Treatment options for reuse of frac flowback and produced water from shale

MARK KIDDER, TOR PALMGREN, ADRIANA OVALLE and MUKESH KAPILA, M-I Swaco, a Schlumberger company

Horizontal drilling followed by multi-stage fracturing is the most prevalent mode of gas extraction from shale. Hydraulic fracturing of a well encompasses, on the average, 12 frac stages, with each stage using about 10,000–12,000 bbl of water. According to a November 2010 market study by Cap Resources, water usage in the main US shale plays is projected to increase from about 450 million bbl in 2010 to about 675 million bbl by the end of 2015.

The need for such vast amounts of water in hydraulic fracturing significantly affects water availability and sourcing for all uses, and also the cost and logistics of accessing and trucking the water to points of use and disposal sites. Furthermore, regulations designed to protect communities and the environment from potential sources of contamination are becoming increasingly stringent.

About 10%–30% of the frac water injected into a well returns to the surface as flowback, along with various amounts of produced water, which will continue to flow to surface over the well’s lifetime. In the interest of conservation and sustainability, it is highly desirable to maximize any opportunities to reuse the flowback water and to use some of the produced water in subsequent drilling and fracturing operations. Water recycling in shale operations has many benefits, including reduced freshwater use, reduced CO2 footprint and overall environmental impact, reduced volume of transportation and, ultimately, reduced cost.

To this end, companies specializing in water treatment continue to develop fit-for-purpose onsite means of treating flowback water and produced water from shales. There are wide variations in target specifications for recycled water, depending on use requirements, location and formation characteristics. Similarly, regulations for water sourcing, use, reuse and disposal vary widely across state boundaries. Consequently, a host of new technologies are being developed and implemented to address specific requirements.

Typical shale flowback/produced water recycling challenges include solids and oil removal, softening or scale minimization, disinfection and desalination. Approaches include mechanical and chemical precipitation, filtration devices, absorption media, cross-flow membrane technologies, and various bacterial dis-infection methods based on ozone, chlorine compounds and ultraviolet light.

There are a few distinct categories that summarize the treatment requirements of shale flowback and produced water for reuse in fracing and/or drilling. Key treatment approaches can be divided into the following four types: 1) total suspended solids (TSS) and bacteria; 2) hardness and oil and grease (O&G); 3) total dissolved solids (TDS); and 4) central water treatment facilities plus conveyance.

TSS AND BACTERIA

Total suspended solids are generally managed by settling/sedimentation systems using coagulants and flocculants, filtration or hydrocyclones. Based on current operational experience, filtration systems are the most efficient and practical means for TSS removal in land-based operations. Hydrocyclones are well-suited for offshore applications where space is limited for both equipment and consumables.

The predominant method deployed onshore for TSS removal is basic sock and/or cartridge filtration, Fig. 1. Typical throughputs are in the 5,000–10,000-bpd range. Advantages of this method include simplicity of design and a relatively small footprint. Disadvantages of sock and cartridge filtration are that these systems tend to be labor intensive and that they often require large amounts of consumables. However, system and labor costs are minimal compared with those of more rigorous treatment options. Due to its relative compactness, sock and cartridge filtration lends itself well to both mobile land operations and offshore applications.

Fig. 1. Filtration pods typically used in shale operations for TSS removal.

Disinfection can be accomplished by various technologies using ultraviolet light, ozonation, chlorinated compounds or chemical bactericides. Methods commonly used in field operations include ozonation and chlorine dioxide generation and injection at the treatment site. These two onsite generation technologies require minimal chemical transportation and provide “bacteria-free” control, which is generally preferred by operators.

Usually generated from air using an ozone generator, ozone has been shown to accomplish flowback water disinfection meeting or exceeding the desired level of 1,000 colony-forming units/ml at concentrations as low as 0.5%–3%. Test assays conducted in conjunction with 200-gal/min. process flowrates include heterotropic plate count, acid-producing bacteria and sulfate-reducing bacteria. The mechanism by which ozone compromises bacteria growth is based on rupturing the cell membrane, resulting in cellular fluid leakage and imbalances leading to functional shutdown of the cell.

Chlorine dioxide acts on bacterial cell membranes in a conceptually similar manner to ozone. Experiential evidence on the effectiveness of chlorine dioxide has been similarly compelling when compared to ozone disinfection, based on the same types of bacteria testing. Chlorine dioxide is produced onsite by electrolytic conversion of sodium chlorite. Field testing of flowback and produced water has been conducted at flowrates up to about 500 gal/min. with effective chlorine oxide concentrations as low as 10–30 ppm.

HARDNESS AND O&G

Hardness is a description of the concentration of scale-forming ions present in the water. In produced and frac flowback water, the most prevalent elements of concern are calcium, magnesium, iron and barium. These ions are commonly removed by lime softening, ion exchange or nanofiltration.

One of the most common methods deployed in shale operations is cold lime softening, involving the addition of lime (Ca(OH)2) to the water, where it dissociates into Ca2+ and OH− ions that eventually form calcium carbonate. Calcium carbonate is much less soluble than lime, so precipitation ensues, thus reducing the calcium concentration in the water.

Oil and grease (O&G) may be present in produced water in free, emulsified or dissolved form. The bulk of the O&G can be extracted using a variety of mechanical devices including hydrocylones, dissolved air flotation systems, and specialized media filters containing porous hydrophobic absorptive substances. These basic approaches remove free oil, whereas added chemical treatment (demulsifiers or gas injection) is typically needed to remove trace amounts of emulsified or dissolved O&G. Specialized absorptive media are also very effective in this regard without requiring chemicals.

One common method for O&G removal in shale operations is compact flotation. This technology combines the concepts of hydrocyclone and gas flotation, with nitrogen as the injected gas. During this process, minute oil droplets are forced to agglomerate and coalesce, facilitating separation from the water. The separation process is aided by baffles inside the chamber, which add to the gas flotation mechanism by causing the release of residual gas from the water. Flocculants can be added for enhanced performance. The separated oil and gas is removed continually via an outlet pipe at the top of the vessel, Fig. 2. 

Fig. 2. Compact flotation process schematic.

Single-step separation by compact flotation has been shown to reduce oil-in-water content to less than 20 mg/L, while simultaneously degassing the water. Two compact flotation units in series have achieved oil-in-water content less than 10 mg/L. A major benefit of this technology is that it can handle large volumes of water within a small equipment footprint. For example, a 528-gal vessel can treat water at throughputs of 1,570 bbl/hr (26 bbl/min.).

TOTAL DISSOLVED SOLIDS

All particles that are less than 0.45 microns in size fall into the category of TDS, including macromolecules, polymers and both multivalent and monovalent ions. Removal of these species can be accomplished by either membrane filtration (micro-, ultra- or nano-filtration and reverse osmosis) or thermal technologies (distillation/evaporation). TDS removal methods are being deployed worldwide in produced water and seawater desalination, mostly in centralized systems. However, mobile versions of these technologies are increasingly common, especially for treating flowback water in shale operations. This is because, unlike the typical case of produced water from one or more producing wells, frac flowback treatment is only required at any given location during the completion phase. Membrane treatments typically result in water salinities less than 1,000 mg/L, similar to drinking water, with capabilities for salinities as low as 500 mg/L.

When TDS concentrations are less than 40,000 mg/L, reverse osmosis (RO) is used. A commonly deployed method for flowback water desalination uses an integrated three-stage mobile RO system. The first module provides pretreatment using chemical flocculation, clarification and oil removal; the second performs cold lime softening; and the third employs micro-, ultra- and nano-filtration as well as reverse osmosis. The equipment is being used to evaluate different water qualities and treatment options.

When TDS concentrations exceed 40,000 mg/L, distillation is one of the most effective ways of purifying water. In this process, the incoming water is boiled to produce steam, while all dissolved solids remain in the concentrate. The resulting steam is then condensed into pure water.

CENTRAL TREATMENT

Centralized treatment facilities and supporting water conveyance systems are constructed to manage and treat large volumes of water for a specified process for reuse, or when discharge and disposal of water are required. Typically, water is treated for disposal and discharge when two conditions exist: produced water volumes exceed potential reuse capacity, and low-cost transportation and disposal options are scarce. 

Central treatment facilities for produced water combine several of the treatment methods previously discussed; the final disposition of the water determines the level of treatment required. These facilities are engineered to support requirements for water reuse for steam injection, waterflooding projects, surface discharge or disposal in injection wells. When surface discharge is required, they often require a membrane or evaporation/distillation system to remove TDS. Pretreatment steps for TSS, O&G and hardness removal are typically required prior to RO or other membrane treatments.

In addition to the challenges of developing a large, permanent facility to manage large volumes of produced water, planning and designing of conveyance systems are equally important. A key decision is how to transport the water to be treated and discharged. In the long term, trucking is almost always the most expensive option. Even though pipeline networks initially require substantial capital to construct, the long-term savings are often significant when compared with trucking. Additionally, pipeline conveyance has a much smaller environmental impact than trucking, eliminating both exhaust pollution and degradation of road infrastructure. Once the necessary transportation infrastructure has been established, centralized facilities have a clear advantage in reducing overall transportation and treatment costs.

Current upstream research and development carried out by SCNGO within NETL is primarily conducted in two programmatic areas: research by the Research Partnership to Secure Energy for America (RPSEA) funded by NETL, and a complementary research program that augments RPSEA research activities, conducted by NETL’s Office of Research and Development (ORD). A third programmatic area is referred to as the “core program” and includes various areas of research supported by prior DOE annual appropriations in areas such as hydrates and the environment. To see details of all current NETL research, visit netl.doe.gov/technologies/oil-gas/index.html. For details on all completed NETL research and development projects, visit the NETL Knowledge Management Database at netl.doe.gov/kmd. All reports are downloadable at no charge.