Field case studies of borehole stability have shown that some failures, previously unexplained, were due to thermal effects, such as heating the upper part of open hole sections by mud circulation when drilling deeper or reheating of the bottomhole when mud circulation is stopped.

A complete analysis of the thermal regime in boreholes was performed and as a consequence, cooling of mud appeared as a means to mitigate these effects.

A series of tests were then carried out to check the practicality of installing such mud cooling systems. Many other advantages then appeared: decrease of the temperature of the borehole allowing better operation of the logging tools, better control of the mud rheology with less additives, extended use of MWD/LWD devices. But the most significant advantage is for the oil based muds which can be maintained at surface below their flash point, improving the safety of operations.

This paper recalls and summarizes the results of observations, measurements and studies performed to determine the feasibility of such systems. Operational results are given for several field cases with emphasis on safety. The use of these very simple devices, which have been field proven on normal and high temperature (BHCT = 150°C), is now contemplated for future HP-HT wells.

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