This paper describes the dynamic and static filtration characteristics of four different drilling fluids under downhole conditions. Filtration rates were measured over two-, four- or five-day periods in a simulated sandstone formation. The drilling fluids studied had a wide range of API fluid loss and rheological characteristics.

The API fluid-loss for all drilling fluids tested were generally higher than the filtrate loss observed in the sandstone formation. The largest static and dynamic filtration rates were seen in drilling fluids having uncontrolled fluid-loss properties.

A large spurt loss volume, varying in duration from 30 seconds to five minutes occurred upon initial contact of the drilling fluid with the formation. A steady state filtration rate was quickly reached following the initial spurt loss. Dynamic filtration rates and the percentage of circulatable hole varied as a function of the drilling fluid flow rate in the annulus. Increased annular flow rates produced higher dynamic filtration rates. The dynamic filtration rates decreased prior to day four of the tests, afterwhich no significant changes were observed. Removal of the external wellbore filter cake using mechanical scrapers did not significantly change the dynamic filtration rate.

The results presented can be used to estimate the effect of flow rates, fluid loss, and rheology of the drilling fluid on the amount of drilling fluid filtrate lost to the formation and on the percentage of the annulus that is circulating.

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