Drilling high quality wells requires the use of high-performance drilling muds that can support and protect the wellbore during the drilling process. Drilling mud will create an internal and external filter cake to protect the well during drilling. Although the external filter cake is usually removed before or during cementing jobs, the internal filter cake remains in the near-wellbore formation throughout the life of the well. As thermal well design relies on maximizing the heat delivered to the reservoir, minimizing heat losses to top formations while also protecting casing and cement layers is mandatory.

A literature review shows that the effects of temperature on drilling fluids or cements have been intensively investigated. For example, high temperature generates a decrease of rheological properties of drilling muds, as well as increasing fluid loss. When heat transfer estimations are required, the drilling mud properties are not important, unless a thick filter cake is considered.

This is why laboratory investigations on filter cake thermal capacity and conduction, together with theoretical estimations of the effect of filter cake on the wellbore heat transfer process make the subject of this paper. For this, the overall heat transfer coefficient has been revisited and additional effects of cement layers and filter cake have been incorporated. The results show that conventional water base muds can reduce the heat transfer from and to the wellbore. Two mud samples have been tested and an increase of the thermal conductivity with temperature is observed. Adding barite to the drilling mud will increase the thermal diffusivity of the filter cake. The thermal diffusivity of the drilling mud is higher than that of its produced filter cake, but can change the heat distribution in the well and the cement temperature levels. Such effects, together with cement layer conductivity and size are usually neglected in conventional heat transfer models and a reassessment of conventional models while considering such parameters is presented in this work.

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