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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).

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_d). The percentage of pore volume saturated by mercury at the maximum pressure is the ''maximum saturation'' (S_max). The unsaturated pore volume at that pressure is the ''minimum unsaturated pore volume'' (u_min) (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<ref name=Wardlaw_etal_1976 />.

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_d). The percentage of pore volume saturated by mercury at the maximum pressure is the ''maximum saturation'' (S_max). The unsaturated pore volume at that pressure is the ''minimum unsaturated pore volume'' (u_min) (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 />.

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_R).

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_R).

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_E) is defined as the ratio of the mercury saturation in the sample at minimum pressure after pressure is reduced (S_R) to the saturation at maximum pressure (S_max) (Figure 3) (Wardlaw and<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_E) is defined as the ratio of the mercury saturation in the sample at minimum pressure after pressure is reduced (S_R) to the saturation at maximum pressure (S_max) (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>\mathbf{Equation}</math>

:<math>\mathbf{Equation}</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_b/r_t); 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_b/r_t); and the number of throats connecting each pore (coordination number). Table 1 shows the effect of these parameters on withdrawal 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 (Wardlaw and<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 (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.

==Reservoir applications==

==Reservoir applications==

Capillary pressure data can also be applied to help distinguish reservoir from nonreservoir and pay from nonpay (see [[Effective pay determination]]). Several workers have attempted to correlate capillary pressure data and brine or air permeabilities. Purcell related capillary pressures empirically to air [[permeability]] through the graphical integral of the curve of mercury saturation versus reciprocal capillary pressure squared. Swanson (1981)<ref name=Swanson_1981>Swanson, R. G., 1981, Sample examination manual:AAPG Methods in Exploration Series, 35 p.</ref> proposed a simple nomograph whose application improved estimation of brine [[permeability]] from capillary pressure measurements on sidewall cores and ditch cuttings.

Capillary pressure data can also be applied to help distinguish reservoir from nonreservoir and pay from nonpay (see [[Effective pay determination]]). Several workers have attempted to correlate capillary pressure data and brine or air permeabilities. Purcell related capillary pressures empirically to air [[permeability]] through the graphical integral of the curve of mercury saturation versus reciprocal capillary pressure squared. Swanson (1981)<ref name=Swanson_1981>Swanson, R. G., 1981, Sample examination manual:AAPG Methods in Exploration Series, 35 p.</ref> proposed a simple nomograph whose application improved estimation of brine [[permeability]] from capillary pressure measurements on sidewall cores and ditch cuttings.

Another type of mercury test involves injecting mercury to a saturation less than the maximum, withdrawing the mercury to some residual wetting phase saturation, and then reinjecting the mercury. This process, repeated several times to progressively higher maximum pressures, produces hysteresis loops. These loops, wherein mercury is partially withdrawn and then reinjected, can be used to investigate withdrawal efficiency at various initial saturations (Morrow, 1970<ref name=Morrow_1970>Morrow, N. R., 1970, Irreducible wetting phase saturations in porous media: Chemical Engineering Science, v. 25, p. 1799-1815.</ref>; Melrose and Brandner, 1974<ref name=Melrose_etal_1974>Melrose, J. C., and C. F. Brandner, 1974, Role of capillary forces in determining microscopic displacement efficiency for oil recovery by [[waterflooding]]: Journal Canadian Petroleum Technology, v. 13, p. 54-62.</ref>; Wardlaw and Taylor, 1976<ref name=Wardlaw_etal_1976>Wardlaw, N. C., and R. P. Taylor, 1976, Mercury capillary pressure curves and the interpretation of pore structures and capillary behavior in reservoir rocks: Bulletin of Canadian Petroleum Geology, v. 24, p. 225-262.</ref>; Wardlaw and Cassan, 1978<ref name=Wardlaw_etal_1978>Wardlaw, N. C., and J. P. Cassan, 1978, Estimation of recovery efficiency by visual observation of pore systems in reservoir rocks: Bulletin of Canadian Petroleum Geology, v. 26, p. 572-585.</ref>; Wardlaw et al., 1988<ref name=Wardlaw_etal_1988>Wardlaw, N. C., M. McKellar, and Y. Li, 1988, Pore and throat size distribution determined by mercury porosimetry and by direct observation: Carbonates and Evaporites, v. 3, p. 1-15.</ref>). Results suggest that the higher the initial saturation of the nonwetting phase, the greater the withdrawal efficiency. Porosimetry uses hysteresis loops to interpret pore body and pore throat size distributions and their patial arrangement (Dullien and Dhawan, 1974<ref name=Dullien_etal_1974>Dullien, F. A. L., and G. K. Dhawan, 1974, Characterization of pore structure by a combination of quantitative photomicrography and mercury porosimetry: Journal Colloid and Interface Science, v. 47, p. 337-349.</ref>; Wardlaw et al., 1988<ref name=Wardlaw_etal_1988 />). Pore sizes have also been evaluated with rate-controlled mercury injection (Yuan and<ref name=Yuan_etal_1986>Yuan, H. H., and B. F. Swanson, 1986, Resolving pore space characteristics by rate-controlled porosimetry: 5th Symposium on [[Enhanced oil recovery]] of the Society of Petroleum Engineers and the Department of Energy, April, SPE/DOE 14892, 9 p.</ref>.

Another type of mercury test involves injecting mercury to a saturation less than the maximum, withdrawing the mercury to some residual wetting phase saturation, and then reinjecting the mercury. This process, repeated several times to progressively higher maximum pressures, produces hysteresis loops. These loops, wherein mercury is partially withdrawn and then reinjected, can be used to investigate withdrawal efficiency at various initial saturations (Morrow, 1970<ref name=Morrow_1970>Morrow, N. R., 1970, Irreducible wetting phase saturations in porous media: Chemical Engineering Science, v. 25, p. 1799-1815.</ref>; Melrose and Brandner, 1974<ref name=Melrose_etal_1974>Melrose, J. C., and C. F. Brandner, 1974, Role of capillary forces in determining microscopic displacement efficiency for oil recovery by [[waterflooding]]: Journal Canadian Petroleum Technology, v. 13, p. 54-62.</ref>; Wardlaw and Taylor, 1976<ref name=Wardlaw_etal_1976>Wardlaw, N. C., and R. P. Taylor, 1976, Mercury capillary pressure curves and the interpretation of pore structures and capillary behavior in reservoir rocks: Bulletin of Canadian Petroleum Geology, v. 24, p. 225-262.</ref>; Wardlaw and Cassan, 1978<ref name=Wardlaw_etal_1978>Wardlaw, N. C., and J. P. Cassan, 1978, Estimation of recovery efficiency by visual observation of pore systems in reservoir rocks: Bulletin of Canadian Petroleum Geology, v. 26, p. 572-585.</ref>; Wardlaw et al., 1988<ref name=Wardlaw_etal_1988>Wardlaw, N. C., M. McKellar, and Y. Li, 1988, Pore and throat size distribution determined by mercury porosimetry and by direct observation: Carbonates and Evaporites, v. 3, p. 1-15.</ref>). Results suggest that the higher the initial saturation of the nonwetting phase, the greater the withdrawal efficiency. Porosimetry uses hysteresis loops to interpret pore body and pore throat size distributions and their patial arrangement (Dullien and Dhawan, 1974<ref name=Dullien_etal_1974>Dullien, F. A. L., and G. K. Dhawan, 1974, Characterization of pore structure by a combination of quantitative photomicrography and mercury porosimetry: Journal Colloid and Interface Science, v. 47, p. 337-349.</ref>; Wardlaw et al., 1988<ref name=Wardlaw_etal_1988 />). Pore sizes have also been evaluated with rate-controlled mercury injection (Yuan and Swanson, 1986)<ref name=Yuan_etal_1986>Yuan, H. H., and B. F. Swanson, 1986, Resolving pore space characteristics by rate-controlled porosimetry: 5th Symposium on [[Enhanced oil recovery]] of the Society of Petroleum Engineers and the Department of Energy, April, SPE/DOE 14892, 9 p.</ref>.

It should be noted that although these more specialized procedures are quite informative, especially for approximating hydrocarbon recovery efficiencies, they are also relatively labor intensive and expensive when compared to routine mercury injection tests.

It should be noted that although these more specialized procedures are quite informative, especially for approximating hydrocarbon recovery efficiencies, they are also relatively labor intensive and expensive when compared to routine mercury injection tests.