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BACKGROUND 
     Several types of screens are used downhole to filter produced fluids of formation particulates, such as sand. The screens can include wire-wrapped screens, metal-mesh screens, and expandable screens, among others. The screens can be used downhole in a number of completion systems to control sand. In a gravel pack operation, for example, gravel is placed in the annulus around the screen in an open hole. Alternatively, the screen can be run in a stand-alone application without a surrounding gravel pack in either a cased or an open hole. 
     A stand-alone screen can become plugged and/or may erode rapidly as formation sand and other produced particulates pass through the screen during production. When plugging or erosion occurs, operators need to take remedial steps to clean out and/or replace the screen, which can be time-consuming and costly. Plugging and erosion can be especially problematic when the stand-alone screen is run in a cased hole. For this reason, a stand-alone screen is only rarely run in a cased hole. Yet, being able to run a stand-alone screen in a cased hole may be beneficial in some circumstances and may also be beneficial when using screens in open hole applications. 
     The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     SUMMARY 
     A borehole completion method treats a formation surrounding a borehole with a chemical treatment. A standalone screen deploys downhole in the borehole (either before, during, or after the treatment) on a downhole string. Any suitable type of standalone screen can be used, including a wire screen, a mesh screen, a sintered metal screen, a perforated pipe, an expandable screen, a gravel pack screen, or a combination thereof. Typically, packers disposed on the string are used to isolate the screen to particular portions of the borehole. 
     When fluid is produced from the formation through the screen, formation particulates treated with the chemical treatment are produced with the fluid from the formation, and they agglomerate in the annulus surrounding the screen in permeable structures to form a type of “gravel pack” structure. With the permeable structures formed in the annulus, operators do not need to actively pack the annulus with gravel. 
     The chemical treatment to agglomerate formation particulates can be especially useful in a cased hole having perforations, but the process may also be beneficial for open hole applications. A standalone screen in a cased hole can be prone to clogging and erosion. Thus, the chemical treatment can be passed through perforations in the casing to treat the surrounding formation. This can be accomplished by injecting the chemical treatment directly in the borehole through the screen, by capillary string, or other conveyance. 
     The chemical treatment includes an inner salt adapted to modify the zeta potential of the formation particulates. As discussed herein, zeta potential of a particulate refers to the electrokinetic potential of the particulates and is represented by a charge of the particulates&#39; surfaces. To agglomerate the particulates, the chemical treatment neutralizes the zeta potential of the formation particulates so they aggregate into one or more permeable structures in the annulus. 
     The foregoing summary is not intended to summarize each potential embodiment or every aspect of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a borehole of a formation having a completion string with multiple stand-alone screens. 
         FIG. 2  illustrates one technique for injecting chemical treatment into the formation. 
         FIG. 3  illustrates agglomerating of formation particulates treated with the chemical treatment and produced with the fluid from the formation in an annulus surrounding the screen. 
         FIG. 4  illustrates a process for chemically treating a formation so plugging and erosion can be reduced for stand-alone screens deployed downhole. 
         FIG. 5  shows the change of zeta potential in silica and ground coal samples when treated with a Zeta Potential Altering System. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a completion string  20  has a number of stand-alone screens  30  deployed in a cased hole  10 . Packers  22  disposed at various intervals between zones of interest isolate the annulus  14  between the casing  12  and the string  20 , and the cased hole  10  has perforations  16  communicating with the surrounding formation of these zones. As fluid is produced from the formation, the produced fluid can pass through the perforations  16  into the borehole annulus  14 . In turn, the produced fluid can enter the screens  30  and be produced up the string  20  at various wellhead components  26 . As shown, a mechanical barrier  24  can be disposed downhole of the string  20  to isolate the bottom of the cased hole  10 . 
     The screens  30  used can include any of the conventional screens used for gravel pack operations, frac pack operations, or wellscreen operations. Therefore, the screens  30  can use wrapped wire, sintered metal, mesh, perforated pipe, ceramic screens, and other components. 
     During production ( 60 ), fluid is produced from the formation through the casing&#39;s perforations  16 . As this process proceeds, formation sand and other particulates may tend to plug and/or erode the screens  30 , and this may be accelerated by virtue of the perforations  16  in the cased hole  10 . To reduce the chances of plugging and erosion, the completion has a chemical treatment ( 50 ) applied to surrounding portions or areas  40  of the formation according to the procedures disclosed herein. ( FIG. 2 , which is discussed below, shows one technique for treating areas  40  of the formation with the chemical treatment ( 50 ).) These treated areas  40  can extend into the surrounding formation as shown. The actual extent of these treated areas  40  may vary depending on how much chemical treatment is applied, characteristics of the formation, and other factors. 
     In any event, as shown in  FIGS. 1 and 3 , produced fluid  60  exits the treated formation area  40  through the perforations  16 , sand and other particulates produced with the fluid will tend to collect in the annulus  14  surrounding the screen  30  and the casing  12 . Left alone, these formation particulates would tend to plug and erode the screens  30 . Being chemically treated, however, the collected formation particulate is intended to have a significant amount of permeability that tends to reduce plugging and erosion. Moreover, the chemically treated formation particulate agglomerates together in the annulus  14  to form one or more permeable structures  42  for filtering produced fluids and reducing plugging and erosion of the screens  30 . In other words, these permeable structures  42  can act as a gravel pack formed from the produced sand and particulate in the annulus  14  around the screen  30  without the structures  42  being formally placed there through gravel packing operations. 
     Although the chemical treatment ( 50 ) is applied to the cased hole  10  in which the stand-alone screens  30  are used, the teachings of the present disclosure can be used in open holes in which stand-alone screens are used. Moreover, the borehole  10  may have a combination of cased and open hole sections as found in the art. 
     Still referring to the components in  FIGS. 1 and 3 , discussion now turns to the flowchart in  FIG. 4 , which shows a process  100  for chemically treating the surrounding formation to reduce plugging and erosion for stand-alone screens  30 . Initially, the completion string  20  is deployed in the borehole  10  and has a number of packers  22  and stand-alone screens  30  on the production tubing (Block  102 ). The packers  22  can then be activated to isolate the zones of interest in the formation from one another according to customary procedures (Block  104 ). 
     A chemical treatment ( 50 ) is then applied downhole so that it permeates into the surrounding formation (Block  106 ). As noted above, the borehole  10  through the formation may have a cased hole with perforations  16  or may be an open hole. In general, the treatment ( 50 ) can be applied before, during, and/or after the screens  30  and completion string  20  have been deployed. Accordingly, the procedure for treating the formation can use any of the available methods depending on what tools can be deployed, how the chemical treatment ( 50 ) can be conveyed downhole, and other factors known in the art. Thus, standard chemical injection procedures can be used to apply the chemical treatment ( 50 ). Some of these standard chemical injection procedures can involve pumping the treatment ( 50 ) directly down the completion string  20 , applying the treatment ( 50 ) with a capillary or workstring deployed in the completion string  20 , or other techniques. 
     When the chemical treatment ( 50 ) is applied after the completion string  20  is run, for example, the chemical additive of the treatment ( 50 ) can be pumped down the tubing string  20  so that it exits the screens  30  and enters the formation through the cased hole perforations  16 . This chemical additive can even be part of a frac operation used to stimulate the formation. 
     As one example placement technique shown in  FIG. 2 , chemical injection uses a “self-diverting” fluid for the chemical treatment  50 . This fluid is designed to be very thin and easy to inject into the formation. A capillary or workstring string  28  deployed in the completion string  20  injects the thin fluid for the chemical treatment  50  downhole, and the injected fluid passes out of the screen  30  and through the perforations  16 . Entering the formation through the perforations  16 , the injected fluid migrates into the surrounding area  40  of the formation. As the thin fluid migrates, the velocity and shear rate of the fluid is reduced, causing the fluid to become more viscous. In turn, the increasing viscosity of the migrating fluid causes the following fluids being injected behind it to be diverted to other parts of the formation in a self-diverting process. 
     Returning back to  FIG. 4 , the chemical treatment ( 50 ) treats the formation substrate (sand, particulates, etc.) with the chemical additive that allows the formation particulates, if free, to flow or otherwise move towards the screens  30 . Yet, as fluids are produced and enter the screens  30  (Block  108 ), the migrating formation particulates collect in the annulus  14  around the screens  30 . However, the previously applied chemical additive prevents the formation particulates from substantially plugging the screens  30  or otherwise preventing the well from flowing by causing the formation particulates to agglomerate and form stable and permeable structures (e.g.,  42  in  FIG. 3 ) around the screens  30  (Block  110 ). 
     One suitable chemical additive that can be used for this purpose includes a Zeta Potential Altering System (hereafter called ZPAS). This type of chemical additive alters the Zeta potential of the downhole formation substrate so that formation particulates are attracted to each other. Zeta potential refers to the electrokinetic potential of the particulates and is represented by a charge of the particulates&#39; surfaces. 
     The Zeta Potential Altering System (ZPAS) used for the chemical treatment ( 50 ) of the present disclosure can be a chemical additive based on an inner salt that modifies the zeta potential of the particulates. In particular, the system changes the particulates&#39; charge towards neutral values, which enhances the agglomeration of the particulates. 
     Further details of the chemical additive for the Zeta Potential Altering System can be found in D. Johnson, et al., “Enhancing Gas and Oil Production With Zeta Potential Altering System,” SPE 128048 (2010), which is discussed below. Other possible chemical additives could be used that alter the electrokinetic potential of the particulates. 
     As specifically discussed in SPE 128048, a Zeta Potential Altering System (ZPAS) can be used in hydraulic fracturing treatments. The system minimizes proppant flow back, controls fines migration, enhances fluid load recovery, and inhibits calcium carbonate scale formation. The Zeta Potential Altering System is based on an inner salt and modifies the zeta potential of particles such as fracture sand and formation substrate, changing the charge towards neutral values and therefore enhancing particle agglomeration. As also discussed in SPE 128048, formations can be treated by incorporating the chemical additive into stimulation fluids, and the chemical additive can be applied using several fluid systems to deliver the product. 
     As discussed in SPE 128048, Zeta Potential is defined by the charge that develops at the interface in the boundary of hydrodynamic shear between solid surfaces as a product of the electrostatic repulsion and the attractive forces related to the Van der Waals&#39; forces. Therefore, zeta potential is a function of the surface charge of the particle, any adsorbed layer at the interface, and the nature and composition or the surrounding suspension medium. In other words, zeta potential can be affected by changes in pH, conductivity of the medium (salinity and kind of salt), and concentration of particular additives (polymer, non-ionic surfactants, etc.). Particles with zeta potential values between −20 and 20 mV have an effective charge low enough that the repulsion between them is lowered to a point where aggregation occurs. 
     As discussed in SPE 128048, the active ingredient of the Zeta Potential Altering System is an inner salt of a very low-molecular weight polymer. When added to fracture water as discussed in SPE 128048, the inner salt disperses and rapidly coats any metal oxide substrate, such as proppant or subterranean formation. The system also contains a penetrating alcohol capable of disrupting the water layer that coats solid surfaces in the formation. The system does not modify the chemical structure of friction reducers and gelling systems, such as non-ionic, cationic, and anionic polyacrylamide and guar gums and derivatives so the system is compatible with slick-water systems and borate-based crosslinked gels. 
     SPE 128048 provides a Figrure, reproduced here as  FIG. 5 , showing the change in the zeta potential in 325 mesh silica and in ground coal samples when treated at concentrations of 6 gal of ZPAS per 1,000 lb of silica or of coal material. In both cases, the ZPAS increases the mean zeta potential of the particles towards more neutral values with a lower standard deviation. The resulting values are in the zeta potential range where higher agglomerating effects are expected. 
     The particular aspects of the chemical additive applied in the chemical treatment  50  may depend on the expected chemistry downhole, including considerations of temperature, pressure, type of produce fluid, expected size of formation particulates, expected types of formation substrate, etc. Being able to treat the formation so that formation particulates form permeable, stable structures around the stand-alone screens  30  can eliminate the need to actively pack the annulus with gravel in a gravel pack operation. Moreover, the disclosed techniques can allow expandable sand screens (ESS) to be run in a cased hole, which can have advantages in some implementations. Use of the chemical treatment can also allow stand-alone screens  30  that have larger outside and inside dimensions to be installed downhole. 
     Treating the formation with chemical additive according to the present disclosure can preferably be done before or at the time of first production. Depending on the implementation, additional additive may be needed to continue to create or maintain the permeable structure in the annulus. 
     The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.

Summary:
A borehole completion method treats a formation surrounding a borehole with a chemical treatment that alters how formation particulates interact. A standalone screen deploys downhole in the borehole (either before or after the treatment) on a downhole string. When fluid is produced, formation particulates treated with the chemical treatment agglomerate in the annulus surrounding the screen in permeable structures. This can be especially when the standalone screen is useful in a cased hole having perforations. The chemical treatment includes an inner salt adapted to neutralize the zeta potential (i.e., electrokinetic potential) of the formation particulates so they aggregate into one or more permeable structures in the annulus.