Disproportionate Permeability Reduction (DPR) / Relative Permeability Modifiers (RPM)

X-ray computed microtomography (XMT) establishes that pore-filling Cr(III)-acetate-HPAM gels reduced water permeability to low values because water must flow through gel itself, while oil pressing on gel in Berea sandstone or porous polyethylene forced pathways by dehydrating the gel — leading to relatively high oil permeability. In very permeable sandpacks, ripping or extrusion mechanisms create oil pathways. XMT also reveals unexpected behavior in imbibition and drainage before gel placement: residual wetting-phase (water) saturations in Berea were surprisingly low in small pores, attributed to surface roughness from clay coating allowing efficient water drainage from small pores during oil injection.

X-ray computed microtomography (XMT) in Berea sandstone reveals that a Cr(III)-acetate-HPAM gel creates DPR by trapping substantial volumes of oil that remain immobile during subsequent water flooding. Residual oil saturation increased from 43.5% before gel placement to 78.7% after — nearly doubling the trapped oil. With this high trapped oil saturation, water is forced to flow through narrow films, the smallest pores, and through the gel itself. In contrast, during oil flooding, oil pathways remain relatively free from constriction by the gel. This mechanism — oil trapping by gel leading to water being forced into the most restricted pathways — provides the first pore-scale mechanistic explanation for DPR consistent with direct XMT observation.

Synchrotron X-ray computed microtomography (XMT) was used to investigate why Cr(III)-acetate-HPAM gels reduce water permeability 80–90 times more than oil permeability in both strongly water-wet Berea sandstone and oil-wet porous polyethylene. Despite different porosities (22% vs. 40%), pore size and aspect ratio distributions were similar between the two media. The gel caused comparable DPR ratios in both. Water and oil saturation distributions versus pore size were substantially different before, during, and after gel placement, providing pore-level insight into the mechanism. This first application of synchrotron XMT to DPR characterization opened the door to direct pore-scale observation of the gel-fluid interactions that cause the asymmetric permeability reduction.

Disproportionate permeability reduction (DPR) refers to the phenomenon where polymer solutions and gels reduce permeability to water more than to oil/gas. This study investigates DPR mechanisms via core- and micro-model-scale experiments using Cr(III)-acetate-HPAM gels. Core-scale NMR detected changes in trapped/free water during oil and water flooding, while micro-model tests recorded oil/gas invasion behavior visually. Results show that gel-displacement initiates oil flow paths, followed by gel dehydration under pressure gradients. During water flooding, gel rehydration blocks channels, significantly reducing water permeability. Mechanisms contributing to DPR include channel segregation, gel rehydration, residual oil effects, and gel’s intrinsic low permeability to water.

Pore-scale XMT images track oil and water saturations in individual pores as a function of oil throughput after pore-filling Cr(III)-acetate-HPAM gel placement, in both water-wet Berea sandstone and hydrophobic porous polyethylene. In hydrophobic polyethylene, oil saturations increased and gel was destroyed quickly in the smallest and largest pores, but most slowly in intermediate-size pores — suggesting a balance between gel dehydration by oil film growth versus gel extrusion. In water-wet Berea, oil saturation increased evenly across all pore sizes — consistent with imbibition behavior before gel placement, suggesting oil had uniform access to Berea pores of all sizes and uniformly dehydrated gel. Gel extrusion did not appear significant in Berea.

This paper proposes a combined wall-effect and gel-droplet model to explain DPR. The wall-effect model applies when gelant matches the wetting phase — the gel forms a thin coating on pore walls, selectively blocking water flow while oil flows through the gel-free pore center. The gel-droplet model applies when gelant matches the nonwetting phase — gel forms discrete droplets that are more easily displaced by the nonwetting oil phase. The combined model predicts that DPR should increase with increasing residual nonwetting-phase saturation. New experimental results support this prediction, providing the first quantitative model that correctly captures the dependence of DPR magnitude on residual oil saturation.

Several previous researchers reported that polymers or gels can reduce permeability to water more than to oil. However, a plausible explanation for the phenomenon is not yet available. This property is critical to the success of gel treatments in production wells if zones cannot be isolated during gel placement. We examined how different types of gels reduce oil and water permeabilities in Berea sandstone. The gel formulations that we investigated included (1) resorcinol-formaldehyde, (2) Cr³⁺(chloride)-xanthan, (3) Cr³⁺(acetate)-polyacrylamide, and (4) colloidal silica. Several new methods were applied to obtain a better understanding of why gels can reduce water permeability more than oil permeability. First, before gel placement in cores, multiple imbibition and drainage cycles were performed in both flow directions. Results from these studies established that hysteresis of oil and water relative permeabilities was not responsible for the behavior observed during our subsequent gel studies. Second, several gels clearly reduced water permeability significantly more than oil permeability. Whereas previous literature reported this phenomenon for polymers and “weak” polymer-based gels, we also observed the disproportionate permeability reduction with a monomer-based gel (resorcinol-formaldehyde), as well as with both “weak” Cr³⁺ (chloride)-xanthan and “strong” Cr³⁺(acetate)-HPAM gels. In contrast, a colloidal-silica gel reduced water and oil permeabilities by about the same factor. Residual resistance factors for several gels were found to erode during multiple cycles of oil and water injection. In spite of this erosion, the disproportionate permeability reduction persisted through the cycles for most of the gels. Studies using both oil and water tracers provided insight into the fraction of the pore volume occupied by gel. The strongest gels appeared to encapsulate the original residual oil saturation—thus rendering the residual oil inaccessible during subsequent oil flooding.

3D pore-scale characterization using synchrotron X-ray computed microtomography combined with X-ray attenuating dopants provides pore network characterization and fluid displacement data in a Berea core. Results cover pore volume, surface area, throat dimensions, and Lattice Boltzmann-computed absolute permeabilities for individual throat structures. Oil and water distributions at residual conditions are reported before and after water-based gel injection. The gel caused oil trapping (higher residual oil saturation after gel), and results provide supporting evidence for gel compaction (dehydration) and oil trapping as contributors to DPR, while discounting gel blockage in throats as a mechanism. Published in Advances in Water Resources.

This paper investigates how different gel types reduce permeability to water and compressed gases (CO₂ and N₂) in porous rock, including a weak resorcinol-formaldehyde gel, a strong resorcinol-formaldehyde gel, a Cr(III)-xanthan gel, a Cr(III)-acetate-HPAM gel, and a colloidal-silica gel. An inline high-pressure spectrophotometer allowed tracer studies without depressurizing. Analogies are noted between gas and water permeability reduction results from a parallel oil study. Results cover permeability reduction characteristics and stability to repeated water-alternating-gas (WAG) cycles at pressures up to 1,500 psi — directly relevant to gel treatments for channeling control in high-pressure gas and CO₂ floods.

This paper provides guidance on when and where RPM/DPR water-shutoff treatments can be successfully applied in oil or gas production wells. When properly designed, these treatments can address a limited range of excess water production problems using bullhead injection without mechanical zone isolation. However, substantial limitations exist. For unfractured wells with radial matrix flow, oil and water zones must not be in pressure communication, and oil-producing zones must be producing at 100% oil cut (dry oil) for full drawdown. For fractured wells, pore-filling gels in matrix rock adjacent to fractures offer an attractive application when oil and water are co-produced through the fracture. Transient RPM/DPR treatments with short-term water/oil ratio improvement are possible in many unfractured wells but require custom engineering and often have marginal economics.

This paper examines multiple possible explanations for why gels reduce water permeability more than oil permeability. Experiments show the effect is not caused by gravity, lubrication effects, gel shrinking/swelling, or wettability differences as root causes. Results from an oil-based gel experiment suggest that microscopic segregation of oil and water pathways through the porous medium may play the dominant role — water-based gels preferentially block water pathways while leaving oil pathways relatively unobstructed. The study does not definitively prove this mechanism but identifies microscale pathway segregation as the most promising hypothesis for subsequent investigation.

Pore-filling Cr(III)-acetate-HPAM gels are investigated as a more reliable approach to DPR. For porous media with pre-gel water permeability at Sor ranging from 120 to 6,500 md, a single pore-filling gel formulation consistently reduced water permeability to approximately 0.24 md (range 0.12–0.37 md) — essentially independent of initial permeability. In contrast, a commercially available relative-permeability modifier showed a much wider range of post-treatment water permeability (0.75–202 md) in Berea cores at 222–363 md. Several pore-filling gel formulations provided water residual resistance factors above 2,000 with ultimate oil residual resistance factors of 2 or less. Significant oil throughput is required to achieve low oil residual resistance factors, suggesting gelant penetration must be small (a few feet or less) for effective DPR.

A mobility-ratio model predicts cleanup times for gel-treated oil zones in both fractured and unfractured production wells. Key findings: (1) cleanup time is similar for radial vs. linear flow; (2) cleanup varies approximately with the cube of gel penetration distance — shallow penetration cleans up orders of magnitude faster; (3) cleanup varies inversely with pressure drawdown; (4) cleanup varies inversely with water permeability in the gel-treated region; (5) cleanup is not sensitive to the final oil permeability at residual water saturation after gel dehydration. Although oil permeability after gel dehydration does not affect cleanup time, it strongly determines how much of the original oil productivity is ultimately recovered.