Investigation of the Effect of Casing-to-Wellbore Clearance on Equivalent Circulating Density During Casing Running

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As drilling depth increases, the demand for a multi-layer wellbore structure grows. However, with the upper wellbore diameter fixed, reducing the casing-to-wellbore clearance becomes necessary to increase the number of casing strings. This poses a significant challenge to maintaining adequate casing-to-wellbore clearance. However, in actual drilling operations, the casing-to-wellbore clearance cannot be continuously reduced. Equivalent circulating density (ECD) during running casing becomes a critical factor to consider. When the casing-to-wellbore clearance is too small, excessive surge pressures can occur, increasing the risk of lost circulation. In this study, a surge pressure calculation model was developed for the casing running process to analyze the variation in ECD under 10,000 m well conditions in China. Focusing on key formations with narrow density windows, the effects of different casing-to-wellbore clearances on ECD were evaluated. The analysis determined that under the constraints of fracture pressure, the minimum allowable casing-to-wellbore clearance for this casing string is 27 mm. Subsequently, using response surface methodology, a fitting formula was derived to describe the relationship between the selected variables—drilling fluid density, casing running speed, and casing-to-wellbore clearance—and the ECD during casing operations. This provides a theoretical basis for the optimal matching of casing-to-wellbore clearance.