(403f) Scaleup of Membrane Distillation for Desalination of Produced Water from Unconventional Reservoirs.

In recent years, unconventional reservoirs are being widely exploited for oil and gas extraction by hydraulic fracturing. With increase in the production of these useful resources, there is an exponential growth in the generation of produced water, which is a by-product of the extraction process. The produced water typically contains chemicals that were added to the fracturing fluid as well as salts and hydrocarbons present in the geologic formation. The composition of produced water changes with the lifetime and location of the well but it generally contains high concentrations of various organic and inorganic chemicals, bacteria, sand or mud, oil and grease, Naturally Occurring Radioactive Materials (NORM) and is not amenable to conventional wastewater treatment processes.

Membrane Distillation (MD) can potentially be used for treatment of such complex wastewaters because it employs the vapor pressure difference across the membrane as a driving force for separation, which is not significantly affected by the salinity. The presence of high concentrations of organics can be a challenge for MD technology because some compounds can cause membrane fouling and wetting, which can reduce separation efficiency. This study evaluated the feasibility of MD for desalination of produced water from Permian Basin, TX and Marcellus Shale region, PA. Permian Basin is one of the most productive tight oil plays in the US, whereas Marcellus Shale is the most productive gas play. Produced waters obtained from these two shale plays varied greatly in composition and represent a range of produced waters that may be encountered in unconventional reservoirs.

Laboratory-scale experiments with produced water from Permian Basin showed an increase in permeate conductivity with water recovery and pretreatment using extended biological degradation, chemical oxidation and activated carbon adsorption did not improve permeate conductivity. Further investigation revealed that the ammonia present in the produced water is transferred to the permeate side where hydrolysis results in conductivity increase. Long-term experiments at 50% water recovery revealed scale formation on the membrane, which was mostly comprised of anhydrite and barite salts. While this scale was sufficiently porous to allow unimpeded vapor transport, simple washing with alkaline solution was effective in cleaning the membrane. Similar tests with produced water from Marcellus Shale play revealed the impact of ammonia on permeate quality. The scale formed on the MD membrane treating this water was primarily composed of iron oxides and had virtually no impact on permeate flux. Washing the membrane with acidic solution effectively removed these scales.

These laboratory tests showed promise to conduct further exploration of MD technology for produced water treatment. Air-gap membrane distillation (AGMD) pilot-scale system equipped with internal heaters and external chiller capable of processing 1 gpm of produced water with 50% water recovery is currently being installed at a field site in Permian Basin. The pilot plant consists of two polytetrafluorethylene (PTFE) modules with 26 m² of membrane area in each module. The pilot-scale system will be operated continuously for at least 200 hrs using baghouse filtration and cartridge filtration as the only pretreatment to remove particulates from the raw produced water. Comparison between the permeate flux and quality obtained in the laboratory- and pilot-scale systems will facilitate validation of the scale-up protocol developed in this study.