Calculation Model Of Collapse Pressure Of Directional Well Based On Shaft Wall Caving

Usually there is some degree of wellbore enlargement during an actual drilling process, which will cause no wellbore instability and the related underground complexities. The present calculation model of collapse pressure does not allow any degree of failure and is significantly conservative in estimating the minimum mud pressure required for ensuring the wellbore stability. A stable wellbore does not mean a complete borehole. The mud density required for maintaining the borehole complete is too high, which may cause formation fracturing and mud loss. Due to the borehole with different deviation angle can afford different degree of damage and the directional wellbore need to support the overlying formation pressure, so issue of wellbore stability is more evident than vertical Wells. Therefore a new calculation model for collapse pressure of directional well is required, which allows some degree of breakout exists in borehole to reduce drilling fluid density while maintain wellbore stable.According to the linear poroelastic theory of rock mechanics, a new calculation model for collapse pressure of directional well based on wellbore breakout is established by using Mohr-Coulomb criterion, Mogi-Coulomb criterion, Modified Lade criterion, Inscribed Drucker-Prager criterion and Circumscribed Drucker-Prager criterion in this paper, respectively. This model can calculate internal borehole pressure, namely the collapse pressure, needed to maintain the breakout width of directional well less than a particular value and optimize the trajectory of directional well based on collapse pressure. In-situ stress is important parameters for the design of drilling fluid density and well trajectory. The method of calculating the magnitude of maximum horizontal stress using wellbore breakout has been widely utilized, but the corresponding model is only suitable for vertical wells. The Baker Hughes GMI software can take advantage of wellbore breakout of arbitrary directional well to estimate the magnitude of the maximum horizontal stress, but its method has not been put forward yet in any literature. During the process of establishing the calculation model for collapse pressure of directional well, the method of calculating the magnitude of maximum horizontal stress using the breakout width of directional well is surprised to be found out in this paper.According to the established calculation model for collapse pressure of directional well, the software for calculating the collapse pressure of directional well based on wellbore breakout is developed with Visual C#.NET. This software can be applied to estimate the magnitude of the maximum horizontal stress using the breakout width of directional well, and the results calculated based on Mohr Coulomb criterion or Modified Lade criterion is in concordance with the result of calculation by GMI software. The collapse pressure of directional well both with and without breakout is calculated in this paper, combining with Mohr-Coulomb criterion, Modified Lade criterion, Inscribed Drucker-Prager criterion and Circumscribed Drucker-Prager criterion, and the results are also in concordance or even exactly the same with the result of calculation by GMI software.The comparison between the results calculated by the software for calculating the collapse pressure of directional well based on wellbore breakout and GMI software shows that the two results are much closed or even exactly the same and allowing wellbore breakout can significantly reduce the collapse pressure of directional well. Therefore, it can be concluded that:The way to estimate the magnitude of the maximum horizontal stress using the breakout width of directional well is correct and reliable The developed software can be used to calculate the collapse pressure of directional well and optimize wellbore trajectory.