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Note that this equation calculates the radius of a cylindrical capillary tube; however, real pore throats have very complex geometries. Therefore, the calculated values represent the ''effective'' radii of the pore throats, which may not equal their actual dimensions. Samples dominated by throats of similar size (well sorted) have broad, flat plateaus (Figure 4). As sorting becomes poorer, the plateau steepens then disappears and the slope of the curve approaches 45° (unsorted).
Note that this equation calculates the radius of a cylindrical capillary tube; however, real pore throats have very complex geometries. Therefore, the calculated values represent the ''effective'' radii of the pore throats, which may not equal their actual dimensions. Samples dominated by throats of similar size (well sorted) have broad, flat plateaus (Figure 4). As sorting becomes poorer, the plateau steepens then disappears and the slope of the curve approaches 45° (unsorted).
Data from the mercury withdrawal (air imbibition) curve can provide information regarding the efficiency with which the nonwetting phase can be withdrawn from the pore system. ''Withdrawal efficiency'' (''W''<sub>E</sub>) is defined as the ratio of the mercury saturation in the sample at minimum pressure after pressure is reduced (S<sub>R</sub>) to the saturation at maximum pressure (S<sub>max</sub>) (Figure 3) (Wardlaw and Taylor, 1976)<ref name=Wardlaw_etal_1976 />. Because few samples reach 100% mercury saturation at routinely available injection pressures, data are normalized by the following equation:
Data from the mercury withdrawal (air imbibition) curve can provide information regarding the efficiency with which the nonwetting phase can be withdrawn from the pore system. ''Withdrawal efficiency'' (''W''<sub>E</sub>) is defined as the ratio of the mercury saturation in the sample at minimum pressure after pressure is reduced (''S''<sub>R</sub>) to the saturation at maximum pressure (''S''<sub>max</sub>) (Figure 3) (Wardlaw and Taylor, 1976)<ref name=Wardlaw_etal_1976 />. Because few samples reach 100% mercury saturation at routinely available injection pressures, data are normalized by the following equation:
:<math>W_\mathbf{E} = \frac{S_\mathrm{max} - S\mathrm{R}}{S_\mathrm{max}} \times 100%</math>
:<math>W_\mathrm{E} = \frac{S_\mathrm{max} - S\mathrm{R}}{S_\mathrm{max}} \times 100%</math>
Withdrawal efficiency in the air-mercury system is controlled exclusively by pore geometry, which can be interpreted from the capillary pressure curves and by direct observation of the rock or an epoxy resin pore cast. Critical pore geometry factors affecting withdrawal efficiency include effective pore throat heterogeneity; the ratio of pore body to pore throat size (r<sub>b</sub>/r<sub>t</sub>); and the number of throats connecting each pore (coordination number). Table 1 shows the effect of these parameters on withdrawal efficiency.
Withdrawal efficiency in the air-mercury system is controlled exclusively by pore geometry, which can be interpreted from the capillary pressure curves and by direct observation of the rock or an epoxy resin pore cast. Critical pore geometry factors affecting withdrawal efficiency include effective pore throat heterogeneity; the ratio of pore body to pore throat size (r<sub>b</sub>/r<sub>t</sub>); and the number of throats connecting each pore (coordination number). Table 1 shows the effect of these parameters on withdrawal efficiency.