High pressure high temperature deep-well drilling is often associated with several unusual problems. Some important factors are pressure control and interpretations of well signals. Mud ballooning, or apparent mud loss during circulation and a pit gain after stopping the circulation, is known to complicate the interpretation of the pore pressure.

In this study, an analytical model is developed by describing the energy balance in a circulating well. Input of energy due to rotation of the drillstring and pumping of the mud are included. The effect of having a riser in offshore applications is also included.

It is shown that for most cases there is a net flow of energy from the formation to the surface. In offshore applications there will always be a heat loss in the riser.

It is also shown that heat transfer from the well is strongly related to the average fluid temperature in the well. Calculating the energy balance provides important information on temperature- and fluid density behavior.

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