U.S. patent application number 12/009915 was filed with the patent office on 2009-07-23 for method of removing dissolved iron in aqueous systems.
This patent application is currently assigned to Total Separation Solutions LLC. Invention is credited to Harry D. Smith, JR., Kevin W. Smith.
Application Number | 20090183922 12/009915 |
Document ID | / |
Family ID | 40875544 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090183922 |
Kind Code |
A1 |
Smith; Kevin W. ; et
al. |
July 23, 2009 |
Method of removing dissolved iron in aqueous systems
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) ; Smith, JR.; Harry D.; (Montgomery, TX) |
Correspondence
Address: |
William L. krayer
1771Helen Drive
Pittsburgh PH
PA
15216
US
|
Assignee: |
Total Separation Solutions
LLC
|
Family ID: |
40875544 |
Appl. No.: |
12/009915 |
Filed: |
January 23, 2008 |
Current U.S.
Class: |
175/66 |
Current CPC
Class: |
E21B 21/068 20130101;
C02F 1/283 20130101; C02F 2103/10 20130101; C02F 1/001 20130101;
C02F 1/34 20130101; C02F 2101/203 20130101; C02F 1/72 20130101 |
Class at
Publication: |
175/66 |
International
Class: |
E21B 21/06 20060101
E21B021/06 |
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.degree. 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.degree. C.
Description
TECHNICAL FIELD
[0001] 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
[0002] 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: [0003] "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. [0004] Depending on the economics, a fluid can
be used in a well and then purchased back to be cleaned and reused
later. [0005] 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. [0006]
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. [0007]
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. [0008] 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."
[0009] 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 Fe.sub.2O.sub.3 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 Fe.sub.2O.sub.3
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.
[0010] 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.
[0011] 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
[0012] 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.degree. 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.
[0013] 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 Fe.sub.2O.sub.3, which may be in
hydroxide form--Fe.sub.2O.sub.3.xH.sub.2O. 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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
[0018] FIGS. 1a and 1b are views of slightly different cavitation
devices useful in our invention.
[0019] FIG. 2 is a flow sheet showing the use of a cavitation
device for treatment of a used oilfield fluid to remove iron.
[0020] FIG. 3 is a flow sheet which includes an activated carbon
unit.
DETAILED DESCRIPTION OF THE INVENTION
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.degree. 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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
Fe.sub.2O.sub.3, which may be in hydroxide
form--Fe.sub.2O.sub.3.xH.sub.2O. 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] Generally, we maintain the temperatures within the
cavitation device at 60.degree. 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
* * * * *