Cavitation process enhances treatment of flowback water

Oil and gas production operations are facing increasingly complex water management issues, both with acquiring freshwater and with proper treatment and/or disposal of produced and flowback water.

In areas like drought-stricken Texas and much of the western US, freshwater acquisition is becoming increasingly difficult. In growing production areas such as the Bakken and the Niobrara, disposal well capacities and locations may not be able to keep pace with new production wells and rising water volumes.

Permitting requirements and a changing regulatory environment compound water management problems. In Texas, special permits are required to store or transport untreated produced water, and the wait time for permits can exceed a year or more in some cases.

One solution to ease water management problems is to treat the water, which can be reused in the oil field, used as agricultural water, or discharged. However, treating oil-field water efficiently is a very complex process.

Treatment requirements vary based on water chemistry, type of reuse, and/or location of discharge. Water chemistry varies wildly from basin to basin and can even change quickly within a single basin from well to well. Thus, in the realm of produced and flowback water treatment, adaptability is a must.

In response to these challenges, Produced Water Solutions Inc. (PWS) has developed a patent-pending in-line reactor system called pHyX that delivers greater flexibility and adaptability in oilfield water treatment. Its design is simple and robust. The system was successfully deployed commercially in September 2011. While new applications are being developed, four areas for commercial application have been identified.

The simple, robust pHyX system consists of a nozzle, reaction chamber, and flow deflector. The system can be operated in a variety of modes depending on desired treatment and is scalable to nearly any flowrate while maintaining a very small footprint. (Image courtesy of Produced Water Solutions)

Cavitation/advanced oxidation

Removing organic constituents, heavy metals, and salt concentrations from produced and flowback water provides a challenge to producers. However, using a Venturi effect in the fluid stream to create cavitation can result in an environment where multiple chemical reactions can occur.

Cavitation is a process by which a void (bubble) is produced in a fluid stream and immediately implodes. Inertial (or transient) cavitation occurs when the pressure in the fluid stream decreases below the saturated vapor pressure of the fluid, creating a void where the matter phase has changed from liquid to gas.

This effect is usually localized since the bubble created within the stream needs a surface to nucleate – such as the sidewall of a container, the tip of a pump impeller vane, solid impurities in the liquid, or undissolved microbubbles.

According to a simplified Bernoulli equation, energy must be conserved. Thus, assuming no change in the fluid’s elevation or density, cavitation can occur due to the resulting pressure drop as a fluid’s velocity increases through a constriction, such as a nozzle or control valve, or as a fluid accelerates around a pump impeller vane.

The bubbles created during cavitation immediately begin to collapse due to the higher pressure of the surrounding liquid and the “drag” resistance at the liquid-gas interface, which slows the velocity of the bubble.

This collapse forces energetic fluid into a small cavity where the forces created by the bubble collapse can create pressures as high as 25,000 psi and temperatures as high as 4,727°C (8,540°F). The shockwave released by many bubbles continuously collapsing also can be heard as the telltale “flowing gravel” sound in a pump, pipe, or valve, which is indicative of cavitation.

These forces can cause multiple chemical reactions, one of which is the dissociation of water into hydrogen and hydroxyl radicals. Hydroxyl radicals are powerful oxidizers and can be used to destroy organic constituents such as hydrocarbons.

Also, above 3,205.3 psi and 374°C (705.2°F), water is in a supercritical state. While the supercritical effect is localized to the area of bubble collapse, there are three unique traits of supercritical water that can be used in produced water treatment: organic phases become completely soluble in supercritical water; oxygen is completely soluble in supercritical water and behaves as a strong oxidizer; and inorganic constituents become largely insoluble.

These traits can be used to reduce organic concentrations through oxidation and potentially to reduce heavy metals and salt concentrations. Note that the water shown in the table was passed through the pHyX system, then treated with ferric chloride (a common coagulant for solids settling). The “treated water” results are from the supernatant of the sample after solids settled; no distillation or membranes were used.

Bench-scale testing was performed on water from the Bakken formation in North Dakota. Analytical testing was performed by an independent lab. For this test, untreated water was run through the pHyX system, then treated with coagulant and allowed to settle. Treated water samples were collected from the supernatant of settled samples. (Data courtesy of Produced Water Solutions)

Disinfection

Many of the same oxidative reactions that destroy organic chemicals also can contribute to disinfection of the fluid stream. Oxidizers work as a biocide by oxidizing the cell membrane of the microorganism that leads to a structural collapse of the cell, resulting in cell lysis and total biological failure.

Additionally, when the system is operated at higher pressures, it is postulated that the shear induced at various points in the system by turbulent flow, cavitation, and the force from impact on the deflector may serve to physically lyse cells, further contributing to biocidal effects.

In the table, the change in adenosine triphosphate (ATP) results between the untreated and treated waters indicates disinfection occurring within the pHyX system. ATP is a functional nucleoside triphosphate that behaves as a coenzyme within cells. ATP serves as a shuttle, delivering chemical energy from storage locations to areas where energy is required. Thus, a reduction in ATP indicates a reduction in living microorganisms.

Platform, in-line reactor

All chemical reactions depend on one piece of matter bumping into another piece of matter (with the required energy present) for the reaction to occur.

To improve matter-on-matter contact, several methods can be used: increasing the amount of mixing (or turbulence); the available surface area for contact (by shrinking the volume of the particle to be contacted, or “atomizing” the fluid); and/or the amount of energy in the system (by increasing the pressure or temperature). The pHyX system can accomplish all of these.

Using the Venturi effect within pHyX, nearly any gas or liquid reagent can be introduced into the process stream. The process stream also can be operated under pressure (up to 1,000 psi or more) to increase reagent solubility.

The turbulent flow within the reaction chamber, the flow along the deflector, the “atomizing” effect of the deflector, and mixing within the containment chamber all serve to decrease reaction time.

Dissolved air flotation systems

Traditional dissolved air flotation (DAF) systems recycle only a small portion of water under pressure; air is injected into the recycle stream and dissolved under pressure.

When the water reenters the DAF system at atmospheric pressure, the oversaturated air comes out of solution as very fine bubbles. These fine bubbles attach to solids, floating them to the surface of the containment tank for removal by skimming.

In comparison with traditional DAF systems, using a pHyX-in-DAF system eliminates the need for both an air compressor and a pressurized recycle line.