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Organic Compounds Separation From Produced Water By Membrane Filtration

Congress: 2015
Author(s): Andrea Chiodini (Novara, Italy), Roberta Miglio, Lino Carnelli, Aurora Bassan
Research Center for Non Conventional Energy – Eni Donegani Institute1, ENI Spa2

Keyword(s): Sub-theme 2: Surface water and groundwater,
AbstractINTRODUCTION Shale gas is natural gas trapped within shale formations. Shale gas has become an increasingly important source of natural gas in the United States since the start of this century and interest has spread to other potential gas shales in the rest of the world. Shale has low matrix permeability, so gas production in commercial scale requires fractures to provide permeability. The shale gas boom in recent years has been due to modern technology in hydraulic fracturing (fracking), able to create extensive artificial fractures around well bores. Fracking is a technique in which water, mixed with sand and chemicals, is injected at high pressure into a wellbore to create fractures, along which gas may migrate to the well. The most common fluid system is water that has been "slickened" with either a friction reducer or low concentration linear gel (Slick water). From 1000 to 20 000 m3 of water can be used for each well. Treatments with cross-linked gels (Gel water) will carry more proppant and transport it further than slick water. The main component of gel water is the gelling agent: it consists of a natural polymer, generally guar gum. Guar gum is a polysaccharide composed of galactose and mannose monomers. Other chemicals are present in the fracturing water. They are for instance cross linked agents and breakers. Cross linked agents form complex, high viscosity, pseudoplastic fracturing fluids when added to linear gel. Generally boron or bivalent cations are used as cross linked agent. Breakers (as Persulfate or other oxidant agent), once a fracture has effectively created, must break (deviscosify) the gels. Guar gum is just one of the possible gelling agent. After being injected, part of the fracturing fluid is returned at surface as flow-back in the days and weeks that follow. Flow-back water (FBW) contains residues derived from the chemicals used in the hydraulic fracturing process. A design of water treatment includes the removal of residual polymers (i.e guar gum). A reduced amount of polymer results in an advantage for the performance of the hardness removal water treatments and produces salted streams with different characteristics and valorization/disposal options. The target of the present study is the evaluation of membrane filtration performances in V*Sep configuration for guar gum separation from water. The aim of the experimental part is to select the correct membrane type and estimate its performances in term of operative pressure, recovery and specific flux. The aforementioned data are the base for a preliminary technical and economical feasibility estimation of filtration unit operation. METHOD/MATERIALS V*SEP configuration has been selected as it represents a system with minimization of fouling due the vibration of membrane during operations. It is a system selected among emerging technology. V*SEP® means Vibratory Shear Enhanced Processing, this unit uses flat organic membranes. Its peculiarity is that flat membranes are held in vibrations to reduce fouling. The oscillation frequency is approximately 53 Hz. The disk stack is oscillated above a torsion spring that moves the stack back and forth approximately 2.2 cm. The oscillation produces a shear at the membrane surface of about 150,000 inverse seconds (equivalent to over 200 G's of force), which is approximately ten times the shear rate of the best conventional cross-flow systems. This technique permits to feed liquid with solid particles with dimension up to 400 micron and conduct microfiltration and reverse osmosis in the same time. VSEP is a lab scale unit with recirculation of concentrate flow. Permeate flow can be separated or re-circulated to feed tank. The main parts are: - 50 liters feed tank - Membrane pump HYDRA CELL for viscous fluids - Membrane holder for flat polymeric membrane of 0.0445 m2 - A valve at the exit of the concentrate for pressure regulation - Two pressure indicators before and after the membrane holder - One indicator for the concentrate flow rate - One vibration system (1HP Baldor Engine) Guar gum concentration was estimated by total organic carbon (TOC) and viscosity analysis of water samples taken from both permeate and concentrate fluids. Results and Discussion Pressure of 20 bar was selected for the concentration study. Temperature varied between 20 and 45°C. Permeate was collected and weighted R is over 90% for guar gum while other elements has low retention factor. The Retention factor (R) (100%: when the component is completely segregated in the concentrated phase and 0%: when the components has the same concentration both in concentrate (retentate) and permeate). CONCLUSION In the preliminary screening phase of the study, NF membrane in V-Sep configuration was effective in removing total organic carbon (TOC) of FBW with clear, colorless permeate and with the same viscosity of salted water. The system with NF membrane showed an important flux decrease. Some cleaning procedures have been tested with good recovery of the membrane capacity. Recovery rate (RR) of over 90% was reached with T= 30°C, P = 20 Bar Permeate specific flow rate (J) resulted as average in: 35-20 kg/h/m2 at 30°C. Temperature raise could be a way to increase the membrane flux of the system, 40-50°C were also explored. References Cooley H. et al. 2012 -Hydraulic Fracturing and Water Resources: Separating the Frack from the Fiction www.pacinst.org/reports/fracking/full_report.pdf www.vsep.com
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