Analysis of the External Pressure on Casings Induced by Salt-Gypsum Creep in Build-up Sections for Horizontal Wells

To alleviate the problems of casing collapse induced by the coupling effect of salt creep movement, as it relates to the curved sections of horizontal wells, an experimental approach was taken to determine the creep (visco-elastic) property of the salt-gypsum formation, using finite-difference software to establish a creep model with curved casing, wellbore and salt formation. With this model, the effects of borehole curvature, drilling direction and casing thickness on external pressure on casing were analyzed, and casing deformation in the non-uniform in situ stress field was simulated. For horizontal wells drilled through salt-gypsum formations, this analysis led to the following conclusions: (1) the casing tends to be much safer when drilling is undertaken along the direction of the minimum horizontal stress; (2) casing deformation occurs and stress increases as the borehole curvature increases; (3) for the same curvature and drilling direction, thicker casings are safer; (4) as the creep time increases, the external pressure on the casing rapidly increases until it reaches its maximum value, whereupon it stays at that value; (5) under the effect of non-uniform in situ stress, a larger non-uniform coefficient would result in greater external pressure on the casing. This model has been successfully applied to analyze the external pressure on curved casings in a build-up section for horizontal wells drilled through salt-gypsum formations in an oilfield in Northwest China. The model has helped to improve the casing design and reduce drilling downtime with greater wellbore stability in salt-gypsum formations.