Experimental study of the membrane behavior of shale during interaction with water-based and oil-based muds
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Three integrated experimental studies were carried out in order to study the membrane behavior of shale when interacting with water-based and oil-based muds. Results confirmed the belief that shales act as leaky semi-permeable membranes. Measured membrane efficiencies were low and ranged from 0.18 % to 4.23 % when shales interacted with water-based muds. Independently, measured ion selectivities (modified diffusion potentials) indicated that shales behaved as ion-selective membranes that restrict the flow of anions. In addition, results showed that both the membrane efficiency and the ion selectivity of shales increase with decreasing shale permeability and increasing cation exchange capacity. Our results also showed a good correlation between the membrane efficiency and the ion selectivity of shales. A gravimetric test was developed that allows us to measure the flux of water and ions into or out of a shale. Results from this test show that the flux of ions depends on the ionic radii and the shale permeability and CEC. These results are consistent with the ion exclusion and membrane potential measured for the shale. The membrane efficiency of oil-based muds was high compared to that obtained for water-based muds. However, the measured membrane efficiency was not 100 %. Results obtained from immersion tests also showed that the oil-based mud was not a perfect ionic barrier since it allowed ions to exchange. A capillary threshold pressure was measured which must be overcome before oil-based mud flows through a shale. Results showed that this capillary entry pressure increases as the shale permeability decreases and the interfacial tension between non-wetting fluid and shale pore fluid increases.