Analysis of wellbore instability while drilling exploratory wells in Bangladesh

Abstract

Wellbore instability is one of the most prevalent downhole challenges since oil well drilling began. The difficulties become more profound and unpredictable when the wells are exploratory. Wellbore Instability issues related to exploratory wells causes significant amount of non-productive time (NPT). These problems could lead to fishing operation and sidetrack which may cause greater financial loss. In Bangladesh wellbore related issues especially stuck pipe cases were experienced from time to time. This work analyzes the wellbore instability incidents of two exploratory wells, Mubarakpur-1 and Sunetro-1.

Based on available data like Geological Technical Order (GTO), Daily Drilling Reports, Well Logging Reports, Mud Reports, XRD (X-ray Diffraction) reports, Cavings samples with pictures, Well Completion Report etc. this work presents the sequence of events leading to the stuck pipe incident on Mubarakpur-1 and Sunetro-1 wells. Based on available information the root cause(s) of wellbore instability issues of the said wells have been analyzed and suggestions have been made on how to drill similar wells while tackling such issues in future.

Mubarakpur-1 and Sunetro-1 are different in geological aspects. Formation pressure and wellbore stresses were higher in Mubarakpur-1 than Sunetro-1. Shale formation showed non-reactive character in Mubarakpur-1, whereas Sunetro-1 suffered from shale swelling effect. Both wells encountered stuck pipe incidents but the sequences of events were different. Mubarakpur-1 faced compressive failure due to stress concentration around the wellbore. And Sunetro-1 crossed the reactive shale instability which resulted in hole pack-off. Some operational mishaps were also responsible for both stuck pipe incidents. The analysis showed that focus should be given on understanding the in-situ stress condition while drilling, identifying formation type. So that appropriate mud weight selection, BHA design can be implemented for trouble free operation.