An experimental apparatus and a statistically-based test matrix have been designed for use in an on-going test program to determine the extent of formation damage which may be caused by oil-base 'drilling fluids on sandstone cores at elevated temperatures and pressures. Tests with a mineral oil-base synthetic filtrate on Berea sandstone cores have shown that of the three independent variables tested (temperature, flowrate and confining pressure), temperature has the most significant effect on the degree of reduction of relative mobility over the range tested.1  The synthetic filtrate, containing the maximum amount of surfactants expected in a comparable 12 ppg mud, caused severe damage (97-100%) in all cases. The mineral oil, also used as a test fluid to show the minimum damage effect corresponding to the same mud, resulted in little change in relative mobility.

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