The drilling of deep wells requires the use of performance muds that can support and protect the wellbore during the process of ‘making hole’. Before and during cement jobs, the goal is to remove as much as possible of the existing filter cake (a by-product of the drilling mud) in order to enhance the quality of the annular cement sheath. Yet, experiments have shown that often a thin filter cake stubbornly remains between cement and formation. This is a cause for concern as the effectiveness of geothermal wells relies on maximizing the heat mining of the reservoir, and optimizing the heat transfer process is a major consideration. The effect of temperature on drilling fluids has undergone thorough investigations which show that high temperatures cause a degradation of rheological properties, as well as increasing fluid loss. In heat transfer estimations for geothermal well design, the required inputs relating to the drilling mud properties will be flawed, unless the filter cake is taken into consideration. This paper focuses on laboratory investigations of filter cake thermal capacity and conduction, followed by a theoretical estimation of the effect of filter cake on the wellbore heat transfer process. The results show that conventional water based muds can impair the heat transfer from and to the wellbore. Two mud samples were tested and it was observed that their thermal conductivities increased with temperature. It was noted that adding Barite to the drilling mud increased the thermal diffusivity of the filter cake, but it was found to be lower than that of the drilling fluid that produced it.

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