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The Permian volcanic reservoir in Sichuan Basin is mainly composed of three types of volcanic rocks (basalt, tuff and breccia), showing an intertwined distribution of their characteristics. The mechanical properties of these rocks, affected by the invasion of water-based drilling fluid, are significantly different, resulting in frequent and complex wellbore instability problems during drilling operations. Thus, using compression tests and scanning electron microscope tests, the mechanical properties and microscopic failure mechanisms of these rocks are studied considering the effect of water absorption, and their implications for wellbore stability are discussed. The results demonstrate the following points. (1) The strength, elastic modulus, cohesion and internal friction angle of basalt are significantly greater than those of tuff and breccia, and these values of tuff are the smallest, but the axial strain of tuff at peak stress is the largest. (2) With the effect of hydration, the elastic deformations of the three kinds of volcanic rocks would weaken, and the plastic deformations are enhanced. These trends for tuff and breccia are more obvious than those of basalt. With the increase in water absorption time, the strength, strains at peak stress, elastic modulus, cohesion and internal friction angle all decrease significantly, but Poisson's ratios change irregularly. (3) The hydration of clay minerals will lead to the surface expansion of mineral particles in these rocks, especially the mineral particles of tuff and breccia will be fragmented and disintegrated to form some loose particle structures in local areas. (4) The collapse mud weight for the tuff well section is the highest, followed by that of the breccia well section and that of the basalt well section, but it decreases most significantly for the breccia well section after water absorption. The collapse mud weights determined by the result of tuff well sections agree with the practical situation. [ABSTRACT FROM AUTHOR]