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The microscopic structure of drilling fluid filter cakes made under both static and dynamic conditions were studied using freeze-dry techniques and the scanning electron microscope. Four different filtration control additives in bentonite suspensions were evaluated.
This investigation was performed so as to obtain a better understanding of the microscopic structure of filter cakes. Hopefully, such information could lead to improved filtration control additives which would prevent differential pressure sticking. prevent differential pressure sticking. API (American Petroleum Institute) standard filtration tests were run at room temperature on various mud compositions. The filter cake was then removed and "shock" freeze-dried using iso-pentane cooled to -140 degrees C. The frozen water in the filter cake was then removed by vacuum sublimination. The structure of the dried cake was then studied using the SEM (scanning electron microscope).
The recent development of filter cake preparation methods have made it possible preparation methods have made it possible to examine the undisturbed filter cake structure using the scanning electron microscope. This technique has been used in this work to examine both static and dynamic filter cakes formed for bentonite based muds which contain the drilling fluid filtration control additives, sodium polyacrylate, carboxymethyl cellulose, starch, and lignosulfonate.
Before the filter cakes can be viewed using the SEM, water was removed from the filter cake using a freeze-drying technique. The water in the filter cake was first frozen and then removed by sublimation, so that a rigid structure was maintained throughout the procedures if the water were removed by evaporation, very large capillary forces would cause the collapse of the filter cake structure. In such cases, it was obvious to the unaided eye that evaporation alters the filter cake structure. The thickness of the evaporation-dried filter cake was much smaller than the thickness of the wet cake.
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