Filtration from drilling fluids is a time-dependent and complex process. Mud composition, formation pressure, and temperature and permeability are some of the key factors affecting filtration behavior. Formation damage control and wellbore stability are the critical issues dealing with drilling fluid's filtration. The effects of mud cake build up and filtration with time aids the understanding the physical mechanisms. The increase of drilling in high temperature and pressure zones (HTHP) necessitates studies that can predict filtrate invasion, and particle bridging. Decreasing the near wellbore permeability by forming an ultra-low 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 effect of different mud particles and bridging solids can be analyzed.

This paper focuses on experimental methods quantifying water based muds used by the industry and investigating effects on particle bridging, filtrate invasion and permeability. To show the particle bridging effect, high temperature and pressure (HTHP) filtration tests were conducted at different time steps on sandstone cores with permeability ranging from 10 mD to more than 1100 md. The study also utilizes analytical models to calculate mud cake permeability for the tests using different mix design. The Scanning Electron Microscope (SEM) analysis conducted on the cores presents an expected trend of particle invasion with depth and time. The results from this study can be applied for designing wellbore strengthening fluids to mitigate wellbore stability and formation damage.

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