Field cases studies of borehole stability showed that some failures were due to thermal effects, heating the upper part of open holes by mud circulation or reheating of the bottomhole when mud circulation cooling is stopped for instance.
A complete analysis of the thermal regime in boreholes was performed and as a consequence, cooling of mud appeared as a mean to mitigate these effects.
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 devices. But the most significant advantage is for the oil based suds which can be maintained at surface below their flash point, improving the safety of operations.
The 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 typical and 150C wells, is now contemplated for future HP-HT wells,
Till the beginning of 80's, little attention had been paid to the thermal effects in wellbores. Except in Russian literature, practical consideration had been given to mud temperature and mud cooling in the cases of geothermal drilling and drilling through Permafrost only.
Nevertheless, thermal effects are commonly considered in mining and underground engineering, for heavy fuel oil storage and nuclear waste disposal. Some data are here recalled on the thermal effects in the wellbores.
Even though this subject has been presented many times in the past, a good understanding of the temperature regime in a wellbore while drilling is not common in practice. This is mostly due to the lack of interest for this parameter, except in very specific cases such as mentioned above. Nowadays, PC software is easily available to provide a good estimate of the temperature profile in the well. At the same time, a larger access to the bottomhole temperature has been obtained through the use of MWD.
When mud is circulated in a wellbore, a balance is reached between the different heat transfers. Bottomhole is cooler than the formation temperature, whereas top hole is warmer than the formation temperature. The amplitude of this deviation and also the temperature difference between bottom and top of the formation depend essentially of the mud flow rate.
In turn, this depends on the borehole diameter, as the flow rate is usually decreased with the bit size. On fig.1, typical temperature profiles are given for the same wellbore for different drilling phases.