Performance Of Ecofriendly Polymers Biology Essay
The demand for fossil fuels particularly rough oil every bit good as its monetary value is increasing twenty-four hours by twenty-four hours and have led to the hunt for newer methods and techniques for oil extraction worldwide and these are jointly known as enhanced oil recovery methods. This paper reviews the present position of the different types of organic and inorganic cross-linked polymers used by the Oil Industry for increasing the oil production. The paper trades with the chronological advancement made in this way. Surveies indicate the suitableness, virtues every bit good as disadvantages of a peculiar cross-linker with mention to the reservoir features of different oil Fieldss where the polymer can be successfully used.
In order to run into the invariably increasing demand of energy particularly of fossil fuels, new methods and techniques are being developed continuously for the extraction of rough oil. This has become even more indispensable in position of the hapless recovery of oil, which is between 30-35 % ofthe oil modesty. With the of all time fluctuating monetary value of rough oil in the universe market with the tendency being largely upward, it has become imperative to better the oil recovery rate by using different techniques and procedures that are used worldwide and are known as enhanced oil recovery ( EOR ) methods. Enhanced oil recovery is achieved by gas implosion therapy, chemical implosion therapy, thermic recovery, supersonic stimulation or microbic injection.
A brief description of the same is presented below:Gas implosion therapy: -It is soon the most-commonly used attack to heighten oil recovery. A gas is injected into the oil-bearing stratum under high force per unit area. This force per unit area pushes the oil up to the surface. In add-on to the good consequence of the force per unit area, this method sometimes aids recovery by cut downing the viscousness of the rough oil as the gas mixes with it. Gases normally used for this purpose include C dioxide, natural gas or N. Carbon Dioxide EOR is giving first-class consequences. For illustration at present the United statesgreen goodss 245,000 barrels of oil per twenty-four hours byutilizing this method [ 1 ] .Chemical implosion therapy: -There are several possible methods that have been practiced by oil industry.
Some successful applications are injection of polymers, which can either cut down the petroleum ‘s viscousness or increase the viscousness of H2O which has besides been injected to coerce the petroleum out of the stratum. Detergent-like wetting agents such as rhamnolipids are injected to take down the capillary force per unit area that impedes oil droplets from traveling through a reservoir. Another method of chemical implosion therapy is alkali / caustic implosion therapy which is used in such reservoirs which are susceptible to H2O implosion therapy and have temperature less than 2000F.
Thermal implosion therapy: – In this technique, either the oil is heated during its flow upward in the well bore, or in the pool, which would let it to flux more easy towards the drill caput. Other discrepancies of this engineering include hot H2O injection, uninterrupted steam injection ( besides known as steam implosion therapy ) , in situ burning or cyclic steam injection. The chief aim of all thermic methods is to cut down the viscousness of the in-place oil, which is accomplished by heating. A serious disadvantage of steam implosion therapy is that a major portion of heat supplied by injected steam is transferred to the reservoir stone.
Microbial injection: – Microbial injection is a portion of microbial enhanced oil recovery. This technique is non really much in usage at present due to its higher cost but offers a batch of promise. Strains of bugs have been both discovered and developed ( utilizing cistron mutant ) which map either by partly digesting long hydrocarbon molecules, by bring forthing biosurfactants, or by breathing C dioxide.Supersonic stimulation: – Application of high-power supersonic quivers from a piezoelectric quiver unit shakes the oil droplets from the stone matrices, leting them to travel more freely towards the drill caput. This technique is most effectual instantly around the drill caput [ 2 ] .
The first research lab survey sing the influence of supersonic stimulation in heightening oil recovery was carried out by Duhon ( 1964 ) and applied practically in sandstone reservoirs.This paper aims to stress on the recent progresss in the usage of polymer / wetting agent implosion therapy and more on ecofriendly polymers. This technique falls in the class of chemical implosion therapy. Chemical EOR methods and techniques are advancement on conventional H2O implosion therapy processes. Chemical EOR methods include polymer implosion therapy, surfactant implosion therapy and alkaline implosion therapy procedures and the most widely applicable among these is polymer /surfactant implosion therapy.
Water production is a serious job during oil production. Water production has to be controlled in order to cut down runing disbursals such as pumping costs, oil/water separation costs, equipment costs, corrosion, graduated table & A ; sand production intervention costs and environmental damage/liability. Furthermore managing and disposal of produced H2O involves batch of investings as per regulative Torahs of the land and to fulfill Enforcement bureaus [ 3, 4 ] .Normally control of H2O production is carried out by following methods like cement/sand stoppers, mechanical baggers, Na silicon oxide gels, rosins, polymer gels etc. These methods are fundamentally being used to halt or cut down H2O cut and therefore bettering sweep efficiency, commanding conformity which will improve/increase the economic life of the reservoir.
Mechanical plugging and cementing are the lone options for high salt and high temperature reservoirs ; it is so due to the fact that their belongingss are independent of the presence of multivalent ions in H2O every bit good as the temperature of the reservoir. The present survey will besides concentrate chronologically on the function of polymer gels covering into H2O production job [ 5-9 ] . In order to undergo polymer deluging the production Wellss must hold atleast one of the undermentioned features:High Water Oil Ratio ( WOR ) . The economic bound of WOR is about 50:1Excessive unproductive H2O productionSignificant movable oil impregnation in the well form being treatedUnexpectedly low oil recoveryEarly H2O or gas discoveryHigh fluid degree in wellboreEqually early as the beginning of 20th century Petroleum Development Companies were utilizing beans and flax to except bottom H2O from oil Wellss in Kern River Oil Field. During 1920 ‘s oil industries started utilizing a mixture of rosin and coarse sand.
From the early 1920 ‘s through the mid 1950 ‘s cementing engineering was used. A major portion of this cementing information nevertheless, represented patterns used after World War II. Cementing method was being used in about 2000 Wellss out of the entire 3.
5 million Wellss present worldwide before 1990 [ 5 ] . Initially manus commixture of cement was practiced for H2O shut off which was later replaced by the mechanical sociable or jet sociable. During 1920 ‘s lone about 250 bags of cement & A ; waste slurry was needed for each well which increased to 2500 bags of cement subsequently on. No uncertainty cement has more strength than polymer gel and will defy high force per unit area gradients and is besides acerb resistant but cementing has certain restrictions as follows: -Cement slurry is made up of suspended solids, and therefore it functions as cement Bridgess and cement slurries do non perforate the formation or gravel battalion.There is ever a danger that some productive sites may be for good closed.
Resins have been used by the Petroleum industry in position of the fact that they provide belongingss such as the following which makes it utile to the industry.Resin is a liquid unlike cement.Resin unlike cement penetrates permeable formations and crushed rock battalions and can hold fluid and gas migration among formations.
Resin is most successful when emplaced under inactive conditions. It can be considered as a good campaigner when used in squashing off pressured gas zones [ 10 ] .
By the center of 20th century, Oil Industry started utilizing polymers such as polyacrylamides and polyoses for enhanced oil recovery. Polymers are supermolecules, made up of reiterating units joined together.
Molecular weight of the polymers ranges up to several million Daltons. The physical size of the polymer molecule can change from sub micrometer to micron scope. Polyacrylamides have been the most widely used polymers for H2O shut-off in production Wellss and for profile alteration in injection Wellss ; nevertheless, they can go unstable in high salt H2O at high temperatures due to car hydrolysis. These polymers can be cross-linked with passage metal ions, and the resulting gels are known to be more stable in rough environments than the individual uncross-linked polymer itself [ 5, 11, 12 ] .The polymer gels have certain advantages as compared to cement bullet and they are:The polymer is injected as a solutionwhich penetrates the reservoir stone andreduces permeableness in the close good bore country and at the perforations.The solution can travel up and down the exterior of the well bore, sealing clefts & A ; bing micro ring within the original cement sheath.The gel left in the well can be cleaned out by gushing with either a mild acerb solution or H2O whereas cement is much more hard to take as it sets.Different polymer systems have been synthesized which are capable of sealing off the more permeable beds so that the implosion therapy liquid could be diverted to the underswept, tighter parts of the reservoirs.
The polymer gels increase the evident viscousness of H2O which increases opposition to flux of H2O in the swept zones. Once the gels block these parts oil easy flows out [ 13, 14 ] . The polymers used for this intent must hold the undermentioned belongingss:Should be thermally stableShould be stable plenty to go on to hinder flow for long periods at peculiar temperature, pH & A ; saltShould be mixable in H2OShould possess sufficient mechanical strengthShould be ecofriendlyA gel consists of a web of cross-linked polymers. Polymer solutions and cross-linking agents are assorted together in order to organize a gelant solution. The gelant with clip and chemical cross-linking develop a 3-D construction.
This 3-D construction including encapsulated H2O is referred to as gel. The gels prepared for heightening oil recovery consists of about 0.5-3 % cross-linked polymer which forms a three dimensional web keeping about 99.
5-97 % H2O in an equilibrium province. In fractured reservoirs, a partly formed gel is injected. If any perturbation occurs in the equilibrium of gel due to alterations in nature or extent of cross-linking so synaeresis ( ejection of liquid from gel and attendant shrinking in gel volume ) takes topographic point [ 15 ] .
Some of import features of a good gelant are: -sufficient gelation hold so that the gel is formed after making the formationproper injectivitygood extension of gelant constituentslastingness of the permeableness decrease
Features of gelled polymers
Gels are created when dry polymer is mixed in H2O and cross-linked with either a metal ion ( normally chromium triacetate or aluminium citrate ) or any organic compound.Gelation clip is governable, runing from a few hours to hebdomads ; slower gelation clip allows for more volume and deeper arrangement.Gels have viscousness and elasticityranging from somewhat greater than fresh H2O to rubber like.Gels can wholly barricade the flow of fluid through all reservoir stone or they can preferentially cut down permeableness and fluid flow through merely the most permeable and conductive tracts.
Gels can be created with polymer concentration runing from a few hundred to more than 50,000 ppm ; low polymer concentration means less gel strength and higher concentration means more gel strength.Gels are every bit applicable to sandstoneand carbonate reservoirs.Gels are comparatively cheap because they contain 98 % or more H2O.
The metallic cross-linkers map through the formation of ionic bond between multivalent metal ions and the negative sites of a polymer such as partly hydrolyzed polyacrylamide or biopolymers.
Aluminium ( III )
The gelation of polyacrylamide utilizing aluminium citrate as cross-linking agent was foremost reported in 1974 [ 16, 17 ] . It has been applied in many Fieldss with impressive economic success. The citrate ion protects the Al ( III ) from hydrolysis which could hold otherwise produced the Al ( OH ) 3 doing the metal ion unavailable for cross-linking with polymers. Gelation occurs best in fresh H2O due to the fact that Ca2+ & A ; Mg2+ nowadays in saline H2O compete with Al3+ for citrate ions.
AdvantagesThis system of PAM/Al3+ is of lowtoxicity.DisadvantagesDue to rapid rate of gelation, the deepness of incursion into the formation will be limited.The divalent ions ( Ca2+ , Mg2+ ) nowadays in H2O can do Aluminium /PAM to precipitate instead than gel.This polymer system of PAM/Aluminium citrate is applicable merely up to 850C.Gelation can be achieved merely when seawater bullet is removed from the bullets ofPAM and aluminum citrate.Al3+ cross-linked gels are more stable at low pH and low temperature.
Zirconium ( IV )
Zirconium ( IV ) type gels are reported as being utile to accomplish higher expanse efficiency [ 18 ] .
AdvantagesNon-toxic.Interact strongly with carboxylate group to organize composites which are more stable than those formed by utilizing Cr ( III ) or Al( III ) .Therefore, gels capable of defying hightemperature than Chromium gels may beprepared by Zirconium cross-linking.
DisadvantagesThese gels are hard to be prepared.Lumpy and heterogenous gels may beformed by cross-linking Zr ( IV ) reagents with carboxylate incorporating polymers.This job of chunky gel formation can be minimized by utilizing little sums of sulfate or carboxylate reaction moderators in the cross-linked composing which provides homogeneous, strong and stable gels in a short clip.
Titanium ( IV )
Another gel system includes Titanium ( IV ) -Hydrolyzed Polyacrylamide.This gel can keep its strength at moderate temperature ( 500C ) for a long clip therefore doing it suited to be used in a moderate temperature oil reservoir [ 19 ] .
AdvantagesRelatively less expensive compared toZirconium.Adaptable to difficult seawater content, impersonal pH conditions and moderate temperature oil reservoirs.DisadvantagesSyneresis occurs at high temperature due to hydrolysis of the amide group.
Ti-HPAM is a weak metal ion cross-linked gel that is merely suited for application in a moderate temperature oil reservoir.
Colloidal silicon oxide gel system
Colloidal silicon oxide gel system refers to stable scatterings of distinct nonporous atoms of formless Si dioxide ( SiO2 ) . This polymer gel got importance due to its non-toxic nature and minimum environmental hazard. The proper concentration of colloidal silicon oxide required to acquire the best consequence is between 6-15 wt % . It has comparatively short gelation times. Greater the figure of silica atoms in solution lesser the gelation clip. However due to miss of mechanical strength it ca n’t be used for permeableness decrease of breaks. Another major disadvantage is the high cost of the system and it ‘s incapableness for in depth permeableness alterations.
In this system gelation clip is controlled by accommodation of pH and salinity.The advantage of this system is that it exhibits stableness over a broad scope of temperature [ 20 ] .Chromium ( III )Another polymer system includes Polyacrylamide cross-linked with either inorganically complexed or organically complexed Chromium ( III ) .
There are reserves sing the usage of Cr ( IV ) and Cr ( VI ) as these are carcinogenic whereas Cr ( III ) is safe and used as a cross-linker [ 21, 22 ] .Furthermore inorganically complexed Cr ( III ) has control over gelation rate and can be used in seawaters incorporating important hardness at temperature even up to 660C. Organically complexed Cr ( III ) protects extremely active Cr ( III ) by organizing compositesdetaining gel formation.The differences between organically and inorganically complexed Cr ( III ) are: -Chemical reaction of organically complexed Cr ( III ) is comparatively much slower at ambient temperature in the pH scope 4-7 compared with the ligand exchange procedure between inorganic salts of Cr ( III ) and polymer under similar temperature and pH conditions. Therefore, organically complexed Cr ( III ) have longer gelation clip.
Organically complexed Cr ( III ) compounds seem to defy gel synaeresis at high temperature.Organically cross-linked Cr ( III ) is more effectual and dependable in H2O shut-off interventions compared to inorganically cross-linked Cr ( III ) .DisadvantagesThe job with Cr ( III ) cross-linked PAMgels is that due to the nature of adhering between Cr ( III ) and the pendent carboxylate group, formation of indissoluble Crspecies can happen at high pH.Thermal instabilityUnpredictable gelling times and gel instability in the presence of some potentially active chemical species.The gelling times can be controlled by the add-on of stuffs that chelate with Cr in competition with the polymer edge carboxylate group. One manner to command gelation clip is presenting chromium ion as Cr6+ ion along with some cut downing agent such as Na thiosulphate to cut down Cr6+ to Cr3+ which consequences in slower rate of cross-linking reaction. Inspite of its disadvantages Cr ( III ) has been extensively used due to its high success rate and comparatively low cost.
Despite several reservoir related jobs such as salt, pH etc Cr ( III ) acetate cross-linked PAM composings are reported to supply dependable public presentation in the field [ 23-26 ] .
Restrictions and disadvantages of metallic cross-linkers
Each metal has affinity for differentfunctionalities. For illustration Al, Cr & A ; Zr arereactive to amide and carboxyl groups while Titanium is reactive to hydroxyl group.Proper pH control necessary for gelation.Ligand metal bond formation and stableness may be affected due to unfavourable conditions such as high ionic strength and the temperature of reservoir seawater.
Unsuitable for high temperature applications [ 16 ] .A figure of cross-linking agents are effectual merely under certain defined conditions of pH which may be different from alkalic conditions under which the polymers function most efficaciously.Some extremely charged metal ions such as Cr3+ have high charge denseness which tends to falsify the polymer as a consequence of which the H2O may be squeezed out of the gel construction so that it becomes less stable than desirable.Certain metal cross-linking agents are comparatively toxic and unfriendly to the environment, peculiarly due to the fact that the metal may come in aquifers from the oilbearing formations.Some metal cross-linking agents such as Cr3+are rather dearly-won and it would be desirable to happen inexpensive options.They have low thermic stableness.Increased responsiveness of metal cations is another cardinal drawback.
Excess cross-linking occurs in some multivalent cations ensuing in synaeresis.Have shorter gelation times.The metallic cross-linker systems have several defects and restrictions in comparing to their advantages.
Thus they are easy being replaced by the more efficient organic cross-linker systems, which are being quickly used in high temperature formations [ 27 ] .
The organic cross-linkers have easyreplaced the metallic cross-linkers, thoughorganically complexed Chromium ( III ) is still giving first-class consequences. These organic cross-linkers involve covalent bonding with the polymer molecule.
Advantages of organic cross-linkers
Have high thermic stableness. This stableness is enhanced by the covalent cross-linking bonds.Elastic modulus is higher for organic cross-linked gels.Normally the N incorporating cross-linkers have a comparatively lower charge denseness than many metal cations therefore morestable gel construction is obtained.Less sensitive to pH and petrology.
They are stable under a broad scope of pHconditions hence are capable of efficaciouslycross-linking a broad assortment of polymers.Less dearly-won and toxic than many metal compounds hence can be used more readily in formations linking with aquifers and at lower cost.Insensible to ionic forces, since they are formed through covalent bondingOrganic gels are easy to implement andcontrol during field operations due to flexibleness in injection clip [ 13, 28 ] .
Amino rosins such as cyanuramide /Formaldehyde are normally used as organic cross-linkers since they have the undermentioned advantages -Economically feasible.Applicable to a broad assortment of polymers.Form thermally stable, brine tolerant gels that are stable even at low pH.
No demand of any acid or baseaccelerator [ 29 ] .
The polymer gel system dwelling of Polyacrylamide and glyoxal is present as a colloidal suspension ever at low polymer concentration irrespective of the polymer concentration. With the increasing concentration of glyoxal the strength of gel additions. The gelation clip of this system is strongly dependent on pH. Generally synaeresis occurs at high concentration of glyoxal ( & gt ; 0.
65g/l ) [ 30 ] .
Aldehydes such as glutaraldehyde have besides gained attending when blended with polyvinyl intoxicant. These gels are insensitive to the hardness degree of H2O and can be used under rough conditions such as high salt, hardness and elevated temperatures. However these gels show inordinate synaeresis and gel shrinking at high temperature [ 10 ] .
Polyacrylamide-t-butyl acrylate/polyethyleneimine system is stable at high temperature. PEI cross-linker has low toxicity and was approved for nutrient contact in the USA.PAtBA hydrolyzes to organize partly hydrolyzed polyacrylamide and 30-butyl intoxicant under conditions of low pH values and temperature and thermolyses at high temperature and pH values to bring forth partly hydrolyzed polyacrylamide and isobutene gas [ 31, 32 ] .
PAM/PEI gels are stable up to a temperature of about 1000C. Polyethyleneimine as cross-linker is considered to be environmentally sensitive in some parts of the universe, and hence other options were explored. This system is being replaced by less caustic, less toxic, less bioaccumulating and by systems which show better biodegradation [ 28 ] .Phenol/Formaldehyde The Phenol/Formaldehyde system is used for high temperature applications.
It is stable up to a temperature of about 1500C.These phenolic gels can be applied under rough conditions such as high temperature, high salt and high pH.It is besides desirable due to its low viscousness and higher thermic stableness. A major disadvantage of this system is that it is non ecofriendly [ 33, 34 ] . Gelation is slightly sensitive to pH and salt and hence was modified by sulphomethylating the phenolic compound. Another major drawback of this system is the loss of phenol through breakdown in rough oil [ 11 ] .A figure of derived functions of Phenol andFormaldehyde has been used in order to cut down the toxicity of the polymer system. A polymer system dwelling PAM cross-linked with Resorcinol/Formaldehyde, gels in shorter clip at low temperature compared to Phenol/HCHO cross-linked PAM which gels in the same clip at temperature greater than 800C.
The advantage of the Resorcinol /Formaldehyde system is the H2O like viscousness of the gelant which makes it desirable for gel intervention. Gelling is sensitive to pH of H2O and therefore used in limited applications [ 3, 33 ] .Some late used organic cross-linker systems alternatively of phenol/HCHO include combination of hydroquinone with hexamethylenetetramine, phenyl ethanoate / HMTA, resorcinol/HMTA etc.
Formaldehyde is non ecofriendly therefore researches till now have proved HMTA to be the lone option for it. Some alternate cross-linkers with lower degrees of toxicity bring forthing stable gels have been suggested for phenol which includes o- and p-amino benzoic acid, m- aminophenol salicylic acid, phenyl salicylate etc. Among these, two systems i.e. HQ/HMTA and Phenyl acetate/HMTA have shown singular effectivity due to their high mechanical strength and cost effectivity. [ 35-39 ]Aspirin/HCHO, Anthranilic acid /HCHO etc have besides been used but have slower gelation rates and therefore take longer gelation times compared to Phenol/HCHO at temperature of 700C and supra.
The ground might be the slow rate of hydrolysis in-situ.In malice of holding several advantages the organic cross-linkers excessively suffer from certain drawbacks as mentioned below:Must be stable non merely at high temperature but besides at high pH.Stability to assorted oil field seawaters.Systems at lower temperature require high cross-linker concentration.Syneresis is another major job which if minimized would heighten theeffectivity of gels.Organic cross-linkers may be capable to partitioning in oil stage.
High TEMPERATURE GEL SYSTEMS
Nowadays selected higher temp gel systems used include -CC/AP ( Chromium ( III ) carboxylate / acrylamide polymer ) gels. These gels are chemically strong and organize a comparatively simple gel engineering. They are cheap and can be applied either as gelants or as partly preformed gels.Co-polymer of acrylamide & A ; t-butyl propenoate cross-linked with Polyethylene -imine. This system has chiefly two advantages due to which it additions attention-firstly, due to the fact that the PAtBA-PEI bonds are formed as a consequence of a nucleophilic onslaught by an amine N from the PEI on the carbonyl C of the polymer, this reaction does non necessitate hydrolysis ( or thermolysis ) of the cross-linking site under reservoir conditions and secondly in PEI cross-linking, unlike many metallic cross- linkers, it is non susceptible to hydrolysis which helps it to propagate more expeditiously through the reservoir.
Phenol/formaldehyde cross-linked acrylamide polymer gels including the possible usage of less toxic derived functions of phenol and methanal as the cross-linking agents. The derived functions that can be used include phenyl ethanoate, hydroquinone, resorcinol salicylic acid, phenyl salicylate etc in-situ. The lone derivative that can be used alternatively of HCHO is HMTA [ 10, 40 ] .
Lignosulfonate gels are simple, cheap & A ; environmentally friendly. The features of the gel formed are independent of the formation seawater or C dioxide nowadays. This system has high temperature tolerance and has long gelation times at high pH.A new tendency for gelation is being used in some oil reservoirs. It involves the application of gel as preformed atom gel ( PPG ) . They are formed at surface prior to injection. This technique can get the better of some distinguishable drawbacks built-in in unmoved gelation system such as deficiency of gelation clip control, uncertainty of gelling due to shear debasement, alteration of gelant composings etc.
PPG has been successfully synthesized and applied to command extra H2O production. Surveies carried out stated that preformed gel had better arrangement than unmoved gel and could efficaciously cut down gel harm on low permeableness unswept oil zones. For their application the PPG atoms are added to the injection H2O for some period of clip, and so followed by normal H2O injection. Advantages of the PPG attack include that the merchandise added has a known chemical composing, a PPG suspension in the injection H2O created at the surface will hold predictable physical belongingss, and these PPG suspensions can be stable and execute their desirable partial stop uping action in the highest permeableness zones of the reservoir at harsher reservoir conditions. The PPG engineering, nevertheless, has two of import restrictions. First of wholly, these PPG atoms swell about instantly when exposed to H2O.
Second, the PPG atoms commercially available have a comparatively big size ( 100s of micrometers to millimetres in diameter ) which limits their application to stop uping merely really high permeableness beds [ 41, 42 ] .Another promising agent for EOR application is micro gel. These micro gels are more tolerant to high temperature and salts and more stable against debasement. These are used either as permeableness qualifiers or as viscousness foils. As the polymer micro-gel scattering is injected into oil-bearing reservoirs the rheological belongingss and the flow belongings of the scatterings affect the profile control and the enhanced oil recovery significantly [ 43-45 ] .
A batch of different polymer gel systems have synthesized boulder clay day of the month but each of them suffer from certain restrictions. It has been seen while reexamining the public presentation of different gel systems that they can be classified into two wide classs. Thereare some systems, which can defy high temperature, and some, which have opposition to high salt. Till day of the month there is no such system that has the ability to stand both high temperature and high salt.Therefore there is a broad range of research and challenge to scientists involved in these countries to develop such cross-linked polymers, which can defy both high temperature and high salt.