Study On Wellbore Stability Of Gas Underbalanced Drilling

Gas underbalanced drilling has advantages of high ROP, little damage to the reservoir etc. In recent years, the technology has been widely applied in oil &gas exploration. Gas underbalanced drilling adopts gas as annulus fluid, which causes no hydrostatic pressure to support sidewall and balance formation pressure during drilling. Formation fluid flows into wells under the negative pressure difference, which changes the distribution of stresses around borehole, so wellbore stability problem is more prominent, and it has become an important factor restricting gas underbalanced pressure drilling technology application. Although at present, much research on wellbore stability of underbalanced drilling has been done, the study on wellbore stability considering fluid-solid interaction effect is much less. So the paper has done some research on wellbore stability of underbalanced drilling.Based on classic fluid-solid coupling theory, fluid-solid coupling model of sidewall rock and formation fluid is established, and the mechanical parameters and failure criteria of sidewall rock is determined in the study. Watering out of gas underbalanced drilling is analyzed and the computation equation for watering out is established by applying empirical formula. By using FEM software ANSYS, fluid-solid coupling and non-fluid-solid coupling of wellbore stability for three working patterns were compared and analyzed. We can conclude from the paper: because wellbore rock is subjected to tensile stress during gas underbalanced drilling, low-tensile strength is easy to cause wellbore instability. As fluid-solid interaction effect formed by drag force of fluid has great influence on wellbore stability, it can not be ignored during underbalanced drilling. When underbalanced drilling passes to overbalance drilling, the tensile stress of wellhole gradually transits to the compressive stress. So fluid-solid interaction effect is dropping and wellbore stability is revising.