Incompatibilities between drilling fluid filtrates and formation connate fluids are believed to cause changes in rock wettability and hence impair production for a given reservoir rock.

This paper summarizes test results and analysis of an experimental study to systematically assess the effect of incompatibilities on rock wettability and in turn on its permeability. The rocks considered are dolomite, limestone and sandstone. The parameters that were studied are the ph, monovalent to divalent cation ratio, total cation concentration (salinity) and the drilling fluid surfactants. The Dynamic Wilhelmy Plate Technique was used in the measurement of wettability changes. The analysis of test data was conducted using a defined deviation number that allowed quantitative evaluation of the various effects that were considered in the study. Results have shown that the most significant in rock wettability changes are the surfactants in the drilling fluids.

Dolomite rock had the maximum reduction in water wetness when surfactants are absent, while the reduction is about the sameforall rocks when surfactants are present. Also, among all five water-base muds studied, the calcium treated gypsum mud had the least water wetness.

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