CA2133707A1

CA2133707A1 - Method of reducing the level of contaminant materials in produced subterranean reservoir fluids

Info

Publication number: CA2133707A1
Application number: CA002133707A
Authority: CA
Inventors: Philip King; John A. Connor; Dennis A. Clifford
Original assignee: Individual
Current assignee: Groundwater Services Inc
Priority date: 1992-04-09
Filing date: 1993-03-16
Publication date: 1993-10-28
Also published as: AU669361B2; AU3923893A; US5196124A; WO1993021112A1

Abstract

A method is provided for reducing the concentration of radioactive materials or other trace contaminants in fluids withdrawn from subterranean reservoirs. The method involves deposition of sorbent solids within the reservoir matrix surrounding a production well to act as an in-situ filter for dissolved radionuclides or other contaminants present in reservoir pore waters. For this purpose, the sorbent solid is either a) precipitated within the reservoir matrix by the reaction of two or more carrier solutions or b) directly introduced into the reservoir as a solid component of a solid-liquid slurry using high-pressure injection techniques.
Thereafter, fluids produced by the well contain significantly lower concentrations of contaminants which are removed from the inflowing reservoir fluids by means of ion exchange, adsorption, chelation, chemisorption, or coprecipitation with the in-situ filter medium.

Description

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2 1 3 ~3 !^~ ~ 7 Pc~r/us93/o249s ~. :
Title.METHC)D OF REDUCING THE LEVEL OF CON~AMINANT
MAI~ERIALS IN PRODUCED SUBTERRANEAN
RESERVOIR ~LUID~
SPECIFICATI(:)N

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Serial No. 07/866,341 filed April g, 1992.

BACKC:ROUN~ OF THE I~ENTION

1. Field of the Inv~n~ion The invention relates to methods for reducing the concentration of radioactive materials or other trace contaminants in fluids withdrawn from subterranean reservoirs or "aquifers" by means of wells.

1~ 2. Descri~ti~n of the Prior Art Naturally-occurring radioactive materials (NORM) and other trace contaminants are present at varying concentrations in groundwater produced from water supply wells, oil production wells, and gas production wells. ln addition, radioactive materials and other contaminants have been artificially introduced into groundwater reservoirs due to leaching or spills associated withradioactive waste disposal, testing, and mining activities. Production of fluidscontaining such contaminants poses both health-related and environmental problems. The invention provides a new method of reducing the production of such radioactive materials and other trace contaminants from wells.
i 2 ~ ~ ~ 7 1~
wO 93/~ll12 PCr/US93/0249~

Radioactive materials contained in fluids produced from subterranean reservoirs can pose a significant risk to human health and the environment.
Due to toxic and carcinogenic effects, strict health-based lirnits have been established for human exposure to radium, radon, and other ~aturally-occurring radionuclides. However, within fluids produced from water, oil, or gas production wells, radioactive material concentrations significantly exceeding acceptable levels are not uncommon. In addition to human exposure, production of such radioactive fluids can contaminate distribution piping and equipment and the environment.
Various water treatment technologies have been shown capable of removing radionuclides from produced fluids; however, such treatment facilities invar.ably involve generation of a concentrated radioactive waste residue, presenting a significant waste management and disposal problem.
The following prior art patents are directed, generally, to the rernoval of dissolved contaminants from produced potable groundwa~er: U.S. Patent No.

3,136,715; U.S. Patent No. 3,449,065; U.S. Patent No. 3,803,208; U.S. Patent No.3,896,045; U.S. Patent No. 4,054,320; U.S. Patent No. 4,636,367; U.S. Patent No.4,664,809; U.S. Patent No. 4,404,498.

Patent No. 4,664,809, entitled "Groundwater Pollution Abatement" is directed to a method for pollution abatement in groundwaters whereby a series of wells are drilled ~n the path of the advancing front of contaminants in an aquifer. A
particulate absorbent material, such as activated carbon, treated clay, inorganic oxides, silicates, alumino silicates, carbonaceous materials, organic-polysners, and the like is introduced through the wells. These methods are not, however, adequate for removal of radioactive materials from such aquifers and do not address ~eatment of production wells.
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Patent No. 4,054,320, entitled "Method for the Removal of Radioactive Waste During In-Situ Leaching of Uranium" is directed to a leachmg process in mining operations wherein a "pre-pack" of sand or other particulate matter is placed around the exterior of the wellscreen of the well through which the leaching water is produced. The sand, or other particulate matter comprising the pre- !

21337~)7 : - WO 93/21 1 12 PC~/US93J02495 ` -pack, has deposited thereon a barium-containing ion exchange material designed to extract and concentrate the dissolved uranium contained in the produced waters.
.~ , The '320 patent's method may reduce the radionuclide content of produced waters to some degree but poses si~snificant disadvantages relative to the present invention. First, due to the limited sor~ent mass provided by the sand "pre-pack", the sorbent capacity of this medium would be consumed in a relatively short period, requiring redrilling of the well and replacement and disposal of 'che "pre-pack" material. Secondly, the "pre-pack" provides a relatively small surface area for contact with produced fluids. Therefore, radionuclide removal efficiencies would be less than those achieved by a larger in-situ zone that filters out or traps radionuclides in place. Finally, this prior method is intended for use in extraction of uranium ions from leaching fluids used in uranium mining operations, whereas the invention is directed to removing lowe r levels of radionuclides of concem from water supply wells, oil production wells, or gas production wells.
What is yet needed are methods for treating produced fluids for the removal of radionuclides, including radium and uranium, that are simple, relatively inexpensive, and do not generate a radioactive waste product that poses disposalproblems.

SUMMARY OF THE INVENTION
The present invention provides simple, inexpensive me~hods for removing or significantly reducing the concentration of radioactive materials or other tracecontaminants in subterranean reservoir fluids prior to entry of the fluids into the production well. The invention methods retain these contaminants within the subsurface by means of an in-situ contaminant "filter' or "trap" that is deposited within the reservoir matrix surrounding the well. Consequently, the contaminant material concentrations in the reservoir fluids produced at ground surface are significantly reduced, providing significant benefits in terrns of reduced risk of human exposure or environrnental contamination. Specifically, for water supply wells, subsurface removal of radionuclides provides a useable 2 ~ 3 c~ 7 ~) 7 WO 93J21112 PCr/US93/024~
-water resource while avoiding the problem of radioactive waste generation commonly associated with water treatment technologies. For oil and gas production wells, the present invention provides a means for reducing radioactive scale formation Oll distribution piping and equipment and for 5mitigating environmental contamination associated with brine storage and disposal. In addition, the rate of radon gas emanation from produced waters i5 reduced due to the reduced concentration of dissolved radium present in the production fluid.

10Conceptually, the invention is directed to the in-situ treatment of subterranean fluids for the removal of trace contaminants. Therefore, the techniques described herein may be utilized, using different scavenging chemical reactants,to remove trace contaminants other than radionuclides from subterranean fluids by an in-si~ treatment process which involves injecting the scavenging 15chemical into the subterranean reservoir for removal of contamin,as~ts.

Non-radioactive trace contaminants to which the invention can be applied include, but are not limited to, barium, arsenate, fluoride, and selenite, present as dissolved constituents within the reservoir pore fluids.
To remove radionuclide contamination, the invention provides for the placement of a sorbent solid within the reservoir matrix to act as an "in-situ filter" for dissolved radionuclides present ~n the reservoir (or aquifer) fluids.
For this purFose, the sorbent solid is supplied in one of two embodiments; it is25either a) precipitated within the reservoir matrix by the reaction of tuo or more carrier solutions or b) directly introduced into the reservoir as a solid phase component of a solid-liquid slurry using high-pressure injection techniques.

In the first embodirnent, a carrier solution, containing a dissolved component of 30the sorbent material, is introduced into the reservoir zone surrounding the production well via the production well itself or one or more injection wells.
An adequate volume of carrier solution is introduced to ensure contact of the carrier solution with reservoir matrix solids over a predetermined area.
Thereafter, the second (and subsequent) carrier solution(s) is (are) introduced, i~
35containing appropriate dissolved components to cause precipitation of the :` WO~3/21117 PCr/US93/0249~ i-sorbent solid within the reservoir matrix. Following such precipitation, excess carrier fluids are removed from the reservoir. The precipitated sorbent solids remain in place ~n the reservoir matrix to ser~e as an in-sihl filter or trap for dissolved radionuclides contained in the reservoir fluids flowing into the J
production well. This filtration or trapping effect is due to ion exchange, adsorption, chelation, chemisorption, or coprecipitation of the radionuclides onto the precipitated sorbent solid. Likewise, by selecting suitable dissolved components in carrier solutions, sorbent solids may be placed ~n a subterranean reservoir to trap barium, arsenates, selenites or fluorides, as desired and necessary.

In the second embodiment, the sorbent solid is directly introduced into the reservoir matrix as a solid component of a slurry. For this purpose, the sorbentsolid and/or particulate solids to which the sorbent solid has been either addedor affL~ced are combined with an appropriate injection flu~d, and subsequently introduced into the reservoir via the production well itself or one or more injection wells, using conventional high-pressure injection, gravel packing, or "hydraulic fracturing" techniques. The particulate solids to which the sorbent solid may be mixed or grafted include sand, proppants, galss beads, ceramic beads, and the like that are commonly used in gravel-packing and hydraulic fracturing operations to prop open subterranean fractures. Hydraulic fracturing processes produce secondary porosity features (e.g., fractures, fissures, partings, etc.) in the reservoir zone surrounding the well are expanded and partially infilled with the injected solids or "proppan~s", which serve to preserve the induced fracture structure following relief of injection pressure. Following such fracture treatment, the excess injection fluid is removed from the reservoir.
However, the sorbent solid remains in place in the fracture zone, to serve as anin-situ filter or t~ap for dissolved radionuclides contained in reservoir fluidsflowing toward the production well. This filtration or trapping effect is due toion exchange, adsorption, chelation, chemisorption, or coprecipitation of the radionuclides onto the sorbent solid.

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ln this embodiment of the invention, the sorbent solid can also be placed around the wellbore using well gravel-packing procedures. For this method, the ~ -sorbent solid is affixed to the gravel-pack material and subsequently placed 2 1 ~ ~ i t, 7 ~ l WO 93/21 1 12 PCI /US93/0~4 within and around the perforated or screened sections of the well using conventional inside-casing or open borehole placement techniques.

The placement of the sorbent solid in the reserYoir by injection with~ a slurry may be used to reduce radionuclide, barium, arsenate, seleni~e, fluoride, or other trace contaminant concentrations in produced reservoir fluids.

The invention is applicable to a broad variety of reservoir conditions and well operating schemes. In-situ precipitation of the sorbent solid can be best applied within reservoirs containing fresh to moderately saline groundwater, such as those which are cornmonly exploited for water supply purposes. Within oil or gas production reservoirs containing complex, highly reducing aqueous solutions, direct injection of the sorbent solid using hydraulic fracturing or gravel-packing tecluliques provides practical advantages. At a given well site/
the chemical composition, concentration, and dimension of the in-situ filter or trap can be adjusted. to achieve trace contaminant removal for an extended time - period. Following eventual saturation of the sorbent solid with radionuclides or other contaminants, the in-situ contaminant filter can be reformed by repeating the initial trea~nent process.
BRIEF DES~IPIION OF THE PRAWINGS

Figure 1 is a graph showing the filtration perforrnance of untreated sand in removing dissolved radionuclides from groundwater.
Figure 2 is a graph showing the radiom~clide filtration performance of sand treated by the invention method.

DESCRIPIION OP THE PRE~ERRED EMBODIM~
1. Gen~ ~ethQçl and ~licabilit~

The invention provides a means of reducing the concentration of radioactive materials or other trace contaminants in fluids produced from subterranean reservoirs (or "aquifers") by means of wells. Naturally-occurring radioactive 2133 ~(~7 ~
` WO 93t21112 PCr/lJS93/07495 materials (NORM) have been found to be present at concentrations exceeding health-based standards in groundwater produced l~y water supply wells, oil production wells, and gas production wells. In addition, radioactive materials have been artificially introduced into groundwater reservoirs due to~leaching orspills associated with radioactive waste disposal, testing, and mining activities, resulting in contamination of drinking water resources. Further, in some areas the ground water supplies are contaminated with unacceptably high leveis of fluoride, arsenate, selenite, or barium. To address these problems, the invention involves placement of "sorbent solids" within the reservoir matrix to act as an "in-situ filter or trap" for dissolved radionuclides or other contaminants present in the reservoir fluids. Radionuclides or other contaminants flowing past the in-situ filter are removed by means of ion exchange, adsorption, chelation, chemisorption, or coprecipitation onto the sorbent solids, resulting in a significant reduction in the contaminant content of the fluids produced from the treated well. Various sorbent solids have be~n identified which can be effectively deposited within the reservoir matrix and which serve as efficient scavengers for radium, uranium, or other contaminants.

For the purpose of the present discussion, the term "sorbent solid" is intended to comprehend a solid material that effects the removal of dissolved radionuclides or other dissolved contaminants from reservoir fluids by ion exchange, adsorption, chelation, coprecipitation, chemisorption, or other physiochemical mechanism. This effect is herein referred to as "in-situ filtration" or "trapping" of the dissolved contaminants within the reservoir matrix.

2. Detailed Description nf the Method To establish an in-situ filter for contaminant ions present within the reservoirfluids, ~e invention provides two altemate procedures for placement of a sorbent solid within the reservoir matrix surrounding a production well~ In the first embodiment of the invention process, the sorbent solid is precipitated within the reservoir matrix by the reaction of two or more carrier solutions Alternatively, the sorbent solid may be directly introduced into the reservoir 21~3707 WO 93/21112 PCr/US93/0249~

matrix as a solid component of a solid ancl liquid slurry. In a particular species of the second embodiment, the "sorbent solid" may be a functional group capable of removing trace contaminants (such as crown eithers, chelating agents,ion exchangers, and the like), bonded to a proppant or gravel-pack material, such as sand, that may be Lntroduced into the reservoir in a slurry to react with and remove radionuclides or other trace contaminants from fluids to be produced. Further description of these alternate embodiments, as well as the critical characteristics of the sorbent solids and injection fluids, is providedbelow.
First Embodiment~ 5itu Pre~pitation of Sorbent Solid For the purpose of in-situ precipitation of the sorbent solid, two or more carrier solutions are prepared contairing complementary components of the sorbent compound, which, upon mixing, result in precipitation of the sorbemt solid. ln a typical application, the first of these carrier solutions is introduced into the reservoir zone surro~mding the screened or perforated portion of the production well by either 1) directly injecting the solution down through the casing of the production well, using appropriate packers, injection tubing, and pumps, as needed, or 2) injecting the solution into the reservoir by means of one or more injection wells appropriately located with respect to the productionwell requiring trea~nent. An adequate volume of this first carrier solution is introduced to permeate the desired portion of the reservoir matrix surrounding the production well. Following such injection, the injection equipment and ~5 piping is purged using clean water, and the second (and subsequent) carrier solution(s) is ~are~ then injected in a sufficient quantity to achieve precipitation of the sorbent solid over the portion of the reservoir matrix permeated by the first carrier solution. Following a time period sufficient for precipitation and/or crystallization of the sorbent solid within the reservoir matrix, any excess carrier solutions that may remain within the reservoir are removed by pumping of the production well. This trea~nent cycle may be repeated as needed to establish thedesired mass and distribution o~ the sorbent solid within the reservoir matrix or to extend the functional life of the in-situ filter medium.

2 ~ 3 3 7 ~ 7 .~` :
WO 93/21112 PCr/US93/0249 Alternative Embodiment: Direct lnjection of Sorbent Sulid In an alternate approach, the sorbent solid is directly introduced into the reservoLr matrix as a solid component of a slurry. For ~is purpose, t~e sorbent solid is either 1) prepared as sand-sized or smaller particles, 2) affixed to orintermixed with particles of sand, ceramics, or other such proppant materials appropriate to hydraulic fracturing applica~ions, or 3) chemicaliy bonded to such proppant materials to functionalize the proppant material surface for removal of dissolved radionuclides or other trace contaminants from the reservoir fluids. The prepared solids are subse~uently combined with an injection fluid in appropriate proportions and by such means as to provide a slurry suitable forinjection into the reservoir using conventional hydraulic fracturing techniques.In a typical application, the sorbent solid is introduced into the reservoir zone surrounding the screened, perforated, or unc~secl portion of the production wellby injecting the slurry down through the casing of the production well using appropriate packers, injection tubing, high-pressure pumps, ancl ancillary equipment.
The hydraulic fracturing process involves application of an injection pressure sufficient to enhance the secondary porosity features (e.g., fractures, fissures, partings, etc.) of the reservoir matrix surrounding the well. The injected proppant solids partially infill these induced fractures, serving to maintain anenhanced secondary porosity within the reservoir matrix following relief of injection pressures. In the current application, sufficient injection pressure is applied and an adequate volume of the slurry introduced so as to permeate the desired portion of the reservoir matrix surrounding the production well.

Following such fracture treatment, excess injection fluid is removed from the reservoir; however, the sorbent solid remains in place within the induced fracture zone, comprising an in-situ filter or trap for dissolved radionuclides or other trace contaminants contained in reservoir fluids flowing toward the well.
This treatInent cycle may be repeated as needed to establish t,he desired mass and distribution of the sorbent solid within the reservoir matrix or extend the functional life of the in-situ filter medium.
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Direct Injection of the sorbent solid can also be achieved using well gravel-packing procedures. For this purpose, the sorbent solid is affixed to, intermixed with, or chemically bonded to appropriate gravel-pack materials, and the prepared solids subsequently combined with a gravel-packing " fluid in appropriate proportions and by such means as to provide a slurry suitable for :
gravel-pack placement within and around the perforated or screened section of the well using conventional inside-casing or open borehole piacement techniques.

Charactenstics of Sorbent Solid Sorbent solids utilized by either embodiment of this invention may be any compound or combination of compounds that serve as an efficient scavenger for dissolved radionuclides or other trace contarninants, either by ion exchange, chelation, adsorption, chemisorption, or coprecipitation. Further, it is desirable that the scavenger may be precipitated as a stable solid from aqueous solutions containing dissolved components of the compound in a subterranean environrnent. Alternatively, the scavenging agent must be capable of direct injection into a subterranean environment, either as a stable solid component of a solid-liquid slurry or in a form wherein it is chemically attached to the surface of proppant or gravel-pack material to functionalize the surface of thatmaterial for contaminant removal.

Sorbent solids that may be employed for removal of dissolved radionuclides ~-include but are not limited to manganese dioxide, barium sulfate, barium carbonate, iron oxide, iron hydroxide, al~ninum hydroxide, fine-grained ion-exchange resins, carbonaceous adsorbents, functionalized and non-functionalized polymeric adsorbents, zeolites, and activated carbon, as well as phosphates of zirconiusn and calcium, and oxides of zirconium, titanium, antimony, and tin. Functional groups that exhibit a high affinity for the contaminant(s) to be removed may be bonded to the surface of proppant or gravel pack particulates (e!g., sand, and the like) for injection into a reservoir.
The sorbent solid deposited within the reservoir matrix surrounding a given production well may be composed of one or more such compounds, as required to remove the principal radionuclides present within the local reservoir fluids. ,`
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21~37~)7 W{) 93/21 1 12 PCr/US93/02495 :
.. . ", These same sorbent solids may be employed for removal of non-radioactive trace contaminants, such as use of manganese dioxide for removal of dissolved barium and use of either iron (III) hydroxide or aluminum (III) hydroxide for rems)val of dissolved arsena~e, fluoride, or selenite from reseryoir fluids.
Sorbent solids such as carbonaceous adsorbents, polymeric adsorbents, and activated carbon can be ernployed for removal of organic contaminants.

To facilitate in-sihl precipitation of the sorbent solid (as described by the first embodiment above), carrier solutions must be devised to segregate two or more water-soluble components of the sorbent compound, which, upon mixing, precipitate the eompound. Such solutions can be readily devised by persons of ordinary skill in the art. Carrier solutions which have been demonstrated for this purpose include: barium chloride and sodium sulfate solutions for deposition of barium sulfate; or potassium permanganate and manganese sulfate solutions for deposition of manganese dioxide. Acidic solutions of aluminum (m) chloride or aluminum (m) sulfate can be contacted by alkaline solutions of sodium, potassium, or calcium hydroxide for deposition of ~ `
aluminum (III) hydroxide. Similarly, ac~dic solutions of iron (III) chloride or iron (III) sulfate can be contacted by aL~caline solutions of sodium, potassium, or calcium hydroxide for deposition of iron (m) hydroxide.

In general, given the trace-level mass concentrations of dissolved radionuclidesor other such contaminants commonly present in reservoir fluid, a relatively small mass of sorbent solid dispersed within the reservoir matrix will suffice to achieve a significant reduction in the concentration of radioactive materials orother trace contaminants in fluids flowing through the treated reservoir zone.
The composition of the carrier solutions (first embodiment) or solid-liquid slurries (alternative embodiment) must be established on a case-by-case basis todeposit an appropriate mass of sorbent solid within the reservoir matrix, as maybe determined by persons of ordinary skill in the art.

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213370~

Characteristics of Functionali~ed Paclcs A sorbent solid rnay be a functional group bonded to a proppant or gravel-pack material. These functional groups include chelating agents, ion exchangers, and the like. The crown ethers are one of the preferred chelating agents for metals removal, and the mode of functionali~ing with this compound is explaLned herebelow.

Crown ethers belong to a large and diverse class of organic compounds characterized by a central ring of ether molecules bound together. Attached to this ring may be one or more functional groups, e.g, a benzene ring or an amine group, or the like. Crown ethers may be synthesized to allow for the incorporation of particular cationic or anionic species into the center of the ring.
In this way, crown ethers bound to a gravel pack material such as silicate sand or synthesized silica particles may be used to remove cationic or anionic radionuclides or other contaminates from subterranean fluids.

The process for attaching a crown ether to silica particles involves (1) activating the silica; ~2) attachirlg a spacer compound which comprises an anchoring ~sroupand a bridge to the silica; and (3) attaching a crown ether to the spacer compound. Ideally, the modified crown ether sand will have the following characteristics: a high capacity for the attac}unent of crown ether molecules;
high chemical and physical stability; good ion separation; and rapid reaction kinetics.
The surface of silica contains both silanol groups and siloxane bridges. The hydroxyl groups which are characteristic of the silanols enable the attachment of a spacer group to the sand. Activation of the sand surface increases the number of silanol groups, and consequently the number of available hydroxyl groups for attaching to the spacer.

In order to increase the accessi~ility of the crown ether for removing ions of concern, the own ether is attached to a spacer which is in turn attached to thesilica sand, or other packing. The opt~mum length of the spacer group is the equivalent length of about six carbon atoms. Organotriethoxysilane has been ' 12 '`

~` W O 93/21112 213370~ PCT/US93/02495 1`';

used with success as an anchoring group to connect the hydroxyl groups on sand to a spacer.

A method for attaching benz~18-crowIl-6 to silica sand is shown il~ Lauth, M.
and Grama~n, P. "(Ben2:o-18-Crown-6)-Modified Silicas: Comparative Synthesis and Cation E~inding Properties, Reactive Polvmers an~I~n Exchan~e Sorbents", Volume 4, pp. 25'~-~67 (November 1986), which is hereby fully incoIporated by reference.

Crown ethers represent only one example of f~ctional groups, such as chelating agents, ion exchangers, and the like, which can be bonded to the surface of a sand particle or other injected solid for the purpose of establishing an in-situ contaminant filter medium. Examples of sisnple ion exchangers include sulfonic acid, carboxylic acid, tertiary amines, polyamines, quaternary amines, and the like. Useful chelating groups, other than crown ethers, include aminodiacetic acid, aminophosphoric acid, polyethyleneimines, carbamates, polyisothiouronium, polybenzisnidazole, polycarboxylic acids, and the like. Por ease of nomenclature, these functional groups are referred to as "functional groups capable of removirg trace contaminants." Methods of attaching any of -`
these functional groups to sand particles or other injected solids will ~e found by one of skill in the art without unreasonable experimentation.

Characteristics of Injection Fluid The fluid used for preparation of carrier solutions (first embodiment) or solid-liquid injection slurries (alternative embodiment) may be any liquid which facilitates the deposition of the sorbent solid within the reservoir matrix without adversely affecting fluids to be produced from the well. Most commonly, fresh water, saline water, or brines available at the site of the production well will be employed for this purpose. For the first embodiment, the injection fluid must be capable of dissolving the components of the sorbent compound~ at the cdncentrations required for precipitation of the necessary mass of sorbent solid within the reservoir. For the second embodiment, the injection fluid may be augmented with various additives serving to enhance ~ `
the hydraulic fracturing or gravel-packing processes~ .

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2 1 3 3 5 ~ 7 WO 93/21 1 12 PCr/U~93/0249'- --, ., ~elevant Reservoir Properties The optimal procedure for deposition of the sorbent solid within the reservoir will depend upon the chemical and physical characteristics of the reservoir porefluids and solid matrix. In-situ precipitation of the sorbent solid (first embodiment) can best be applied within reservoirs containing fresh to moderately saline groundwater, such as those which are commonly exploited for water supply purposes. Within oil or gas production reservr irs containing complex, highly reducing aqueous solutions and/or multiphase fluids, direct injection of the sorbent solids (second embodirnent) using hydraulic fractur~g techniques can provide practical advantages in terms of the distrib1ltion and stability of the sorbent solid within the reservoir matrix.
Suhterranean reservoirs or "aquifers" exploited for production of water, oil, orgas often consist of sand or sandstone deposits with sufficient effective porosity to store and transmit fluids or gasses. ln such reservoirs, the reservoir solid matrix is composed prirnarily of silica sand particles intermixed with varying concentrations of silt, clay, and other minerals. The sorbent solids deposited within such reservoirs by the in-situ precipitation method of the first embodiment are most likely retained by adsorption to the clay mineral fraction of reservoir matrix, as well as direct adsorption to the surface of the sand particles. In reseNoirs consisting of fractured granite, basalt, dolomite, limestone, or other rocks, the carrier solutions can be expected to permeate thefracture network of the reservoir, resulting in direct precipitation and adsorption of the sorbent solids onto the rock surface or onto minerals previously deposited within the fracture network.
Sorbent solids introduced by means of direct injection are retained within the existing and induced fracture network of such reservoirs as a component of the solid proppant media deposited during the hydraulic fracturing process. Sorbent solids placed as a component of gravel pack materials occupy the space within orinunediately surrounding the perforated or screened portion of the well casing.

It is important to note that, due to the low mass concentrations of radionuclides conunonly present in reservoir pore fluids and the high sorbent capacity of the 2 1 3 3 ~ ~3 ~ ~
- WO 93/21 1 12 PCr/US93/02495 ..

selected in-situ filter materials, a relatively small mass of sorbent solids dispersed within the reservoir pore space will suffice to achieve a significant reduction in the radionuclide content of fluids flowing through the treated reservoir zone. For example, manganese dioxide concentrations of 1~ to ~ g/kg within the reservoir matrv( will comprise an effective in-situ filter for dissolved radinnuclides. At such minor concentrations, the deposited sorbent solids do not measurably alter the effective porosity of the reservoir matrix or significantly diminish the hydraulic yield oÇ the production well.
The following examples illustrate the invention but do not define the limits of the invention as Aisclosed above and claimed herebelow.

Example of Metho~ Application For a 1 ft diameter water production well with a 100 ft length of screened section, in-situ precipitation of a manganese dioxide filter medium would involve several steps. First, a carrier solution having an 18 g/l concentration of manganese sulfate dissolved in water would be injected into the well casing in amarner causing the solution to flow out through the well screen and into the surrounding reservoir. A fluid volume of approximately 360,000 gallons would be injected into the reservoir in order to permeate the reservoir within a 23-ftradius surrounding the well bore over the 100 ft length of the screened section.Fresh water would then be pumped through the injection piping to flush the system of the ca~Tier solution and minimize subsequent precipitation of sorbent solids within the injection piping or wellbore. Next, a 360,000-gallon volume ofthe second carrier solution, consisting of a 12.6 g/l solution of potassium permanganate dissolved in water, would be injected through the well casing to contact the first carrier solution and result in the precipitation of solid manganese dioxide at a bulk concentration of approximately 2 g/kg within the reservoir matrix over a 23 ft radius surrounding the well. If necessary, the efficient mixing of the carrier solutions and resultant deposition of the in-situ filter medium can be enhanced by introducing the carrier solutions in a cyclic 3' manner, alternating small quantities of the two solutions each followed by a fresh water flush of the injection lines, until a sufficient fluid quantity has been injected to treat the reservoir over the desired radial distance surrounding the $
production well.

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In a sand reservoir in which the pore fluids contained a dissolved radium ion content of 25 pCi/l, deposition of manganese dioxide, as described in the above example, would provide an in-situ filter capable of removing radlum from billions of gallons of water produced from the well. Following saturation of thesorbent solid with radium ions or other radionuclides, the in-situ filter could be readily reformed by repeating the initial treatment process.

3. Demonstration of the Method The effectiveness of the method provided by the present invention has been demonstrated by laboratory procedures that simulate the environrnent of a subterranean reservoir or "aguifer." Por this purpose, glass columns were paclced with a natural reservoir material, such as sand; treated to deposit an appropri~te sorbent solid within this reservoir matnx by means of in-situ precipitation; and subsequently contacted with water containing dissolved radioactive materials to test the performance of the in-si~u filter. A summary of the procedures and results of this laboratory demorlstration is providecl below.
For the purpose of this laboratory program, groundwater with an average 226Ra (radium~ concentration of 23 pCi/L was obtained from an out-of-service water supply well located in the Chimney Hill subdivision in Houston, Texas. A
series of soil column studies was conducted to evaluate the performance of a sorbent-treated reservoir sand as an in-situ filter of dissolved radium from these reservoir pore waters. A glass column was loaded with a natural silty sand soil in accordance with procedures specified under ASTM Method No. D2434, treated to deposit a radionuclia~sorbent material (i.e., manganese dioxide) by means of in-situ precipitation, and subsequently rinsed with groundwater containing naturally-occurring radium. As an experimental control, a second glass column was loaded with the same sand soil but was not treated by the invention me~od prior to filtration of groundwater.

The soil columns employed in this demonst~ation consisted of glass columns approximately 1 inch in diameter and 2 feet in length and loaded with , I
approximately 300 ml of sand. After loading, the sand-filled columns were wO 93/~11 12 2 1 3 3 7 0 7 PCI /US93/02495 placed under a vacuurn and backwashed with approxirnately 3 bed volumes of deionized water to evenly saturate t~e sand matrix, remove air bubbles, and remove loose soil fines. One of the soil columns was retained as a control whilemanganese dioxide was deposited in the other column. In order to deposit the manganese dioxide, the column was alternately rinsed with 30 ml of 0.12 molar concentration manganese sulfate, followed by 30 ml of deionized water, followed by 30 ml of 0.08 molar concentration potassium permanganate. After 500 ml of each of manganese sulfate and potassium permanganate had been washed through the column at an average flow rate of about 30 ml per minute, the sequence was stopped. The reactants produced a clearly-visible black manganese dioxide precipitate on the sand.

A sample of Chimney Hill groundwater was then passed through the treated sand filter at a fluid velocity of 1.5 cm/sec while effluent samples were collected every 2 hours for the first 10 hours, every 12 hours through day 2, daily thereafter through day 7, and every alternate day until termination of the 4~daytest period. Similarly, groundwater was passed through the untreated column and effluent samples were collected for analysis for a period of 3 days.

Column effluent fluids were analyzed to compare the radium removal efficiencies achieved by the treated sand column to that achieved by the untreated sand. For this purpose, dissolved 226Ra concentrations in the groundwater effluent samples were analyzed in accordance with EPA Method No. 900.1, using a scintillator detection system.
The concentrations Of 226Ra measured in groundwater effluent (Ce) from the untreated and treated sand columns are plotted on Figures 1 and 2, respectively,as a proportion of initial concentration (Ci). As shown, the 226Ra concentrationin groundwater effluent from the untreated column reached a value equal to approximately 75% of the influent radium concentration after 1200 bed volurnes of the radionuclide-containing groundwater had been passed through the column. In contTast, the radium concentration in the effluent of the MnO2-~reated column was still non-detectable after over 11,000 bed volumes (BV~ of groundwater had been passed through the column. For a production well, this 11,000 BV capacity of the MnO2-treated reservoir zone would provide radium ;

. . .. . . . ... . .. . . . ... ... ... .. .... . ..... . . . ... .

2~37l37 WO 93/21112 PCr/US93/0249~

removal from of a very large volume of water prior to radium breakthrough.
For example, in the case of a well with a 100 ft screen length treated over a 25 ft radius surrounding the well, the MnO2 -treated reservoir zone would reduce radium concentrations to non-detectable levels in over 4 billion gallons of produced fluids.

ln this test program, deposition of solid MnO2 within a reservoir matrix was shown to establish an effective filter for dissolved 226~a contained in groundwater passing through the treated reservoir zone.
Although ~e invention has been described in terms of specified embodiments which are set forth in detail, it should be understood that this is by way of illustration only and that the invention is not necessarily limited thereto, since alternative embodiments and operating techniques will becorne a]pparent to those skilled in the art in view of the disclosure. Accordingly, modifications which can be made without departing from the spirit of the invention as described above are within the scope of the invention claimed herebelow.

Claims

WHAT IS CLAIMED:

1. A method for reducing trace contaminant content of fluids withdrawn from subterranean reservoirs, the method comprising:

a) introducing a first carrier solution, comprising a dissolved component of a solid scavenger for trace contaminants found in the reservoir fluids, into the reservoir in the area surrounding a production well, the quantity and concentration of the carrier solution being sufficient to ensure deposition of a mass of sorbent solid sufficient to reduce the dissolved trace contaminant content of withdrawn fluids, over a predetermined area when ions are precipitated from the carrier solution;

b) introducing a second carrier solution, comprising a complementary component of the solid scavenger for trace contaminants, into the reservoir area to commingle with ions of the first carrier solution;

c) reacting the dissolved component of the first carrier solution and the complementary component of the second carrier solution to precipitate a solid scavenger able to remove trace contaminants from fluids in the reservoir, when the fluids contact the precipitated solvent; and d) allowing the solid scavenger to contact fluids containing dissolved trace contaminants in the reservoir to thereby remove these contaminants from the fluids.

2. The method of claim 1, wherein the reservoir fluids are selected from the group consisting of fresh water, saline water, and hydrocarbons.

3. The method of claim 1, wherein the reservoir fluids comprise brine fluids in a geothermal reservoir.

4. The method of claim 1, wherein the introducing of carrier solutions is via a production well.

5. The method of claim 1, wherein the introducing of carrier solutions is through at least one injection well.

6. The method of claim 1, wherein the trace contaminants are selected from the group consisting of barium, arsenates, fluorides, selenites, and trace organic compounds.

7. The method of claim 6, wherein the solid scavenger for removal of trace contaminants produced by the reacting step is manganese dioxide when the contaminant is barium, and is selected from the group consisting of iron hydroxide and aluminum (m) hydroxide when the contaminant is arsenite, fluoride, or selenite.

8. A method for reducing the dissolved trace contaminant content of fluids withdrawn from subterranean reservoirs, said method comprising:

(a) injecting into a reservoir at an injection point a particulate solid material comprising a functional group capable of removing trace contaminants grafted to the surfaces of the particulate solid material;

(b) pressuring the injected particulate solid material to distribute the material in subterranean fractures over a predetermined area surrounding the injection point; and (c) contacting the injected solid particulate material with fluids to be withdrawn from the reservoir zone containing dissolved trace contaminants to remove the dissolved trace contaminants from the fluids.

9. The method of claim 8, wherein the functional group capable of removing trace contaminants is grafted to the surface of a gravel-packing material by a process comprising the steps of: activating the surface of the gravel-pack material; bonding a coupling agent to the acid-treated gravel-pack surfaces;
and grafting a functional group capable of removing trace contaminants to the coupling agent on the gravel-pack material.

10. The method of claim 9, wherein the trace contaminant is a dissolved radionuclide.

11. The method of claim 8, wherein the functional group capable of removing trace contaminants is a crown ether grafted to a proppant material used to prop open subterranean fractures produced by subterranean hydrofracturing.

12. The method of claim 8, wherein the functional group capable of removing trace contaminants is selected from the group consisting of chelating agents and ion exchangers.

13. A method for reducing trace contaminant content of fluids withdrawn from subterranean reservoirs, the method comprising:

(a) introducing into the subterranean reservoir a particulate solid material comprising a sorbent solid, in a sufficient quantity to ensure deposition of a mass of sorbent solids sufficient to reduce the dissolved trace contaminant content of withdrawn fluids, over a predetermined area within the reservoir; and (b) allowing ions or molecules of the trace contaminants in subterranean fluids in the reservoir to react with the sorbent solid thereby removing the trace contaminants from the fluids.

14. The method of claim 13, wherein the trace contaminant is selected from the group consisting of radionuclides, barium, arsenates, fluorides, and selenites.

15. The method of claim 14, wherein the sorbent solid is selected from the group consisting of manganese dioxide, iron (m) hydroxide, and aluminum (m) hydroxide, zeolites, polymeric adsorbents, carbonaceous adsorbents, fine-grained ion-exchange resins, and activated carbon.

WO 93/21112 PCT/US93/0249?

16. The method of claim 13, wherein the particulate solid material is selected from the group consisting of silica sand, gravel, ceramic beads, glass beads, and proppants.

17. The method of claim 13, wherein the sorbent solid is selected from the group consisting of: barium sulfate, barium carbonate, iron oxide, phosphates of zirconium and calcium, and oxides of zirconium, titanium, antimony, and tin.