Gel Treatments in Fractures

A laboratory study has shown improved performance for fracture-problem water-shutoff polymer gels that are formulated with a combination of high- and low-molecular-weight (MW) polymers. These gels are intended for application to fractures or other high-permeability anomalies that are in direct contact with petroleum production wells. More specifically, we focused on evaluating the mechanical strength and improved performance of these water-shutoff gels for use when exceptionally large fracture apertures or large drawdown pressures are encountered.

This paper investigates whether gel behavior in rheometers correlates with that examined during extrusion through fractures. Although qualitative similarities were noted, the pressure gradients during gel extrusion through fractures were substantially greater than the values expected from rheological measurements. Also, the pressure gradient for gel extrusion through fractures varied in an unexpected manner with fracture width.

Crosslinked polymers extrude through fractures during placement of many conformance-improvement treatments, as well as during hydraulic fracturing. Dehydration of polymer gel during extrusion through fractures has often been observed and was extensively investigated during recent decades. Injection of highly viscous gel increases the pressure in a fracture, which promotes gel dehydration by fluid leakoff into the adjacent matrix. The present comprehension of gel behavior dictates that the rate of fluid leakoff will be controlled by the gel and fracture properties and, to a lesser extent, be affected by the properties of an adjacent porous medium. However, several experimental results, presented in this work, indicate that fluid leakoff deviates from expected behavior when oil is present in the fracture-adjacent matrix. We investigated fluid leakoff from chromium (Cr)(III)-acetate hydrolyzed polyacrylamide (HPAM) gels during extrusion through oil-saturated, fractured core plugs. The matrix properties were varied to evaluate the effect of pore size, permeability, and heterogeneity on gel dehydration and leakoff rate. A deviating leakoff behavior during gel propagation through fractured, oil-saturated core plugs was observed, associated with the formation of a capillary driven displacement front in the matrix. Magnetic resonance imaging (MRI) was used to monitor water leakoff in a fractured, oil-saturated, carbonate core plug and verified the position and existence of a stable displacement front. The use of MRI also identified the presence of wormholes in the gel, during and after gel placement, which supports gel behavior similar to the previously proposed Seright filter-cake model. An explanation is offered for when the matrix affects gel dehydration and is supported by imaging. Our results show that the properties of a reservoir rock might affect gel dehydration, which, in turn, strongly affects the depth of gel penetration into a fracture network and the gel strength during chase floods.

This work investigated the blockage performance of a Cr(III)-acetate-hydrolyzed polyacrylamide (HPAM) gel after placement in open fractures, with emphasis on the effect of gel maturity during placement. Polymer gel is formed through a chemical reaction between a polymer and a crosslinking agent (in a gelant solution) that occurs during the gelation time. In field applications, gelant is generally pumped from the surface, but gelation may occur during injection because of high-temperature conditions and longer pumping times; hence, partially or fully mature gel may exit the wellbore during polymer-gel injection in a fractured reservoir.

Using wide ranges of gel age, gel velocity, and fracture conductivity or tube diameter, Cr(III)-acetate-HPAM gels were studied as they extruded through fractures and tubes. Gels exhibited shear-thinning behavior in fractures and tubes that correlated with the gel superficial velocity and the fracture width or tube diameter. In fractures with sufficiently small widths, gels dehydrated during extrusion, thus reducing the rate of gel propagation. This effect was more pronounced as the fracture width decreased. Using the experimental results, a numerical study was conducted to compare placement of preformed gels and water-like gelants.

Polymer gel is frequently used for conformance control in fractured reservoirs, where it is injected to reside in fractures or high-permeability streaks to reduce conductivity. With successful polymer-gel conformance control in place, increased pressure gradients across matrix blocks may be achieved during chase floods, diverting water, gas, or enhanced oil recovery (EOR) chemicals into the matrix to displace oil. Knowledge of gel behavior during placement and chase floods is important because it largely controls the success of subsequent injections. Polymer-gel behavior is often studied in corefloods, where differential pressure and effluents from fracture and matrix outlets give information about gel deposition during placement and flow paths during chase floods.
The work presented in this paper uses complementary positron emission tomography (PET) chromatographic tomography (CT) imaging to quantify the behavior and blocking capacity of Cr(III)-acetate hydrolyzed polyacrylamide (HPAM) gel during chase waterflooding. In-situ imaging provides information about changes that may not be extracted from pressure measurements and material balance only, such as changes in local fluid saturations and dynamic spatial flow within the fracture and within the structure of the gel network.

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 pressure gradient observed during gel placement. The mechanism of gel failure involved the displacement of relatively mobile gel from wormholes. Generally, only a small fraction of the gel (<5%) was displaced during the washout process. Resistance to washout can be increased by injecting a more concentrated gel. However, this approach exhibits significantly higher pressure gradients during gel placement. The presence of a constriction in a fracture inhibited gel washout during the first pulse of brine flow after gel placement. However, during subsequent brine flow, gel erosion occurred upstream of the constriction to the same extent as downstream.

In many successful conformance control treatments, large volumes of gels were extruded through fractures during placement. The pressure gradient for gel extrusion depends strongly on fracture width and gel composition. Extrusion experiments directly measure gel properties in fractures, but they are both expensive and time-consuming. In this work, we investigated whether using rheology measurements to assess gel properties in fractures might prove a good substitute for the extrusion experiments, at a much more reasonable cost.
The rheology behavior of the gels tested showed a strong parallel to the results obtained from previous gel extrusion experiments. However, for a given aperture (fracture width or plate-plate separation), the pressure gradients measured during the gel extrusion experiments were much higher than anticipated from rheology measurements.
Extensive experiments established that wall slip and first normal stress difference were not responsible for the pressure gradient discrepancy. Steady shear and oscillatory shear data were collected with a rheometer using both smooth and rough parallel-plate geometries and employing various gap heights. Wall-slip effects were present with smooth plates but negligible with rough plates.

This experimental study investigates how Cr(III)-acetate-HPAM gels propagate through fractures. Key findings show that gels extrude as concentrated plugs without progressive plugging or increasing pressure gradients. Water leakoff into surrounding porous rock causes gel dehydration along the fracture, but gel composition remains stable. Pressure gradients were found to be independent of porous media permeability and varied inversely with the square of fracture width. The work supports a model where gels move as semisolids with limited internal viscous dissipation, leading to stable placement behavior under appropriate conditions.

This study demonstrates that low-salinity chase water can enhance the performance of Cr(III)-acetate-HPAM gels in fractured reservoirs by inducing gel swelling. Using coreflood experiments, the authors show that low-salinity brines increase differential pressure and reduce flow through reblocked fractures. In some cases, blocking capacity exceeded the original gel performance. The results were stable and reproducible across both sandstone and carbonate cores, making this a promising approach in integrated EOR (IEOR).