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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, after which 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.


One of the most important functions of drilling fluids is to minimize the amount of drilling fluid filtrate entering the hydrocarbon bearing formation. Drilling fluids and drilling fluid filtrates may cause formation damage due to fines migration, rock wettability changes, drilling fluid solids plugging and formation water chemistry incompatibilities. The current API RP13B-1 specifications for fluid-loss control conducted on filter paper do not reflect the downhole filtrate loss and filter cake deposition-erosion process. The API test, at best, gives a qualitative comparison of fluid-loss properties.

A number of laboratory studies have been conducted to emulate filtration losses under downhole pressure and temperature conditions. Laboratory drilling fluid circulation equipment has been designed to provide borehole annular shear rates, temperatures and pressures. These laboratory designs, however, have always been limited by formation length (less than 1 m) and have not duplicated downhole functions such as pipe movement and mechanical filter cake removal under flow conditions. Recently, the first attempt at downhole fluid-loss measurement was described for a series of four wells in the Mississippi delta. These wells were shut in and the amount of filtrate loss was determined by the annular volumes lost to sandstone formations.

As a prelude to this investigation, Smith and Ravi described cement displacement efficiency as a function of drilling fluid properties in a full scale wellbore.

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