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 possible to reap the benefits of reduced filtrate invasion.


The Petroleum industry has long appreciated that filtration of drilling fluid is a major cause of permeability damage to sensitive formations. permeability damage to sensitive formations. Everyone concerned realizes that minimizing filtration will be beneficial to production, but where do you reach the economical optimum? The fact that static fluid loss is a vague indication of down hole filtration was established years ago. Yet the drilling engineer is left with some rules of thumb and a few bench tests to guide him through decisions which may ultimately cost tens of thousands of dollars in mud additives, stimulation treatments and lost production. In order to intelligently attack this Problem, it appeared necessary to obtain a better understanding of the enemy. Hence, an extensive review of literature scanning more than two decades of research was undertaken.


Downhole filtration is a function of many things in addition to the physical and chemical nature of the drilling fluid. Other items of importance down hole are the temperature, total pressure and differential Pressure, circulation rate, components and movement of the drilling assembly, and rock type. Test data in the available literature seldom had more than two of these parameters in common. Meaningful comparisons were therefore difficult to draw without some method of normalizing the data. it was, therefore, undertaken to unravel the relationships between the parameters. Theoretical, mathematical, and empirical analysis of the data provided a set of equations which could be used to accommodate most down hole filtration conditions. The equations and relationships used are outlined in the following four sections:

Static Filtration

Filtration under static fluid conditions is called "static" filtration. Static filtration rate declines with time according to equation 1.


Volume of static filtrate increases with time much as described in equation 2.


Where C2 is one half of C1 and C3 represents spurt loss. The constants can be obtained from static tests.

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