This paper was presented as part of the student paper contest associatedwith the European Petroleum Conference.
The operational margins in high pressure high temperature (HPHT) wells areoften very small. Therefore, it may be critical to separate between ballooningeffects and responses from having a kick, or having a mud loss.
We present a new analytical model for the density-pressure- temperaturedependence for drilling fluids.By coupling this to a temperature model for thecirculating fluid temperatures, we can study the temperature effects on returnmud volume variations. In the application of the model we simulate a North Seawell to study the effect of transient temperature behavior.
During drilling it is often observed that the return mud volume varies tosome extent, giving either a too low or a too high return rate. Also, when thepumps are turned off, a mud backflow results. This phenomenon is often calledborehole ballooning or breathing. Different interpretations of this phenomenainclude ballooning shales, fractures being opened and closed, andcompressibility of the drilling fluid. Unfortunately, these effects oftencomplicate interpretation of the pore pressure, as seen in several criticalhigh pressure high temperature (HPHT) wells in the North Sea.
Under critical circumstances it may be difficult to separate the ballooningeffect from responses from taking a kick, or having a mud loss. And since theoperational margins in HPHT wells are very small, a misdiagnosis may leadto unwarranted well control procedures that can be very costly. Therefore,borehole stability, well control and interpretation of well signals are keyissues related to this phenomenon. Since the change in temperature is nearlyalways transient or pseudo steady state, the temperature effect is critical tothe analysis of mud volume variations.
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