Process stabilizes frac water supply by intentionally blending disparate waters to take advantage of natural precipitation.
In unconventional shale plays, water is used to drive well stimulation. Frac supply water can be sourced from rivers, groundwater and produced water. These water sources are blended and consolidated at surface impoundments, then made available to completion crews to stimulate wells into production.
Once in the formation, the source water takes on the character of the zone and typically increases in salinity over time. Salinity can range from 80,000 ppm to 220,000 ppm of total dissolved solids. The longer the source water stays in the ground, the more it picks up the character of that formation.
Beneficial reuse of produced water is a developing trend. The Delaware Basin located in western Texas and southern New Mexico, for instance, yields many barrels of water for every barrel of oil produced. Disposal capacity is becoming constrained and costs are increasing. It makes economic sense for operators to recycle more produced water.
The industry is incorporating nonpotable water sources as an alternative to freshwater for completions. As stewards of freshwater resources, industry and regulators have recognized the benefits of transitioning to alternative water sources.
Scale formation inside pipes and tank walls from production activities is a potential risk well known to operators. Managing scale formation is made more difficult as different water types are blended together and produced to the surface. What is stable in the formation may not be stable at the surface.
Once a well is completed, water is released during flowback and production. As the water transitions to the surface, the scaling potential changes. Scale formation is costly to production operations, as it causes a reduction in flow through transfer lines, builds on the surface of equipment, restricts mechanical actions, and accumulates in tanks and impoundments.
Reduction of scaling can be achieved by reducing concentrations of hardness, alkalinity and silica. The Multiflo™ system of enhanced treatment equipment from Veolia allows operators to intentionally blend disparate water types to take advantage of natural precipitation tendencies, thus stabilizing frac supply water.
As water is produced to the surface, temperature and pressure are reduced, potentially leading to scale formation. For example, both silica and barium sulfate solubility decrease with decreasing temperature. Anhydrite (CaSO4) solubility decreases with decreasing pressure.
The Permian Basin offers a vast supply of brackish groundwater that can be used as frac supply water. However, some water sources can be high in carbonate or sulfate, thus promoting scale formation when blended with other water sources having high concentrations of certain cations.
The above image illustrates produced water carbonate scale. High sulfate in the source water can lead to barite (BaSO4) and celestite (SrSO4) scale given the presence of barium and strontium.
There are several mineral scales that can potentially form as a result of blending different produced water sources. The table below provides some examples. Groundwater from the Santa Rosa formation (Dockum Aquifer) or Rustler formation, both located in western Texas, can exhibit sulfate levels greater than 1,000 ppm.
Scaling reduction is achieved with reduced concentrations of hardness, alkalinity and silica.
When injected into the Permian Avalon Shale or Wolfcamp formations, precipitation can result.
Potential Scale Types from Blended Permian Water
As shown in the above table, the greater Permian Basin in west Texas and southeast New Mexico is organized across the horizontal and includes sub-basins spanning several hundred miles. Across the vertical are systems and zones that span geologic time of more than 400 million years.
Some zones are water-rich aquifers, some are conventional production and others are targeted shale zones. The ionic make-up in each zone is similar; however, once a threshold is crossed into a different zone, temperature, pressure and the ratio between ions changes. Mixing dissimilar water can lead to precipitation.
The above illustration shows a histogram of total dissolved solids concentrations in produced water samples from Guadalupian to Ordovician reservoirs. Gray windows show trends in salinity with reservoir age drawn using modes of the histograms.
Besides reducing flow through transfer lines, scale formation reduces conductivity through the proppant pack. Treatment and remediation, which includes solvents, scale inhibitors, milling and jetting, are costly. Even when scale inhibitors are used, their effectiveness eventually lessens over time, and scale begins to form.
By blending disparate water types, the system developed by Veolia promotes the natural tendency of scale formation in a dedicated reaction vessel. Precipitation can be accelerated by changing the pH or temperature of the water, adding salt or salts, or recirculating the sludge to provide nucleation sites that promote crystal growth. The growth of crystals reduces the potential for scale formation inside pipes and on tank walls. After generating and removing the precipitated solids within the process, the potential for post-precipitation downstream of the system is minimized.
Promoting the growth of crystals reduces potential scale formation inside pipes and on tank walls.
The Veolia precipitation system consists of the following stages:
• Dynamic mixing stage destabilizes particulate solids by adding metallic coagulant salts, if necessary;
• Enhanced precipitation reactor improves reaction of ions with the hardness and alkalinity of the water to form insoluble compounds;
• Accelerated flocculation stage allows rapid development of large settleable flocs;
• A lamella settling unit removes suspended matter in two steps: natural sedimentation of flocs and enhanced lamella clarification;
• Sludge recirculation system allows re-injection of sludge in crystallization reaction for improved performance and optimized chemical consumption.
Using a patented mixing technology to maintain a slurry of precipitated solids in a relatively homogenous suspension is a key feature of the company’s system. A reactor provides nucleation sites that encourage further precipitation into solids from larger crystals that can be recovered as a dry cake for disposal.
In the reactor, an impeller inside the draft tube provides high pumping rates with low shear to minimize the attrition of crystals. The system performance can be optimized via independent control of the solids wasting rate and solids recycling rate. The recycled solids provide secondary nucleation sites that accelerate salt precipitation in supersaturated solutions. The reactor design eliminates dead zones to ensure maximum solids contact and to minimize induction time.
Design of the reactor provides a much higher driving force for the chemistry and precipitation reaction, resulting in:
• Reduced reaction time;
• Increased particle size;
• Minimized potential for scaling and post precipitation;
• Improved separation of solids and liquids;
• Homogeneous solid suspension with lower energy and reduced breakage of precipitated solids;
• Axial flow increases pumping flow.
Once the precipitation reaction is driven to completion, stabilized treated water has lower potential for post-precipitation in transfer lines or in the formation.
Where water types are marginally stable or a more complete reaction is required, precipitation reactions can be promoted using chemical-based softening. Additives including lime, soda ash, caustic soda, magnesium salts, and sodium sulfate, can be used to remove hardness, silica, iron, manganese, barium and strontium from the water, along with particulate solids and free oil.
The Veolia system is designed to treat water with average to high suspended solids levels between 10 to 4,000 ppm TSS and produce water with a turbidity of less than 3 NTU, depending on raw water characteristics. Chemical-based softening agents can be used to drive precipitate reactions to completion, yielding a high-quality effluent.
Some typical water quality levels include:
• Total Hardness <35 ppm as CaCO3, depending on water temperature and ionic strength;
• Iron <1 ppm; • Barium <2 ppm ;
• Strontium <10 ppm;
• Silica <15 ppm, depending on magnesium concentration or use of magnesium salt additives;
• Free Oil <10 ppm;
• Suspended Solids: <15 ppm.
The system produces a dense sludge, typically with dry solids content of 6 to 10 percent mass fraction.
Factory-assembled package plants are pre-engineered and transportable with minimal hook-up needed. The plants feature a modular design that permits movement from site-to-site and have capacity up to 30,000 BWPD.
Where a more complete reaction is required, precipitation can be promoted using chemical-based softening.
The oil and gas industry recognizes the benefits of produced water reuse and it is becoming more common to blend different water types. However, scale formation is a potential risk that could lead to increased costs. Operators require a water treatment solution that allows them to use these disparate waters while lowering the risk of scale formation in transfer lines or impoundments.
By reducing hardness, alkalinity and silica, Veolia’s water softening system stabilizes frac supply water. As operators intentionally blend disparate water types, the system allows natural precipitation and solids are removed under controlled conditions to minimize potential post-precipitation in downstream transfer lines, tanks, and impoundments.
With its distinctive impeller design, the in-line reactor provides a much higher driving force for chemical precipitation reactions to go to completion. The high-quality effluent produced by the water softening system allows operators to focus on production, instead of time-consuming and costly line and equipment maintenance and remediation.
Ultimately, improved water economics are vital to any operator’s business outlook.
Authored by J.P. Welch, Director of Upstream Business Development, + John Korpiel, Principal Engineer, Veolia Water Technologies