This paper presents a comprehensive set of experimental data for the membrane efficiency of four shales when interacting with dif- ferent water-based and oil-based muds. Pressure transmission tests were used to measure the membrane efficiency using three differ- ent cations and two different anions at different concentrations (water activities).

It was found that the measured membrane efficiencies of shales, when exposed to salt solutions, were low—ranging from 0.18% to 4.23%. Useful correlations are presented between the membrane efficiency and other shale properties. Results suggest that the membrane efficiency of shales is directly proportional to the ratio of the cation exchange capacity and permeability of shales. Higher cation exchange capacities and lower permeabilities correlate very well with higher membrane efficiencies. Moreover, the ratio of the hydrated solute (ion) size to shale pore throat determines a shale’s ability to restrict solutes from entering the pore space and controls its membrane efficiency. Cations and an- ions with large hydrated radii yielded higher membrane efficien- cies, compared to ions with small hydrated diameters. Thus, the formulation of drilling fluids must take into account the types of cation and anion in the water-based fluid.

It was also found that the membrane efficiency of oil-based muds was high, however, these membrane efficiencies were not 100 % as postulated by many researchers.