US20140202683A1

US20140202683A1 - Ultrasonic wellbore dewatering device, system and method

Info

Publication number: US20140202683A1
Application number: US14/161,622
Authority: US
Inventors: Cindy Wolfe; Ken Lowe
Original assignee: Cenovus Energy Inc
Current assignee: Cenovus Energy Inc
Priority date: 2013-01-22
Filing date: 2014-01-22
Publication date: 2014-07-24
Also published as: CA2840419A1

Abstract

An ultrasonic device and system is provided for specific application to unloading non-gaseous production (typically mineralized water which may or may not be associated with produced solids and/or hydrocarbon liquids) from gas producing wells. In one embodiment, the system comprises an ultrasonic particle generator bank, including a transformer as needed (geometry of bank varies depending on down hole configurations) with multiple ultrasonic sources for redundancy/longevity and particle formation rate control. The multiple ultrasonic sources may be powered electrically from the surface, or by other means, with a length management conveyance system. The ultrasonic sources may be buoyed at substantially optimal depth below the surface of the non-gaseous production being particlized.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 61/755,262, filed Jan. 22, 2013, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention is related to the dewatering of gas producing wellbores including methods, devices and systems therefor and more specifically to ultrasonic devices, methods and systems therefor.
2. Description of the Related Art
Gas producing wells, such as natural gas and including those wells that have concurrent production of liquids or non-gaseous products such as water, oil or condensates, are often incapable of clearing these liquids from the well bore. This is especially true in depleted reservoirs and low-rate gas producing wells. Liquids can accumulate in the well bore as gas is produced. Accumulated liquid exerts backpressure on the producing formation such that flow of gas is reduced or completely restricted.
Natural gas producing wells can experience sporadic or permanent cessation of production if the gas flow velocity is too low to lift slugs or large drops of non-gaseous production from the wellbore. This process is known in the field as loading. Typically, the non-gaseous production comprises mineralized water which may or may not be associated with produced solids and/or hydrocarbon liquids.
Typical dewatering methods for reducing liquid accumulation in the well bore and re-establishing a viable gas production rate usually involve using the reservoir energy to lift out the liquid or by using external energy to lift it out. Reservoir energy is used by concentrating the reservoir pressure using mechanical means (i.e. plunger lift) or by taking advantage of critical flow properties (i.e. velocity strings). These methods are limited in application by the reservoir pressure and flow rates. External energy is used to activate pumps or to periodically bail, swab or perform coiled tubing cleanouts (CTC). These methods are not as limited by reservoir pressure or flow rates but may be too expensive (in the case of pumps) or technically inadequate (in the case of periodic removal methods) to be considered an optimal solution for gas producing wells including shallow gas producing wells.
A further problem with external energy sources such as down-hole pumps is that many pumping methods are labour intensive, require regular attention and generally use expensive equipment to provide an external source of lifting capacity to clear the well bore of the liquids. As a result, these technologies are cost prohibitive, and are often not economically viable for low production wells.
Other dewatering technologies have a narrow operating range, and must be suited to each individual well based on well characteristics such as water gas ratio (WGR), well pressure, and gas flow rate. This information is often unavailable, and can be highly variable over time. These technologies generally require regular attention from operations staff which can be problematic in areas of limited or restricted lease access. The narrow operating range of these dewatering technologies means that they usually fail when well conditions change in such a way that they are outside of the operating range.
Failure of these technologies results in down time and lost production, and can also require attention from operations staff in order to resume production.
A need therefore exists for a well dewatering method and system that overcomes at least one of the above mentioned shortcomings associated with existing technologies or at least overcomes at least one shortcoming inherent to existing and potential well dewatering systems further to those described above.

SUMMARY OF THE INVENTION

This invention uses an ultrasonic particle generator to create ultra fine particles of the non-gaseous production thereby allowing gas producing wells with low gas flow velocities to remain unloaded. The smaller the particle of non-gaseous production, the less velocity is required to transport it in the gas stream and out of the well bore. Devices in accordance with the invention may also potentially be used to enhance the utility of velocity string tubulars by allowing the velocity string to sit with its entrance high above the gas/non-gaseous interface thereby reducing the risk of flooding the velocity string during a period of shut-in.
An ultrasonic device and system is provided for specific application to unloading non-gaseous production (typically mineralized water which may or may not be associated with produced solids and/or hydrocarbon liquids) from gas producing wells. In one embodiment, the system comprises an ultrasonic particle generator bank, including a transformer as needed (geometry of bank varies depending on down hole configurations) with multiple ultrasonic sources for redundancy/longevity and particle formation rate control. The multiple ultrasonic sources may be powered electrically from the surface, or by other means, with a length management conveyance system. The ultrasonic sources may be buoyed at substantially optimal depth below the surface of the non-gaseous production being particlized to be dewatered.
In one embodiment, the present invention provides for an ultrasonic dewatering system for use in a gas producing well to particlize non-gaseous production for removal from a wellbore, the ultrasonic dewatering system comprising:
an ultrasonic particle generator bank for positioning in the non-gaseous production for particlizing at least a portion of the non-gaseous production, the ultrasonic particle generator bank comprising:
an ultrasonic source for positioning at or below the surface of the non-gaseous production for particlizing at least a portion of the non-gaseous production; and
a power source in communication with the ultrasonic source.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the ultrasonic particle generator bank further comprises a buoyancy control for positioning the ultrasonic source a desired depth below the surface of the non-gaseous production.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the desired depth is between about 0 mm and 4 mm.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the desired depth is about 1.0 mm or less.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the system or systems further comprise:
an electrical conveyance electrically connecting the power source and the ultrasonic particle generator bank.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the electrical conveyance further comprises a length management system for controlling the length of the electrical conveyance to maintain a length of the electrical conveyance at a length suitable to maintain the buoyancy of the ultrasonic source at the desired depth.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the ultrasonic particle generator bank comprises multiple ultrasonic sources and a transformer in communication with each ultrasonic source.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the wellbore further includes a velocity string mounted at a height that is raised above the surface of the non-gaseous production relative a typical velocity string.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the ultrasonic source is a directly coupled ultrasonic atomizer, a horn nebulizer, or a mesh nebulizer.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the wellbore is a horizontal wellbore.
In yet another embodiment, the present invention provides for an ultrasonic dewatering system for use in a gas producing well comprising a wellbore to particlize non-gaseous production for removal from the wellbore, the ultrasonic dewatering system comprising:
an ultrasonic particle generator bank in the non-gaseous production for particlizing at least a portion of the non-gaseous production, the ultrasonic particle generator bank comprising:
an ultrasonic source below the surface of the non-gaseous production for particlizing at least a portion of the non-gaseous production; and
a power source in communication with the ultrasonic particle generator bank.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the ultrasonic particle generator bank further comprises a buoyancy control positioning the ultrasonic source a desired depth below the surface of the non-gaseous production.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the desired depth is between about 0 mm and 4 mm.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the desired depth is about 1.0 mm or less.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the system or systems further comprise:
an electrical conveyance electrically connecting the power source and the ultrasonic particle generator bank.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the electrical conveyance further comprises a length management system for controlling the length of the electrical conveyance to maintain a length of the electrical conveyance at a length suitable to maintain the buoyancy of the ultrasonic source at the desired depth.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the ultrasonic particle generator bank comprises multiple ultrasonic sources and a transformer in communication with each ultrasonic source.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the wellbore further includes a velocity string mounted at a height that is raised above the surface of the non-gaseous production relative a typical velocity string.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the ultrasonic source is a directly coupled ultrasonic atomizer, a horn nebulizer, or a mesh nebulizer.
In another embodiment of the ultrasonic dewatering system or systems outlined above, the wellbore is a horizontal wellbore.
In yet another embodiment, the present invention provides for a method of dewatering a gas producing well comprising:
positioning a ultrasonic particle generator bank in the non-gaseous production of the wellbore; and
particlizing at least a portion of the non-gaseous production for upward flow with the gas and eventual evacuation of the wellbore.
In yet another embodiment, the present invention provides for the use of an ultrasonic device at a wellhead of a producing well, the ultrasonic device comprising
an ultrasonic source for positioning at the wellhead; and
a power source in communication with the ultrasonic source;
wherein the ultrasonic source is for particlizing water condensing at the wellhead into ultrafine particles for re-vaporization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrative of one embodiment of an ultrasonic dewatering system utilizing one embodiment of an ultrasonic dewatering device in a wellbore of a gas producing well; and

FIG. 2 is a schematic drawing illustrative of one embodiment of an ultrasonic dewatering system utilizing one embodiment of an ultrasonic dewatering device in a wellbore of a gas producing well having a velocity string.

FIG. 3 is a plot showing surface tension energy of 1 L of pure water as a function of the droplet size;

FIG. 4 is a plot showing energy requirements of the commercial mist generators normalized to 1 L of water and superimposed with the scaled surface tension energy curve;

FIG. 5 is a schematic illustration of the interfacial profile as a cavitation bubble near the interface leads to its destabilization and hence pinch-off of an aerosol droplet.⁴;

FIG. 6 shows one embodiment of an ultrasonic atomizer from TDK;

FIG. 7 shows one embodiment of an ultrasonic fountain in water;

FIG. 8 is a schematics of one embodiment of an ultrasonic horn;

FIG. 9 shows an embodiment of an ultrasonic horn atomizer;

FIG. 10 shows an embodiment of a metal mesh nebulizer;

FIG. 11 shows liquid dispersing by an embodiment of a mesh nebulizer;

FIG. 12 A & B shows one embodiment of an experimental setup for testing the mesh nebulizer wherein droplets are generated perpendicularly to the mesh;

FIG. 13 is a chart illustrating evaporation rate from the mesh nebulizer as function of the depth with the mesh oriented at 45 degrees;

FIG. 14 shows one embodiment of a step horn, design #1;

FIG. 15 shows another embodiment of a step horn, design #2;

FIG. 16 shows yet another embodiment of a step horn, design #3;

FIG. 17 is a photograph showing atomization by step horn, design #4;

FIG. 18 is a plot showing a minimum gas (methane) velocity;

FIG. 19 is a plot showing the Reynolds number at minimum velocity;

FIG. 20 is a plot showing the gas flow profile in 4.5″ pipe where V=V_min, only droplets in the central zone are suspended (A); V_min=V_bulk=V_max/2, half of droplets can be lifted (B); V_min=V_max/10, 90% of droplets can be lifted (C)

FIG. 21 is a plot showing gas velocity capable of lifting 90% of water droplets

FIG. 22 is a schematic illustration of nebulizer lifted above the water level to disperse water into CBM flow;

FIG. 23 is a schematic illustration of the placing of the mesh nebulizers in a CBM well;

FIG. 24 is a schematic illustration of the lifting of the water to the level of the gas stream may optionally require spreading the water to the pipe wall;

FIG. 25 is a schematic of one type of a horn type nebulizer in the 4.5″ pipe; and

FIG. 26 is a schematic of a set of TDK type nebulizers in 4.5″ pipe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An ultrasonic device and system is provided for specific application to unloading non-gaseous production (typically mineralized water which may or may not be associated with produced solids and/or hydrocarbon liquids) from gas producing wells. This process will be referred to as “dewatering” throughout this specification. Although the term “dewatering” is used, it will be appreciated that in addition to water generated by the hydrocarbon well, oil and/or condensates are also encompassed by this term and may be particlized and drawn upwards or in the general direction of gas flow and evacuated from the gas producing well.
It will be appreciated that reference herein to a “gas producing well” encompasses but is not limited to those wells that produce a gas, optionally in addition to a further hydrocarbon optionally in a liquid form or in the form of a liquid and solid composition such as an oil sand, wherein such gas generally has a sufficient pressure or density to flow from the formation to the surface.
It will also be appreciated that reference herein to an “ultrafine” particle encompasses those particles that are of a sufficient diameter to be transported by the displacement of the gas within the wellbore of a gas producing well. Such particles include but are not limited to those that behave in accordance to Stokes law. It will be appreciated that the diameter of the particles may vary based on the velocity and/or density of the gas in the wellbore.
One example of an ultrasonic dewatering system for use in a gas producing well is illustrated in schematic form in FIG. 1. The ultrasonic dewatering system is shown installed in a wellbore 10 of a gas producing well, for example a natural gas well. The wellbore 10 is shown as having a region wherein gas flows generally upwards from the bottom of the well and the gas producing reservoir and out of an outlet 60 where it may be collected, filtered, separated, or otherwise processed. The wellbore 10 includes a non-gaseous production 30 generally collected towards the bottom or the lower region of the wellbore 10. The non-gaseous production 30 may be in the form of water, oil or condensates and generally a large portion of the non-gaseous production is in the form of water or mineralized water.
An ultrasonic particle generator bank 20 is positioned in the non-gaseous production 30 for particlizing the non-gaseous production 30. The non-gaseous particles are drawn upwards and out of the wellbore 10 via the outlet 60 by the upward flow of the gas. This process may be facilitated by the production of smaller particles which are more easily drawn upwards with the gas flow or which require a lower rate of gas flow to be drawn upwards for dewatering of the wellbore 10. The ultrasonic particle generator bank 20 shown in FIG. 1 is powered by a power source which may be positioned on the surface outside of the wellbore 10. It will be appreciated that a down bore power source, such as a battery, may be used as a power source for the ultrasonic particle generator bank 20.
In the embodiment shown in FIG. 1, an electrical conveyance 50 transfers power from the power source 40 to the ultrasonic particle generator bank 20. In one embodiment, the electrical conveyance 50 further includes a length management system for extending and withdrawing the conveyance as the level of the non-gaseous production changes over time.
The ultrasonic particle generator bank 20 includes a transformer as needed as the geometry of bank may vary depending on the down hole configurations. For example, the ultrasonic disk or disks may be aligned in a vertical orientation and strung vertically or at a desired orientation. The ultrasonic particle generator bank 20 further includes an ultrasonic source, and generally multiple ultrasonic sources for providing redundancy while increasing longevity and allowing for particle formation rate control. The multiple ultrasonic sources may be powered electrically from the surface, or by other means, for example a battery or battery pack. In addition, in a further embodiment, the ultrasonic particle generator bank 20 includes a buoyancy control for controlling the depth at which the ultrasonic sources may be buoyed below the surface of the non-gaseous production. An optimal depth may be selected to more effectively particlize the non-gaseous production for dewatering. For example, the ultrasonic sources may be submerged about 0-60 mm, or from 0-4 mm or from 1 mm or less below the surface of the non-gaseous production depending on the type of atomizer, for example, a directly coupled ultrasonic atomizer, a horn nebulizer, or a mesh nebulizer. The buoyancy control is more effective when used together with a length management system for the electrical conveyance so that the electrical conveyance does not significantly alter the buoyancy of the ultrasonic particle generator bank 20 by adding further weight or by pulling upwards on the ultrasonic particle generator bank 20.
Using the ultrasonic sources, ultrafine non-gaseous particles are produced that can be transported with the gas phase out of the wellbore. It will be appreciated that the particlized non-gaseous particles have not undergone a phase change and are not gas vapour but are non-gaseous particles and so phase change of the particles from a gas vapour back into a water droplet is not observed as the particlized non-gaseous particles travel upwards with the gas flow. Without wishing to be bound by theory, it is predicted that some particles will coalesce on the walls of the tubulars in the wellbore. However, in order for precipitation to form in the free flowing space, it is predicted that particles will not spontaneously precipitate under static conditions until they are about 500 microns in size, and to coalesce the droplets should be in a net downdraft condition and of varying droplet diameters. Since their targeted creational size is <30 microns the particles can impact and join with a significant number of other particles before they get to a raindrop size and precipitate in the wellbore. Since the particles are in an updraft condition this will reduce their tendency to precipitate. It will be noted that the higher the velocity the greater the momentum (proportional to velocity squared) and the greater the likelihood that a collision will occur and will result in a coalescing of particles. There is therefore an increased risk of coalescing associated with the use of velocity strings which is balanced off by reduced potential contact time associated with the increased velocity. The velocity should therefore be monitored to determine if flow volume has decreased to a point where a velocity string is required. A tubingless open wellbore, for example, will typically have the lowest momentum but the longest exposure time for potential collisions. Optimization will require the monitoring of the relationship between flow velocity and droplet growth.
It will be appreciated that dewatering of a gas producing well may be carried out both in gas producing wells of higher and lower gas flow volume. In gas producing wells wherein gas flow velocity is lower, smaller particle size will allow for dewatering as lower gas flow will necessitate a smaller particle size generation for allowing upward flow of the non-gaseous particles with the lower flow velocity of gas.
One typical method of dewatering includes the installation of a velocity string in an effort to increase the rate at which gas flows upwards and out of the wellbore. The increase in gas flow velocity allows for the gas to carry the liquid or non-gaseous production upwards or in the general direction of the flow of the gas such as either a traditional vertical wellbore or a horizontal wellbore. As gas flow volume decreases, a velocity string may be used to increase the velocity of the gas flow and continue operation of the gas producing well. An ultrasonic dewatering system may also be used in conjunction with a wellbore including a velocity string as shown in FIG. 2. A wellbore 10 includes a velocity string 70. As described with reference to FIG. 1, the ultrasonic dewatering system includes an ultrasonic particle generator bank 20 positioned in the non-gaseous production 30 and powered by a power source 40. The power source 40 may be connected to the ultrasonic particle generator bank 20 by an electrical conveyance 50 if the power source is external the wellbore 10 and may include a length management system. In addition the ultrasonic particle generator bank 20 may optionally include a buoyancy control for positioning the ultrasonic sources at or near optimal depth in the non-gaseous production for particle generation.
The particlized non-gaseous production flows upward with the gas flow up the velocity string 70 where gas flow velocity is increased allowing for dewatering of the non-gaseous production in the wells of lower or reduced gas flow volume where a velocity string 70 has been installed.
Typically, a velocity string is positioned at the interface of the gas/fluid (top of perforations). One issue with the installation of a velocity string in a wellbore is that the velocity string can be flooded if the level of non-gaseous production rises higher than the bottom of the velocity string. By using an ultrasonic dewatering system the velocity string may be raised so that the distance between the bottom of the velocity string 70 and the surface of the non-gaseous production is increased or maximized. This reduces the risk of flooding of the velocity string 70 but still allows for dewatering of the non-gaseous production in a particlized form.
It is predicted that an ultrasonic dewatering system may extend reserve life beyond those expected from reservoir energy unloading methods, install and operate more cheaply than pumps and provide continuous unloading as compared to periodic removal methods.
In addition to those applications outlined above, an ultrasonic particle generator for the particlization of the non-gaseous production for removal of the non-gaseous production may also be applied to other scenarios such as to blowdown to generate vapour (as compared to flashing by pressure drop). Ultrasonic particlization may also be applied as a means for water treatment (similar to evaporation technology) wherein it may be used to create an ultrafine mist from the produced water which would then require less energy to vaporize the mist than the produced water itself.
Further, an ultrasonic particle generator may be used in combination with steam generators, which may be distant, for example several kilometers, from the wellhead. An ultrasonic particle generating may be positioned at or near the well head to convert condensed water (generally at very high pressure), which forms when the steam travels long distances in pipes and cools. The ultrasonic device could convert the condensed water to ultrafine particles. The energy within the steam flowing in the pipe may be sufficient to re-vaporize the ultrafine particles at the well head back to steam, thus increasing the steam quality being injected into the wellbore. Ultrasonic devices may be placed along the pipe at locations where condensation is typically observed.
Furthermore, ultrasonic particlization of the non-gaseous production may be used at tailings ponds, in an oil-water separation application wherein ultrasonic devices may be used on or near the surface to cause coalescence of emulsions and enhance separation. The ultrasonic particle generator may be operated to generate particles of water leaving the oil in a liquid state, or vice-versa.
In addition, an ultrasonic particle generator may be used to generate a mist within a steam separator to increase the surface-area of droplets, and thus induce more evaporation of vapour from the blowdown liquid.
Further, an ultrasonic particle generator may be employed by a lower pressure mist generator vessel after the steam separator to recover a low-pressure water vapour stream from the hot boiler blow-down. Such an application may be used to bring the blowdown flashing step closer to thermodynamic equilibrium by increasing the liquid surface area.
In addition, an ultrasonic particle generator may be used when introducing diluent into process emulsion (before a free-water knockout and/or treater) to finely disperse condensate as a “pseudo-mist in a liquid”. This may reduce localized areas of high condensate concentration that could lead to asphaltene precipitation from the bitumen.
It will be appreciated that various modifications, revisions and/or additions may be made to the systems and methods outlined herein without departing from scope of the invention and these modifications, revisions and/or additions are within the contemplated scope of the invention. In addition, it will be appreciated that the embodiments outlined above are merely illustrative of various contemplated embodiments and are not intended to be limiting in any way.

Experimental Work

It will be appreciated that the following experimental section is directed to illustrative embodiments of various aspects of the invention which are not intended to be limiting but merely illustrative of the concept of particlization or atomization of a liquid such as water using various ultrasonic devices and implementation methods. The invention is not limited by these devices and methods or the implementation techniques outlined in this section. All indication of volume, performance, orientation, dimensions, power output and materials is simply intended as illustrative and should not be deemed as essential or as a promise or guarantee or utility or performance.

Mist Generation

Starting Point: Calculated Energy for Water Dispersion

The energy required for mechanical generation of the mist from water can be calculated based on surface tension. The surface tension of the bulk water which is contained in a volume unit (1 L) is fairly low due to small surface area. The surface area is increased while this volume is dispersed into miniature droplets. The volume of the ordinary droplet is proportional to R³, subsequently the number of droplets is changed as 1/R³, where R is the radius of the droplet. As the surface of the ordinary droplet is proportional to R², the total surface area of the mist is changed with R as 1/R. Thus the finer the mist, the more energy is required to produce it. FIG. 3 represents the absolute surface tension energy of the 1 L of water as a function of the droplet size. For practicality, the data is presented in kW*h as oppose to scientific values in Joules.

Real Power Consumed, Real Efficiency

The calculated data in FIG. 3 is fairly optimistic; only 0.04 W*h is needed for dispersing of 1 L of water into a 3 μm mist. This represents only 0.8 W of power for permanent evaporation of required 20 L per hour which is the power of one LED lamp. The practical data is very different though.
The Table 1 presents data for several commercial ultrasonic mist generators.


			Mist
	Power,	Mean droplet	rate,
Manufacturer	W	diameter, μm	L/h	Note

TDK

	30	3	0.45	1.6 MHz
TDK	13	2	0.20	2.4 MHz
STULZ-Ultrasonic	495	3	10
New Tech Trading	1800	3	18
Hangzhou Success	500	62	150	YPW63, 15 kHz
Ultrasonic	300	51	70	YPW61, 20 kHz
Equipment Co.	100	39	50	YPW59, 30 kHz
	60	32	10	YPW57, 40 kHz
	30	28	0.5	YPW51, 50 kHz
	30	28	2	YPW55, 50 kHz

FIG. 4 superimposes power of the listed above devices normalized to one liter of water and scaled to the surface tension energy curve. The scale factor is 1700 giving us efficiency of about 0.06%.

Efficiency

The estimated efficiency of less than 0.1% is in accordance with data presented in the recent monograph.²
The heat vaporization of pure water is 2260 kJ/kg which is equivalent to 627 W*h or 0.6 kW*h. That is still much higher than any real mechanical atomizer consumes. Another words, ultrasonic atomizers are much more efficient than thermal vaporization.
The mechanism of ultrasonic atomization has not been fully understood yet. This phenomenon is fairly complex from an engineering point of view as it is influenced by properties of liquid, its amount and temperature, geometry of the transducers, operating frequency, applied voltage, etc. The complexity is coming from a not-fully known physics of the atomization process. According to the classic hypothesis, the generation of the aerosol droplets involves cavitation which occurs when vapor bubbles are formed in the liquid as the pressure in a localized region of the fluid suddenly decreases owing to the periodic disturbances introduced by the sound excitation.²If the local pressure falls below the vapor pressure during the negative half cycle of the oscillation, the liquid in that region essentially “boils” to form a vapor pocket. On the positive half cycle of the oscillation, these bubbles suddenly collapse with such intensities that extremely high instantaneous pressures and accelerations are generated in the form of shock waves. According to a second theory, the disturbance shock waves resulting from the implosion of the bubbles during cavitation lead to the excitation of finite amplitude capillary waves that result in droplet ejection.³This theory dominates in explanation of the aerosol formation by ultrasonic transducers placed in the liquid and is illustrated in FIG. 5.
Whichever mechanism is responsible for the excitation of capillary wave on the interface, a critical threshold amplitude of the waves must be exceeded before the onset of nebulization resulting from their destabilization. The critical amplitude α_cis given by:
$α_{c} = \frac{2 μ}{ρ} {(\frac{ρ}{π γ ω})}^{\frac{1}{3}}$
Where μ, ρ, γ—is the viscosity, density and surface tension of the liquid and ω=2πf. For water at 10° C. to 20° C. and frequency f of 10 kHz, 100 kHz, and 1 MHz, the critical amplitude α_cwill be equal to 0.13, 0.06 and 0.03 μm, respectively.

Ultrasonic Versus Hydraulic Atomization

A hybrid solution which represents ultrasonic atomizer together with the air supply (or, in general, gas carrier) is effectively used in ultrasonic sprayers providing satisfactory droplets size and large volume rate. However, this solution may not be practical for UDWW due to substantial gas consumption.
Another option is hydraulic dispersion in which the liquid is passed under considerable pressure through a nozzle. The pressure in hydraulic atomizers is about 80 to 100 bar. The advantage of the hydraulic dispersion is its high efficiency. In fact, this is the most efficient method of water atomization as it allows dispersing of 1 m³of water by using only 2 kW of power.5
The practical merit of such an economical approach, however, requires further investigation due to the need of having a miniature high-pressure downhole hydraulic pump. Also, the droplet size may vary and as result Prof. R. Christiansen observed about 70% of hydraulically formed water droplets were stuck to the pipe wall.¹
Shalunov A. V. claimed achieving the efficiency above 0.5% by directing the water stream horizontally on a vertically placed large (42 cm) piezo disk.⁵However, this approach is not applicable for ultrasonic wellbore dewatering devices (UWDD).

Methods for Ultrasonic Mist Generation

Ultrasonic atomizers can be divided in three groups:

- Transducers immersed in the liquid, direct energy transfer;
- Transducers displaced from the liquid, the energy is transferred trough special attachments (horns); and
- New type of nebulizers with mesh actuators.

Transducer Immersed in the Liquid

An ultrasonic atomizer developed by TDK was used. Direct coupling was used between the piezo element and the vaporizing liquid (water).
The method requires an active control of water level above the ultrasonic transducer to avoid overheating the piezo element. The optimum level is from 40 to 45 mm. A very similar product is offered by the APC with optimum water level of about 30 mm. The atomization rate is from 300 to 500 mL/h, as illustrated in FIG. 6.
This type of ultrasonic nebulizers has been popular in medical application for several decades. The ultrasonic fountain in the water (FIG. 7) can be cost-efficiently achieved, and it is widely used for decoration.
The method, however, has not found any valuable application in the industry. The reason is its limited power. The transducer cannot be simply scaled up to get higher atomization rate as water starts sporadically dispersing thus reducing the water level from its optimum. The atomizer becomes a demonstrator of a volcano eruption.
Placing number piezo elements of 20 mm to 25 mm in diameter allows a parallel work of several transducers simultaneously. That is one potential approach for UWDD. The number of elements is limited though by the pipe diameter, about 22 of elements will fit into the 4.5″ pipe.
Transducers Displaced from the Liquid, Half Wave Couplers
High volume atomizers can be built based on coupling the piezo element to the liquid via a probe (a horn) which length is equal to half of a wavelength. The probe is made from low-density metals such aluminum or titanium. These types of atomizers operate at a frequency range from 20 to 100 kHz, consequently the probe length is from 2.5 to 12.5 cm. Horns are designed to achieved the maximum vibration amplitude at the tip (FIG. 8).
Most of technological ultrasonic equipment such as welders, sprayers, mixers, etc. is built this way. The horn is bolted to a powerful Langevin ultrasonic transducer. For atomizing, the liquid is delivered through the transducer or through the base of the horn as shown in FIG. 9.
Some horn atomizers have the liquid feed channel passing near the node instead of going along the axis of the device.
Horns transfer the power of transducers into a smaller area at the horn tip. As oppose to direct coupling, all processes occur in the proximity of the horn surface. The active depth depends on amplitude of the vibration and is typically less than 1.0 mm.

Thin Plate Couplers, Mesh Nebulizers

Piezo elements operate more efficiently in a radial mode rather than in axial mode. Conversion of the radial vibration into axial displacement is therefore necessary. The most compact solution can be achieved using a metal mesh nebulizer. The active element in this device represents a thin (0.1-0.2 mm) metal foil with a number of miniature holes (10 to 200 μm diameter). The foil is placed between two piezo elements as shown in FIG. 10.
Radial vibration of thin piezo elements translates into the longitudinal displacement of the foil. Small amount of water placed on the top of the foil is dispersed effectively by the vibration; the mesh structure of the foil enhances the effect due to the surface acoustic waves (SAW), as shown in FIG. 11. Such design is used in miniature, low-power consumer sprayers.⁶
The size of actuators may be about 12×12 mm, the mesh nebulizers cannot be scaled up as they disperse water only at the edge of the mesh. Full covering of the mesh even with a thin water layer will result in immediate interruption of the atomization due to disappearance of the SAW.

Testing

Metal Mesh Nebulizer

A metal mesh nebulizer with a 12×12 mm mesh was fixed in a 3D optical translation stage as shown in FIG. 12. The nebulizer was powered from a custom generator with tunable frequency from 100 to 150 kHz. The peak-to-peak voltage was constant and equal to 12V. A water container was placed on a digital scale with resolution of 0.01 g. The nebulizer was oriented at 45 degree angle to the water surface.
FIG. 13 shows the experimental data on evaporation rate as a function of the depth. The maximum rate was recorded at the depth of 0.3 mm which was equal to 1.05 g/min. Deeper placement of the mesh into the water drastically reduced the atomization. The evaporation was entirely stopped at the depth of less than 1.0 mm. The maximum consumed power was 0.75 W and it remained practically unchanged with the depth. Generated droplets were oriented perpendicularly to the mesh.
Changing the orientation of the mesh from 45 degrees reduced the evaporation rate. The maximum rate dropped to 0.5 g/min at 30 degree and below 0.2 g/min at 60 degree. Placing the mesh horizontally stopped the evaporation entirely.
The data above is equivalent to the maximum Watt*Hour production of 84 g. Evaporation of 1 L and 20 L of water will need 12.0 and 240 W*h, respectively. Therefore, the mesh nebulizer appears to be about 5 times more efficient than the direct coupled nebulizer from TDK.

Horn Atomizers

The first model of the step-type horn was prototyped from 6061 aluminum, the diameter of the bottom and upper parts was 25.4 and 12.7 mm, respectively. The horn was firmly bolted to a 40-kHz 60 W Langevin transducer. The transducer was powered with 300 Vp-p. An acrylic pipe was mounted in the location of the node (FIG. 14). The pipe was filled with water to simulate water collection from the pipe wall.
The vaporization started after reducing the water layer to approximately 1.0 mm above the tip. The atomization was accompanied with sporadic eruptions which were spreading large water aggregates. The appearance of the tip was changed from shiny & polished to dull due to degradation of the surface under cavitation.
A second horn was machined with an extension at the tip, FIG. 15. The extension had the same diameter as the bottom part of the horn, 25.4 mm.
The second horn had a secondary resonance peak at approximately 45 t kHz which was the result of superposition of the original 40 kHz transducer peak and overtones of the conical extension of the horn.
The water was delivered on the tip from the top. The second horn dispersed the water more intensively; however, the intensity depended on location of the water and its amount. Larger droplets of water were collected closer to the center without getting dispersed for a long time.
The third step horn has a delivery channel through the center as shown in FIG. 16. The radial part of the channel was drilled close to the step where minimal displacements are taking place. The outer dimension of the horn matched the transducer diameter of 44.40 mm (1.75″) and the thinner part was 22.20 mm. The horn ended up with a 150-degree cone.
Preliminary testing shows that the main transducer resonance of 40 kHz split by several overtones. The water dispersing depended on the amount of water and orientation of the setup. A slight deviation of the horn from the vertical orientation changed the amount of water on the corresponding side of the cone and that was accompanied with the sporadic atomization. The test indicated, therefore, that taking more power from the transducer does not necessarily mean a better performance of the nebulizer if the power is spread over the large dispersing tip.
The prototyping of the forth horn included the modification of the basic 28-kHz Langevin transducer. Its base was machined out to OD of 25 mm at the length of 26 mm. The step horn had same diameter at the bottom (20 mm long) and 12.5 mm diameter upper part. The delivery channel of 2.0 mm diameter was drilled along the axis, its radial part was machined close to the location of the step, FIG. 17.
Atomization from the forth step horn was the most consistent from all horn type nebulizers above. The resonance frequency increased to 33 kHz, such a high shift was not anticipated based on modeling in Solid Works although it may be influenced by reducing the counter-mass of the transducer and location of the delivery channel. The measured actual power was 24 W and the dispersion rate was near 10 mL/min or 600 mL/hour. This efficiency was better than one can get from the TDK type of nebulizers although it was still lower than we achieved from the mesh nebulizers.
The horn type nebulizers are particularly sensitive to horn geometry, type of transducers and their coupling to the horn, material, etc. They can reach efficiency of 0.1% by generating water droplets of 30 μm.5 The evaporation rate of 20 L/hour will require only 40 W of power which is less than 1/30 of the standard TDK nebulizers. Scaling up the power is not exactly linear, one should expect the real output of the horn type nebulizers as 150 W for 20 L/hour.

Gas Lift

Droplet Size and Minimum Gas Flow

Condensed water can be lifted by methane up to the wellhead according to the UWDD concept. This paragraph is devoted to the estimation of the minimum gas velocity needed for lifting the water droplets. Such velocity can be directly calculated as a terminal velocity from Stokes law:
$V_{m i n} = \frac{2}{9} \frac{ρ_{w} - ρ_{g}}{μ} g r^{2}$
Where: ρ_wis the density of the water; ρ_gis the density of the gas; μ is the dynamic viscosity of the gas; g is the gravitational acceleration; r is the radius of the droplet.
Properties of pure methane under normal conditions (temperature 20° C. and atmospheric pressure) are: ρ_g=0.664 kg/m³; μ=1.1*10⁻⁵Pa s.
Calculated minimum gas velocity is presented in FIG. 18 as a function of the droplet size.
Droplets will be suspended in the gas which moves with minimum velocity V_min. The flow regime is expected to be laminar due to low value of V_minand a small pipe diameter. Reynolds number for a bulk velocity of V_min/2 and low amount of water droplets in the flow is equal to:
$Re = \frac{V_{m i n}}{2 μ} D ρ_{g}$
and is plotted in FIG. 19.
As it seen from FIG. 19, flow regime will be laminar over the entire range of droplet size. This means that a correction factor×2 should be applied for V_minto account for the bulk velocity. Otherwise, only a central part of the gas flow will be able to suspend droplets (FIG. 20).
Assuming the uniform distribution of droplets in the pipe, the gas flow of V=2Vmin (correction factor×2) should lift half of droplets in the pipe. Correspondently, selecting the correction factor×10 should assure lifting of 90% of all droplets.
FIG. 21 presents calculated data of Vmin multiplied by the correction factor×10. The underlined value of 2.0 m/s corresponds to droplet size of approximately 60 μm.
The required dewaterization rate of 20 L/h increases the density of the mixture by approximately 0.27 kg/m³at velocity of 2.0 m/s. This corresponds to a higher Reynods number of about 18,000 which indicates that flow regime become turbulent. Turbulence increases the friction between gas and water droplet by a factor C_t:⁷
C _t=1+0.16Re ^3/2
Correspondently, one should expect a better gas lifting due to turbulence when the water content increases.

Location of the Nebulizer

Gas flow in the CBM well begins from the lowest level of the coal seam which is located above the bottom of the gas producing well. As the dispersed water droplets have almost the same temperature as the surrounding gas, they do not spread too far from the nebulizer without being picked up by the gas flow. Therefore, the nebulizer should be lifted up to the level where gas flows at least with minimum velocity V_min, FIG. 22.
Lifting the nebulizer to the height H from the water surface (or a bottom of the gas producing well) may be done using for example a small water pump producing at least 20 L/h at pressure of a few bars.

Practical Considerations, Recommendations and Conclusions

Mesh Nebulizers

Mesh nebulizers represent an elegant and efficient ultrasonic atomizing solution. They operate at frequencies above 100 kHz, which allow generation of quite small and easy to lift water droplets.
Several rows of nebulizers may be required to achieve a desired particlization rate by placing the nebulizers along the pipe wall (FIG. 23).
Mesh nebulizers generate water mist generally perpendicular to the mesh, and a 45 degree orientation of the mesh was found to be optimal in terms of generating efficiency. Therefore, the mist will be directed under 45 degree to the pipe instead of being dispersed vertically up. One should expect some mist deposition on the pipe wall before some droplets will be drafted up by the gas flow.
3) Water lifted from the bottom at the height H may be dispersed to the pipe wall in order be picked up and dispersed by the mesh actuators (FIG. 24)

Horn Nebulizers

Horn nebulizers are a robust and versatile atomizing tool. They can disperse high power within a limited volume and they do not suffer from overheating when the water level is low. Horn type nebulizers generally pair with a pump, such as a micropump, as they do not disperse bulk water. However, the pump is needed in the UDWW anyway for lifting the water level at the height H. In one embodiment, the nebulizer may be positioned in the center of the pipe. It will occupy the space of approximately 70 mm in diameter which is approximately 40% of the pipe cross section (FIG. 25.)

High-Frequency Direct Coupling

High-frequency (for example 1.6 or 2.4 MHz) ultrasonic atomizers based on direct coupling of piezo elements to the water provides the tiniest water droplets which can be lifted by very low gas flow as shown in FIG. 26. Their maximum power is generally limited. Given a production rate of a single unit of approximately 0.5 L/hour, a total of 40 devices would be needed for a volume of 3 bbd.

REFERENCES

1. Christiansen R. L. New Technologies for Lifting Liquids from Natural Gas Wells, report for DOE, Colorado School of Mines, 2003
2. Plesset M. S., Prosperetti A. “Bubble Dynamics and Cavitation”. Annu. Rev. Fluid Mech. 1977, 9; 145-185
3. Boguslayskii Y. Y., Ekhadyosants O. K. “Physical Mechanism of the Acoustical Atomization of a Liquid” Sov. Phys. Acoust. 1969, 15, 14-21.
4. Boulton-Stone J. M., Blake J. R. “Gas Bubble Bursting at a Free Surface” J. Fluid Mech. 1993, 254: 457-66.
5. Khmelev V. N., Shalunov A. V., Shalunova A. V. “Ultrasonic Dispersion of Liquids”—Altai Tech Univ., 2010, (in Russian).
6. Yeo L. Y., Friend J. R., etc. “Ultrasonic Nebulization Platforms for Pulmonary Drug Delivery”. Expert Opin. Drug Deliv., 2010, 7: 663-679.
7. Van Boxel J. H. “Numerical Model for the Fall Speed of Raindrops in a rainfall simulator”—Workshop on Wind and Water Erosion, 1997, p. 77-85

Claims

What is claimed is:

1. An ultrasonic dewatering system for use in a gas producing well to particlize non-gaseous production for removal from a wellbore, the ultrasonic dewatering system comprising:

an ultrasonic particle generator bank for positioning in the non-gaseous production for particlizing at least a portion of the non-gaseous production, the ultrasonic particle generator bank comprising:

an ultrasonic source for positioning at or below the surface of the non-gaseous production for particlizing at least a portion of the non-gaseous production; and

a power source in communication with the ultrasonic source.

2. The ultrasonic dewatering system according to claim 1, wherein the ultrasonic particle generator bank further comprises a buoyancy control for positioning the ultrasonic source a desired depth below the surface of the non-gaseous production.

3. The ultrasonic dewatering system according to claim 2, wherein the desired depth is between about 0 mm and 4 mm.

4. The ultrasonic dewatering system according to claim 2, wherein the desired depth is about 1.0 mm or less.

5. The ultrasonic dewatering system according to claim 1, further comprising:

an electrical conveyance electrically connecting the power source and the ultrasonic particle generator bank.

6. The ultrasonic dewatering system according to claim 5, wherein the electrical conveyance further comprises a length management system for controlling the length of the electrical conveyance to maintain a length of the electrical conveyance at a length suitable to maintain the buoyancy of the ultrasonic source at the desired depth.

7. The ultrasonic dewatering system according to claim 1, wherein the ultrasonic particle generator bank comprises multiple ultrasonic sources and a transformer in communication with each ultrasonic source.

8. The ultrasonic dewatering system according to claim 1, wherein the wellbore further includes a velocity string mounted at a height that is raised above the surface of the non-gaseous production relative a typical velocity string.

9. The ultrasonic dewatering system according to claim 1, wherein the ultrasonic source is a directly coupled ultrasonic atomizer, a horn nebulizer, or a mesh nebulizer.

10. The ultrasonic dewatering system according to claim 1, wherein the wellbore is a horizontal wellbore.

11. An ultrasonic dewatering system for use in a gas producing well comprising a wellbore to particlize non-gaseous production for removal from the wellbore, the ultrasonic dewatering system comprising:

an ultrasonic particle generator bank in the non-gaseous production for particlizing at least a portion of the non-gaseous production, the ultrasonic particle generator bank comprising:

an ultrasonic source below the surface of the non-gaseous production for particlizing at least a portion of the non-gaseous production; and

a power source in communication with the ultrasonic particle generator bank.

12. The ultrasonic dewatering system according to claim 11, wherein the ultrasonic particle generator bank further comprises a buoyancy control positioning the ultrasonic source a desired depth below the surface of the non-gaseous production.

13. The ultrasonic dewatering system according to claim 12, wherein the desired depth is between about 0 mm and 4 mm.

14. The ultrasonic dewatering system according to claim 13, wherein the desired depth is about 1.0 mm or less.

15. The ultrasonic dewatering system according to claim 11, further comprising:

16. The ultrasonic dewatering system according to claim 15, wherein the electrical conveyance further comprises a length management system for controlling the length of the electrical conveyance to maintain a length of the electrical conveyance at a length suitable to maintain the buoyancy of the ultrasonic source at the desired depth.

17. The ultrasonic dewatering system according to claim 11, wherein the ultrasonic particle generator bank comprises multiple ultrasonic sources and a transformer in communication with each ultrasonic source.

18. The ultrasonic dewatering system according to claim 11, wherein the wellbore further includes a velocity string mounted at a height that is raised above the surface of the non-gaseous production relative a typical velocity string.

19. The ultrasonic dewatering system according to claim 11, wherein the ultrasonic source is a directly coupled ultrasonic atomizer, a horn nebulizer, or a mesh nebulizer.

20. The ultrasonic dewatering system according to claim 11, wherein the wellbore is a horizontal wellbore.

21. A method of dewatering a gas producing well comprising:

positioning a ultrasonic particle generator bank in the non-gaseous production of the wellbore; and

particlizing at least a portion of the non-gaseous production for upward flow with the gas and eventual evacuation of the wellbore.

22. Use of an ultrasonic device at a wellhead of a producing well, the ultrasonic device comprising

an ultrasonic source for positioning at the wellhead; and

a power source in communication with the ultrasonic source;

wherein the ultrasonic source is for particlizing water condensing at the wellhead into ultrafine particles for re-vaporization.