Thermodynamic models for major electrolytes and swelling of hydrogels in high salinity produced water



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Over the past decade, hydraulic fracturing has been the most common technique for the production of oil and gas from shale and other tight rocks. The fracturing process requires an enormous amount of fracturing fluid which consists of primarily water and a small amount of chemical additives. The process not only uses water for fracturing, it also generates a large amount of impure water with the production of oil and gas. This byproduct water, generally known as produced water, may have salinity seven times higher than the sea waters. Due to water demand for hydraulic fracturing and environmental regulations on disposing the produced water outside the well site, recycling the produced water to further use for the fracking process is essential.

To aid in heat and mass balance calculations for the desalination process, a comprehensive thermodynamic model is required that can provide accurate representation for the wide range of thermodynamic properties and salt solubility of the major electrolytes present in high salinity waters. Thermodynamic modeling for aqueous electrolytes play a key role not only in the desalination process, but also in many natural and industrial processes, including salt extraction from the salt-lake brines, electrodialysis, solubility prediction in geothermal energy processes, nuclear waste processing, and water-pollution control.

This dissertation is primarily focused on developing predictive tools for calculating phase equilibria, calorimetric properties, and salt solubility of the high salinity water systems. Use of an appropriate liquid activity coefficient model is crucial to describe the non-ideal nature of the aqueous electrolyte systems. We use the electrolyte nonrandom two liquid (eNRTL) activity coefficient model for thermodynamic modeling of high salinity brines. The semi-empirical model has strong thermodynamic foundations, and to account for solution non-ideality, the model requires only two interaction parameters per water-electrolyte and electrolyte-electrolyte pair.

Using the eNRTL framework, we first develop a thermodynamic model for the aqueous Mg2+-Na+-K+-Cl− quaternary electrolyte system and its subsystems. Next, we develop a thermodynamic model to predict aqueous Na+-K+-Mg2+-Ca2+-SO42− quinary and its subsystems. This quinary highlights the systems involving (Ca2+ SO42−), which are the primary concerns in scale formation during oil and gas production. After that, we consider interactions involving Cl− with the quinary to report thermodynamic model for the aqueous hexary system of Na+, K+, Mg2+, Ca2+, Cl−, and SO42−, the major ionic species present in high salinity waters. These developed models are validated with the electrolyte concentrations up to salt saturation and temperatures from 273 to 473 K.

We also extend the thermodynamic model developed for the aqueous electrolyte systems to predict swelling nature of hydrogels present in the brine solution. Hydrogels are crosslinked polymers that are used with fracturing fluid to enhance oil and gas production. Presence of salinity in the water affects the swelling of hydrogels used in the hydraulic fracturing fluid. In the dissertation, we present the thermodynamic modeling of poly (N-isopropyl acrylamide) hydrogels in aqueous NaCl solutions at room temperature. The models developed in the dissertation, altogether, should be a clinical tool in process research, and simulation of novel treatment processes for high salinity produced waters.



Thermodynamic model, Electrolytes, Swelling, Hydrogel, Produced water