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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 uid optimization studies in order to de ne the proper mud type, mud density, salt type and concentration for inhibition. Through the analysis of o set wells, they are interpreting data about mud ltrate breakouts into the rock formations and chemical potential mechanisms to understand their in uence on the time-dependent 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 di erent shale formations in the gas condensate eld D in the Persian Gulf. The importance of drilling uid design optimization and solutions applied to overcome hole instability problems were analysed and highlighted. Besides the development of a model for mud density calculations, a concept of e ective hoop stress and its in uence on time dependent failure mechanisms is discussed. As a contribution to the method improvement, mud density calculation is veri ed by taking into the consideration the relationship between pore pressure and e ective hoop stress and it is based on measured data from Well A in gas condensate eld D from the Persian Gulf.

CONTRIBUTION TO THE METHODOLOGY OF DETERMINING THE OPTIMUM MUD DENSITY - A CASE STUDY FROM THE OFFSHORE GAS CONDENSATE FIELD D IN THE PERSIAN GULF