Pore throats

Laminae of clay and clay drapes act as vertical or horizontal baffles or barriers to fluid flow and pressure communication. Dispersed days occupy pore space-which in a clean sand would be available for hydrocarbons. They may also obstruct pore throats, thus impeding fluid flow. Reservoir evaluation, is often complicated by the presence of clays. This is particularly true for the estimation of hydrocarbon saturation. 

Finally, it is worth remembering the sequence of events which occur during hydrocarbon accumulation. Initially, the pores in the structure are filled with water. As oil migrates into the structure, it displaces water downwards, and starts with the larger pore throats where lower pressures are required to curve the oil-water interface sufficiently for oil to enter the pore throats. As the process of accumulation continues the pressure difference between the oil and water phases increases above the free water level because of the density difference between the two fluids. As this happens the narrower pore throats begin to fill with oil and the smallest pore throats are the last to be filled. 

Nearly all reservoirs are water bearing prior to hydrocarbon charge. As hydrocarbons migrate into a trap they displace the water from the reservoir, but not completely. Water remains trapped in small pore throats and pore spaces. In 1942 Arch/ e developed an equation describing the relationship between the electrical conductivity of reservoir rock and the properties of its pore system and pore fluids. 

The change of permeability caused by precipitation in pore throats... 

The relaxation time for each pore will still be expressed by Eq. (3.6.3) where each pore has a different surface/volume ratio. Calibration to estimate the surface relaxivity is more challenging because now a measurement is needed for a rock sample with a distribution of pore sizes or a distribution of surface/volume ratios. The mercury-air or water-air capillary pressure curve is usually used as an estimator of the cumulative pore size distribution. Assuming that all pores have the same surface relaxivity and ratio of pore body/pore throat radius, the surface relaxivity is estimated by overlaying the normalized cumulative relaxation time distribution on the capillary pressure curve [18, 25], An example of this process is illustrated in Figure 3.6.5. The relationship between the capillary pressure curve and the relaxation time distribution with the pore radii, assuming cylindrical pores is expressed by Eq. (3.6.5). 

In addition, mercury intrusion porosimetry results are shown together with the pore size distribution in Figure 3.7.3(B). The overlay of the two sets of data provides a direct comparison of the two aspects of the pore geometry that are vital to fluid flow in porous media. In short, conventional mercury porosimetry measures the distribution of pore throat sizes. On the other hand, DDIF measures both the pore body and pore throat. The overlay of the two data sets immediately identify which part of the pore space is the pore body and which is the throat, thus obtaining a model of the pore space. In the case of Berea sandstone, it is clear from Figure 3.7.3(B) that the pore space consists of a large cavity of about 85 pm and they are connected via 15-pm channels or throats. 

Hg data indicates a pore throat size of 15 pm. The overlay of the two results identify the pore throat and pore body. (C) Optical microscopy of the 30-pm thin section of the Berea sample. The pore spaces are indicated by the blue regions, which were impregnated with blue epoxy prior to sectioning. Figure from Ref. [51] with permission. 

The pressure fluctuations were analyzed to identify pore bodies and pore throats. 

For example, the sudden drop in pressure indicates the mercury meniscus entering a wide region (pore body) from a narrow region (pore throat). One of the parameters obtained from such an experiment is the distribution of the pore body volumes, shown in Figure 3.7.4 with a pronounced peak at around 20 nL. The... 

The pore geometry described in the above section plays a dominant role in the fluid transport through the media. For example, Katz and Thompson [64] reported a strong correlation between permeability and the size of the pore throat determined from Hg intrusion experiments. This is often understood in terms of a capillary model for porous media in which the main contribution to the single phase flow is the smallest restriction in the pore network, i.e., the pore throat. On the other hand, understanding multiphase flow in porous media requires a more complete picture of the pore network, including pore body and pore throat. For example, in a capillary model, complete displacement of both phases can be achieved. However, in real porous media, one finds that displacement of one or both phases can be hindered, giving rise to the concept of residue saturation. In the production of crude oil, this often dictates the fraction of oil that will not flow. 

Figure 1. Micrograph of foam in a 1.1 pm, two dimensional etched-glass micromodel of a Kuparuk sandstone. Bright areas reflect the solid matrix while grey areas correspond to wetting aqueous surfactant solution next to the pore walls. Pore throats are about 30 to 70 /xm in size. Gas bubbles separated by lamellae (dark lines) are seen as the nonwetting "foam" phase.

Figure 9 reports the effect of the pore-body to pore-throat radius ratio, R /R, on the critical capillary pressure for... 

Figure 9 teaches that large pore-body to pore-throat radii ratios lead to a more unstable foam. The effect is more dramatic for higher capillary numbers. 

In a given porous medium there will be a distribution of pore-body and pore-throat sizes. If a lamella exits a constriction into a pore body whose ratio, R /R, is less than... 

Thus, particular combinations of pore throats and pore bodies (i.e., those with R /R, greater than critical) can be classified... 

For transporting foam, the critical capillary pressure is reduced as lamellae thin under the influence of both capillary suction and stretching by the pore walls. For a given gas superficial velocity, foam cannot exist if the capillary pressure and the pore-body to pore-throat radii ratio exceed a critical value. The dynamic foam stability theory introduced here proves to be in good agreement with direct measurements of the critical capillary pressure in high permeability sandpacks. 

Flow properties of macroemulsions are different from those of non-emulsified phases 19,44). When water droplets are dispersed in a non-wetting oil phase, the relative permeability of the formation to the non-wetting phase decreases. Viscous energy must be expended to deform the emulsified water droplets so that they will pass through pore throats. If viscous forces are insufficient to overcome the capillary forces which hold the water droplet within the pore body, flow channels will become blocked with persistent, non-draining water droplets. As a result, the flow of oil to the wellbore will also be blocked. 

Retardation. A mobilized particle inherently falls behind the moving fluid since the drag force experienced by the particle is proportional to the relative velocity. Hydrodynamic conditions around the tortuous geometry of the pore space, gravity, inertial effects, high flow velocity and collisions enhance retardation. Retardation increases the local concentration of particles near pore throats. 

As shown in the studies commented below, the most important about the mechanism of foam in EOR applications are the connectivity and geometry of medium (a size distribution of pore bodies of the order of 100 pm in diameter and pore-throats of the order of 10 pm in diameter) the distribution of the two-phase systems (liquid-gas) in pores which depends on the wetting of pore walls and the volume ratio of the liquid and gas phases the regulating capillary pressure the mode of foam generation and foam microstructure. 

The basic mechanism of foam degradation in porous medium is film coalescence. It depends on film thickness and capillary pressure. In the process of advancement the film thickness changes considerably thickens in the narrow parts (pore throats) and thins in the wider parts (pore bodies). Visual observations of such a stretching-squeezing mechanism are reported by Huh et al. [178]. Therefore, the film thickness would depend on the liquid/gas ratio, the rate of movement and the ratio of pore-body to pore-throat. When the critical capillary (disjoining) pressure is reached, the film will rupture. 

Dorsch, J. et al., Effective Porosity and Pore-Throat Size of Conasauga Group Mudrock Application, Test and Evaluation of Petrophysical Techniques, Oak Ridge National Laboratory, Oak Ridge, TN, ORNL/GWPO-021, 1996. 

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