Analysis Of Wellbore Instability Mechanisms In Shale Gas Well

During the drilling process of shale gas horizontal well, wellbore instability problem happens frequently. Especially in the horizontal interval, shale which has high levels of gamma is more likely to collapse. Long construction time and the existence of certain collapsed period are severely restricting the development of shale gas exploitation. Existing studies on wellbore instability mechanisms of shale formation are mostly providing only qualitative description, so the further research on the formation process of cracks network around the wellbore is needed. Based on the calculation, the influence of geometrical morphology of the fissure (width, height and inclination) and the drilling fluid properties (interfacial tension and wetting angle) on the stress intensity factor of fissure are analyzed, and the formation mechanism of cracks network around the wellbore in shale gas reservoirs during drilling process is investigated. The geometrical morphology of fissure plays a decisive role of the strength of shale. Variation of drilling fluid properties will change the stress intensity factor of fissure tip, make the propagation of the crack deviate from its original direction, and consequently different extension and cross between cracks are generated. It results in the formation of cracks network around the wellbore inside shale formation, and further leads to wellbore instability. During the drilling process of shale gas well, the density of drilling fluid, the height and opening of fissure as well as the interfacial tension of drilling fluid, should be rationally controlled to prevent wellbore instability.