Evaluation and application of wellbore stability in deep water

The deep water formation has low overburden pressure, low fracture pressure and narrow safety window of mud density. So the risk of wellbore instability is very high in this kind of formation. It is significantly important to accurately evaluate the safety window of mud density. The difficulties of wellbore stability analysis in deep water are how to accurately predicate the overburden pressure and establish the calculation model of caving and fracture pressure in the shallow part of the deep water formation. The published foreign models, for calculating the overburden pressure in the shallow part of the deep water formation, have precision problems in density evaluation. In this paper, we begin from the properties of deepwater formation, and suggest that different models should be used according to the different diagenetic stages in the shallow and deep part of the deep water formation.The wellbore stability is researched for shallow and deep parts separately, and different models are obtained. The model is used to evaluate the safety window of mud density in a deep water gas field in South China Sea. The result is coincide with the really drilling condition, which indicates the model is accurate and reliable and can supply useful information for mud density design in deep water drilling.