next the rotor was removed from the naf and left to drain for two minutes the bottom of the rotor was wiped clean and then weighed w 1 the rotor was then remounted on the rheometer and immersed in a cup containing the treatment fluid such that the naf layer was just covered by the treatment fluid the rotor was rotated for 10 minutes at 60 rpm the rotor was then removed from the treatment fluid and left to drain for two minutes the bottom of the rotor was wiped clean and weighed w 2 the naf removal efficiency r was then determined by eq 2 the tests were repeated at least twice and the results were averaged to obtain a final result it is desirable to achieve an r value higher than 75 experiments were performed to evaluate the effect of fiber diameter on cleaning efficiency the ezeflo u2122 surfactant was present at a concentration of 23 8 vol 1 gal bbl the fiber length was 6 mm and the fiber concentration in the treatment fluid was 3 6 kg m 3 1 25 lbm bbl the results are presented in table 1 fibers with diameters between 5 microns and 30 microns showed better cleaning efficiencies.