Produced water desalination using high temperature membranes
Introduction
Produced water is the type of water produced as a byproduct during oil and gas production activities [1]. In heavy oil operations, steam is often injected to reduce heavy oil viscosity and improve oil recovery [2]. Most of the time, produced water can be recycled to make soft water for steam generation. However, the volume of produced water can increase rapidly if the volume of steam injected declines and the oilfield matures, which may result in a large amount of excessive produced water [3]. This excessive produced water can be a great challenge for heavy oil operations. In many cases, there are drivers to manage excessive produced water through desalination:
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Reducing and avoiding downhole injection when downhole disposal capacity is insufficient or unavailable in the long term [4];
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Increasing reuse opportunities for produced water including beneficial reuse;
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Reducing the use of freshwater in a drought region;
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Meeting future production water handling needs for field operations;
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Extending the life of the asset by removing reservoir capacity constraints.
Produced water composition is largely dependent on the geological formation, injection water and steam, gas and hydrocarbon properties from the reservoir; leading to a very diverse water chemistry which poses significant challenges for developing treatment processes [5]. Table 1 lists the representative produced water chemistry from different heavy oil production regions and geological formations. For steam flood in sandstone reservoirs such as California and Canada, produced water chemistry may possess some unique features including high silica, high boron and relatively high hardness [6]. For beneficial reuse, the water quality specifications can be stringent, and may apply to some common regulated components in the produced water such as oil and grease, pH, total dissolved solids (TDS), boron, and ammonia. For example, California's local Regional Water Quality Control Board specification regulate the fresh water discharge limit to TDS <500 mg/L and boron <0.5 mg/L. [7]
While there may be variations in composition across the regions, this scenario shown in Table 1 indicates that the produced waters from many heavy oil operations are in general brackish in nature with relatively low TDS. This makes the reverse osmosis (RO) technology more suitable for produced water desalination due to its low energy demand compared to thermal desalination. One of the key features of produced water from steam flood operations, which is integral to most heavy oil operations, is its high temperature due to steam injection. In mature steam floods, produced water temperature can be as high as 90 °C which requires significant cooling if commercial RO membranes are used. Due to high silica in such produced water, any substantial cooling will result in silica precipitation which can foul the heat exchanger. Therefore, the emerging high temperature RO technology is an attractive option if proper pretreatment can be achieved.
A commercial high temperature produced water desalination process includes the following unit operations: de-oiling, cooling, silica and hardness removal and membrane desalination [8]. Conventional de-oiling treatment includes gravity separation, skimming, flotation, and nutshell filtration [9]. After de-oiling, high silica and hardness can usually be managed by the lime softening process followed by the media or membrane filtration. Finally, the water needs to be cooled down to 45 °C or below to be desalinated by commercial reverse osmosis process [10]. Although the process has been successfully deployed, there are several drawbacks associated with these unit operations:
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Conventional de-oiling treatment usually reaches <5 mg/L of oil and grease in the effluent which is insufficient to meet RO feed water specifications. It can result in organic fouling of the RO membrane in the long run [11].
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The lime softening process is chemical-intensive using magnesium oxide and lime with a large production of solid waste that requires proper management. Some of the solid waste can be hazardous for disposal [12].
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Lime softening alone may not be sufficient in removing hardness, and additional softening may still be needed.
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The cooling step adds high operating cost and consumes large amount of energy especially in warm climate regions.
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Organic fouling can always be a challenge in the RO process due to both suspended and dissolved hydrocarbon.
The emerging high temperature RO membrane technology is of great interest to address these drawbacks of the conventional technologies as it has the following benefits:
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The high temperature RO membrane can be operated up to 80 °C; therefore, the energy consumption during cooling of produced water can be significantly reduced or avoided.
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Silica and organic acid solubilities are higher at high temperatures, which can mitigate membrane fouling [13], hence the feed silica concentration specification can be relaxed.
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Energy cost can be reduced for the RO process at high temperatures due to lower pressure operations.
Some traditional applications of high temperature RO membranes are in the fields of pharmaceutical, biotech, electronics, power plant and food production [14]. However, using high temperature RO membranes for treating produced water is new for oil and gas operations. In this study, the performance of high temperature RO membrane was evaluated using both synthetic water and field produced water. The high temperature RO membranes were obtained from Hydranautics and produced water sample was received from heavy oil fields with steam flood. pH and temperature were proven to be the key factors determining membrane performance. It was found that the current membrane simulation model did not accurately simulate trace water constituents at the high temperature range (>45 °C) for boron. Simulation software was calibrated using synthetic water test results. Follow-up single element tests with field produced water were designed to simulate full scale membrane operation at enhanced recoveries. Full scale membrane system performances were simulated using Hydranautics's proprietary software, which matched well with the single element lab test results using field produced water.
Section snippets
Produced water composition
Produced water sample was collected directly from the steam flood field in California at a temperature of 87.8 °C. The composition was analyzed using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Ion Chromatography (IC) as shown in Table 2.
The synthetic produced water studies were conducted using the above formula as a reference to make the laboratory recipe. Silica composition was omitted in the synthetic formula, since the main goal was to evaluate boron rejection at
Synthetic water testing
Synthetic water testing was conducted at four different temperature set points (25 °C, 40 °C, 55 °C and 60 °C) to evaluate the rejection of various dissolved salts/ions at the selected temperature conditions. pH was kept in the range of 10.9–11.3 to maximize the borate speciation in the water and to enhance the boron rejection by the RO membrane [15]. Fig. 3 plots the membrane feed pressure in response to feed water temperature for two sample elements tested at Hydranautics's lab with synthetic
Conclusions
Synthetic and field produced water desalination was studied with high temperature RO membranes using small-scale element tests at two different lab facilities. The selected high temperature membrane module demonstrated a stable and high rejection of boron and TDS in the tested temperature range from 25 °C to 60 °C. Boron rejection was above 98% at all testing temperature conditions at an elevated pH of 11. The overall TDS rejections were above 96% at 60 °C. Operation condition for field
CRediT authorship contribution statement
Cheng Chen: Conceptualization, Methodology, Chevron Lab Testing, Writing- Original draft preparation.
Xiaofei Huang: Conceptualization, Methodology, Hydranautics Lab Testing, Writing- Reviewing and Editing.
Prakhar Prakash: Result discussion, Sampling collection and data analysis, Writing- Reviewing and Editing.
Satish Chilekar: Modeling, Writing- Reviewing and Editing.
Rich Franks: Result discussion, testing design, Writing- Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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