Porous media permeability reduction

This mechanism is similar to that of a deep-bed filtration process with some differences (12). In the filtration process the particle-size to pore-size ratio is small, and the particles are mostly captured on the media surface. Thus interceptive capture dominates, and this capture does not alter the flow distribution in the porous medium. Permeability reduction is not significant and is ignored. On the other hand, the emulsion droplet size is generally of the same order of the pore size, and the droplets are captured both by straining and interception. This capture blocks pores and results in flow redistribution and a reduced permeability. 

The main result of the particles trapping is a fooling effect for the porous medium that leads to a reduction of the material permeability. As shown on Figure 7.7, this fooling... 

These tests show that CC -foam is not equally effective in all porous media, and that the relative reduction of mobility caused by foam is much greater in the higher permeability rock. It seems that in more permeable sections of a heterogeneous rock, C02-foam acts like a more viscous liquid than it does in the less permeable sections. Also, we presume that the reduction of relative mobility is caused by an increased population of lamellae in the porous medium. The exact mechanism of the foam flow cannot be discussed further at this point due to the limitation of the current experimental set-up. Although the quantitative exploration of this effect cannot be considered complete on the basis of these tests alone, they are sufficient to raise two important, practical points. One is the hope that by this mechanism, displacement in heterogeneous rocks can be rendered even more uniform than could be expected by the decrease in mobility ratio alone. The second point is that because the effect is very non-linear, the magnitude of the ratio of relative mobility in different rocks cannot be expected to remain the same at all conditions. Further experiments of this type are therefore especially important in order to define the numerical bounds of the effect. 

The wettability of the porous medium was found to have a significant effect on foam flow as early as 1966 (D. C. Bond and G. G. Bernard, AICh 58th Annual Meeting, Dallas, February 7-10, 1966). Later, Kanda and Schechter showed that a foam produced a large reduction of permeability only if the aqueous phase wet the porous medium (64). Thus, various flow studies confirm the importance of wettability. 

To calculate the reduction in the concentration of surfactant in the fluid by adsorption it is necessary to have an estimation of the inner surface area of the reservoir. This parameter is related to the porosity of the medium and to its permeability. Attempts have been made to correlate these two quantities but the results have been unsuccessful, because there are parameters characteristic of each particular porous medium involved in the description of the problem (14). For our analysis we adopted the approach of Kozeny and Carman (15). These authors defined a parameter called the "equivalent hydraulic radius of the porous medium" which represents the surface area exposed to the fluid per unit volume of rock. They obtained the following relationship between the permeability, k, and the porosity, 0 ... 

This research has shown that emulsions which are stable at elevated temperatures and survive dilution with fresh water can be formed. They have drop sizes appropriate for blocking pores in a porous medium at elevated temperatures and in the presence of saturated steam. Emulsion blocking also occurs in the presence of residual oil. It was also demonstrated that injection of a small slug of emulsion made from oil and water available in a specific field caused a significant reduction in the permeability of core material from that field. 

Soo and Radke (11) confirmed that the transient permeability reduction observed by McAuliffe (9) mainly arises from the retention of drops in pores, which they termed as straining capture of the oil droplets. They also observed that droplets smaller than pore throats were captured in crevices or pockets and sometimes on the surface of the porous medium. They concluded, on the basis of their experiments in sand packs and visual glass micromodel observations, that stable OAV emulsions do not flow in the porous medium as a continuum viscous liquid, nor do they flow by squeezing through pore constrictions, but rather by the capture of the oil droplets with subsequent permeability reduction. They used deep-bed filtration principles (i2, 13) to model this phenomenon, which is discussed in detail later in this chapter. 

Figure 16 shows the results when 20 pore volumes of an emulsion having a 3.1- xm mean droplet size is injected into an 1170-mD sand pack and is followed by several pore volumes of water (ii). After emulsion injection, a permeability reduction of about 50% is observed. With water injection, the effluent concentration drops to 0 after one pore volume, whereas the permeability is unaltered. For this dilute emulsion, the droplets are captured in the porous medium, and this capture leads to blocking of the flow paths. Figure 16 shows that once the droplets are captured, they do not re-enter the flow stream, velocity being constant. Soo and Radke (ii) proposed the following physical interpretation for the results of Figure 15. Initially oil droplets are preferentially captured in the small-size pores, and as injection proceeds, more and more of the small pores become blocked. This blockage leads to a flow diversion toward larger size pores, and the rate...

The results of Figure 13 suggest that as the droplet size increases, the emulsion retention increases. The large droplets have a higher capture probability and fill up more of the pores faster, a result that explains why they elute later than the smaller droplets. Emulsions with small droplet size diameters elute with essentially the inlet size distributions. Two factors control permeability reduction the total volume of droplets retained and the effectiveness of these droplets in restricting fiow. For a given porous medium, a critical mean droplet size of the emulsion controls permeability reduction. Below this value, retention of oil in porous media is dominant, and above the critical mean droplet size, their obstruction ability is pronounced. This situation explains the trends shown in Figure 13 for the effect of droplet size on permeability reduction. These conclusions are valid for stable, very dilute OAV emulsions and are based on a few experiments. 

Permeability of the porous medium is not affected by the flow of emulsion through it. Alvarado and Marsden (25) and Ali and Abou-Kassem (id) account for permeability reduction that is observed by using the flowing permeability as a parameter. 

The model correctly describes the permeability reduction as a function of pore volume injected and takes into account the effect of emulsion droplet saturation and droplet-size to pore-size ratios. The main drawbacks of this theory are that the permeability reduction is caused as long as the emulsion is flowing and that the initial permeability is restored once the emulsion injection is followed by water alone. In other words, the emulsion droplets all pass through the porous medium, and none of them is captured inside. However, experimental evidence 9,11) suggests that the permeability reduction cannot be restored after subsequent water injection (Figure 16). 

Re-entrainment of liquid droplets that are captured can also occur as a result of squeezing when the local pressure drop is increased to overcome the capillary resistance force. The shape of the liquid droplets depends on the wettability of the rock. On the basis of this physical picture, Soo and Radke 12) proposed a model to describe the flow of dilute, stable emulsion flow in a porous medium. The flow redistribution phenomenon and permeability reduction are included in the model. Both low and high interfacial tension were considered. 

Investigations to determine the leak-off control mechanisms of foam have shown (26—29) that the effective permeability of a porous medium is greatly reduced in the presence of foam. Some basic assumptions were used during the testing to determine the leak-off control mechanisms of foamed fracturing fluids. The first assumption was that the liquid or continuous phase moves freely, and permeability reduction is a function of the liquid saturation. The other assumption was that the gas or discontinuous phase flows only by rupture and reformation of the foam film. The resistance of foam to flow through porous media is a function of the stability of the foam. 

It remains to establish, the pressure trend throughout the porous medium as the particles filter out of solution. As long as no filtration has taken place, Darcy s law can be applied. However, the pressure profile will change according to deposition of filtrate on the matrix of the porous medium. The permeability reduction can be approximated with the Carman-Kozeny relation ... 

The permeability reduction occurs via two mechanisms (1) trapping (particles larger than the pore throat size) and (2) deposition (particles attached to the pore walls). The same two mechanisms also contribute to pore throat plugging. The porous medium is characterized by the number density of unplugged pore throats, Np, and the size distribution of open throats, . 

The emulsion behaviour in porous media is discussed in [235]. O/w emulsions with volume fractions of up to 50% show Newtonian behaviour, whereas those with more than 50% are non-Newtonian liquids, the apparent viscosity of which depends on the shear rate. The viscosities of such emulsions are more than 20 times that of water and sometimes can be even comparable with that of oil. When the emulsion is moving, a temporary permeability reduction of the reservoir may occur due to the capture of small droplets by the surface of the porous medium. In this case, stable o/w emulsions may flow not as a continuous liquid, i.e. the emulsion flow largely depends on the nature of the porous medium. Therefore, it is necessary to know about the structure and physicochemical characteristics of the oil reservoir (porous medium) porosity, the mean pore diameter, the mean pore size and pore size distribution, chemical composition of the minerals ( acidic , basic , neutral ), the nature of the pore surface, first of all wettability, for a successful application of the emulsion flooding method. 

Flow Characteristics. Permeability Reduction. Polymer retention reduces the apparent permeability of the rock. Pemiea-bility reduction depends on the type of polymer, the amount of polymer retained, ttie pore-size distribution, and the average size of the polymer relative to the pores in the rock. Permeability reduction is determined experimentally by first displacing polymer solution through a porous medium and then displacing die polymer with brine and measuring the permeability to brine after all mobile polymer has been displaced. Fig. 5.28 illustrates the effect of initial rock permeability on the permeability reduction of Berea sandstone cores by partially hydrolyzed polyacrylamide in 3 % NaCl. The trend in permeability reduction in Fig. 5.28 is consistent with the trend of increased retention as permeability decreases shown in Fig. 5.22. 

Prediction of the permeability reduction from properties of the porous rock and the polymer is not possible at this time. Experimental measurement with the rock and polymer of interest is necessary. It is often possible, however, to correlate permeability reduction for the same polymer in the same type of porous medium and use the resulting correlation for interpolation and extrapolation. Gogarty37 correlated the permeability of consolidated porous rocks after contact with polyacrylamide by use of an empirical relationship. 

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