摘要：Drilling the wells using water based mud through shale formations, causes their exposure to serious time-dependent wellbore instability due to shale swelling. Operating companies, before drilling operations through demanding shale formations, usually conduct drilling fluid optimization studies in order to define the proper mud type, mud density, salt type and concentration for inhibition. Through the analysis of offset wells, they are interpreting data about mud filtrate breakouts into the rock formations and chemical potential mechanisms to understand their influence on the time-de- pendent wellbore instability. The main objective of this paper is to give an insight in time-dependent and mechanical wellbore instability problems faced while drilling the wells through different shale formations in the gas condensate field D in the Persian Gulf. The importance of drilling fluid design optimization and solutions applied to overcome hole in- stability problems were analysed and highlighted. Besides the development of a model for mud density calculations, a concept of effective hoop stress and its influence on time dependent failure mechanisms is discussed. As a contribution to the method improvement, mud density calculation is verified by taking into the consideration the relationship be- tween pore pressure and effective hoop stress and it is based on measured data from Well A in gas condensate field D from the Persian Gulf.
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