When planning the development of a new field, the potential of horizontal wells is systematically investigated from a reservoir engineering point of view. Yet, from a drilling point of view, the feasability of such wells must be questionned. When reservoir temperature and pressure are high, the margin of admissible mud weight which will avoid kicks and yet not lead to mud losses becomes very narrow and may even prohibit the drilling of such wells. This paper describes such a study and shows how log and core data from vertical appraisal wells can be interpreted and used in various models to determine such a margin.

After estimating precisely, in situ stresses and rock mechanical and thermal properties, a first margin of admissible mud weight is calculated.

This first result is then modified to account for the effects of mud temperature and circulation which are also modelled. The model results are calibrated against an FMS log on a vertical well showing drilling induced hydrofractures which corresponded to mud losses in certain formations. The paper concludes at the feasability of the well provided certain tripping practices and mud monitoring techniques are used.

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