Abstract
Abstract Wellbore collapse is an instability-event that occurs at low mud density and leads to unfavorable economic project, reaching billions of US dollars. Thus, it is important to accurately determine its value, especially in deepwater horizontal wellbores. The main reasons for nontrivial problems with such wellbores are evident: the shale encountered are anisotropic in nature and possess planes of weakness; they react with water-based mud, generate osmotic stresses, swell, and fall unto the wellbore bottom, thereby increasing the non-productive time. To this end, salts are added to reduce the collapse tendency, but it is not currently known what amount of salt addition maintains stability, and does not lead to wellbore fracture; in deepwater, the current trend in global warming means there is a future concern to the industry. As the climate temperature increases, more ice melts from the polar region, the seawater expands and the sea level rises. How to incorporate the corresponding effect on collapse gradient is scarcely known. This study captures the major concerns stated above into wellbore stability analysis. Following the classical approach for geomechanical analysis, Mogi-Coulomb criterion was combined with a constitutive stress equation comprising contributions from mechanical and osmotic potentials of mud and shale. A sophisticated industry model was used to consider the deepwater effect. The results show significant reduction in collapse gradient as the water depth increases, also, larger difference between the mud and shale chemical activities represents higher complexities in the wellbore. In addition, the reduction in the chemical activities of mud limited to 37.5% of the initial value can be practically safe.
Highlights
One of the critical components required in order to carry out safe and stable drilling of horizontal wellbores is the selection of accurate mud weight and chemistry
Wellbore collapse is an instability event that occurs at low well pressure, where rock materials fall into the wellbore, and through shear failure (Mitchell and Miska 2011)
Sea level indicators are required to be installed into the drilling platform, so that changes in the water depth can be measured and noted, currently, it is very minimal
Summary
One of the critical components required in order to carry out safe and stable drilling of horizontal wellbores is the selection of accurate mud weight and chemistry. The effective principal stresses are used when applying the failure criterion because it is the failure at the matrix that is of importance While this classical approach is sound, it has not been used to show whether a rise in water depth significantly affects collapse pressure during offshore drilling of horizontal wells, neither has it been used to specify a range of values for mud chemistry that can help to maintain stability especially in shale formations. For stress concentration at the walls of the horizontal wellbore, an elasto-plastic model can be applied, but the most common approach is to use elastic models because of the fewer input parameters needed (Gholami et al 2013) In generating such models the hollow cylinder model is act as a pressure increase resulting from the movement of water molecules from the mud into the shale or vice versa. This shows that higher mud weight is needed to successfully drill through weak planes
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