POLYACRYLAMIDE DEGRADATION DURING HYDRAULIC FRACTURING AND ITS IMPACT ON MEMBRANE FOULING DURING WASTEWATER TREATMENT. PDF Download

Author: Boya Xiong
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Languages : en
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Book Description
The high intensity of unconventional oil and gas development nationwide and at global scale has an enormous impact on local and regional water resource and water quality. High volume hydraulic fracturing (HVHF) utilizes a wide range of proprietary chemicals and more than a million liters of water per well, generating around 100 billion liters of flowback and produced water annually in the U.S. HVHF wastewater contains high levels of salinity, turbidity, organic matter and radioactivity; posing technical and economic challenges to wastewater treatment. Current practices are unlikely to manage the growing volume of wastewaters in a sustainable and economically feasible manner. More importantly, many environmental impacts of HVHF wastewater contamination remain unclear due to the unresolved organic components in wastewater, many of which originate from injected chemicals. The goal of work was to better quantify the environmental risks of HVHF activities by identifying and analyzing the fate and characteristics of these injected chemicals that cause challenges in subsequent membrane treatment and also have the potential to release toxic byproducts.High molecular weight (106 3107 Da) polyacrylamide (PAM) and its copolymers are heavily used as friction reducers in HVHF. Extractions in the Marcellus shale alone are estimated to have consumed 5000-140,000 tons of PAM. However, PAM molecules are not characterized by the current organic analyses that utilize advanced chromatography and mass spectrometry techniques due to their hydrophilic nature and large size. In this work, it was identified that under simulated HVHF deep subsurface conditions, PAM (1.5107 Da) is susceptible to significant free radical induced chemical degradation, with the final MWs ranging over three orders of magnitude from 8103 - 1.5107 Da as quantified by size exclusion chromatography. The degradation kinetics are governed by the formation temperature, shale mineralogy and dissolved oxygen concentration in the initial fluids. The above information on operating conditions is readily available from drilling logs and geological surveys of gas reservoirs - thus these results make it possible to predict the extent of polymer degradation at a specific fracturing site. In addition, my PhD work extends the main work on chemical degradation to quantify the mechanical degradation of polyacrylamide using a high-pressure capillary flow set up. The experimental setup simulates the high strain rates similar to that at the entrance flow through small pores and fractures in the formation face during the fracture propagation phase where the hydraulic pressure can reach as high as 700 bar. Here I report a non-chemical pathway- purely physical transformation of fracturing chemicals under HVHF conditions that is unique to high molecular weight polymers.The objectives of this work also included evaluation of membrane treatability of actual flowback and produced waters from the Marcellus shale play. The data show severe fouling during microfiltration membrane treatment. Surprisingly, the high variation in fouling behavior for different water samples cannot be correlated with their levels of total organic carbon or suspended solids, both commonly measured water quality parameters, suggesting a high level of complexity in the fouling components of the feed water matrix in these waste streams. The fouling rates of some wastewaters were dominated by the presence of colloidal and organic matter; however, their origin and detailed characterization remains unclear. Experiments performed with a synthetic fracturing fluid demonstrated that out of 10 different fracturing chemicals, the PAM-based friction reducer is the primary contributor to fouling rates during microfiltration treatment of the wastewater. More importantly, fouling rates were well correlated with the hydrodynamic size of PAM present in the wastewater. These results indicate that a higher fouling rate of membrane treatment will occur during the treatment of wastewaters containing large size polymers, when specific fracturing and formation conditions are favorable for limited polymer degradation.These degraded polymer molecules, which have unknown toxicities, are present in the wastewater and could reach downstream water supplies. Furthermore, complete degradation of PAM would result in release of the neurotoxic monomer acrylamide. This work, for the first time, provides detailed information on the downhole transformation and membrane treatability of PAM, a heavily used chemical in the HVHF process. This work enables the development of treatment strategies to minimize waste volume, toxicity, and broader environmental impacts of HVHF wastewaters that will be continuously generated in upcoming years as a part of the energy production trajectory of many nations worldwide.