehole trajectories of horizontal wells in coal measure strata.