Title:
Method of removing dissolved iron in aqueous systems
Kind Code:
A1


Abstract:
Oilfield completion, drilling and workover fluids containing iron are treated to remove the iron by passing them through a cavitation device together with an oxidizing agent. The cavitation device intimately mixes the oxidizing agent with the fluid while increasing the temperature of the fluid, thus promoting the oxidation reaction. Ferric hydrate and other solids or colloidal iron are removed in a filter capable of removing particles as small as 0.5 micron. The system may be enhanced by the addition of a bed of activated carbon capable of catalyzing the oxidation reaction.



Inventors:
Smith, Kevin W. (Huston, TX, US)
Smith Jr., Harry D. (Montgomery, TX, US)
Application Number:
12/009915
Publication Date:
07/23/2009
Filing Date:
01/23/2008
Assignee:
Total Separation Solutions LLC
Primary Class:
International Classes:
E21B21/06
View Patent Images:



Primary Examiner:
HRUSKOCI, PETER A
Attorney, Agent or Firm:
William L. krayer (Pittsburgh PH, PA, US)
Claims:
1. Method of treating a used oilfield fluid containing iron to remove iron therefrom comprising (a) passing said used oilfield fluid through a cavitation device in the presence of added oxygen, thereby mixing said oxygen with said oilfield fluid, elevating the temperature of said oilfield fluid, and forming iron oxide solids therein, and (b) passing said used oilfield fluid through a filter capable of removing said iron oxide solids.

2. Method of claim 1 including recycling at least a portion of said used oilfield fluid from the outlet of said cavitation device to the inlet thereof.

3. Method of claim 1 including (c) passing at least a portion of said used oilfield fluid to a flash tank from said cavitation device, and (d) recycling at least a portion of said used oilfield fluid from said flash tank to said cavitation device, wherein oxygen is incorporated into said used oilfield fluid in said flash tank from the atmosphere therein

4. Method of claim 1 wherein at least a portion of said oxygen is added in the form of air.

5. Method of claim 1 wherein at least a portion of said oxygen is added in the form of hydrogen peroxide.

6. Method of claim 1 wherein said filter is capable of removing particles as small as 0.5 micron.

7. Method of claim 1 including maintaining temperatures of at least 60° C. within said cavitation device.

8. Method of claim 1 including maintaining a pH of at least 2.5 within said cavitation device.

9. Method of claim 4 wherein said air enters said fluid in a pump.

10. Method of claim 1 wherein said filter is a crossflow filter.

11. Method of claim 1 wherein said used oilfield fluid also contains a viscosity-enhancing polymer, and including impairing the viscosity-enhancing effect of said polymer in said cavitation device.

12. Method of claim 2 wherein about 10% to about 90% of said fluid is substantially continuously recycled from the outlet of said cavitation device to its inlet.

13. Method of claim 1 including passing said fluid from said cavitation device to a flash tank, evaporating at least some water from said fluid to achieve a fluid of less volume, and passing said fluid of less volume to a filter capable of removing particles as small as 1 micron.

14. Method of claim 1 which is substantially continuous and wherein the concentration of oxygen in said fluid is maintained at 2 mg/L or greater.

15. Method of treating an oilfield drilling, workover or completion fluid to remove iron therefrom comprising adding an oxidizing agent to said fluid and passing said fluid through a bed of activated carbon capable of enhancing the oxidation of ferrous iron.

16. Method of removing iron from an oilfield drilling, completion or workover fluid containing iron comprising (1) passing said fluid through a cavitation device in the presence of an oxidizing agent (2) controlling the operation of said cavitation device to maintain it effective to (a) elevate the temperature, (b) dissolve and mix said oxygen with said fluid, and (c) achieve the reaction of said oxidizing agent and said iron to form insoluble iron oxide, and (3) separating said insoluble iron oxide from said fluid in a filter.

17. Method of claim 16 wherein said filter is capable of removing particles of 0.5 micron.

18. Method of claim 16 including passing said fluid containing iron and also containing said oxidizing agent through an activated carbon bed capable of enhancing the oxidation reaction between said oxidizing agent and said iron.

19. Method of claim 16 including recycling at least a portion of said fluid through said cavitation device.

20. Method of claim 16 wherein said temperature is elevated to at least 60° C.

Description:

TECHNICAL FIELD

Dissolved iron is removed from an aqueous solution by passing the solution through a cavitation device while feeding an oxidizing agent into the solution, mixing and heating the solution in the cavitation device to oxidize ferrous iron to ferric iron, optionally increasing the pH to form solid iron oxide, and separating the solid iron oxide from the solution in a filter. The process is particularly useful for removing iron from oilfield completion, drilling, and workover fluids

BACKGROUND OF THE INVENTION

Iron dissolved in various kinds of aqueous solutions has caused many undesirable effects, and its removal has long been a vexing problem. As applied to workover and completion fluids used in hydrocarbon recovery, sometimes called clear completion brines, used in oil recovery, the background of the problem has been well described by Qu et al in U.S. Pat. No. 7,144,512:

    • “High density brines (completion brines) have been widely used in well completion and workover operations in oilfields in the past several decades. The completion brines are salt solutions typically having fluid densities ranging from about 8.4 ppg (pounds per gallon) to about 20 ppg. Depending on the density desired, a completion brine can be a one salt solution (e.g. NaCl, NaBr, CaCl2, CaBr2, ZnBr2 or formate salt in water), a two salt solution (e.g. CaCl2/CaBr2 or ZnBr2/CaBr2), or a three salt solution (e.g. ZnBr2/CaBr2/CaCl2). The composition of the brines determines the fluid properties such as pH, density, etc.
    • Depending on the economics, a fluid can be used in a well and then purchased back to be cleaned and reused later.
    • At the conclusion of any completion or workover project, a substantial volume of ‘contaminated’ or unneeded completion/workover fluid typically remains. Such fluids may be contaminated with any or all of the following: water, drilling mud, formation materials, rust, scale, pipe dope, and viscosifiers and bridging agents used for fluid-loss-control pills. Depending on their composition and level of contamination, these fluids may or may not have further practical or economic value. If it is deemed that the fluids have future use potential, they may be reclaimed. Conversely, if they are determined to have no further use, they must be disposed of in an environmentally responsible way.
    • The benefits derived from the use of solids-free fluids, and especially high-density brines, for completion and workover operations have been extensively documented in the literature. Unfortunately, the costs associated with the initial purchase and subsequent disposal of such brines has been a hindrance to their universal acceptance especially since the “use once and dispose” means of disposal is neither prudent nor economically sound.
    • Because of the relatively high cost and limited worldwide natural mineral resources available for producing medium- and high-density completion/workover fluids, it is essential that their used fluids be reclaimed. The reconditioned fluids must meet the same specifications as those of ‘new’ or ‘clean’ fluids. With respect to completion/workover fluids, the term ‘clean’ denotes not only the absence of suspended solids but also the absence of undesirable colloidal or soluble species which are capable of undergoing adverse reactions with formation, formation fluids or other completion fluids to produce formation-damaging insoluble substances.
    • There are many known methods for removing contaminates from a brine solution. One approach is to remove suspended solids by filtration. Simple filtration processes, wherein the brine is filtered through a plate and frame type filter press with the use of a filter aid such as diatomaceous earth and then through a cartridge polishing filter, are effective to remove solid contamination but they have no effect on removing other types of contamination such as colloidal or soluble species. This is the case since colloidally dispersed and soluble contaminants cannot be removed by filtration without first treating the fluid to change the chemical and/or physical properties of the contaminants. The treatments required to salvage the fluid depend on the nature of the contaminants incorporated and their chemical and physical properties.”

Almost all used clear completion fluids, and also many drilling fluids, contain iron, which has historically been extremely difficult to remove in the process of cleaning and preserving the fluids for reuse. Iron is generally in the form of FeO, which is soluble in the low pH common in completion fluids. Dissolved iron in the form of FeO cannot be filtered unless it is oxidized to a higher oxidative state. Simply raising the pH means the useful zinc and calcium bromide salts will also precipitate. The fluid incorporates dissolved oxygen from the air with normal pumping and handling, which converts the iron to Fe2O3 in the form of a 0.5 micron colloidal suspension, but the quantity of oxygen dissolved in this manner is seldom enough. Such small colloidal suspensions are very difficult to filter. Leaving 0.5 micron solids downhole is a problem since the formation is essentially a porous medium that cannot be backwashed. Everyone knows about iron, but until now no one has developed a practical solution for iron removal. One can add oxygen scavengers to try to keep the iron in solution, but that masks the problem and is never a permanent solution. One cannot add enough oxygen scavenger to prevent the iron from precipitating in the formation. There is simply too much oxygen. In addition, iron oxidation is a relatively slow process. One can filter a fluid today and it will be crystal clear, but tomorrow one will start seeing rust or Fe2O3 dropping out of solution. Thus, the problem has been that the ubiquitous iron is usually in solution in a used clear completion fluid, but it will damage the formation if it is not removed; removal without diminishing the other components of the fluid, or undertaking an enormous expense, has been elusive.

Various methods of oxidizing iron in water are reviewed by Schlafer et al in U.S. Pat. No. 5,725,759. See also Maree, U.S. Pat. No. 6,419,834. Hydrogen peroxide is one of several oxidizing agents proposed to oxidize iron in well servicing fluids to a higher oxidation state; the oxide is stabilized at a higher pH, and the fluid is then filtered, in Darlington et al U.S. Pat. No. 4,465,598. Particles as small as 0.1 micrometer are said to be filtered from oil and gas well fluids by Abrams et al in U.S. Pat. No. 4,436,635.

As none of these processes has achieved commercial success, there is a need in the industry for a practical way to prepare used completion, workover, and drilling fluids for reuse, including removing iron from them.

SUMMARY OF THE INVENTION

The invention involves passing the iron-containing completion, drilling, or workover solution, in the presence of added oxidizing agent, through a cavitation device, followed by filtration using a filter capable of removing particles as small as 0.5 micrometers. Concentration of dissolved oxygen or other oxidizing agent is maintained within the cavitation device at levels of at least 2 mg./L, and the temperature within the cavitation device is maintained at least at 60° C. The elevated temperature promotes iron oxidation. The pH is beneficially increased by any convenient means, such as the addition of lime or alkali metal hydroxides, to at least 2.5.

The cavitation device is operated so that oxygen or other oxidizing agent is thoroughly mixed and/or dissolved in the fluid and the temperature of the fluid is increased to the point at which the ferrous iron is converted to ferric iron, forming a colloidal-size precipitate of Fe2O3, which may be in hydroxide form—Fe2O3.xH2O. Colloidal iron is typically about 1 micron in size. Residence time in the cavitation device may be enhanced by recycling. The solution, now containing colloidal solids, is removed from the cavitation device and the solids are separated by a filter, preferably capable of removing particles as small as 0.5 micrometers.

The solution may be monitored for iron content before entering the cavitation device, and the introduction of oxygen controlled to supply the amount required to oxidize the iron or slightly more. The oxygen may be introduced in the form of air, oxygen, ozone, or a chemical oxidizing agent such as hydrogen peroxide, chlorine-containing bleaches, various carbamates, or any other suitable oxidizing agent. Generally also it may be expected that air may enter the system through seals and/or the ordinary action of centrifugal or other pumps that move the fluid into the cavitation device and elsewhere in the system; the pumps may introduce air into the fluid in amounts approaching or even in excess of the 2 milligrams per liter usually sufficient to oxidize the iron present.

When processing used completion and workover fluids, we do not require filtration before passing the fluid into the cavitation device, since its operation is unaffected by undissolved solids which may be found in used workover or completion fluids After passing through the cavitation device, a coarse filter may be used to remove larger particles before iron is removed in a microfilter. In processing used drilling fluids it may also be desirable to filter or screen the fluid before passing the fluid into the cavitation device.

The cavitation device has a distinct advantage in the common situation where polymeric viscosifiers, or other polymers, are present in the fluid to be treated for iron removal. Water-soluble polymers of almost all varieties are notorious for their tendency to plug filters, and this is especially true where the pore size of the filter is small. Subjecting the viscosity-enhancing polymers to the cavitation process and its accompanying temperature increase, however, will physically destroy the polymer molecules and render their remnants filterable without plugging the filters. The heat generated within the cavitation device during its normal operation also assists in reducing the detrimental effects of polymers via breakdown and/or viscosity reduction.

Our invention benefits from the additional use of certain types of activated carbon which have been found to rapidly decompose peroxides or otherwise catalytically enhance the oxidation rate of the iron species in the liquid. The liquid is beneficially contacted with the activated carbon immediately downstream from the cavitation device, but may be used anywhere in the system to enhance the reaction of a peroxide with the iron species in the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are views of slightly different cavitation devices useful in our invention.

FIG. 2 is a flow sheet showing the use of a cavitation device for treatment of a used oilfield fluid to remove iron.

FIG. 3 is a flow sheet which includes an activated carbon unit.

DETAILED DESCRIPTION OF THE INVENTION

We use a cavitation device to increase the temperature of the completion, drilling, or workover fluid while also mixing it with an oxidizing agent to oxidize the iron. A cavitation device heats a solution within it by generating shock waves within the solution and also by friction within the device. The term “cavitation” derives from pockets or cavities which are filled by shock waves of fluid.

We use the term “cavitation device” or to mean and include any device which will impart thermal energy to flowing liquid by causing bubbles or pockets of partial vacuum to form within the liquid it processes, the bubbles or pockets of partial vacuum being quickly imploded and filled by the flowing liquid. The bubbles or pockets of partial vacuum have also been described as areas within the liquid which have reached the vapor pressure of the liquid. The turbulence and/or impact, which may be called a shock wave, caused by the implosion imparts thermal energy to the liquid, which, in the case of water, may readily reach boiling temperatures. The bubbles or pockets of partial vacuum are typically created by flowing the liquid through narrow passages which present side depressions, cavities, pockets, apertures, or dead-end holes to the flowing liquid; hence the term “cavitation effect” is frequently applied. Steam or vapor generated in the cavitation device can be separated from the remaining, now concentrated, water and/or other liquid which frequently will include significant quantities of solids small enough to pass through the reactor. We prefer to use cavitation devices made by Hydro Dynamics, Inc., of Rome, Ga., most preferably the device described in U.S. Pat. Nos. 5,385,298, 5,957,122 6,627,784 and particularly U.S. Pat. No. 5,188,090, all of which are incorporated herein by reference in their entireties. In recent years, Hydro Dynamics, Inc. has adopted the trademark “Shockwave Power Reactor” for its cavitation devices, and we use the term SPR herein to describe the products of this company and other cavitation devices that can be used in our invention. The term “cavitation device” includes not only all the devices described in the above itemized patents U.S. Pat. Nos. 5,385,298, 5,957,122 6,627,784 and 5,188,090 but also any of the devices described by Sajewski in U.S. Pat. Nos. 5,183,513, 5,184,576, and 5,239,948, Wyszomirski in U.S. Pat. No. 3,198,191, Selivanov in U.S. Pat. No. 6,016,798, Thoma in U.S. Pat. Nos. 7,089,886, 6,976,486, 6,959,669, 6,910,448, and 6,823,820, Crosta et al in U.S. Pat. No. 6,595,759, Giebeler et al in U.S. Pat. Nos. 5,931,153 and 6,164,274, Huffman in U.S. Pat. No. 5,419,306, Archibald et al in U.S. Pat. No. 6,596,178 and other similar devices which employ a shearing effect between two close surfaces, at least one of which is moving, such as a rotor, and/or at least one of which has cavities of various designs in its surface as explained above. The cavitation process also causes intimate mixing of the fluid constituents as they pass through the device, and additional heating is provided as a result of friction generated as the fluid and the rotor move within the housing.

FIGS. 1a and 1b show two slightly different variations, and views, of a cavitation devices sometimes known as a cavitation pump, or a cavitation regenerator, and sometimes referred to herein as an SPR, which we use in our invention to regenerate solutions comprising heavy brine components.

FIGS. 1a and 1b are adapted from FIGS. 1 and 2 of Griggs U.S. Pat. No. 5,188,090, which is incorporated herein by reference along with related U.S. Pat. Nos. 5,385,298, 5,957,122, and 6,627,784. As explained in the U.S. Pat. No. 5,188,090 patent and elsewhere in the referenced patents, liquid is heated and mixed in the device without the use of a heat transfer surface, thus avoiding the usual scaling problems common to boilers and distillation apparatus.

A housing 10 in FIGS. 1a and 1b encloses cylindrical rotor 11 leaving only a small clearance 12 around its curved surface and clearance 13 at the ends. The rotor 11 is mounted on a shaft 14 turned by motor 15. Cavities 17 are drilled or otherwise cut into the surface of rotor 11. As explained in the Griggs patents, other irregularities, such as shallow lips around the cavities 17, may be placed on the surface of the rotor 11. Some of the cavities 17 may be drilled at an angle other than perpendicular to the surface of rotor 11—for example, at a 15 degree angle. Liquid (fluid)—in the case of the present invention, a used workover, drilling, or completion fluid containing iron,—is introduced through port 16 under pressure and enters clearances 13 and 12. As the fluid passes from port 16 to clearance 13 to clearance 12 and out exit 18, areas of vacuum are generated and heat is generated within the fluid from its own turbulence, expansion and compression (shock waves). As explained at column 2 lines 61 et seq in the U.S. Pat. No. 5,188,090 patent, “(T)he depth, diameter and orientation of (the cavities) may be adjusted in dimension to optimize efficiency and effectiveness of (the cavitation device) for heating various fluids, and to optimize operation, efficiency, and effectiveness . . . with respect to particular fluid temperatures, pressures and flow rates, as they relate to rotational speed of (the rotor 11).” Smaller or larger clearances may be provided (col. 3, lines 9-14). Also the interior surface of the housing 10 may be smooth with no irregularities or may be serrated, feature holes or bores or other irregularities as desired to increase efficiency and effectiveness for particular fluids, flow rates and rotational speeds of the rotor 11. (col. 3, lines 23-29) Rotational velocity may be on the order of 5000 rpm (col 4 line 13). The diameter of the exhaust ports 18 may be varied also depending on the fluid treated. Pressure at entrance port 16 may be 75 psi, for example, and the temperature at exit port 18 may be as high as 300° F. Thus the heavy brine components containing solution may be flashed or otherwise treated in and/or following the cavitation device to remove excess water as steam or water vapor. Note that the position of exit port 18 is somewhat different in FIGS. 1a and 1b; likewise the position of entrance port 16 differs in the two versions and may also be varied to achieve different effects in the flow pattern within the SPR.

Another variation which can lend versatility to the SPR is to design the opposing surfaces of housing 10 and rotor 11 to be somewhat conical, and to provide a means for adjusting the position of the rotor within the housing so as to increase or decrease the width of the clearance 12. This can allow for different sizes of solids present in the fluid, to reduce the shearing effect if desired (by increasing the width of clearance 12), to vary the velocity of the rotor as a function of the fluid's viscosity, or for any other reason.

Operation of the SPR (cavitation device) is as follows. A shearing stress is created in the solution as it passes into the narrow clearance 12 between the rotor 11 and the housing 10. This shearing stress causes an increase in temperature. The solution quickly encounters the cavities 17 in the rotor 11, and tends to fill the cavities, but the centrifugal force of the rotation tends to throw the fluid back out of the cavity, which creates a vacuum. The vacuum in the cavities 17 draws fluid back into them, and accordingly “shock waves” are formed as the cavities are constantly filled, emptied and filled again. Small bubbles, some of them microscopic, are formed and imploded. All of this stress on the fluid mixes the constituents of the fluid and generates heat which increases the temperature of the fluid dramatically. The design of the SPR ensures that, since the bubble collapse and most of the other stress takes place in the cavities, little or no erosion of the working surfaces of the rotor 11 takes place, and virtually all of the heat generated remains within the fluid.

Temperatures within the cavitation device—of the rotor 11, the housing 10, and the fluid within the clearance spaces 12 between the rotor and the housing—remain substantially constant after the process is begun and while the feed rate and other variables are maintained at the desired values. There is no outside heat source; it is the mechanical energy of the spinning rotor—to some extent friction, as well as the above described cavitation effect—that is converted to heat taken up by the solution and soon removed along with the solution when it is passes through exit 18. The rotor and housing indeed tend to be lower in temperature than the liquid in clearances 12 and 13. There is little danger of scale formation even with high concentrations of heavy brine components in the solution being processed.

Any solids present in the solution, having dimensions small enough to pass through the clearances 12 and 13 may pass through the SPR unchanged. This may be taken into account when using the reconstituted solution in for oil well purposes. Subjecting water-soluble polymers that may be present in the solution to the localized cavitation process and heating will tend to break them down, shear them, or otherwise completely destroy them; in any case they will not be likely to foul or plug the filters set up to remove precipitated iron compounds.

Hudson et al U.S. Pat. No. 6,627,784, one of the patents incorporated by reference above, describes the introduction of a gas to a fluid just prior to entering a cavitation device. Gas such as air is injected into the conduit leading to port 16, as depicted herein in FIGS. 1a and 1b. There may be more than one port 16, not all of which need necessarily contain both liquid and gas. As explained in the Hudson et al patent, the cavitation process, acting on the crude mixture of liquid and gas—for example, air—breaks down the air bubbles into a large number of very small bubbles, thus greatly increasing the surface area of the bubbles and greatly increasing the likelihood of contact by the air with a species susceptible to oxidation. The air may be dissolved in the liquid. Hudson et al describe specifically the oxidation of sodium sulfide in black liquor, a byproduct of cooking wood chips.

For the present invention, it should be understood that the oxidation of iron, and FeO, present in a used workover, drilling or completion fluid, requires not only simple contact with an oxidizing agent, but a facilitating temperature and a residence time sufficient to bring about oxidation in the practical amounts.

Referring now to FIG. 2, the iron-containing used completion, drilling or workover fluid enters cavitation device 30 through conduit 31, being propelled by a pump not shown. An oxidizing agent is introduced to conduit 31 through line 32. The oxidizing agent may be oxygen, air, a solution of hydrogen peroxide, sodium or ammonium persulfate, or any of various carbamates known as oxidizing agents, or any other convenient oxidizing agent such as a chlorine-containing bleach. The oxidizing agent immediately begins mixing with the fluid and the mixing effect is greatly enhanced within the cavitation device as explained above, bringing about intimate contact between the oxidizing agent and the iron species in the fluid under elevated temperatures due to the cavitation effect. If the oxidizing agent is a gas, such as air or oxygen, bubbles formed in the conduit 31 will immediately be dispersed and greatly divided into microbubbles, to the point of dissolution, similar to the effect described in the above cited Hudson et al patent U.S. Pat. No. 6,627,784. The dispersion and intimate contact of the oxidizing agent with the iron species causes oxidation and formation of Fe2O3, which may be in hydroxide form—Fe2O3.xH2O. These oxides are in solid or colloidal form, generally from 0.5 to 1.5 micron in size and are filtered out by a filter 33 capable of removing such materials. Where chloride oxidizing agents are used, the precipitates may be somewhat larger.

Line 34 passes from exhaust port 18 (FIGS. 1a and 1b) to filter 33. Filter 33 is desirably a nanofiber medium of Nylon 66 or materials having similar properties, and desirably such a filter medium made and sold by DuPont under the trademark HMT. The filter may be operated in the dead-end or cross-flow mode. For cross-flow, a beneficial filter medium is a sintered 904 stainless steel metallic membrane or a sintered ceramic membrane; porous plastic filters having a membrane coating of an appropriate pore size may also be used. Membrane and other filters able to remove particles of size 0.5 are readily available commercially. We may use any filter capable of removing particles as small as one micron and preferably as small as 0.5 micron. The retentate in filter 33 may be disposed of in any convenient manner; desirably the filter will be capable of convenient cleaning or backwashing for reuse, but disposable filters are also contemplated. Permeate of greatly reduced iron content passing through filter 33 is taken in line 29 to a holding tank for reuse or recirculation as a workover or completion fluid, or can be sent directly to such use.

Optionally, the system also utilizes flash tank 36. Flash tank 36 is used to enhance the removal of water from the completion, drilling or workover fluid in a manner similar to that shown and described by Smith and Sloan in U.S. Pat. No. 7,201,225. As shown in the '225 patent, upper outlet 39 from flash tank 36 contains vapor or steam which may be vented or condensed to make clean water for use elsewhere; its removal may be enhanced by an applied vacuum. Removal of water from the input solution in conduit 31 means that less fluid must be handled by the filters. This somewhat concentrated fluid 37 is supplied through line 34b from flash tank 36 to filter 33. Liquid in line 34 can be sent entirely to the flash tank through line 34a, or directly to the filters, or partially to each, within the discretion of the operator. If the flash tank 36 is used, oxygen from the air will be entrained in the somewhat concentrated fluid 37 in the bottom of the flash tank, and this fluid 37 may be recycled to the cavitation device through line 38, thus providing more oxygen for use in oxidizing the iron in the liquid while also providing another opportunity for oxidation of any yet unoxidized iron. In some situations, the flash tank may be used as the source of all the oxygen in the system.

The system of FIG. 2 is provided with recycle capabilities as well as pH-adjusting capabilities. The pH is generally beneficially increased by introducing a base through line 35, so that it will be intimately mixed along with the oxidizing agent. As is known in the art, a pH higher than about 2.5 is necessary for ferrous oxide to achieve a colloidal, filterable state. Accordingly, where the pH is lower than 2.5, addition of a pH-increasing agent is recommended.

Generally, we maintain the temperatures within the cavitation device at 60° C. or higher. Maintenance of the temperature, and consequent enhancement of the oxidation reaction, is benefited by a significant percentage of recycling through the cavitation device. Recycle line 28 accordingly returns a portion of the liquid in line 34 to conduit 31 for reintroduction to cavitation device 30. Although in some situations recycling may not be necessary, the process may benefit from recycling as little as 10% of the fluid in line 34 and as much as 90%. Specifications of the cavitation device should be reconsidered when recycling a very large volume of fluid is contemplated.

FIG. 3, in many respects similar to FIG. 2, is a flow sheet illustrating the use of activated carbon to enhance the oxidation reaction. An alternate line 40 carries the output liquid from an exhaust port 18 (FIG. 1a or 1b) of cavitation device 30 directly to a container 41 for a bed of activated carbon capable of enhancing the oxidation of the iron species present in the liquid by an oxidizing agent in the liquid. A catalytic activated carbon made by Calgon Carbon Corporation and sold under the trademark CENTAUR has been found satisfactory. See U.S. Pat. No. 5,356,849, which explains that activated carbon made in a certain way will accelerate the decomposition of hydrogen peroxide, thus making the reactive oxygen more readily available for reaction with ferrous iron. See also Hayden U.S. Pat. No. 5,637,232 and the prior art reviewed in relating to catalytic oxidation by activated carbon. It is recommended that the operator review the specifications of the activated carbon with respect to the particulars of the type of oxidizing agent used. The activated carbon container 41 may also be fed by line 27 from the flash tank 36, which has the advantage that less liquid must be handled by the activated carbon than otherwise would be the case, since fluid 37 is somewhat concentrated. After passing through the activated carbon bed in container 41, where additional colloidal iron is created, the liquid is passed through line 43 to the filter 33, similar to the filter 33 in FIG. 2.

The configuration of FIG. 3 is not the only one in which an activated carbon unit may be used. For example, a unit such as activated carbon container 41 could be placed upstream of cavitation device 30 at any point along conduit 31. If it is placed upstream of line 32, which introduces the oxidizing agent, it could have its own intake for oxidizing agent. An activated carbon container 41 could be placed in recycle line 28 or 38 as well—it should be remembered that performance of the cavitation unit 30 is not impaired by the presence of solids in the fluid it handles.

It is seen, therefore, that our invention comprises a method of treating a used oilfield fluid containing iron to remove iron therefrom comprising (a) passing the used oilfield fluid through a cavitation device in the presence of added oxygen, thereby mixing the oxygen with the oilfield fluid, elevating the temperature of the oilfield fluid and forming iron oxide solids therein, and (b) passing the used oilfield fluid through a filter capable of removing the iron oxide solids.

It also includes a method of treating an oilfield drilling, workover or completion fluid to remove iron therefrom comprising adding an oxidizing agent to the fluid and passing the fluid through a bed of activated carbon capable of enhancing the oxidation of ferrous iron.

In addition, our invention includes a method of removing iron from an oilfield drilling, completion or workover fluid containing iron comprising (1) passing the fluid through a cavitation device in the presence of an oxidizing agent (2) controlling the operation of the cavitation device to maintain it effective to (a) elevate the temperature, (b) dissolve and mix the oxidizing agent with the fluid, and (c) achieve the reaction of the oxidizing agent and the iron to form insoluble iron oxide, and (3) separating the insoluble iron oxide from the fluid in a filter.