Produced-water (PW) and flowback-water (FW) quality in shale projects is influenced not only by the formation, but also by the fracturing fluid introduced to the formation during hydraulic stimulation. The water produced by shale wells can contain suspended solids, dissolved solids, organics that include hydrocarbons and residual-fracturing-fluid chemicals, and bacteria. This paper describes functional water-treatment steps that target the most common removal of suspended solids and oil or condensate from PW and FW for recycling or disposal operations.
Introduction
Rapid industry expansion into shale-gas and -oil development requires a reconsideration of how water management and technology are applied. What may be acceptable and fit for purpose at a smaller scale may become unsatisfactory when the business grows significantly. Water footprint and the food/stress nexus are critical concerns requiring the industry to improve continuously upon the approach followed in the past.
Selection of external water sources, securing access rights, and establishing water-offtake limits are the focus areas of water management. In some areas, freshwater resources may be scarce and the water for fracturing may need to be transported over long distances, adding to operational intensity and transportation-safety risks. Multiple steps are required to move the water for multiple consumers and to multiple locations. Storage facilities are required to create buffer capacity between water supply and water demand (Fig. 1 above).
Recycling of PW and FW for fracturing will reduce the consumption of new water from external sources and reduce the volume of wastewater disposal. While disposal of PW into subsurface formations can be relatively inexpensive, requirements to obtain permits for this injection are becoming more stringent. On the other hand, if suitable injection zones are not available near the producing shale development, surface discharge needs to be considered. If surface discharge is the only option, then evaporative or zero-liquid-discharge (ZLD) technologies could be required, but this could be cost-prohibitive.
Technical Challenges
Water Characterization. For the purposes of this paper, the two water categories can be distinguished as follows:
- PW is managed by the production team controlling the wells for the remainder of well lifetimes.
- FW is produced when the completions team controls the well. This period is typically days or, at maximum, a few weeks after fracture stimulation.
Water Production. In some plays, a significant portion of the fluids injected into shale formations during fracture stimulation is retained in the subsurface formation and never returns for reuse or disposal. Beyond that, water production varies depending on the formation type, subsurface pressures and temperatures, the organic maturity of the source rocks, presence of and connectivity to an adjacent aquifer, and other factors. Some of the characteristics of water production in different unconventional assets are covered in the complete paper.
Mineral Composition. Field monitoring suggests that the recovered fluids are likely to have picked up substantial amounts of formation mineral content from water/rock interactions. The dissolved elemental content of the recovered waters is controlled by a combination of connate-formation-water chemistry, dissolution of minerals by the fracturing fluids, and possible water influx from adjacent water-bearing formations.
Residual Polymers and Emulsion Formation. Polymers are added to fracturing fluids to decrease friction or to increase viscosity. The degree to which the organic-polymer material breaks down before returning to the surface depends upon polymer concentration, reservoir temperature, residence time in the formation, effectiveness of the breaker, and other factors. In combination with the presence of condensate, unbroken polymer results directly in the generation of emulsions, creating additional challenges with condensate removal.
Slickwater fracturing fluids contain a drag-reducing agent (DRA), typically polyacrylamide polymer. The concentration of DRA in the fracturing fluid is typically less than approximately 2 lbm/1,000 gal (ppt), and no residual is usually detected in the PW and FW. Finally, although slickwater formulations with DRA polymers are usually simpler than crosslinked-gel fluids, they contain other additives such as biocides, surfactants, clay stabilizers, and scale inhibitors.
In crosslinked-fluid or hybrid fracturing, the guar-gum-polymer concentration is significantly higher, up to 30 ppt. In contrast with the slickwater system, in a crosslinked-fluid or hybrid system, the FW and PW may contain broken or unbroken polymer gel or a combination thereof. The form of the residual polymer is subject to many variables such as polymer grade, breaker type, formation temperature, and downhole residence time.
Another important challenge in water treatment is bacterial activity. Biopolymers such as guar gum, especially in the broken (i.e., monomeric) form, are excellent food sources for bacteria in the formation, the wellbore, and the surface facilities. Bacteria in the wellbore and surface facilities may result in the formation of biomass and generation of hydrogen sulfide and additional solids such as iron sulfide, as well as microbiologically induced corrosion.
Variations in Solids Production. Suspended solids, including bacteria biomass, proppant, and colloidal mineral particles from the formation, may be present in the produced-hydrocarbon phase and the water phase at different concentrations. Solids content plays an important role in water disposal and water reuse for fracturing applications. Proppant returns can be expected mainly during the first weeks of the well-production cycle and can be removed relatively easily with conventional desander equipment or even simple gravity separation. The other types of solids are of colloidal nature, and removal is more difficult. Removal of these solids is very important as part of the water-treatment process.
Water-Treatment Processes
Critical success factors for shale developments include the implementation of an effective health, safety, and environment (HSE) program; proper planning to allocate water sourcing; and responsible management of fracture-water disposal. A workflow and toolbox of water-treatment technologies and processes have been developed, and these methods can be categorized into four main types: primary, secondary, tertiary, and ZLD. The selection of any of these categories is made on the basis of careful consideration of project-specific characteristics, economics and logistic drivers, options, and opportunities.
Primary Treatment. Primary treatment removes suspended solids, oil and condensate, iron, unbroken polymers, and bacteria, or combinations thereof. The generic primary-treatment process involves several steps:
- Separate the fluid phases, removing as much of the oil from the water phase as possible.
- Equalize the water stream to offset dynamic changes in volume and concentration.
- Consider the separate treatment of PW and FW separately, depending upon observed differences in their composition.
- Coagulate or electrocoagulate, then flocculate, suspended solids, emulsified oil, and broken organic polymer/gels.
- Remove the formed solids in a flotation or gravity-separation unit.
- Perform filtration, which can comprise multimedia filtration followed by cartridge/bag filtration.
- Perform sludge treatment and dewatering of solids to minimize waste hauling and off-site disposal of liquids.
- Consider that primary-treatment systems often target iron removal based on chemical oxidation in tandem with chemical coagulation and flocculation.
The extent of the application of these steps depends on the level of PW and FW quality required for subsequent reuse in the development of, or for injection into, disposal wells, or as a pretreatment for further treatment. When comparing the costs of primary treatment with those of other required treatments, the costs are not burdensome in terms of overall water-treatment costs.
Effect of Residual Polymer on Primary-Treatment Process. The presence of residual unbroken polymer gel in the PW imposes significant negative effects on the conventional primary-treatment technologies, such as increase of the oxidant demand; interference with coagulation/flocculation, flotation, and gravity settling; and plugging of the plate-and-frame dewatering equipment.
Treatment of biologically active waters also requires intensive bacteria control to minimize problems either in the treatment step itself or further downstream in the treatment process. Bacteria control in PW and FW containing residual polymers is also important for the following reasons:
- To ensure no interaction of the bacteria with the fracturing-fluid additives if the treated water is reused.
- To avoid negative impact in the reservoir in case the treated water is either disposed of or reused (if bacteria growth is not controlled, the reservoir may produce hydrogen sulfide, negatively impacting well integrity, HSE integrity, or product specification).
- To mitigate bacteria growth in the surface facilities, which may result in the formation of biomass or extra solids such as iron sulfide at various stages in the process (if bacterial activity is not controlled, suboptimal surface-separation performance may occur).
For a discussion of primary-treatment field-test results, please see the complete paper.
Other Water-Treatment Processes
Secondary Treatment. The objective of this treatment step is to remove divalent cations, such as calcium, magnesium, barium, or strontium, to address the following issues in part or combination:
- Limited ionic compatibility of fracturing-fluid additives
- Scaling potential in the producing or water-disposal formation
- Scaling potential in wells and surface facilities
After secondary treatment, the treated effluent can then be used as source water for fracturing-fluids preparation (water reuse), as a feed for evaporation or crystallization processes, or for subsurface disposal. However, secondary treatment adds operational intensity and costs. Depending on the dissolved-salt content of the PW or FW, secondary treatment may generate sludge containing technologically enhanced naturally occurring radioactive material, which may require special handling and disposal.
Secondary treatment needs to be minimized where possible. Removal of divalent cations is a proven technology in downstream, industrial, and municipal applications. However, at high total-dissolved-solids levels in combination with fracturing-organic residuals in the PW and FW, both contaminants may act as inhibitors for precipitation reactions.
Tertiary Treatment. When removal of dissolved salt is required, either for compatibility with fracturing-fluid additives or for discharge at surface (when deep-well disposal is not feasible), then a desalination process would be required. Some of the tertiary processes may not require primary- and secondary-treatment processes beforehand.
Selection of effective desalination technology depends mainly on the salt concentration and the presence of other contaminants in the influent water. Existing technologies are applicable such as ion exchange, reverse osmosis, and mechanical-vapor-recompression evaporation. All tertiary technologies generate a reject concentrated-brine stream, which requires disposal in a deep well or by means of a ZLD treatment.
In contrast to the tertiary “hardware-facility solutions,” a chemical solution may be less costly when PW or FW reuse is being considered. Ionic compatibility of the fracturing-fluid formulation is the main driver that determines the treatment requirements for PW or FW reuse. Continuous technological advances in fracturing-fluid formulations increase their tolerance to dissolved salts, leading to relaxation of the water-quality requirements, which allows the use of produced brines for fracturing without costly salt-removal treatment.
ZLD. If a disposal route for the liquid-waste stream is not available, ZLD facilities are needed. ZLD treatment minimizes the liquid-waste stream and converts the brine into a salt cake for industrial applications or landfill disposal, and also into distillate water. However, the ZLD process requires the use of thermal crystallizers, which are highly energy-intensive to operate. The costs of tertiary and ZLD technologies are high and could render some developments uneconomical.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 167057, “Water-Management Approach for Shale Operations in North America,” by C. Kuijvenhoven, V. Fedotov, D. Gallo, and P. Hagemeijer, Shell, prepared for the 2013 SPE Unconventional Resources Conference and Exhibition—Asia Pacific, Brisbane, Australia, 11–13 November. The paper has not been peer reviewed.