The technology for predicting the safety density window of drilling fluid in deepwater shallow layers and its engineering application

The shallow formations in deep water are of poor diagenesis, so the wellbores tend to suffer collapse and leakage easily and the safety density window of drilling fluid is narrow. For this, the precise prediction on the safety density window of drilling fluid is key to the safety and success of deepwater well drilling operation. The segmented prediction method for deepwater formation density was put forward by analyzing the in-situ stress and lithologic and diagenetic characteristics of shallow formations in deep water. And accordingly, the vertical profiles of three in-situ stresses (overburden pressure, maximum horizontal in-situ stress and minimum horizontal in-situ stress) in deepwater wells were determined. Then, the model for calculating the ultimate strain of the wellbore collapse pressure in deepwater shallow plastic formations was established. And by virtue of this model, the safety density window of drilling fluid in the deepwater shallow formations can be predicted precisely. What’s more, it was applied in the deepwater well (Well S1) which is located in the Gulf of Equatorial Guinea in West Africa. And owing to its application, the safety density window of drilling fluid in Well S1 is determined and the safe and fast drilling of Well S1 is guaranteed.