Misunderstanding of Fractured Shale Collapse Prevention and Plugging Method for Preventing Borehole Collapse

In fractured formations, drilling fluids under the action of pressures inside and outside of the borehole wall are very easy to invade the formation along the fracture channels, thereby increasing the pore pressure in the near-wellbore where fractures are quite developed, causing shale to hydrate and swell and strength decrease, increasing the collapse pressure of the formations, and hence exacerbating borehole wall collapse. In dealing with borehole wall collapse occurred in drilling the Shihezi formation and the Shiqianfeng formation in Yan'an gas field, it was found that the borehole wall collapse became more severe when mud weight was being increased. Study on this situation found that the failure of the borehole wall collapse just came from the lack of effective plugging of the fractures in the two formations. Understanding this, a countermeasure of preventing the collapse of fractured formations was developed, which can be described as "plugging the fracture has the top priority, inhibiting the hydration of the formations and using reasonable mud weight". Based on this measure, an optimized drilling fluid with high plugging capacity was developed for use in drilling operation in the Yan'an gas field. The technical clues in this study can be used as a reference for preventing borehole wall collapse in drilling the similar fractured formations.