This is the fourth article of a series covering water management in hydraulic fracturing (HF) in unconventional resources. In the first article, published in June, water management and planning were discussed. Fluid properties and characterization were discussed in the second article, published in August. In the third article, published in October, suspended solids removal using coagulation/flocculation and electrocoagulation was discussed. An explanation was given as to why those technologies are justified based on the characteristics of flowback fluids. This article discusses the use of mechanical vapor compression (MVC) as a desalination technology.
The series of articles is intended to identify and explain the technologies used in HF and explore whether they are appropriate and cost effective. When talking with operators, their comments often suggest that there are too many technologies from which to choose and little basis upon which to make the selections. Water management for HF has become a magnet for every water treatment scheme imaginable. Thus, it is helpful to look at a few successful technologies in some detail to understand why they are appropriate.
In general, desalination of recycle flowback water is becoming less important. New formulations of salt-tolerant polymers and fluids are being developed and applied. Some of the HF fluids are more expensive on a per pound basis, but become cost-competitive when overall reduction in water source, treatment, and disposal costs are taken into account. Nevertheless, there is still a need in some regions to desalinate. This is particularly true when specific compounds must be removed, such as boron or scaling components.
Desalination Technologies
In industrial and municipal water treatment, two technologies used for desalination are thermal processes and membranes. Thermal desalination processes are much older than membrane processes. Despite the rapid advance of membrane processes in the past 2 decades, at least one-third of the installed worldwide desalination capacity is provided by thermal desalination. Other desalination technologies, such as ion exchange, electrodialysis, and softening, are not applicable for the high salinities of HF flowback fluids.
Because of the high fouling tendency of HF flowback fluids and the high salinity in some regions, membrane-based desalination technology is not viable. Spiral wound nanofiltration and reverse osmosis (RO) membranes are the workhorses for onshore applications and offshore desalination of seawater. Neither can be used when the concentration of organic fouling material exceeds a few tens of mg/L. As discussed in Part 2 of this series, the slickwater formulations contain several hundred mg/L of spent polymer. The linear and cross-linked gels (mostly guar-based polymer) contain a few to several thousand mg/L of organic fouling material.
In industrial and municipal water treatment, the main thermal desalination technologies are multistage flash (MSF), multiple-effect distillation (MED), and MVC. The global market shares of these processes are 87%, 12%, and 0.2%, respectively (Global Water Intelligence 2006). A variation of these technologies is a hybrid combination of MED and thermal vapor compression, which has a high energy efficiency compared with the others. MSF process uses multiple evaporation chambers, each having lower pressure and, therefore, lower temperature. The chambers, or stages, are designed for maximum heat recovery.
Thermal desalination processes consume more energy than the RO processes. Depending on the particular technology, the energy required can be as high as 10 to 15 kWh/m3 of water (1.6 to 2.4 kWh/bbl). This is high compared with 5 kWh/m3 (0.8 kWh/bbl) for RO in a seawater application, which by itself is considered to have a significant energy requirement for pumping. However, the reliability, low fouling tendency, and extensive field experience with the thermal desalination technologies keep them in demand, particularly for large facilities where waste heat is available.
The MSF and MED systems are often applied in cogeneration plants, where power and water are produced simultaneously. Both systems require low-pressure heating steam, which can be easily extracted from the power plant at low cost.
Mechanical Vapor Compression
The MVC system is operated solely on electric power, which can be a benefit or a drawback. It offers an advantage because it can be applied where no waste heat is available. However, the high costs of electricity make it less preferable in the industrial and municipal water industries.
MVC is a niche technology with features that make it appropriate for desalination of flowback fluids in HF. Because it does not require waste heat, it is the preferred desalination technology for use in some HF operations.
Fig. 1 illustrates the MVC process, which includes the following steps:
- Incoming (feed) brine is heated in a waste heat recovery heat exchanger (preheater). The hot effluent brine and fresh water is used to heat the incoming brine.
- The brine enters the MVC unit at the top of the tube bundle where it is sprayed onto the outside of the tubes. The brine flows over the tubes as a thin film.
- Vapor is generated from the brine, which is sucked into the vapor compressor. The compressor has a dual function. It lowers the gas pressure, which promotes evaporation, and it compresses the vapor, which heats it (like a heat pump), and pushes the vapor into the side of the tube bundle.
- The hot vapor exchanges heat with the cooler brine, thus causing the vapor to condense. The condensed fresh water is discharged.
- A stream of brine is discharged from the bottom of the brine sump and is pumped to the top of the tube bundle, together with incoming brine.
- A fraction of the circulating brine is discharged. Referred to as the drawdown, it is expressed as a fraction of the flow rate of the incoming feed brine. If the drawdown is 10%, then its salinity is 10 times that of the incoming brine. In this case, the recovery is 90%.
In any of the desalination processes, the flow rate of concentrated brine is a critical process parameter. The smaller the flow volume, the greater the concentration of waste brine. Scaling potential is the limiting factor. It is possible to further concentrate the brine into a high-solids sludge, which could be dried into a granular solid, and it is possible to produce salt products. However, the additional process steps add significantly to cost and are generally not practiced. Instead, the concentrated waste brine is disposed, and the economics of desalination must also include the cost of the waste disposal.
An important point that is often overlooked in discussions of MVC is that it does not involve distillation. Distillation requires nucleate boiling, in which vapor is generated on the surface of a heat exchange tube. Since vapor is such a poor conductor of heat, local tube surface temperature can be several degrees above the boiling point of the liquid. If the tube is immersed in the boiling liquid, then there is a hydraulic head that must be overcome to form the vapor, which further increases the temperature of boiling. The presence of vapor on the tube surface and the elevated temperatures create a scaling potential for all but the most pure liquids. Thus, distillation is not appropriate for fluids with high scaling and fouling tendency.
The MVC processes are more precisely referred to as evaporation processes. Nucleate boiling is minimized to lower the scaling potential and allow desalination of highly contaminated feed streams. Vapor is generated by heating across a large surface area and with the application of partial vacuum so that the operating temperature is well below the boiling point of the liquid.
MVC has been applied in steamflood (for example, Oxy’s Mukhaizna field in Oman) and steam-assisted gravity drainage projects in Alberta, Canada (Heins 2010).
In oilfield application, typical scale-forming components include the carbonates (calcium, magnesium, and iron carbonate) and silica. The carbonates are problematic since their solubility decreases with higher temperature. As carbon dioxide is vaporized out of the brine, the pH of the brine increases, which causes the carbonates to precipitate.
Other techniques used to prevent scale deposits include large surface area (and low thermal driving force), mist mats to prevent liquid carry-over into the vapor, the use of scale inhibitors, softening, ion exchange, pH adjustment, the use of seeded slurry, or the use of ball pigs. Equipment suppliers, such as Sasakura Engineering, provide scale prevention strategies optimized for oilfield brines. Scaling and fouling are also concerns in the auxiliary equipment, such as the heat exchangers. Companies, such as Alfa Laval, have developed large surface area vertical heat exchangers that reduce fouling.
Among various desalination technologies, mechanical vapor recompression (MVR, or alternately MVC) stands out as appropriate for HF application in a semipermanent or modular configuration.
Stages of Field Development Determine Water Treatment Technologies
To understand where and why MVC is appropriate for HF flowback treatment, the unique aspects of the economics of HF flowback water treatment must be considered.
In the June article, the stages of field development were discussed. A brief review emphasizes the reasons why evaporation technologies are being deployed for modular applications and not being deployed from mobile units.
The three stages of shale field development are defined below in terms of the type of water treating equipment deployed. It is important to make a distinction between the stages of field development because they are critical to the selection of water treatment technology.
The stages of field development and the appropriate water treatment technology are:
Stage 1: Remote and isolated well development—mobile water treating systems
Stage 2: Well clusters with some in-field drilling and completions—modular water treating systems
Stage 3: Extensive in-field development with infrastructure to transport water to and from a centralized treatment facility—centralized water treatment plants
Mobile Stage of Development
In the early stage of development of an unconventional field, a number of individual wells are drilled and completed. In the United States, mineral rights are owned by the land leaseholders. The initial wells in a region will typically be drilled in remote and isolated areas. If water recycling is carried out, the water treating equipment must be mobile. Such equipment is compact and placed on a flatbed truck.
The economics of this kind of water treatment are significantly different from those of industrial water treatment. Capital cost is typically a small fraction of the total cost. Most of the cost of water treatment is due to staff time related to transportation to site, setup of the equipment, operation of the equipment, and demobilization and return transportation. If the equipment is complex, additional operators and time are required to mobilize and set up the equipment, adding to the cost. If the capacity is low, additional time is required to process the water volumes. In general, the water treatment rate must be at least 5 to 7 bbl/min of water to justify the cost. Lower capacity will take too long, and the cost of on-site personnel will be too high. Thus, appropriate equipment in this stage of development is compact, simple, and relatively high capacity. Few technologies meet these criteria. Because MVC does not meet the criteria, there are few, if any, mobile MVC units operating successfully in HF flowback applications, to my knowledge.
Modular Stage of Development
As field development progresses, the leases are secured and the drilling campaign becomes more structured. Clusters of wells are drilled and completed. It is then possible for several adjacent wells to be developed in sequence or simultaneously, thus facilitating the use of a modular water treating system.
A daisy chain or hub-and-spoke type of water piping arrangement can be constructed to feed the water treatment unit and convey treated water to the wells that require it. Lay-flat hose, storage tanks, and pond liners are components of the water management tool kit. In this case, a semipermanent/modular water treatment facility is justified. The equipment is transported on a flatbed truck. It requires a few weeks to prepare the site and erect the equipment. When a few or several wells are involved, the construction cost of a modular treating system is justified.
Aquatech and Fountain Quail are among the companies that provide modular treating systems. Aquatech’s modularized evaporation system is designed for rapid installation. It is transported in modular units and erected with a minimum of field staff. Fountain Quail’s system is an MVR evaporator packaged in self-contained skid-mounted units. It is capable of processing 20,000 BWPD and requires three operators. The capacity of the system must be integrated into the storage capacity of spent HF flowback water, the storage capacity of fresh water, the volume of water required for each HF, and the load recovery. The MVR system is being applied in several shale developments, including the Barnett and Marcellus.
Centralized Stage of Development
Later in field life, there may be many wells in relatively close proximity. Over time, the construction of a water conveyance network together with a centralized water treatment facility becomes justified, as is the current trend in the Marcellus Shale. It has also been successfully implemented in the Pinedale Anticline in southwestern Wyoming (Boschee 2012). The capital costs of the water transport system and the water treatment facility are the main cost drivers and contribute significantly to the overall cost. Because of plant automation and the ability to achieve relatively stable steady-state operation, the number of operators is minimized, compared with the previous stages of field development.
In the centralized application of desalination, MVC is not the only thermal desalination that could be applied. If low-grade steam is available, MSF or MED could be used for reduction of energy use.
For Further Reading
2006. 19th IDA Worldwide Desalting Plant Inventory. Global Water Intelligence.
Boschee, P. 2012. Handling Produced Water from Hydraulic Fracturing. Oil and Gas Fac. 1 (1): 22–26.
Heins, W.F. 2010. Is a Paradigm Shift in Produced Water Treatment Technology Occurring at SAGD Facilities? J. Cdn. Pet. Tech. 49 (1): 10–15.
