Energy consumption in desalinating produced water from shale oil and gas extraction

Abstract

On-site treatment and reuse is an increasingly preferred option for produced water management in unconventional oil and gas extraction. This paper analyzes and compares the energetics of several desalination technologies at the high salinities and diverse compositions commonly encountered in produced water from shale formations to guide technology selection and to inform further system development. Produced water properties are modeled using Pitzer's equations, and emphasis is placed on how these properties drive differences in system thermodynamics at salinities significantly above the oceanic range. Models of mechanical vapor compression, multi-effect distillation, forward osmosis, humidification-dehumidification, membrane distillation, and a hypothetical high pressure reverse osmosis system show that for a fixed brine salinity, evaporative system energetics tend to be less sensitive to changes in feed salinity. Consequently, second law efficiencies of evaporative systems tend to be higher when treating typical produced waters to near-saturation than when treating seawater. In addition, if realized for high-salinity produced waters, reverse osmosis has the potential to achieve very high efficiencies. The results suggest a different energetic paradigm in comparing membrane and evaporative systems for high salinity wastewater treatment than has been commonly accepted for lower salinity water.

Energy efficiency of membrane distillation up to high salinity: Evaluating critical system size and optimal membrane thickness

“…Conventional spiral wound reverse osmosis (RO), the workhorse of the desalination industry today, is typically operated below 70 bar of applied pressure [1]. As a result, RO is not directly applicable to desalination of these saltier streams for which the osmotic pressure can be as high as 300 bar [2].…”

“…At higher salinity levels, a peak value of GOR was observed with changing in feed inlet flow rate. Similarly, Thiel et al [2] used numerical models of PGMD to illustrate the existence of an optimal system size (represented through a terminal temperature difference) at which GOR is maximized when the feed inlet flow rate is held constant.…”

Thermophysical properties of seawater: A review and new correlations that include pressure dependence

“…Osmotically driven processes such as Pressure Retarded Osmosis (PRO) and the more recently introduced Assisted Forward Osmosis (AFO) may experience pressures above 4.7 MPa [10,11] and 0.6 MPa 7 respectively [12]. Unconventional reverse osmosis configurations have also recently been investigated for treating oil and gas produced water to salinities near saturation values of 260 g/kg and pressures near 30 MPa [13]. For accurately modeling conventional and unconventional desalination technologies, there is a need for accurate prediction of the thermophysical properties of seawater at elevated pressures and salinities.…”

Thermodynamic analysis of brine management methods: Zero-discharge desalination and salinity-gradient power production

“…Under reversible conditions, the work required would be the 'least work' thermodynamically possible, which for brine concentration and crystallization we designate asẆ least bc andẆ least crys , respectively. Thus, the least work for brine concentration is the least work for concentrating from a given feed salinity up to saturation, while the least work for crystallization is the least work for taking a saturated solution up to complete separation of water and salt [30].…”

“…The activity coefficients were obtained from an implementation by Thiel et al [30,33] of Pitzer's equations modeling the thermodynamics of aqueous electrolyte solutions [34][35][36][37][38][39]. Thiel et al [30] found that aqueous NaCl serves as a reasonable surrogate for the property estimation in certain high salinity waters.…”