Test Research on Tight Sandstone Wellbore Stability During Gas Drilling

In light of the energy dissipation principle, the mechanism of wellbore instability in a tight sandstone formation during gas drilling was studied through triaxial compression tests. The results of triaxial compression tests were analyzed, and it was found that the energy evolution process of sandstone includes three stages, i.e., the stable accumulation of elastic energy, the slow accumulation of dissipated energy, and elastic energy release with rapid accumulation of dissipated energy. With the decrease in confining pressure, the limit of dissipated energy required to destroy sandstone structure decreased exponentially, while that of the stored elastic energy declined linearly. As the loading rate was enhanced, the dissipated energy required to destroy the sandstone structure first decreased and then increased, with the appearance of a critical loading rate. The conversion rate of dissipated energy of sandstone was positively correlated with confining pressure and loading rate, and a high conversion rate caused the weakening of cohesion and the strengthening of friction. Too fast gas drilling enlarged the wellbore instability area, this was more distinct when drilling high-pressure formations. Therefore, appropriately reducing the drilling speed while giving sufficient pressure relief time to formations is conducive to maintaining wellbore stability during gas drilling. The research results are of great significance for optimizing gas drilling speed.