Abstract:
A sand control screen assembly ( 100 ) positionable within a wellbore ( 26 ) for filtering particulate matter out of production fluids. The sand control screen assembly ( 100 ) includes a perforated base pipe ( 102 ) that allows fluid flow therethrough and a filter medium ( 124 ) disposed exteriorly about the base pipe ( 102 ). A surface-modified agent is disposed on at least a portion of the filter medium ( 124 ). The surface-modified agent is operable to enhance the surface filter characteristics of the filter medium ( 124 ), thereby reducing particulate infiltration into the filter medium ( 124 ).

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    This invention relates, in general, to a sand control screen assembly used in a wellbore that traverse a subterranean hydrocarbon bearing formation and, in particular, to a sand control screen assembly having a surface-modified filter medium and method for making same. 
       BACKGROUND OF THE INVENTION 
       [0002]    Without limiting the scope of the present invention, its background will be described in relation to sand control screen assemblies operating in a wellbore that traverses a subterranean hydrocarbon bearing formation, as an example. 
         [0003]    Since the beginning of oil production from subsurface formations, the industry has been concerned with efficient control of the movement of unconsolidated formation particles, such as sand, into the wellbore. For example, such particle movement commonly occurs during production from completions in loose sandstone or following hydraulic fracture of a formation. Production of these materials causes numerous problems in the operation of oil, gas or water wells. These problems include plugging of formations, tubing and subsurface flow lines, as well as erosion of tubing, downhole equipment and surface equipment. These problems lead to high maintenance costs and undesirable downtime of wells. Accordingly, numerous methods have been attempted to control the movement of unconsolidated particles during the production of fluids. 
         [0004]    One such method is the use of prepacked sand control screen assemblies. Generally, an inner wire screen is affixed to a plurality of ribs that extend longitudinally along the outer surface of a perforated base pipe. An outer wire screen is disposed about the inner screen to create an annular space between the two wire screens into which a resin coated sand may be confined. These prepacked sand control screens may be constructed by first fabricating the metal components, then forcing the pack sand into the desired location such as between the inner wire screen and the outer wire screen of a multi-layer screen as discussed above. 
         [0005]    Prepacked sand control screen assemblies may operate differently based on their design. For example, some prepacked sand control screen assemblies operate substantially as a depth filter. Depth filters typically utilize a filtering medium having a relatively significant thickness that provides for filtering throughout the depth of the filtering medium. In general, larger particles may be trapped at the surface layer, while finer particles are trapped by succeeding inner layers. 
         [0006]    It has been found, however, that depth type prepacked sand control screen assemblies are prone to plugging, especially where a significant amount of fines exists in an unconsolidated formation. As a prepacked sand control screen assembly becomes plugged, production therethrough becomes less efficient. In addition, depth type prepacked sand control screen assemblies are difficult to clean. 
         [0007]    In contrast, surface type filter systems are easily cleaned because the trapped particles may simply be washed off the surface of the assembly. For example, certain sand control screen assemblies that are sometimes referred to as premium screens, utilize a wire mesh or woven mesh type filter medium that operates substantially as a surface filter. Unconsolidated formation particles that are trapped on the outer surface of this type of filtration medium may then be cleaned off by either backwashing or injecting cleaning fluids on the filtering surface. 
         [0008]    It has been found, however, that premium screens may suffer from erosion problems, such as pin hole erosion, created by fine particles or streams of fine particles that travel through the wire mesh layers of these screens. Once a pin hole is formed, there is generally no additional filtering medium inward of the penetrated wire mesh or woven mesh, thus fine particles may go through the pin hole unabated. This fine stream of particles may then further erode the filter medium resulting in a large diameter opening. 
         [0009]    Accordingly, a need has arisen for a sand control screen assembly that is capable of filtering fines out of a production stream from a subterranean hydrocarbon bearing formation. A need has also arisen for such a sand control screen assembly that resists plugging and is operable to be cleaned to remove particle buildup. Further, a need has arisen for such a sand control screen assembly that does not suffer from pin hole erosion. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention disclosed herein is directed to an improved sand control screen assembly having a surface-modified filter medium that provides improved sand control in downhole operations. Specifically, the sand control screen assembly of the present invention is capable of filtering fines out of a production stream from a subterranean hydrocarbon bearing formation. In addition, the sand control screen assembly of the present invention resists plugging and is operable to be cleaned to remove particle buildup. Further, the sand control screen assembly of the present invention does not suffer from pin hole erosion. 
         [0011]    In one aspect, the present invention is directed to a sand control screen assembly that is positionable within a wellbore for filtering particulate matter out of production fluids. The sand control screen assembly includes a perforated base pipe that allows fluid flow therethrough and a filter medium disposed exteriorly about the base pipe. A surface-modified agent is disposed on at least a portion of the filter medium. The surface-modified agent is operable to enhance the surface filter characteristics of the filter medium, thereby reducing particulate infiltration into the filter medium. 
         [0012]    In one embodiment, the filter medium includes prepack sand or prepack gravel. In another embodiment, the filter medium includes fabric mesh and/or wire mesh screening elements. In one embodiment, the surface-modified agent may be an acrylic acid polymer, an acrylic acid ester polymer, an acrylic acid derivative polymer, an acrylic acid homopolymer, an acrylic acid ester homopolymer, an acrylamido-methyl-propane sulfonate polymer, an acrylamido-methyl-propane sulfonate derivative polymer, an acrylamido-methyl-propane sulfonate copolymer, an acrylic acid/acrylamido-methyl-propane sulfonate copolymer, copolymers thereof or mixtures thereof. In another embodiment, the surface-modified agent may be activated by an activator such as an organic acid, an anhydride of an organic acid, an inorganic acid, an inorganic salt, a charged surfactant, a charged polymer or combinations thereof. In yet another embodiment, the filter medium may be pretreated to improve the surface receptivity of the filter medium to the surface-modified agent. 
         [0013]    In another aspect, the present invention is directed to a sand control screen assembly that is positionable within a wellbore for filtering particulate matter out of production fluids. The sand control screen assembly includes a perforated base pipe that allows fluid flow therethrough. A first screen element is disposed exteriorly about the base pipe. A second screen element is disposed exteriorly of the first screen element forming a region therebetween. A prepack filter medium is disposed in the region between the first and second screen elements. A surface-modified agent is disposed on at least a portion of the filter medium. The surface-modified agent is operable to enhance the surface filter characteristics of the filter medium, thereby reducing particulate infiltration into the filter medium. 
         [0014]    In a further aspect, the present invention is directed to a method for making a sand control screen assembly. The method includes providing a base pipe having a plurality of perforation therethrough, positioning a filter medium exteriorly about the base pipe and applying a surface-modified agent to at least a portion of the filter medium, the surface-modified agent operable to enhance the surface filter characteristics of the filter medium, thereby reducing particulate infiltration into the filter medium. 
         [0015]    The method may also include activating the surface-modified agent with an activator selected from the group consisting of an organic acid, an anhydride of an organic acid, an inorganic acid, an inorganic salt, a charged surfactant, a charged polymer and combinations thereof, pretreating the filter medium with one of a cationic compound and a cationic polymer or pretreating the filter medium with one of an anionic compound and an anionic polymer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
           [0017]      FIG. 1  is a schematic illustration of an offshore platform operating a sand control screen assembly having a surface-modified filter medium according to an embodiment of the present invention; 
           [0018]      FIG. 2  is a side elevation view, partially cut away, of a sand control screen assembly having surface-modified prepack sand according to an embodiment of the present invention; 
           [0019]      FIG. 3  is a cross sectional view of the sand control screen assembly having surface-modified prepack sand of  FIG. 2 ; 
           [0020]      FIG. 4  is a side elevation view, partially cut away, of a sand control screen assembly having surface-modified prepack sand according to an embodiment of the present invention; 
           [0021]      FIG. 5  is a cross sectional view of the sand control screen assembly having surface-modified prepack sand of  FIG. 4 ; 
           [0022]      FIG. 6  is a side elevation view, partially cut away, of a sand control screen assembly having surface-modified prepack sand according to an embodiment of the present invention; 
           [0023]      FIG. 7  is a cross sectional view of the sand control screen assembly having surface-modified prepack sand of  FIG. 6 ; 
           [0024]      FIG. 8  is a side elevation view, partially cut away, of a sand control screen assembly having a surface-modified wire mesh screen according to an embodiment of the present invention; 
           [0025]      FIG. 9  is a cross sectional view of the sand control screen assembly having a surface-modified wire mesh screen of  FIG. 8 ; 
           [0026]      FIG. 10  is an enlarged photomicrographic view of surface-modified prepack sand according to an embodiment of the present invention; 
           [0027]      FIG. 11  is an enlarged photomicrographic view of surface-modified prepack gravel according to an embodiment of the present invention; 
           [0028]      FIG. 12  is an enlarged front view of a surface-modified fabric mesh according to an embodiment of the present invention; and 
           [0029]      FIG. 13  is an enlarged cross sectional view of a surface-modified wire mesh according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention. 
         [0031]    In the following description of the representative embodiments of the invention, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used for convenience in referring to the accompanying drawings. In general, “above”, “upper”, “upward” and similar terms refer to a direction toward the earth&#39;s surface along a wellbore, and “below”, “lower”, “downward” and similar terms refer to a direction away from the earth&#39;s surface along the wellbore. 
         [0032]    Referring to  FIG. 1 , a sand control screen assembly having a surface-modified filter medium in use with an offshore oil and gas production platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . Wellhead  18  is located on deck  20  of platform  12 . Well  22  extends through the sea  24  and penetrates the various earth strata including formation  14  to form wellbore  26 . Disposed within wellbore  26  is casing  28 . Disposed within casing  28  and extending from wellhead  18  is production tubing  30 . A pair of seal assemblies  32 ,  34  provides a seal between tubing  30  and casing  28  to prevent the flow of production fluids therebetween. During production, formation fluids enter wellbore  26  through perforations  36  of casing  28  and travel into tubing  30  to wellhead  18 . As part of the final bottom hole assembly, a sand control screen assembly having a surface-modified filter medium  38  is included within tubing  30 . Sand control screen  38  filters the particles out of the formation fluids as the formation fluids are produced. 
         [0033]    Even though  FIG. 1  depicts a sand control screen of the present invention in a cased hole environment, it should be understood by those skilled in the art that the sand control screens of the present invention are equally well suited for use in open hole environments. Also, even though  FIG. 1  depicts a single sand control screen of the present invention, it should be understood by those skilled in the art that any number of sand control screens of the present invention may be deployed within a production interval without departing from the principles of the present invention. In addition, even though  FIG. 1  depicts a sand control screen of the present invention in a vertical wellbore, it should be understood by those skilled in the art that the sand control screens of the present invention are equally well suited for use in well having other directional configurations including horizontal wells, deviated wellbores, slanted wells, multilateral well and the like. 
         [0034]    Referring to  FIGS. 2-3 , one embodiment of a sand control screen assembly having a surface-modified filter medium is depicted and generally designated  100 . Sand control screen assembly  100  includes a base pipe  102  having a plurality of openings  104 . Sand control screen assembly  100  also includes a plurality of ribs  106  that are substantially symmetrically disposed or positioned about the axis of base pipe  102 . A slotted screen  108  including a plurality of slotted openings  110  is wrapped around ribs  106 . Substantially symmetrically disposed or positioned about slotted screen  108  is a plurality of ribs  112 . Wrapped around ribs  112  is a screen wire  114  that forms a plurality of turns such as turns  118 ,  120 ,  122  having gaps therebetween. Disposed in the annular area between slotted screen  108  and screen wire  114  is a filter medium depicted as surface-modified prepack sand  124 . As described above, conventional prepacked sand control screens operate substantially as depth filters wherein larger particles may be trapped at the surface layer, while finer particles are trapped within the filter medium. As the fines begin to plug such screens, production efficiency is reduced. In addition, these depth type prepacked sand control screens are difficult to clean. The surface-modified agent of the present invention is operable to enhance the surface filter characteristics of filter medium  124  which reduces particulate infiltration into filter medium  124 , thus converting filter medium  124 , at least in part, from a depth filter to a surface filter. As such, formation particles that previously infiltrated prepacked filter media now build up on the outer surface filter medium  124  thus enabling cleaning of filter medium  124  by backwashing, chemical injection or the like. A further description of surface-modified prepack sand  124  is provided below with reference to  FIGS. 10-11 . Together, ribs  106 , slotted screen  108 , surface-modified prepack sand  124 , ribs  112 , and screen wire  114  form sand control screen jacket  116 . Sand control screen jacket  116  is attached to base pipe  102  at its upper end by weld  126  and its lower end by weld  128 . 
         [0035]    Referring next to  FIGS. 4-5 , another embodiment of a sand control screen assembly having a surface-modified filter medium is depicted and generally designated  200 . Sand control screen assembly  200  includes a base pipe  202  having a plurality of openings  204 . Sand control screen assembly  200  also includes a plurality of ribs  206  that are substantially symmetrically disposed or positioned about the axis of base pipe  202 . Wrapped around ribs  206  is a screen wire  208  that forms a plurality of turns such as turns  212 ,  214 ,  216  having gaps therebetween. Disposed in the annular area between base pipe  202  and screen wire  208  is a filter medium depicted as surface-modified prepack sand  218 . Use of the surface-modified agent on the pack sand enhances the surface filter characteristics of filter medium  218  which reduces particulate infiltration into filter medium  218 , thus converting filter medium  218 , at least in part, from a depth filter to a surface filter. A further description of surface-modified prepack sand  218  is provided below in reference to  FIGS. 10-11 . Together, ribs  206 , surface-modified prepack sand  218 , and screen wire  208  form sand control screen jacket  210 . Sand control screen jacket  210  is attached to base pipe  202  at its upper end by weld  220  and its lower end by weld  222 . 
         [0036]    Turning now to  FIGS. 6-7 , yet another embodiment of a sand control screen assembly having surface-modified filter medium is depicted and generally designated  300 . Sand control screen assembly  300  includes a base pipe  302  having a plurality of openings  304 . Sand control screen assembly  300  also includes a plurality of ribs  306  that are substantially symmetrically disposed or positioned about the axis of base pipe  302 . A slotted screen  308  including a plurality of slotted openings  310  is wrapped around ribs  306 . A shroud  312  including a plurality of openings  314  is positioned around slotted screen  308 . Disposed in the annular area between slotted screen  308  and shroud  312  is a filter medium depicted as surface-modified prepack sand  316 . Use of the surface-modified agent on the pack sand enhances the surface filter characteristics of filter medium  316  which reduces particulate infiltration into filter medium  316 , thus converting filter medium  316 , at least in part, from a depth filter to a surface filter. A further description of surface-modified prepack sand  316  is provided below in reference to  FIGS. 10-11 . Together, ribs  306 , slotted screen  308 , surface-modified prepack sand  316 , and shroud  312  form sand control screen jacket  318 . Sand control screen jacket  318  is attached to base pipe  302  at its upper end by weld  320  and its lower end by weld  322 . 
         [0037]    Referring to  FIGS. 8-9 , still yet another embodiment of a sand control screen assembly having surface-modified filter medium is depicted and generally designated  400 . Sand control screen assembly  400  includes a base pipe  402  having a plurality of openings  404 . Sand control screen assembly  400  has a multilayer filtration medium including an inner mesh layer  406  having a plurality of openings  408 , an intermediate mesh layer  410  having a plurality of openings  412  and an outer mesh layer  414 . In one embodiment, mesh layer  414  may be a plain Dutch weave or a twilled Dutch weave wire mesh material. Sand control screen assembly  400  further includes a shroud  416  having a plurality of openings  418 . 
         [0038]    Generally, mesh layer  414  is intended to act as a flow diffuser rather than as a filter. In particular, in such a weave, the fill wires are separated by warp wires resulting in spaces and a wavy configuration, so that fluid that passes through openings  418  of shroud  416  is deflected by the wires of mesh layer  414  to prevent direct radial flow of fluid through sand control screen assembly  400 . The deflected fluid flow passes along the wires and then to the mesh layer  410  for filtration of sand and other particles therefrom. Together, mesh layer  406 , mesh layer  410 , mesh layer  414  and shroud  416  form sand control screen jacket  420 . Sand control screen jacket  420  is attached to base pipe  402  at its upper end by weld  422  and its lower end by weld  424 . Preferably, each of the mesh layers  406 ,  410 ,  414  is treated with the surface-modified agent of the present invention. Use of the surface-modified agent on the mesh layers  406 ,  410 ,  414  enhances the filtering characteristics thereof. 
         [0039]    Referring now to  FIG. 10 , an embodiment of a surface-modified filter medium is depicted and generally designated  500 . Surface-modified filter medium  500  includes prepack sand including sand particles  502  having a surface-modified agent  504  coated on at least a portion of sand particles  502 . Surface-modified filter medium  500  further includes matrix pores  506  created by the spaces between sand particles  502  and surface-modified agent  504 . It is noted that the edges of each sand particles  502  and the coating of surface-modified agent  504  may be irregular in shape, and that matrix pores  506 , may also be irregular in shape. 
         [0040]    Referring now to  FIG. 11 , an embodiment of a surface-modified filter medium is depicted and generally designated  520 . Surface-modified filter medium  520  includes prepack gravel including gravel particles  522  having a surface-modified agent  524  coated on at least a portion of gravel particles  522 . Surface-modified filter medium  520  further includes matrix pores  526  created by the spaces between gravel particles  522  and surface-modified agent  524 . It is noted that the edges of each gravel particles  522  and the coating of surface-modified agent  524  may be irregular in shape, and that matrix pores  526 , may also be irregular in shape. 
         [0041]    Even though two exemplary embodiments of surface-modified filter media having two substantially uniform particle sizes have been shown, those skilled in the art will understand that a surface-modified filter medium of the present invention could include particles having any desired size, ratio of sizes or combination sizes without departing from the principles of the present invention. Also, even though the surface-modified agent is depicted as forming a substantially uniform coating on the particle, those skilled in the art will understand that the surface-modified agent may alternatively, form a non-uniform coating on the particles that may fully or partially encapsulate the particles. In addition, it should be noted that the pack sand for prepacked sand control screen assemblies has typically included a consolidating resin that requires a heat treatment to cure. The use of a surface-modified agent of the present invention, however, may eliminate the need for consolidation resins, thus lowering manufacturing costs. 
         [0042]    Referring now to  FIG. 12 , an embodiment of a surface-modified filter medium is depicted and generally designated  540 . Surface-modified filter medium  540  includes surface modified fabric mesh material  542  having a surface-modified agent  544  coated on at least a portion thereof. Surface-modified filter medium  540  further includes matrix pores  546  created by the spaces between the fabric elements of surface-modified filter medium  540 . 
         [0043]    Referring now to  FIG. 13 , an embodiment of a surface-modified filter medium is depicted and generally designated  560 . Surface-modified filter medium  560  includes wire mesh  562  having a surface-modified agent  564  coated on at least a portion of wire mesh  562 . Surface-modified filter medium  560  further includes matrix pores  566  created by the spaces between wire mesh  562 . 
         [0044]    The surface-modified agent of the present invention generally create a surface that attracts and holds silica fines, silica clays, and fine particles produced during operation of any of sand control screen assembly having a surface-modified filter medium. In one instance, the surface-modified agent acts as an electrostatic filter by trapping particles at the surface, thus preventing them from proceeding to the depths of the filter medium of a particular sand control screen. This produces a sand control screen that operates as a surface type filter system, including all of the benefits of such systems, like ease of cleaning the surface of the filter while retaining conductivity through sand control screen assembly. 
         [0045]    In addition, the surface-modified agent of the present invention alters the pore throat size of matrix pores to prevent silica fines, silica clays, and fine particles from entering into the depths of sand control screen assembly. In certain embodiments, the surface-modified agent of the present invention is a polymer that has nodular, hook-like components distributed along the chemically stable polymer&#39;s backbone that attracts or creates attraction to fine particulates. 
         [0046]    Whenever two or more surface-modified agent molecules contact each other, the hook-like components tend to interlock. The forces supporting this interlock are generally weak Van der Waal forces, but these forces are additive, resulting in high cohesiveness. Suitable surface-modified agents are capable of forming at least a partial coating upon the surface of the filter medium. In certain embodiments, the surface-modified agents of the present invention are activated, for example, destabilized, coalesced or reacted, to transform the compound into a sticky, surface-modified agent at a desirable time. Such activation may occur before, during, or after the surface-modified agent is coated onto filter medium. This may be done during any part of the assembly and manufacturing processes of the sand control screen assembly. In some embodiments, a pretreatment may be applied to the surface of filter medium to prepare it to be coated with the surface-modified agent. 
         [0047]    In one embodiment, the surface-modified agent may be an aqueous based or activated molecule. Examples of aqueous surface-modified agents suitable for use in the present invention include, but are not limited to, acrylic acid polymers, acrylic acid ester polymers, acrylic acid derivative polymers, acrylic acid homopolymers, acrylic acid ester homopolymers, such as poly(methyl acrylate), poly(butyl acrylate), and poly(2-ethylhexyl acrylate), acrylic acid ester copolymers, methacrylic acid derivative polymers, methacrylic acid homopolymers, methacrylic acid ester homopolymers, such as poly(methyl methacrylate), poly(butyl methacrylate), and poly(2-ethylhexyl methacryate), acrylamido-methyl-propane sulfonate polymers, acrylamido-methyl-propane sulfonate derivative polymers, acrylamido-methyl-propane sulfonate copolymers, and acrylic acid/acrylamido-methyl-propane sulfonate copolymers and combinations thereof. 
         [0048]    As noted above, suitable aqueous surface-modified agents may be charged polymers that preferentially attach to the surfaces of the filter medium having an opposite charge therefrom. For instance, an aqueous surface-modified agent having a negative charge will preferentially attach to surfaces having a positive to neutral zeta potential and/or a hydrophobic surface. Similarly, using analogous chemistry, a positively charged aqueous surface-modified agent will preferentially attach to negative to neutral zeta potential and/or hydrophilic surfaces. In particular embodiments where the surface being treated lacks an adequately receptive surface, that is, the surface being treated lacks a charge substantially opposite of the chosen aqueous surface-modified agent, a pretreatment fluid may be used to make the surfaces of filter medium more receptive to the aqueous surface-modified agent. 
         [0049]    For example, pretreatments may include a cationic polymer to treat a surface with a negative zeta potential or an anionic pretreatment to treat a surface with a positive zeta potential. As will be understood by one skilled in the art, amphoteric and zwitterionic pretreatment fluids may also be used so long as the conditions they are exposed to during use are such that they display the desired charge. Suitable pretreatment fluids include charged fluids comprising a charged surfactant, a charged polymer or a combination thereof. As will be understood by one of skill in the art, with the benefit of this disclosure, the use of a pretreatment is optional and depends, at least in part, on the charge disparity or lack thereof between the chosen surface-modified agent and the surface of filter medium being treated. 
         [0050]    As mentioned above, an aqueous surface-modified agent may become more effective when contacted with an activator. Typically, the activator is an organic acid or an anhydride of an organic acid that is capable of hydrolyzing in water to create an organic acid, an inorganic acid, an inorganic salt, such as a brine, a charged surfactant, a charged polymer, or a combination thereof, but any substance that is capable of making the aqueous surface-modified agent insoluble in an aqueous solution may be used as an activator in accordance with the teachings of the present invention. The choice of an activator may vary, depending on factors including the composition of the aqueous surface-modified agent. An example of one activator suitable for use in the present invention is an acetic acid/acetic anhydride blend. Other acids, acids salts, anhydrides, and mixtures thereof are also suitable. Suitable salts include, but are not limited to, sodium chloride, potassium chloride, calcium chloride, and mixtures thereof. In another exemplary embodiment of the present invention, the concentration of salts or other activating compounds present in the formation water itself may be sufficient to activate an aqueous surface-modified agent. In such an embodiment it may not be necessary to add an external activator. 
         [0051]    Generally, when used, the activator is present in an amount in the range of from about 0.1% to about 40% by weight of the fluid volume, however, in some cases such as with brines, the activator may be in excess of the aqueous surface-modified agent. Nevertheless, any compound that will cause the activation of the aqueous surface-modified agent, e.g., causing the aqueous surface-modified agent to become insoluble, may be used and is considered within the teachings of the present invention, regardless of the concentration of activator necessary to trigger the activation of the surface-modified agent. 
         [0052]    The suitable activators are substantially the same as the suitable pretreatment fluids when used in suitable amounts and in a suitable time period. For example, where the same chemical or chemicals are used as a pretreatment fluid and as an activator, the pretreatment fluid may make up only from about 0.1% to about 5% of the volume of the total amount used. One skilled in the art will recognize that the pretreatment fluid is primarily used to prepare a surface to accept an aqueous surface-modified agent and, generally, will not be used in an amount sufficient to substantially activate the aqueous surface-modified agent. 
         [0053]    The surface-modified agent is preferably coated onto sand particles  502 , gravel particles  522 , fabric mesh  542  or wire mesh  562  prior to or during assembly of sand control screen. Alternatively, any of the sand control screen assemblies of the present invention may be manufactured with the desired filter medium and then treated or coated with the desired surface-modified agent. In these embodiments, the surface-modified agent may be applied or coated onto all or part of the filter medium by means commonly known to those skilled in the art include, but not limited to, spraying, coating, dipping, flowing, brushing, vapor deposition, electrolysis, vacuum deposition, calendering, roller coating, electrostatic spraying and the like. Further, once the surface-modified agent is applied or coated onto all or a portion of the filter medium, the surface-modified agent may be activated by contact with the activator agent. 
         [0054]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.