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Line 60: − Mercury injection capillary pressure data are acquired by injecting mercury into an evacuated, cleaned, and extracted core plug. Mercury injection pressure is increased in a stepwise manner, and the percentage of rock pore volume saturated by mercury at each step is recorded after allowing sufficient time for equilibrium to be reached. The pressure is then plotted against the mercury saturation (Figure 3), resulting in the injection curve (which is also called the drainage curve because the wetting phase is being drained from the sample). − The pressure at which mercury first enters the sample (after the mercury has filled any surface irregularities on the sample) is termed the ''displacement pressure'' (''P''<sub>d</sub>). The percentage of pore volume saturated by mercury at the maximum pressure is the ''maximum saturation'' (''S''<sub>max</sub>). The unsaturated pore volume at that pressure is the ''minimum unsaturated pore volume'' (''u''<sub>min</sub>) (Figure 3). This is sometimes incorrectly referred to as irreducible saturation. This term is inappropriate for the air-mercury system because saturation depends on applied pressure and on the duration of the experiment (Wardlaw and Taylor, 1976)<ref name=Wardlaw_etal_1976 />.+ − + − + + + + Line 77:
Line 79: − 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). − + + + Line 97:
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→Terminology and mercury injection systematics
==Terminology and mercury injection systematics==
==Terminology and mercury injection systematics==
[[File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_3.jpg|left|thumb|{{figure_number|3}}Mercury injection capillary pressure curve terminology.]]
[[File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_3.jpg|thumb|{{figure_number|3}}Mercury injection capillary pressure curve terminology.]]
Mercury injection capillary pressure data are acquired by injecting mercury into an evacuated, cleaned, and extracted core plug. Mercury injection pressure is increased in a stepwise manner, and the percentage of rock pore volume saturated by mercury at each step is recorded after allowing sufficient time for equilibrium to be reached. The pressure is then plotted against the mercury saturation ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_3.jpg|Figure 3]]), resulting in the injection curve (which is also called the drainage curve because the wetting phase is being drained from the sample).
After the maximum pressure is reached, the pressure is reduced in steps and air (the wetting phase) is allowed to imbibe into the sample. The amount of mercury expelled from the sample at each pressure is expressed as a percentage of total pore volume or bulk volume. Again, pressure is plotted against mercury saturation in the withdrawal curve (Figure 3). The volume of pore space still saturated with mercury after pressure is reduced to the minimum is called ''the residual mercury saturation'' (S<sub>R</sub>).
The pressure at which mercury first enters the sample (after the mercury has filled any surface irregularities on the sample) is termed the ''displacement pressure'' (''P''<sub>d</sub>). The percentage of pore volume saturated by mercury at the maximum pressure is the ''maximum saturation'' (''S''<sub>max</sub>). The unsaturated pore volume at that pressure is the ''minimum unsaturated pore volume'' (''u''<sub>min</sub>) ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_3.jpg|Figure 3]]). This is sometimes incorrectly referred to as irreducible saturation. This term is inappropriate for the air-mercury system because saturation depends on applied pressure and on the duration of the experiment.<ref name=Wardlaw_etal_1976 />
After the maximum pressure is reached, the pressure is reduced in steps and air (the wetting phase) is allowed to imbibe into the sample. The amount of mercury expelled from the sample at each pressure is expressed as a percentage of total pore volume or bulk volume. Again, pressure is plotted against mercury saturation in the withdrawal curve ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_3.jpg|Figure 3]]). The volume of pore space still saturated with mercury after pressure is reduced to the minimum is called ''the residual mercury saturation'' (S<sub>R</sub>).
Data from the mercury injection curve can be used to approximate the distribution of pore volume accessible by throats of given effective radii (in cm) using the following equation:
Data from the mercury injection curve can be used to approximate the distribution of pore volume accessible by throats of given effective radii (in cm) using the following equation:
[[File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_4.jpg|thumb|{{figure_number|4}}Idealized mercury injection capillary pressure curve shapes. Note that all of the curves have identical displacement pressures and minimum unsaturated pore volumes, but that the saturation profiles would differ dramatically due to differences in pore throat size distributions.]]
[[File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_4.jpg|thumb|{{figure_number|4}}Idealized mercury injection capillary pressure curve shapes. Note that all of the curves have identical displacement pressures and minimum unsaturated pore volumes, but that the saturation profiles would differ dramatically due to differences in pore throat size distributions.]]
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:
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 ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_4.jpg|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>) ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_3.jpg|Figure 3]]).<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_\mathrm{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>
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Relating withdrawal efficiency in the air-mercury system to recovery efficiency in the hydrocarbon-water system is dependent on properties of the fluids as well as properties of the pore system. Fluid properties that affect recovery include viscosity, density, interfacial tension, [[wettability]], contact angle hysteresis, and rate of displacement (Wardlaw and Cassan, 1978)<ref name=Wardlaw_etal_1978 />. Nevertheless, for a given range of fluid properties in a water wet reservoir, the same pore geometry factors that contribute to increased mercury withdrawal efficiency also increase hydrocarbon recovery efficiency.
Relating withdrawal efficiency in the air-mercury system to recovery efficiency in the hydrocarbon-water system is dependent on properties of the fluids as well as properties of the pore system. Fluid properties that affect recovery include viscosity, density, interfacial tension, [[wettability]], contact angle hysteresis, and rate of displacement.<ref name=Wardlaw_etal_1978 /> Nevertheless, for a given range of fluid properties in a water wet reservoir, the same pore geometry factors that contribute to increased mercury withdrawal efficiency also increase hydrocarbon recovery efficiency.
==Reservoir applications==
==Reservoir applications==