Peroxidase Catalyzed Degradation of Partially Hydrolyzed Polyacrylamide
Gilbert, William Joe Rivie
University of Kansas
Chemical & Petroleum Engineering
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Recently developed technologies such as directional drilling, multi-stage fracturing, and friction reducing fracturing fluids, have allowed access to previously unattainable resources. These technologies were adapted to deal with the extremely low rock permeability where conventional fracturing practices were unsuccessful. Although there are many similarities in the fracturing ideologies between conventional and unconventional fracturing processes, one main component that differs greatly is the fracture fluid makeup, composition, and quantity. In most conventional fracturing practices, high viscosity cross-linked cellulosic polymers are used in order to transport and suspend proppant in the fracture. On the other hand, high molecular weight partially hydrolyzed polyacrylamide (HPAM) is used as a friction reducer in unconventional “slickwater” fracturing. In both cases, these polymer additives have been shown to decrease the productivity of the well by damaging the fracture conductivity and fracture face permeability. Moreover, water usage and disposal has created social and political pressures for recycling or cleaning reproduced fracture fluids. Major advances have been made to recover damage created in conventional fracturing fluids enzymatically as opposed to chemically. A few studies have investigated the oxidative chemical breakdown of HPAM but no known enzymatic processes have shown to directly degrade the polymer. The purpose of this study was to develop an enzymatic method for degrading HPAM polymer. For this study the oxidative enzyme Horseradish Peroxidase (HRP) Type II was used in conjunction with hydrogen peroxide, and β-nicotinamide adenine dinucleotide (NADH). This system was chosen for its ability to form hydroxyl free radicals though the breakdown of hydrogen peroxide. NADH was added to act as stable electron carrier during free radical formation. In addition, pilot studies, observing HPAM viscosity reduction, using soybean peroxidase was investigated as an economical alternative to HRP. Partially hydrolyzed polyacrylamide (HPAM; MW = 6,000 kDa) Alcoflood 935 was used as a commercially available polymer. Initially, studies investigated the reduction of HPAM viscosity after exposure to the HRP/NADH system using low, moderate and high peroxide concentrations in aqueous solution. Results of this study show that there was a significant reduction in viscosity 17.6 % ± 5.16, 37.7% ± 6.1, and 63.4% ± 3.9 respectively when reaction were conducted at 37C for 24 hours. In addition, studies were conducted to observe viscosity reduction in the absence of NADH and with varying HRP concentrations. Periodic sampling over a 24-hour period showed that most viscosity reduction takes place within the first 4-5 hours for all peroxide concentrations. Size exclusion chromatography was used to confirm that the reduction in viscosity was directly related to reduction in molar mass. It was determined that the weight average molecular weight of Alcoflood 935 is 6.14 ± 0.66106 Da. Reductions of 15.0 ± 3.0%, 40.2 ± 2.7%, and 66.8 ± 11.0% were observed for low, medium, and high peroxide concentrations respectively. In anticipation of core flooding studies, molecular weight reductions were also measured in 2% potassium chloride (KCl) brine. There was no significant difference in molecular weight reduction in brine when compared to studies conducted in water. Periodic sampling of molecular weight revealed that there are two kinetically distinct regions. Further studies investigated the kinetics of the reaction using different polymer concentrations. Filter cake studies were performed using 0.1-micron nylon filter. In this study filter cake was formed using polymer and/or polymer plus different components of the enzyme system to determine the most effective treatment. The resulting damage to flow from filter cake formation was greater than 99%. The best recovery (14.0 % ± 7.4) in flowrate was observed when the filter cake was formed using HPAM and HRP followed by NADH and peroxide treatment. Core flooding studies, using low permeability (1-4 md) Indiana Limestone, investigated the enzyme’s ability to recover HPAM damage to porous media. Permeability to 2% KCl was measured for the undamaged, damaged, and treated cores to determine flow damage and recovery. Recoveries were measured by rigorously flowing all components through the core. This study resulted in an average recovery of 9.7 ± 3.3% using free HRP in solution. Further investigation revealed additional damage (28.0% ± 0.7%) to permeability caused by flowing HRP treatment through the core. For this reason, HRP was covalently immobilized on Ottawa fracturing sand as a means of enzyme application. Enzyme immobilization was achieved by covalently binding HRP to Ottawa sand using (3-aminopropyl)triethoxysilane (APTES) and glutaraldehyde. This technique resulted in an immobilized concentration of 1.03 mg HRP/g sand and a specific activity of 1.1 ± 0.6 U/g sand. Using HPAM solutions, immobilized HRP resulted in a viscosity reduction of by about 65% but to a lesser degree than free enzyme in the presence of sand. Application of this system during core flooding resulted in a mean recovered permeability of 28.0 ± 0.4 percent, which is a considerable improvement, compared to free enzyme treatment (9.7 ± 3.3%). In summary, a novel approach for degrading partially hydrolyzed polyacrylamide was investigated using hydrogen peroxide and horseradish peroxidase. This sustainable HRP/H2O2 system degraded the polymer in solution, reducing its viscosity and molecular weight. Molecular weight measurements confirmed that the viscosity reduction was due to a significant degradation of the polymer backbone and not primarily by amide hydrolysis or rearrangement, etc. Furthermore, the un-optimized treatment method was able to increase the permeability in HPAM damaged cores. Indiana limestone core samples with low permeability (< 4 md) were damaged and then the HRP/H2O2 treatment was used to improve the flow. It was noticed that the enzyme treatment method actually both increases and decreases the damage in unidirectional flow system; which would not occur in an actual field treatment. However, immobilizing the enzyme on sand alleviated any further damage due to the enzyme plugging pores and increased the recovery of the damaged cores. This immobilized system may be a useful platform for remediation of polymer damage in hydraulic fracturing operations.
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