Annular pressure build up (APB) is an important problem in the area of well design. Different solutions have been proposed to mitigate APB, including vacuum-insulated tubing, silicate foam wellbore insulation techniques, and insulating with completion (packer) fluids. However, proposed solutions that include the optimization of annular fluid rheology of drilling fluids trapped in the outer annuli need further investigation.

Convective and conductive heat transfer through fluids placed between the casing strings is a driving force for increasing annular pressure. This study explains how to design and optimize the rheological and thermal properties of drilling fluid to provide a sustainable and reliable insulating performance. The objectives of this study are 1) to develop a better insight of APB in the annulus of casing and 2) to experimentally measure and 3) analytically model the effect of different parameters, such as rheological and thermal properties, on the insulating performance of drilling fluid trapped in annular spaces.

By combining the theory of sedimentation and the model proposed for free convective heat transfer of Yield Power Law fluids, a stepwise guideline is proposed to estimate the Nusselt number and thermal conductivity coefficient of annular fluids. A computer code to perform this procedure has been developed. Additionally, a unique experimental apparatus was designed and constructed to investigate the insulating performance of drilling fluids.

Experimental observation and model results indicate that compressive yield stress, density difference between the solid and liquid phases of a drilling fluid, solid particle size and annular space geometry, are four important factors that determine sedimentation rate. These criteria can be used to characterize YPL annular fluid properties to minimize sedimentation rate and consequently to reduce free convective flow. The magnitude of the convective heat transfer coefficient is affected by the shear yield stress and density of annular fluid, by the flow behavior index (m), consistency index (k), conductivity coefficient and specific heat of the YPL fluid, and by annular space clearance. This conclusion can be used to quantitatively analyze the reliability and sustainability of the insulating performance of drilling fluids.

The sedimentation profile, shear yield stress distribution and effective conductivity coefficient profile as functions of time and depth, as well as the Nusselt number, Nu, as a function of temperature difference in the radial direction can be estimated by the introduced guideline. The most important achievement is that the developed guideline can be used as a design tool in offshore drilling and completion operations to mitigate APB. A practical benefit of this study is demonstrated by the application of its findings in an illustrative example.

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