Abstract: Borehole instability of the Weizhou Member has been encountered during drilling in Block Xuwen, although the mud weight used exceeded the equivalent density calculated from the collapse pressure of the formation, as known from logging data and analyses of 3 pressure profiles. It was found that fractures and fissures were developed in the Weisan Member. In laboratory experiments, cores taken from the formation of interest were immersed in drilling fluid, and changes in collapse pressure were calculated using a simplified equation of calculating mud weight from collapse pressure. It was found that the rock strength decreased with immersion time, resulting in a large increase in collapse pressure of formation; in the experiments, the collapse pressure increased by 12.15% after 5 d of immersion in drilling fluid. Analyses of borehole instability in the Weisan Member indicated that lack of sealing and plugging of formation by drilling fluid resulted in filtrate invasion, which in turn reduced the strengths of formation and increased the pressure of formations near the wellbore. The collapse pressure of formation was thus increased, resulting in borehole instability. Based on these understandings, the formulation of drilling fluid was modified by adding organic salts, encapsulators and amine additives into the drilling fluid, and using ZHFD, QS-4 and NFA-25 as plugging agents. With this optimized drilling fluid, the time required for the borehole to collapse was prolonged, and borehole instability of the Weisan Member was prevented.