Practical Advantages Of Mud Cooling Systems For Drilling

Abstract

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, INTRODUCTION AND BACKGROUND 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. TEMPERATURE REGIME IN A WELLBORE WHILE DRILLING 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.

HPHT Well Basis of Design Case Study: Offshore Thailand Advanced Tubular Analyses Experience

“…The use of a mud cooler has been widely received in providing a practical mud temperature-control solution at the surface when handling a high-temperature fluid system (Maury and Geunot 1993). Drilling data from the 12-¼-in.…”

Maintaining the Stability of Deviated and Horizontal Wells: Effects of Mechanical, Chemical and Thermal Phenomena on Well Designs

“…There have been numerous discussions regarding the thermal effects on wellbore stability. One school of thought has postulated that mud cooling is beneficial to improving wellbore stability [31] . Another school suggests heating the wellbore to prevent lost circulation [32] .…”

Increasing Effective Fracture Gradients by Managing Wellbore Temperatures

“…Thermal effects have been used to explain some cases of time-dependent wellbore collapse resulting from the heating of shallower formations by hotter mud rising up the annulus of the wellbore from deeper formations 6,8,10 , resulting in proposals that mud cooling may be used to reduce the incidence of time-delayed wellbore instabilities 11 . Warm mud rising from deeper formations may also explain evidence within the drilling community of increases in leak-off pressures over time in some wells.…”

“…Comparing the horsepower input from hydraulic and rotational energy sources, hydraulic horsepower input is by far the greater quantity. Maury and Guenot 15 indicate that hydraulic energy usually accounts for ten times more heat than mechanical energy.…”