Analysis of the Contributing Factors to Wellbore Collapse in Shallow Water Flow Formations for Deepwater Drilling

In order to quantitatively evaluate and effectively prevent the damage of shallow water flow during deep water drilling, the contributing factors of wellbore collapse in shallow water flow formations during deep water drilling were analyzed. Based on the fluid-solid coupling theory, the theoretical model of wellbore stability in shallow water flow formation was established. Then they determined the shear and tensile damages of the sand body by the Mohr-Coulomb and maximum tensile stress criteria respectively, as well as analyzed the influence of each factor on maximum damage radius and sand body critical overpressure. The calculation and analysis found that the overpressure would lead to the damage of shallow water flow formation, the maximum damage radius increased rapidly with the increase of overpressure. The greater the ratio of effective horizontal geostress to effective vertical geostress (K₀ ), and the greater the burial depth of sand body, and the less likely the shallow water flow formation is to be damaged; the larger the internal friction angle in the sand body, the more stable the sand body. Likewise, the smaller the internal friction angle, the more significant impact will be. The maximum damage radius around wellbore linearly decreases with the increase of drilling fluid density. There is a relations between critical overpressure found in the shallow water flow. The larger the burial depth, strength and K₀ of sand body, the larger the critical overpressure. The results showed that the risk of wellbore collapse in shallow water flow formation was high. Further, the damage degree around wellbore was related to such factors as overpressure, geostress, sand body depth, drilling fluid density, internal friction angle, etc. When the critical overpressure is exceeded, the entire sand body will be in an unstable state, and the increase of drilling fluid density will be conducive to the wellbore stability in the shallow water flow formation.