It is usually accepted that wellbore instabilities are caused by either or both an excessive stress concentration at the borehole wall and the chemical reactivity of the formation. Assuming good hole cleaning, common cures for such instabilities are therefore mud density increase and/or change of the mud system. However, even though these methods have proved highly successfull when drilling through intact formations, the same may not stand when it comes to highly fractured rocks. The purpose of this paper is to describe the studies performed in order to allow the safe drilling of a particularly troublesome, highly fractured formation which had led previously to several successive side tracks.The efficiency of the different attempts at solving the problem are analysed in the light of the various side tracks. The core taken during one of these side tracks was analysed, showed that the formation was highly fractured and chemically inert, and provided many parameters for the modelling of the problem which was performed by a discrete element programme. Further modelling was performed on the mud hydraulics. Both theoretical and field data show that density increase has, in such a case, a negative role on borehole stability while filtrate reduction and mud rheology enhancement are highly positive.
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 feasabil'ky of such wells must be questionned. When reservolr 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.
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 feasabil'ky of such wells must be questionned. When reservolr 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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