A technique has been developed which calculates filtration of drilling fluids under actual well conditions. Theoretical and empirical equations were derived from the findings of an extensive literature search. These relationships enable comprehension of the many intricacies of downhole filtration far beyond that enabled by the primitive static fluid loss test. With this understanding, it is possible to modify drilling fluids and drilling practices such that invasion of drilling fluid filtrate is minimized.

The model accuracy was verified through comparison with well log results on 21 Canadian wells. Filtration volumes into a particular zone can now be estimated through use of a programmable calculator under conditions where well log data fails to do the job.

This paper outlines the types of filtration relationships derived, examples of the model results and the potential applications of the system. Armed with the capabilities of this system, it becomes possible to reap the benefits of reduced filtrate invasion.

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