Wellbore stability issues are continually plaguing the industry and it is important to understand the mud properties that contribute to these issues. The effects of mud cake build up and filtration with time aids the understanding of formation damage and wellbore stability. The increase of drilling in high temperature and pressure zones (HTHP) necessitates studies that can predict filtrate invasion, and particle bridging. The most common hydraulically challenged zones are: depleted sands or intervals with very low pore pressure and fracture pressure, wellbore ballooning zones encountered in deep water drilling where surge and swab pressures could be a serious challenge, and complete lost circulation/no return zones.

Filtrate invasion and mud cake build up can be considered as primary factors controlling wellbore stability while drilling. Decreasing the near wellbore permeability by forming an ultra low permeability mud cake can strengthen the wellbore and mitigate further lost circulation problems. Very few studies have investigated filtration and filter cake build up under HPHT situations where the effect of different mud particles and bridging solids can be analyzed.

This paper focuses on experimental methods to investigate the effects of particle bridging, filtrate invasion and permeability on some common water based muds used by the industry. The methodology in this paper is based on using high temperature and pressure (HTHP) filtration tests at different time steps on sandstone cores of 50 mD and 750 mD fitted into Particle Plugging Test (PPT) aparatus. The Scanning Electron Microscope (SEM) and elemental mapping was used to characterize filter cake and further investigating the particles invasion in the core samples. Finally, mud cake permeability of the samples was calculated using analytical models from litreture.

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