Gel Filter Cake Research Articles

Unlocking the potentials of gel conformance for water shutoff in fractured reservoirs: Favorable attributes of the double network gel for enhancing oil recovery

The double-network prepared with an in-situ monomer gel and a fast-crosslinked Cr(III) gel is introduced to develop a thixotropic and high-strength gel (THSG), which is found to have many advantages over the traditional gels. The THSG gel demonstrates remarkable thermal stability, and no syneresis is observed after 12 months with high salinity brine (95,500 mg/L). Moreover, the SEM and XRD results indicate that the gel is intercalated into the lamellar structures of Na-MMT, where the gel can form a uniform and compact structure. In addition, the THSG gel has an excellent swelling behavior, even in the high salinity brine. In the slim tube experiments, the THSG gel exhibits high rupture pressure and improves blocking capacity after being ruptured. The core flooding results show that a layer of gel filter cake is formed on the face of the fracture, which may be promoted by a high matrix permeability, a small aperture fracture, and a high injection rate. After the gel treatment, the fracture can be completely blocked by the THSG gel. It is found that a high incremental oil recovery (65.3%) can be achieved when the fracture was completely blocked, compared to 40.2% if the gel is ruptured. Although the swelling of ruptured gel can improve oil recovery, part of the injected brine may be channeled through the gel-filled fractures, resulting in a decrease in the sweep efficiency. Therefore, the improved blocking ability by gel swelling (e.g., in fresh water) may be less efficient to contribute to an enhancement of oil recovery. It is also found that the pressure gradient and residual resistance factor to water (Frrw) are higher if the matrix is less permeable, indicating that the fractured reservoir with lower matrix permeability may require a higher gel strength for treatment. The findings of this study may provide novel insights on designing robust double network gels for water shutoff in fractured reservoirs.

An analytical model for analyzing the impact of fracturing fluid-induced formation damage on rate transient behavior in tight formations

An enormous amount of fracturing fluid is irreducible in the tight formation after hydraulic fracturing stimulation. This will cause serious damage to the fracture face as well as the matrix close to the hydraulic fractures by invading into the matrix. Modeling the effect of water blockage on rate transient behavior is significant in reservoir engineering. This paper presents a modified trilinear model to analyze the rate transient behavior after flowback period. Single-phase flow is assumed in this stage because most injected water is hard to be reduced. For damage to the matrix caused by rock-fluid interaction, clay swelling, and polymer adsorption, a fracturing fluid invasion layer is added to the model between hydraulic fractures and the inner stimulated reservoir volume to capture this kind of damage. A skin factor is used to capture the damage to the matrix-fracture face caused by gel filter cake formation and polymer adsorption. The analytical model is benchmarked using the commercial numerical simulator Eclipse. Several synthetic cases and a field case are studied, the results show that water blockage has a great impact on the productivity of the well and decline curves. The effect of choking effect, convergence flow, and gel filter cake could be incorporated using a skin factor in the model. In addition, two more flow regimes could be observed on the theoretical type curve if the well is affected by water blockage, and the two flow regimes would happen before linear flow dominated by the inner stimulated zone. Besides, the duration of the inner zone linear flow regime could be shortened if the permeability of the fracturing fluid invasion layer is lower and invasion depth is larger. These phenomena can be used in matching the field production data from water blockage affected wells, which exhibit a transition flow regime before the formation linear flow regime. This transition flow regime could be matched quite better using the proposed model than the classical trilinear flow model. The reason is that the linear flow regime could be much longer if the water blockage effect is not considered in the model. On the other hand, the skin factor has a different impact on the flow regimes, so the water blockage effect could be characterized properly only by adding a skin factor to the model. The main contribution of this paper is the provision of an analytical model for evaluating the formation damage caused by irreducible fracturing fluid, analyzing production data and making forecasts.

The Potential of Using Cr3+/Salt-Tolerant Polymer Gel for Well Workover in Low-Temperature Reservoir: Laboratory Investigation and Pilot Test

Summary Using mature Cr3+/partially hydrolyzed polyacrylamide (HPAM) gel can reduce filtration for water shutoff in the fractured reservoir. Whether the mature gel can act as a fluid-loss-control pill for well-workover operation is worth investigating. In this paper, we start a systematic experimental study to reveal the physical process and fluid-loss-control mechanism of the Cr3+/KYPAM (salt-tolerant polymer) gel used for overbalanced well workover. The polymer gel used in this study is formulated with a combination of 0.4 to 0.6 wt% KYPAM and added 0.02 to 0.04 wt% chromium acetate, which can provide a gelation time between 2 and 4 hours, and with a maximum gel strength of Code G at a temperature of 30°C. Results show that the mature Cr3+/KYPAM gel can withstand positive pressure of 10 MPa for a period of 120 minutes with average fluid-loss volume of 15 cm3 for the core permeability between 9.18 and 217 md, indicating a favorable fluid-loss-control performance. The regained-permeability recovery can reach up to 85% for different core permeabilities. Scanning-electron-microscope (SEM) pictures show that a dense structure was formed in the gel filter cake during fluid-loss experiment. The wettability results show that the core has a greater potential to increase its water-wet ability after interacting with Cr3+/KYPAM mature gel. Field test shows that a small amount of gel leakoff was observed during each reperforation process, whereas water cut decreased from 89.1 to 52.1% and oil production increased from 0.15 to 1.11 m3/d. This study suggests that the mature Cr3+/KYPAM gel can act as a fluid-loss-control pill in high-water-cut oil wells, which can provide an avenue to bridge the design philosophy of well workover and water shutoff.

Effects of Fracture and Matrix on Propagation Behavior and Water Shut-off Performance of a Polymer Gel

The influence of reservoir parameters on water shut-off treatment in hydraulic or natural fractured reservoirs remains unclear. In the present study, effects of fracture width and matrix permeability on propagation behavior and water shut-off performance of a polymer gel have been investigated. Results show that the fracture width can greatly affect the injection pressure of the gelant, but the effect of the matrix permeability is relatively slight. During the gelant propagation in fracture, because of the concentration and pressure differences, the gelant loses itself from fracture into matrix to form a gelant filter cake on the fracture face. The smaller the fracture width is, the greater the gelant loses; moreover, a higher gelant loss degree occurs in relatively high permeable matrix. After the cross-linking reaction between polymer and chromium, the gelant filter cake becomes into a gel filter cake. The cementing strength of the gel filter cake and the strength of the gel inside fractures greatly aff...

Laboratory Characterization of Gel Filter Cake and Development of Nonoxidizing Gel Breakers for Zirconium-Crosslinked Fracturing Fluids

Summary Borate or Group 4 metal-crosslinked biopolymer fluids constitute the bulk of gelled-fracturing-fluid volumes used in field-fracture stimulation operations to date. Oxidizing chemicals remain the primary gel breakers to break down the filter cake and establish regained formation permeability or fracture conductivity. These breakers presumably oxidize the polymer backbone, causing chain breakdown and facilitating fluid flowback. Safety hazards associated with the use of bulk quantities are one of the many potential problems associated with the use of oxidizing agents as gel breakers. Proper timing of gel breakdown is of utmost importance with respect to oxidizing gel breakers. Conversely, premature polymer oxidation can cause premature proppant settling. On the other hand, a long delay between the completion of the fracturing operation and activation of the gel breaker could cause the deposition of additional filter cake on fracture faces, which could be difficult to break down. A desirable advancement of designing improved fracturing fluids is to develop nonoxidizing gel breakers that focus on decrosslinking the crosslinked gel structure by reacting with the crosslinking agent, rather than only breaking down the polymer chain. In addition, by insolubilizing such breakers, they can be embedded in the filter cake and, when activated, they can degrade the filter cake completely without leaving any residues. In this paper, synthetic disks of different porosities were used to simulate formation-permeability damage caused by filter cakes formed by Zr-crosslinked, carboxylated guar-based gels, and the results were compared with those measured for uncrosslinked fluids. The filter cakes were formed under different applied-pressure regimes and characterized for their polymer content and filter-cake/filtrate-weight ratios. From flow-rate measurements, it was clear that uncrosslinked base polymer solutions did not cause any permeability damage. Uncrosslinked polymer fluids produced from Zr-crosslinked gels by use of decomplexing, nonoxidizer breakers also did not significantly damage disk permeability compared with the fluids produced from conventional, oxidizer-type breakers. Several ortho-disubstituted aromatic compounds containing polar heteroatoms capable of functioning as potentially strong ligands for zirconium were found to be suitable as decomplexing breakers. Biopolymers containing such functional groups, notably lignosulfonate derivatives, show promise as commercially viable alternatives to oxidizing breakers for metal-crosslinked fracturing fluids.

Influence of Shear History at Bottomhole Temperature on Fracturing-Fluid Efficiency

Summary The effects of downhole environmental conditions on fracturing-fluid rheology and leakoff were evaluated in a flow loop at simulated fracture conditions. Linear and crosslinked gel fluids were subjected to shear rates of 50 to 100 seconds−1 past core for 4 hours at temperatures of 75 to 250°F [24 to 121°C]. Nonlinear effects commonly observed with crosslinked gel fluids diminished as the fluid was sheared. Shearing for 30 minutes to 4 hours at 100 seconds−1 before a fluid-loss test was run resulted in leakoff following square-root-of-time behavior. Spurt loss increased for crosslinked gel fluids as the fluid degraded, whether by enzyme or oxidative mechanism. The leakoff coefficient decreased slightly with time under shearing conditions. Although the polymer was breaking, the absolute amount of polymer available for deposition did not change. The apparent viscosity of fluid remaining within the fracture was also investigated. During initial fluid leakoff, polymer was concentrated as a gel filter cake at the fracture face. Once an equilibrium filter-cake thickness was achieved, however, fluid loss to the formation resulted in an increase in the polymer concentration in the remaining fluid and hence an increase in apparent viscosity.