Wall-Effect/Gel-Droplet Model of Disproportionate Permeability Reduction
- J. Liang (Idaho Natl. Engineering and Environmental Laboratory) | R.S. Seright (New Mexico Petroleum Recovery Research Center)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 2001
- Document Type
- Journal Paper
- 268 - 272
- 2001. Society of Petroleum Engineers
- 4.3.4 Scale, 1.8 Formation Damage, 5.4.1 Waterflooding, 1.6.9 Coring, Fishing, 3 Production and Well Operations, 5.1 Reservoir Characterisation, 5.3.4 Reduction of Residual Oil Saturation, 4.1.2 Separation and Treating, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.2.1 Phase Behavior and PVT Measurements
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Many polymers and gels can reduce the permeability to water more than they can the permeability to oil or gas. However, the mechanism of this disproportionate permeability reduction is not clear. This paper considers a promising potential explanation that is based on a combined "wall-effect" and "gel-droplet" model. Many aspects of the disproportionate permeability reduction can be explained by a wall-effect model if the gelant is prepared from or matches the wetting phase, and by a gel-droplet model if the gelant is prepared from or matches the nonwetting phase. The combined model predicts that disproportionate permeability reduction should increase with increasing residual nonwetting-phase saturation. New experimental results support this prediction.
The objective of polymer and gel treatments in production wells is to reduce water production without damaging oil productivity. Many polymers and gels can reduce the permeability to water more than they can the permeability to oil or gas.1 This property is critical to the success of water-shutoff treatments in production wells if hydrocarbon-productive zones cannot be protected during placement.2,3 However, the magnitude of the effect has been unpredictable from one application to the next. Presumably, the effect would be more predictable and controllable if we understood why the phenomenon occurs. In this study, we first briefly review the validity of several possible explanations for this disproportionate permeability reduction. Then, we investigate a promising mechanism - a combined wall-effect and gel-droplet model. To test this potential mechanism, we examined the effects of residual oil saturation, pressure drawdown, absolute permeability, and core wettability.
Review of Previous Mechanisms
Our previous studies showed that the disproportionate permeability reduction was not caused by simple hysteresis of relative permeabilities or by gel breakdown during successive injection of oil and water banks.1,2 This phenomenon was observed in core experiments using constant-pressure and constant-rate drives. Also, the disproportionate permeability reduction did not vary with core length.4 Finally, this phenomenon was observed not only with polymers or weak polymer-based gels, but also with a resorcinol-formaldehyde gel and strong polymer-based gels.1 Thus, the effect does not appear to be an experimental artifact.
Several theories for the disproportionate permeability reduction were tested previously.1,4-7 Gravity and lubrication effects were discounted as significant mechanisms.6 Some researchers speculated that this phenomenon occurs because water-based gels or polymers shrink when in contact with oil and swell when in contact with water.8,9 Mennella et al.10 proposed a pore-scale model to describe the shrinking/swelling effects. However, our previous study demonstrated that gel shrinking and swelling were unlikely to be the primary mechanism responsible for disproportionate permeability reduction.6 A mechanism involving a balance between capillary forces and gel elasticity was also considered.5,11 Our experimental results suggested that this mechanism was valid only in micromodels and small glass tubes, not in porous rock.5 Experiments revealed that wettability may play a role in the disproportionate permeability reduction.7,12,13 However, wettability effects, by themselves, are insufficient to explain the underlying cause of the phenomenon.5,7 Another promising mechanism relies on oil and water following segregated pathways on a microscopic scale. Although this segregated pathway theory has merit,5 several experimental results appear inconsistent with the proposed mechanism.4,7,14
Zaitoun et al.12 attributed the disproportionate permeability reduction to wall effects resulting from an adsorbed polymer layer on the pore walls. Fig. 1 shows that in a strongly water-wet rock, residual oil droplets at the center of the pores can significantly reduce the effective width of the water channels during waterflooding. In contrast, this restriction may not exist during oilflooding. Therefore, for a given thickness of an adsorbed polymer layer, the permeability reduction for water during waterflooding is greater than the permeability reduction for oil during oilflooding. Following similar logic, if the adsorbed layer on the pore walls is either a polymer or a water-based gel, the wall-effect model could explain why some water-based gels exhibit disproportionate permeability reduction in strongly water-wet cores (Fig. 1).
Row 1 of Table 1 lists an example illustrating this behavior for a water-based gel [0.5% Alcoflood 935 hydrolyzed polyacrylamide (HPAM), 0.0313% Cr(III)-acetate, 0.0121% CrCl3, 1% NaCl, and 0.01% CaCl2] in a strongly water-wet rock (700-md Berea sandstone with a residual oil saturation of Soltrol 130, 41°C). The gel reduced the permeability to water by a factor of 10,100 (i.e., the water-residual resistance factor, Frrw, was 10,100), while the permeability to oil was reduced by a factor of 59 (i.e., the oil-residual resistance factor, Frro, was 59). (Details of the experimental procedures and results for this and other corefloods summarized in Tables 1-5 can be found in Refs. 4, 7, and 14. All results were obtained at 41°C.)
As mentioned above, the wall-effect model was developed after studying the disproportionate permeability reduction associated with adsorbed polymers. In contrast, most of our experiments involved gels. This paper considers whether the model is applicable to gels as well as to adsorbed uncrosslinked polymers. We recognize the possibility that the mechanism for disproportionate permeability reduction may be different for different materials - i.e., for adsorbed polymers, "weak" gels (generally formed by incomplete gelation so most of the aqueous pore space is not filled with gel), and "strong" gels (where gelation in porous rock is fairly complete and most of the aqueous pore space is filled with gel). Nevertheless, at present, no compelling evidence exists that the mechanism for disproportionate permeability reduction in porous rock is fundamentally different for the various materials.
In an oil-wet system, Zaitoun et al.12 proposed that polymer could cover most of the rock surface by anchoring on the small part of the rock surface that remains water-wet. The layer of polymer covering the oil-wet surface would shift the wettability toward water-wet. In this way, the polymer could reduce the permeability to water more than the permeability to oil in an oil-wet core. Zaitoun et al.12 reported that the capillary pressure of a silane-treated oil-wet sandstone core shifted from negative before a gel treatment to positive after treatment. Also, the polymer reduced the permeability to water more than that to oil in the oil-wet core. Based on these findings, they concluded that the adsorbed polymer layer was responsible for the disproportionate permeability reduction in both the oil- and water-wet cores.
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