WO1993021112A1

WO1993021112A1 - Method of reducing the level of contaminant materials in produced subterranean reservoir fluids

Info

Publication number: WO1993021112A1
Application number: PCT/US1993/002495
Authority: WO
Inventors: Philip King; John A. Connor; Dennis A. Clifford
Original assignee: Groundwater Services, Inc.
Priority date: 1992-04-09
Filing date: 1993-03-16
Publication date: 1993-10-28
Also published as: AU669361B2; CA2133707A1; US5196124A; AU3923893A

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A method is provided for reducing the concentration of radioactive materials or other trace contaminants in fluids withdrawn from subterranean reservoirs.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

Title: METHOD OF REDUCING THE LEVEL OF CONTAMINANT

MATERIALS IN PRODUCED SUBTERRANEAN RESERVOIR FLUIDS

SPECIFICATION

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Serial No. 07/866,341 filed April 9, 1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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relates to methods for reducing the concentration of radioactive materials or other trace contaminants in fluids withdrawn from subterranean reservoirs or "aquifers" by means of wells.

2. Description 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. In addition, radioactive materials and other contaminants have been artificially introduced into groundwater reservoirs due to leaching or spills associated wit radioactive waste disposal, testing, and mining activities. Production of fluids containing such contaminants poses both health-related and environmenta problems. The invention provides a new method of reducing the production o such radioactive materials and other trace contaminants from wells. 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 limits have been established for human exposure to radium, radon, and other naturally- 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 invariably 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 removal of dissolved contaminants from produced potable groundwater: 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.4,804,498.

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 in 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-polymers, 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 treatment of production wells.

Patent No. 4,054,320, entitled "Method for the Removal of Radioactive Waste During u -Situ Leaching of Uranium" is directed to a leaching 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- 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 significant disadvantages relative to the present invention. First, due to the limited sorbent 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 the "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 lower levels o radionuclides of concern from water supply wells, oil production wells, or gas production wells.

What is yet needed are methods for treating produced fluids for the removal o radionuclides, including radium and uranium, that are simple, relatively inexpensive, and do not generate a radioactive waste product that poses disposal problems.

SUMMARY OF THE INVENTION

The present invention provides simple, inexpensive methods for removing o significantly reducing the concentration of radioactive materials or other trac contaminants in subterranean reservoir fluids prior to entry of the fluids int the production well. The invention methods retain these contaminants withi the subsurface by means of an in-situ contaminant "filter" or "trap" that i deposited within the reservoir matrix surrounding the well. Consequently, th contaminant material concentrations in the reservoir fluids produced at groun surface are significantly reduced, providing significant benefits in terms o reduced risk of human exposure or environmental contamination. Specifically for water supply wells, subsurface removal of radionuclides provides a useabl 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 on distribution piping and equipment and for mitigating environmental contamination associated with brine storage and disposal. In addition, the rate of radon gas emanation from produced waters is reduced due to the reduced concentration of dissolved radium present in the production fluid.

Conceptually, 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-situ treatment process which involves injecting the scavenging chemical into the subterranean reservoir for removal of contaminants.

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 in the reservoir (or aquifer) fluids. For this purpose, the sorbent solid is supplied in one of two embodiments; it 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 phase component of a solid-liquid slurry using high-pressure injection techniques.

ha the first embodiment, a carrier solution, containing a dissolved component of the 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, containing appropriate dissolved components to cause precipitation of the sorbent solid within the reservoir matrix. Following such precipitation, excess carrier fluids are removed from the reservoir. The precipitated sorbent solids remain in place in the reservoir matrix to serve as an in-situ filter or trap for dissolved radionuclides contained in the reservoir fluids flowing into the 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 in 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 sorbent solid and /or particulate. solids to which the sorbent solid has been either added or affixed are combined with an appropriate injection fluid, 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 "proppants", 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 an in-situ filter or trap for dissolved radionuclides contained in reservoir fluids flowing toward the production well. This filtration or trapping effect is due to ion exchange, adsorption, chelation, chemisorption, or coprecipitation of the radionuclides onto the sorbent solid.

In 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 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 reservoir by injection with a slurry may be used to reduce radionuclide, barium, arsenate, selenite, 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 sorberit solid can be best 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 reservoirs containing complex, highly reducing aqueous solutions, direct injection of the sorbent solid using hydraulic fracturing or gravel-packing techniques 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 treatment process.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing the filtration performance of untreated sand in removing dissolved radionuclides from groundwater.

Figure 2 is a graph showing the radionuclide filtration performance of sand treated by the invention method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. General Method and Applicability

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 materials (NORM) have been found to be present at concentrations exceedin health-based standards in groundwater produced by water supply wells, oi production wells, and gas production wells. In addition, radioactive material have been artificially introduced into groundwater reservoirs due to leaching o spills associated with radioactive waste disposal, testing, and mining activitie resulting in contamination of drinking water resources. Further, in some area the ground water supplies are contaminated with unacceptably high levels o fluoride, arsenate, selenite, or barium. To address these problems, th invention involves placement of "sorbent solids" within the reservoir matrix t act as an "in-situ filter or trap" for dissolved radionuclides or othe contaminants present in the reservoir fluids. Radionuclides or othe contaminants flowing past the in-situ filter are removed by means of io exchange, adsorption, chelation, chemisorption, or coprecipitation onto th sorbent solids, resulting in a significant reduction in the contaminant content o the fluids produced from the treated well. Various sorbent solids have bee identified which can be effectively deposited within the reservoir matrix an which serve as efficient scavengers for radium, uranium, or othe contaminants.

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

2. Detailed Description of the Method

To establish an in-situ filter for contaminant ions present within the reservoi fluids, the invention provides two alternate procedures for placement of sorbent solid within the reservoir matrix surrounding a production well. In th first embodiment of the invention process, the sorbent solid is precipitate within the reservoir matrix by the reaction of two or more carrier solution Alternatively, the sorbent solid may be directly introduced into the reservo matrix as a solid component of a solid and 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 introduced 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 provided below.

First Embodiment: In-Situ Precipitation of Sorbent Solid

For the purpose of in-situ precipitation of the sorbent solid, two or more carrier solutions are prepared containing complementary components of the sorbent compound, which, upon mixing, result in precipitation of the sorbent solid. In a typical application, the first of these carrier solutions is introduced into the reservoir zone surrounding 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 production well requiring treatment. 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 pipmg 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 treatment cycle may be repeated as needed to establish the desired mass and distribution of the sorbent solid within the reservoir matrix or to extend the functional life of the in-situ filter medium. Alternative Embodiment: Direct Injection of Sorbent Solid

In an alternate approach, the sorbent solid is directly introduced into the reservoir matrix as a solid component of a slurry. For this purpose, the sorbent solid is either 1) prepared as sand-sized or smaller particles, 2) affixed to or intermixed with particles of sand, ceramics, or other such proppant materials appropriate to hydrauUc fracturing apphcations, or 3) chemically 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 subsequently combined with an injection fluid in appropriate proportions and by such means as to provide a slurry suitable for injection into the reservoir using conventional hydrauUc fracturing techniques. In a typical application, the sorbent soUd is introduced into the reservoir zone surrounding the screened, perforated, or uncased portion of the production weU by injecting the slurry down through the casing of the production weU using appropriate packers, injection tubing, high-pressure pumps, and 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 partiaUy infill these induced fractures, serving to maintain an enhanced secondary porosity within the reservoir matrix following relief of injection pressures. In the current appUcation, 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 treatment cycle may be repeated as needed to establish the desired mass and distribution of the sorbent solid within the reservoir matrix or extend the functional life of the in-situ filter medium. Direct injection of the sorbent solid can also be achieved using well gravel- packing procedures. For this purpose, the sorbent soUd 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 placement techniques.

Characteristics of Sorbent Solid

Sorbent soUds 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 contaminants, either by ion exchange, chelation, adsorption, chemisorption, or coprecipitation. Further, it is desirable that the scavenger may be precipitated as a stable soUd from aqueous solutions containing dissolved components of the compound in a subterranean environment. Alternatively, the scavenging agent must be capable of direct injection into a subterranean environment, either as a stable soUd component of a soUd-Uquid slurry or in a form wherein it is chemicaUy attached to the surface of proppant or gravel-pack material to functionalize the surface of that material 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, aluminum hydroxide, fine-grained ion- exchange resins, carbonaceous adsorbents, functionalized and non- functionalized polymeric adsorbents, zeolites, and activated carbon, as well as phosphates of zirconium 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 Uke) for injection into a reservoir. The sorbent soUd 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 radionucUdes present within the local reservoir fluids. 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 fo removal of dissolved arsenate, fluoride, or selenite from reservoir fluids. Sorbent solids such as carbonaceous adsorbents, polymeric adsorbents, an activated carbon can be employed for removal of organic contaminants.

To faciUtate in-situ precipitation of the sorbent soUd (as described by the firs embodiment above), carrier solutions must be devised to segregate two or mor water-soluble components of the sorbent compound, which, upon mixing precipitate the compotmd. Such solutions can be readily devised by persons o ordinary skill in the art. Carrier solutions which have been demonstrated fo this purpose include: barium chloride and sodium sulfate solutions fo deposition of barium sulfate; or potassium permanganate and manganes sulfate solutions for deposition of manganese dioxide. Acidic solutions o aluminum (El) chloride or aluminum (III) sulfate can be contacted by alkalin solutions of sodium, potassium, or calcium hydroxide for deposition o aluminum (HI) hydroxide. Similarly, acidic solutions of iron (HI) chloride o iron (m) sulfate can be contacted by alkaline solutions of sodium, potassium, o calcium hydroxide for deposition of iron (HI) hydroxide.

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

Characteristics of Functionalized Packs

A sorbent soUd may be a functional group bonded to a proppant or gravel-pack material. These functional groups include chelating agents, ion exchangers, and the Uke. The crown ethers are one of the preferred chelating agents for metals removal, and the mode of functionalizing with this compound is explained 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 Uke. 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 radionucUdes or other contaminates from subterranean fluids.

The process for attaching a crown ether to siUca particles involves (1) activating the siUca; (2) attaching a spacer compound which comprises an anchoring group and 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 attachment of crown ether molecules; high chemical and physical stabiUty; good ion separation; and rapid reaction kinetics.

The surface of siUca 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 accessibiUty of the crown ether for removing ions of concern, the crown ether is attached to a spacer which is in turn attached to the siUca sand, or other packing. The optimum length of the spacer group is the equivalent length of about six carbon atoms. Organotriethoxysilane has been used with success as an anchoring group to connect the hydroxyl groups on san to a spacer.

A method for attaching benzo-18-crown-6 to silica sand is shown in Lauth, and Gramain, P. "(Benzo-18-Crown-6)-Modified Silicas: Comparative Synthesi and Cation Binding Properties, Reactive Polymers and Ion Exchange Sorbents Volume 4, pp. 257-267 (November 1986), which is hereby fully incorporated b reference.

Crown ethers represent only one example of functional groups, such a chelating agents, ion exchangers, and the like, which can be bonded to th surface of a sand particle or other injected soUd for the purpose of estabUshin an in-situ contaminant filter medium. Examples of simple ion exchanger include sulfonic acid, carboxylic acid, tertiary amines, polyamines, quaternar amines, and the Uke. Useful chelating groups, other than crown ethers, includ aminodiacetic acid, aminophosphoric acid, polyethyleneimines, carbamate polyisothiouronium, polybenzimidazole, polycarboxyUc acids, and the like. F ease of nomenclature, these functional groups are referred to as "function groups capable of removing trace contaminants." Methods of attaching any these functional groups to sand particles or other injected soUds will be found b one of skill in the art without unreasonable experimentation.

Characteristics of Injection Fluid

The fluid used for preparation of carrier solutions (first embodiment) or soli liquid injection slurries (alternative embodiment) may be any liquid whic facilitates the deposition of the sorbent solid within the reservoir matri without adversely affecting fluids to be produced from the well. Mo commonly, fresh water, saline water, or brines available at the site of t production well will be employed for this purpose. For the first embodimen the injection fluid must be capable of dissolving the components of the sorbe compounds at the concentrations required for precipitation of the necessar mass of sorbent soUd within the reservoir. For the second embodiment, t injection fluid may be augmented with various additives serving to enhan the hydraulic fracturing or gravel-packing processes. Relevant Reservoir Properties

The optimal procedure for deposition of the sorbent soUd within the reservoir will depend upon the chemical and physical characteristics of the reservoir pore fluids 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 reservoirs containing complex, highly reducing aqueous solutions and/or multiphase fluids, direct injection of the sorbent soUds (second embodiment) using hydrauUc fracturing techniques can provide practical advantages in terms of the distribution and stability of the sorbent soUd within the reservoir matrix.

Subterranean reservoirs or "aquifers" exploited for production of water, oil, or gas often consist of sand or sandstone deposits with sufficient effective porosity to store and transmit fluids or gasses. In such reservoirs, the reservoir solid matrix is composed primarily of siUca sand particles intermixed with varying concentrations of silt, clay, and other minerals. The sorbent soUds 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 weU as direct adsorption to the surface of the sand particles. In reservoirs consisting of fractured granite, basalt, dolomite, limestone, or other rocks, the carrier solutions can be expected to permeate the fracture 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 soUds introduced by means of direct injection are retained within the existing and induced fracture network of such reservoirs as a component of the soUd proppant media deposited during the hydrauUc fracturing process. Sorbent soUds placed as a component of gravel pack materials occupy the space within or immediately surrounding the perforated or screened portion of the weU casing.

It is important to note that, due to the low mass concentrations of radionucUdes commonly present in reservoir pore fluids and the high sorbent capacity of the 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 2 g/kg within the reservoir matrix will comprise an effective in-situ filter for dissolved radionuclides. At such minor concentrations, the deposited sorbent solids do not measurably alter the effective porosity of the reservoir matrix or significantly diminish the hydrauUc yield of the production weU. The following examples illustrate the invention but do not define the limits of the invention as disclosed above and claimed herebelow.

Example of Method Application

For a 1 ft diameter water, production weU 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/1 concentration of manganese sulfate dissolved in water would be injected into the well casing in a manner 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-ft radius 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 carrier solution and minimize subsequent precipitation of sorbent solids within the injection piping or wellbore. Next, a 360,000-gallon volume of the second carrier solution, consisting of a 12.6 g/1 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 cycUc 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 weU. In a sand reservoir in which the pore fluids contained a dissolved radium ion content of 25 pCi/1, deposition of manganese dioxide, as described in the above example, would provide an in-situ filter capable of removing radium from bilUons of gaUons of water produced from the weU. FoUowing saturation of the sorbent soUd 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 environment of a subterranean reservoir or "aquifer." For this purpose, glass columns were packed with a natural reservoir material, such as sand; treated to deposit an appropriate sorbent soUd within this reservoir matrix by means of in-situ precipitation; and subsequently contacted with water containing dissolved radioactive materials to test the performance of the in-situ filter. A summary of the procedures and results of this laboratory demonstration is provided below.

For the purpose of this laboratory program, groundwater with an average 22°Ra

(radium) concentration of 23 pCi/L was obtained from an out-of-service water supply weU located in the Chimney HiU 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 radionucUde-sorbent material (i.e., manganese dioxide) by means of in-situ precipitation, and subsequently rinsed with groundwater containing naturaUy-occurring radium. As an experimental control, a second glass column was loaded with the same sand soil but was not treated by the invention method prior to filtration of groundwater.

The soil columns employed in this demonstration consisted of glass columns approximately 1 inch in diameter and 2 feet in length and loaded with approximately 300 ml of sand. After loading, the sand-fiUed columns were placed under a vacuum and backwashed with approximately 3 bed volumes of deionized water to evenly saturate the sand matrix, remove air bubbles, and remove loose soU fines. One of the soil columns was retained as a control while manganese 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 HiU 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 45-day test period. SimUarly, groundwater was passed through the untreated column and effluent samples were coUected 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 ^^^Ra concentrations in the groundwater effluent samples were analyzed in accordance with EPA Method No. 900.1, using a scintillator detection system.

The concentrations of 226R₃ measured in groundwater effluent (C_e) from the untreated and treated sand columns are plotted on Figures 1 and 2, respectively, as a proportion of initial concentration (C_j). As shown, the 22^Ra concentration in groundwater effluent from the untreated column reached a value equal to approximately 75% of the influent radium concentration after 1200 bed volumes of the radionuclide-containing groundwater had been passed through the column. In contrast, the radium concentration in the effluent of the Mnθ2- treated column was stiU non-detectable after over 11,000 bed volumes (BV) of groundwater had been passed through the column. For a production weU, this 11,000 BV capacity of the Mnθ2-treated reservoir zone would provide radium removal from of a very large volume of water prior to radium breakthrough. For example, in the case of a weU with a 100 ft screen length treated over a 25 ft radius surrounding the weU, the Mnθ2 -treated reservoir zone would reduce radium concentrations to non-detectable levels in over 4 biUion gallons of produced fluids.

In this test program, deposition of solid Mnθ2 within a reservoir matrix was shown to establish an effective filter for dissolved 226R₃ contained in groundwater passing through the treated reservoir zone.

Although the 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 become apparent to those skiUed 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 weU.

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 soUd 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 Tfl) hydroxide and aluminum (IE) 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 soUd material comprising a functional group capable of removing trace contaminants grafted to the surfaces of the particulate soUd 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 dissolve radionuclide.

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

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

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

(a) introducing into the subterranean reservoir a particulate solid materia comprising a sorbent soUd, in a sufficient quantity to ensure depositio of a mass of sorbent solids sufficient to reduce the dissolved trac contaminant content of withdrawn fluids, over a predetermined are within the reservoir; and

(b) allowing ions or molecules of the trace contaminants in subterranea fluids in the reservoir to react with the sorbent soUd thereby removin the trace contaminants from the fluids.

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

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

16. The method of claim 13, wherein the particulate solid material is selected from the group consisting of siUca 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.