Gel Treatments in Fractures

A laboratory study showed improved performance for fracture water-shutoff polymer gels formulated with a combination of high- and low-molecular-weight (MW) polymers. These gels are intended for applications to fractures or high-permeability anomalies in direct contact with production wells — especially when large drawdown pressures or large-aperture fractures (>1.5 mm) are encountered. The combined high-MW and low-MW gel formulation exhibited an effective viscosity of approximately 500 cp during placement in a 1-mm fracture, withstood 92 psi total differential pressure (26 psi/ft) for 24 hours with no detectable brine flow, and achieved a 30,000x permeability reduction factor when the differential pressure was increased to 175 psi (88 psi/ft).

This paper investigates whether gel behavior in rheometers correlates with behavior during extrusion through fractures. Qualitative similarities were noted between rheometer results and fracture extrusion data, but the pressure gradients during gel extrusion through fractures were substantially greater than values expected from rheological measurements. Also, the pressure gradient for gel extrusion through fractures varied with fracture width in an unexpected manner. These discrepancies point to a mechanism beyond simple bulk rheology — later work by Wang and Seright identified the wormhole flow mechanism as the explanation.

Water leakoff from Cr(III)-acetate-HPAM gels during extrusion through oil-saturated, fractured cores was investigated. Fluid leakoff deviated from expected behavior when oil was present in the fracture-adjacent matrix. A capillary-driven displacement front formed in the oil-saturated matrix, altering the leakoff rate and the depth of gel penetration into the fracture network. Matrix properties (pore size, permeability, heterogeneity) were varied to evaluate their effect on gel dehydration and leakoff rate. MRI was used to monitor water leakoff in a fractured, oil-saturated carbonate core — verifying the displacement front and confirming wormhole presence in the gel during and after placement, consistent with Seright’s filter-cake model.

This work investigates blockage performance of Cr(III)-acetate-HPAM gel placed in open fractures, with emphasis on the effect of gel maturity at the time of placement. In field applications, gelant may partially or fully mature during pumping before reaching the fracture. Mature gel placed in open fractures yielded consistent, predictable rupture pressures following linear trends with placement rate and volume. Immature gel (gelant) gave comparable but less predictable rupture pressures; in some cores with Bentheim sandstone, gel failed to form due to rock-gelant interactions. Average permeability reduction was 5,000 for mature gel and 600 for gelant-treated cores. Placing gelant in both the fracture and adjacent matrix increased rupture resistance.

This paper studies preformed Cr(III)-acetate-HPAM gels using wide ranges of gel age, gel velocity, and fracture conductivity. Gels exhibited shear-thinning behavior in fractures and tubes correlated with gel superficial velocity and fracture width or tube diameter. In narrow fractures, gels dehydrated during extrusion, reducing the rate of gel propagation — an effect more pronounced as fracture width decreased. A numerical study compared placement of preformed gels and water-like gelants, examining ideal placement locations in fractures.

PET (positron emission tomography) and CT (computed tomography) imaging were combined to quantify Cr(III)-acetate-HPAM gel behavior during chase waterflooding in fractured core plugs. Gel was placed in cores with longitudinal fractures, and F-18 labeled water was injected during chase floods to visualize and quantify local flows with PET. The study directly visualized gel rupture and wormhole development as a function of flow rate, providing spatial confirmation of gel behavior models previously inferred from pressure measurements and material balance alone. PET imaging revealed local flow redistribution during gel erosion that cannot be extracted from global measurements.

This paper investigates washout of mature Cr(III)-acetate-HPAM gels from fractures. After gel placement, the pressure gradient for gel washout during brine or oil flow was similar to the gradient during placement. The mechanism of gel failure involved displacement of relatively mobile gel from wormholes — only a small fraction of the gel (<5%) was displaced during the washout process. Washout resistance increases with more concentrated gel. A constriction in a fracture inhibited washout during the first pulse of brine flow but not subsequently. Oil-wet polyethylene cores showed similar washout behavior to water-wet Berea sandstone. Lower placement rate and secondary crosslinking reactions (post-placement Cr(III) acetate) both substantially increased resistance to washout.

This paper investigated whether rheology measurements could substitute for fracture extrusion experiments to assess gel properties in fractures. The rheology behavior showed a strong qualitative parallel to prior gel extrusion results. However, for a given fracture aperture, pressure gradients measured during extrusion were much higher than anticipated from rheology measurements. Extensive experiments ruled out wall slip and first normal stress difference as explanations. The pressure gradient discrepancy was explained by noting that gel flows through narrow wormholes in concentrated immobile gel — the effective aperture for mobile gel is much narrower than the fracture width. Two models using shell momentum balances explain why (1) pressure gradient varies inversely with the square of fracture width, and (2) fracture gradients exceed viscometer predictions.

This paper describes an experimental investigation of the mechanism for propagation of Cr(III)-acetate-HPAM gel through fractures. When large volumes were extruded through a fracture, progressive plugging (continuously increasing pressure gradients) was not observed. Effluent from the fracture had the same appearance and similar composition as the injected gel, even though concentrated, immobile gel formed in the fracture. The concentrated gel formed when water leaked off from the gel along the length of the fracture, driven by the pressure difference between the fracture and adjacent porous rock. A simple model was developed to account for the experimental results. Critically, pressure gradients and dehydration factors were the same for fractures in 650-md sandstone, 50-md sandstone, and 1.5-md limestone.

This paper demonstrates that low-salinity water swells conventional Cr(III)-acetate-HPAM gels, significantly improving gel-blocking performance after gel rupture. Formed polymer gel was placed in fractured core plugs, and chase waterfloods were performed using four brine compositions, including three low-salinity brines. Injection pressure and matrix flow rate both increased with decreasing chase-water salinity. In some cores, the fracture was reblocked during low-salinity waterfloods — gel-blocking capacity was restored above the initial rupture level. Low-salinity water also improved matrix sweep during chase floods. Results were reproducible in both sandstone and carbonate outcrop cores.