Further Investigations of Why Gels Reduce Water Permeability More Than Oil Permeability
- Jenn-Tai Liang (New Mexico Petroleum Recovery Research Center) | R.S. Seright (New Mexico Petroleum Recovery Research Center)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- November 1997
- Document Type
- Journal Paper
- 225 - 230
- 1997. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 4.3.4 Scale, 4.2.3 Materials and Corrosion, 1.6.9 Coring, Fishing, 6.5.2 Water use, produced water discharge and disposal, 2.5.2 Fracturing Materials (Fluids, Proppant), 3 Production and Well Operations
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In this paper, we investigate why some gels can reduce the permeability to water much more than to oil. This property is critical to the success of chemical-based water-shutoff treatments in production wells if hydrocarbon-productive zones cannot be protected during placement. We first briefly review previous findings and the validity of several possible explanations for this disproportionate permeability reduction. Next, we describe experiments that test the validity of a promising mechanism - the segregated pathway theory. This theory speculates that on a microscopic scale, aqueous gelants follow water pathways more than oil pathways. Our experimental results in cores support this mechanism for oil-based gels, but not for water-based gels. We also explore another interesting mechanism that involves a balance between capillary and elastic forces. Results from our experiments support this mechanism for flow in tubes and micromodels, but not in porous rock. Other mechanisms are also discussed.
The ability of blocking agents to reduce the permeability to water much more than to oil is critical to the success of water-shutoff treatments in production wells if hydrocarbon-productive zones cannot be protected during placement.1,2 Results from the literature and our own experimental work3-14 have shown that many polymers and gels exhibit this disproportionate permeability reduction. 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 our previous studies, we examined six possible mechanisms for this disproportionate permeability reduction (see Table 1 ).12-14 Although we have not yet definitively explained this phenomenon, some possible mechanisms have been identified.
In this paper, we first briefly review previous findings and the validity of several possible explanations for the disproportionate permeability reduction. Next, we describe experiments that test the validity of a promising mechanism, the segregated pathway theory (see Fig. 1 and Mechanism 6 in Table 1). Third, we explore another interesting mechanism that was suggested after viewing a videotape of micromodel experiments performed by Dawe and Zhang.15 This mechanism involves a balance between capillary and elastic forces (Mechanism 7). Finally, we examine yet another mechanism that assumes that, during brine injection, polymer leaches from the gel and significantly decreases the brine mobility (Mechanism 8).
Review of Previous Findings
The ability to reduce permeability to water more than that to oil or gas has been reported for some adsorbed polymers and many gels of different strengths and generic types.2-17 Earlier work (at 41°C) showed that this effect was not caused by simple hysteresis of relative permeabilities or by gel breakdown during successive injection of oil and water banks.2,3 Thus, the effect does not appear to be an experimental artifact.
Mechanism 1 in Table 1 speculates that the disproportionate permeability reduction occurs because gels shrink when in contact with oil but swell when in contact with water. This mechanism is counterintuitive because it requires that a hydrophilic gel give up water to a hydrophobic oil.12 Also, experiments in beakers show no tendency for oil to synerese or degrade aqueous gels (assuming that the oil does not contain corrosive agents). Furthermore, in contact with water, gels can shrink, swell, or remain unchanged, depending on the salinity and pH of the water.18 Finally, in cores, the oil/water disproportionate permeability reduction was insensitive to system pressure between 0 and 1,500 psi.12 These facts all argue against the shrinking/swelling mechanism (Mechanism 1).
The disproportionate permeability reduction was not sensitive to flow direction or core orientation (vertical or horizontal) during flooding or to water/oil density differences between 0.12 and 0.24 g/cm3. These facts indicate that gravity effects (Mechanism 2 in Table 1) do not cause the disproportionate permeability reduction.12
The effect was not sensitive to oil viscosity between 1 and 31 cp.12 Also, behavior observed in water/gas experiments (using either N2 or CO2 at 900 to 1,500 psi) was analogous to that in water/oil experiments.14 These facts argue against the importance of lubrication effects (Mechanism 3 in Table 1), where a low-viscosity wetting layer lubricates or reduces the apparent resistance associated with flow of the nonwetting phase.12
Mechanism 4 in Table 1 suggests that the disproportionate permeability reduction should be greatest in water-wet cores. However, cases have been observed where the effect is significantly more pronounced in cores of intermediate wettability than in water-wet cores.3 Thus, although we suspect that wettability may play a role in the disproportionate permeability reduction, its effects are unclear.
In a previous paper,12 we reported evidence that supported Mechanism 6. On a microscopic scale, aqueous gelants follow water pathways more than oil pathways. The main evidence supporting this mechanism was that an oil-based gel reduced permeability to oil much more than that to water. Our previous paper acknowledged that additional work was needed to support Mechanism 6. The remainder of this paper documents our efforts to find that support, as well as our investigation of two new mechanisms (Mechanisms 7 and 8 in Table 1).
Segregated Oil and Water Pathways
If (on a microscopic scale) a water-based gelant follows primarily the pathways available to water, then many of the oil pathways could remain open (relatively gel-free) after treatment while most of the water pathways would be blocked by the gel (Fig. 1). In this way, the water-based gel could reduce permeability to water more than to oil.
Following the same logic, during high oil fractional flow, if an oil-based gel follows primarily the pathways available to oil on a microscopic scale, then many of the water pathways could remain open after treatment while most of the oil pathways would be blocked by the gel. In support of this theory, we found that an oil-based gel (12-hydroxystearic acid in Soltrol 130) reduced permeability to oil much more than to water.12,13
If this segregated-pathway theory is valid, we speculate that the disproportionate permeability reduction could be enhanced by simultaneously injecting oil with a water-based gelant or water with an oil-based gelant. Presumably, simultaneous injection of oil and a water-based gelant should allow a larger fraction of oil pathways to remain open than if a water-based gelant is injected by itself. Using similar logic, simultaneous injection of water and an oil-based gelant should allow a larger fraction of water pathways to remain open than if an oil-based gelant is injected by itself.
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