A Study on the Stability of the Borehole in Shale, in Extended-reach Drilling

摘要

Shale is easy to hydrate and often causes a collapse of the borehole during drilling, especially in drilling extended-reach wells (ERW). In order to solve the problem of collapse of the shale, the changinges in the mechanical properties of shale, as affected by hydration and water absorption, are studied in this paper, through experiments. The relationships between the mechanical properties of shale and the water content are established. The borehole-stability models, which couple chemistry and mechanics are established, by considering the anisotropy of swelling, based on the experimental results. The stability of shale in the borehole is analyzed to obtain the temporal and spatial variations of the mechanical properties of shale and its stress state. Under the action of the drilling fluid, the hydration softens the shale formation, and hence the strength and stiffness of shale decrease with the increase of the openhole time and the Poisson's ratio increases with the increase of drilling time. The shale will swell, and produce the swelling strain after drilling. All these will lead to the change of the maximum tangential stress, from the wall of the borehole into the formation. The collapse pressure of shale reduces in the short time of drilling, and then increases sharply. After several days, the collapse pressure will no longer change. The present results provide a reference for studying the collapse period due to the hydration of shale in ERW drilling.