Fluid loss additives are also called filtrate-reducing agents. Fluid losses mayoccur when the fluid comes in contact with a porous formation. This is relevantfor drilling and completion fluids, fracturing fluids, and cement slurries.

The extent of fluid loss is dependent on the porosity and thus the permeabilityof the formation and may reach approximately 10 t/hr. Because the fluids usedin petroleum technology are in some cases quite expensive, an extensive fluidloss may not be tolerable. Of course there are also environmental reasons toprevent fluid loss.

Mechanism of Action of Fluid Loss Agents

Reduced fluid loss is achieved by plugging a porous rock in some way. Thebasic mechanisms are shown in Table 2-1.

Action of Macroscopic Particles

A monograph concerning the mechanism of invasion of particles into theformation is given by Chin [375].

One of the basic mechanisms in fluid loss prevention is shown in Figure 2-1.The fluid contains suspended particles. These particles move with the lateralflow out of the drill hole into the porous formation. The porous formation actslike a sieve for the suspended particles. The particles therefore will be capturednear the surface and accumulated as a filter-cake.

The hydrodynamic forces acting on the suspended colloids determine therate of cake buildup and therefore the fluid loss rate. A simple model has beenproposed in literature [907] that predicts a power law relationship between thefiltration rate and the shear stress at the cake surface. The model shows that thecake formed will be inhomogeneous with smaller and smaller particles beingdeposited as the filtration proceeds. An equilibrium cake thickness is achievedwhen no particles small enough to be deposited are available in the suspension.The cake thickness as a function of time can be computed from the model.

C h a p t e r 2

F l u i d L o s s A d d i t i v e s

Table 2-1Mechanisms of Fluid Loss Prevention

Macroscopic particles

Microscopic particles

Chemical grouting

Suspended particles may clog the pores, forming afilter-cake with reduced permeability.

Macromolecules form a gel in the boundary layer of aporous formation.

A resin is injected in the formation, which curesirreversibly; suitable for bigger caverns.

Vertical Flow

Lateral Flow

Drill Hole Filter Cake Porous Formation

Figure 2-1. Formation of a filter-cake in a porous formation from suspension(•) in a drilling fluid. The suspended particles can invade the formation to someextent.

For a given suspension rheology and flow rate there is a critical permeabilityof the filter, below which no cake will be formed. The model also suggests thatthe equilibrium cake thickness can be precisely controlled by an appropriatechoice of suspension flow rate and filter permeability.

Action of Cement Fluid Loss Additives

Two stages are considered with respect to the fluid loss behavior of a cementslurry [140]:

1. A dynamic stage corresponding to placement2. A static stage, awaiting the setting of the cement

During the first period, the slurry flow is eroding the filter-cake as it isgrowing; thus a steady state, in which the filtration occurs through a cake ofconstant thickness, is rapidly reached. At the same time, because the slurryis losing water but no solid particles, its density is increasing in line with thefluid loss rate.

During the second period, the cake grows because of the absence of flow.It may grow to a point at which it locally but completely fills the annulus:Bridging takes place and the hydrostatic pressure is no longer transmitted tothe deeper zones. From the typical mudcake resistance it can be estimated thatunder both dynamic and static conditions, the fluid loss could require reductionto an American Petroleum Institutue (API) value lower than what is generallyconsidered a fair control of fluid loss.

Testing of Fluid Loss Additives

Predictions on the effectiveness of a fluid loss additive formulation can bemade on a laboratory scale by characterizing the properties of the filter-cakeformed by appropriate experiments. Most of the fluids containing fluid lossadditives are thixotropic. Therefore the apparent viscosity will change when ashear stress in a vertical direction is applied, as is very normal in a circulatingdrilling fluid. For this reason, the results from static filtering experiments areexpected to be different in comparison with dynamic experiments.

Static fluid loss measurements, which are the present standardized testingmethod, provide inadequate results for comparing fracturing fluid materials orfor understanding the complex mechanisms of viscous fluid invasion, filter-cake formation, and filter-cake erosion [ 1806]. On the other hand, dynamic fluidloss studies have inadequately addressed the development of proper laboratorymethods, which has led to erroneous and conflicting results.

Results from a large-scale, high-temperature, high-pressure simulator werecompared with laboratory data, and significant differences in spurt loss valueswere found [1125].

Static experiments with pistonlike filtering can be reliable, however, toobtain information on the fluid loss behavior in certain stages of a cementationprocess, in particular when the slurry is at rest.

Organic Additives

The properties of the filter-cake formed by macroscopic particles can besignificantly influenced by certain organic additives. The overall mechanismof water-soluble fluid loss additives has been studied by determining the elec-trophoretic mobility of filter-cake fines. Water-soluble fluid loss additives are

divided into four types according to their different effects on the negativeelectrical charge density of filter-cake fines [1891]:

1. Electrical charge density is reduced, such as polyethylene2. Glycol and pregelatinized starch3. Electrical charge density is not changed, such as carboxymethyl starch4. Electrical charge density is increased, such as sulfonated phenolic resin,

carboxymethylcellulose, and hydrolyzed polyacrylonitrile

The properties of filtrate reducers contribute to their different molecularstructures. Nonionic filtrate reducers work by completely blocking the filter-cake pore, and anionic ones work by increasing the negative-charge densityof filter-cakes and decreasing pore size. Anionic species cause further claydispersion, but nonionic species do not, and both of them are beneficial tocolloid stability [1890].

The change of properties of the filter-cake due to salinity and polymericadditives has been studied by scanning electron microscope (SEM) photog-raphy [1438]. Freshwater muds with and without polymers such as starch,polyanionic cellulose (PAC), and a synthetic high-temperature-stable poly-mer, were prepared, contaminated with electrolytes (NaCl, CaCl2, MgC^),and aged at 200° to 350° F. Static API filtrates before and after contaminationand aging were measured. The freeze-dried API filter-cakes were used for SEMstudies. The filter-cake structure was influenced by electrolytes, temperature,and polymers. In an unaged, uncontaminated mud, bentonite forms a card-house structure with low porosity. Electrolyte addition increases the averagefilter-cake pore size. Temperature causes coagulation and dehydration of clayplatelets. Polymers protect bentonite from such negative effects.

Formation Damage

The damage of the formation resulting from the use of a filtration lossagent can be a serious problem for certain fields of application. Providingeffective fluid loss control without damaging formation permeability in com-pletion operations has been a prime requirement for an ideal fluid loss con-trol pill.

Filter-cakes are hard to remove and thus can cause considerable formationdamage. Cakes with very low permeability can be broken up by reverse flow.No high-pressure spike occurs during the removal of the filter-cake. Typicallya high-pressure spike indicates damage to the formation and wellbore surfacebecause damage typically reduces the overall permeability of the formation.Often formation damage results from the incomplete back-production of vis-cous, fluid loss control pills, but there may be other reasons.

Reversible Gels

Another mechanism for fluid loss prevention is caused by other additives,which are able to form gels on a molecular mechanism.

Ultrafine Filtrate-Reducing Agents

Methods are available for reducing the fluid loss and reducing the concen-tration of polymer required to provide a desired degree of fluid loss controlto a drilling fluid and to a well servicing fluid, respectively [500]. The fluidscontain, as usual polymeric viscosifiers, a polymeric fluid loss additive and awater-soluble bridging agent suspended in a liquid in which the bridging agentis not soluble. It is important to add to the fluids a particulate, water-soluble,ultrafine filtrate-reducing agent. The particle size distribution should be suchthat approximately 90% of the particles are less than 10 JLL, the average particlesize being between 3 and 5 |U and the ultrafine filtrate-reducing agent beinginsoluble in the liquid.

Bentonite

Bentonite is an impure clay that is formed by weathering of volcanic tuffs. Itcontains a high content of montmorillonite. Bentonites exhibit properties suchas ability to swell, ion exchange, and thixotropy. Properties can be modified byion exchange, for example, exchange of earth alkali metals to alkali metals. Thespecific surface can be modified with acid treatment. Organophilic propertiescan be increased by treatment with quaternary ammonia compounds.

Polydrill

Poly drill is a sulfonated polymer for filtration control in water-based drillingfluids [1775]. Tests demonstrated the product's thermal stability up to 200° Cand its outstanding electrolyte tolerance. Polydrill can be used in NaCl-saturated drilling fluids as well as in muds containing 75,000 ppm of cal-cium or 100,000 ppm of magnesium. A combination of starch with Polydrillwas used successfully in drilling several wells. The deepest hole was drilledwith 11 to 22 kg/m3 of pregelatinized starch and 2.5 to 5.5 kg/m3 of Polydrill toa depth of 4800 m. Field experience with the calcium-tolerant starch/Poly drillsystem useful up to 145° C has been discussed in detail [1774].

In dispersed muds (e.g., lignite or lignosulfonate), minor Polydrill addi-tions result in a significantly improved high-temperature, high-pressure filtrate.Major benefits come from a synergism of polymer with starch and poly-saccharides. The polymer exerts a thermal stabilizing effect on those polymers.In conventional or clay-free drilling and completion fluids, Polydrill can be

used by itself or in combination with other filtrate reducers for various pur-poses [1436]. Handling and discharge of the product as well as the waste mudcreated no problem in the field.

Bacteria

Instead of using polymers, the addition of bacteria cultures, which mayform natural polymers and could then prevent fluid loss, has been suggested. Inone study, a bacterial culture selected for its abundant exopolymer productionwas added to drilling mud to determine whether the PAC component couldbe replaced without sacrificing viscosity or fluid retention [50]. Drilling mudperformance was tested using a standard API test series. The bacterial inoculumwas not as effective in maintaining viscosity or preventing fluid loss as wasthe PAC. However, the inoculum was capable of reducing the amount of PACrequired in the drilling mud.

The combination of the bacterial inoculum with less expensive start sources,for example, carboxylated methyl cornstarch, crosslinked hydroxypropyl corn-starch, and amine-derivatized potato starch, gave viscosity and fluid loss con-trol as good as or better than PAC alone. The bacterial strain tested was effec-tive over a wide range of drilling mud conditions with growth at varying pH(3 to 11), varying salinities (0% to 15%), and a wide range of temperatures.

Polysaccharides

Cellulose-Based Fluid Loss Additives

Polyanionic Cellulose

A composition containing polyanionic cellulose and a synthetic polymerof sulfonate has been tested for reducing the fluid loss and for the thermalstabilization of a water-based drilling fluid for extended periods at deep welldrilling temperatures [812].

Sulfonate. When a sulfonate-containing polymer is added to a drilling fluidcontaining PAC, the combination reduces fluid loss. Improved fluid loss is ob-tained when PAC and the sulfonate-containing polymer, which has a molecularweight of 300,000 to 10 million Dalton, are combined in a water-based drillingmud after prolonged aging at 300° F.

Carboxymethylcellulose

Certain admixtures of carboxymethylhydroxyethylcellulose or copolymersand copolymer salts of N,N-dimethylacrylamide and 2-acrylamido-2-methyl-propane sulfonic acid (AMPS), together with a copolymer of acrylic acid, may

provide fluid loss control to cement compositions under elevated temperatureconditions [252].

Hydroxethylcellulose

Hydroxyethylcellulose with a degree of substitution of 1.1 to 1.6 has beendescribed for fluid loss control in water-based drilling fluids [1473]. An ap-parent viscosity in water of at least 15 cP should be adjusted to achieve anAPI fluid loss of less than 50 ml/30 min. Crosslinked hydroxyethylcellulose issuitable for high-permeability formations [344,346].

A derivatized hydroxyethylcellulose polymer gel exhibited excellent fluid-loss control over a wide range of conditions in most common completionfluids. This particular grated gel was compatible with the formation mate-rial and caused little or no damage to original permeability [1341]. Detailedmeasurements of fluid loss, injection, and regained permeability were takento determine the polymer particulate's effectiveness in controlling fluid lossand to assess its ease of removal. Hydroxyethylcellulose can be etherified oresterified with long chain alcohols or esters. An ether bond is more stable inaqueous solution than is an ester bond [96].

Starch

Crosslinked Starch

A crosslinked starch was described as a fluid loss additive for drilling flu-ids [632,1626]. The additive resists degradation and functions satisfactorilyafter exposure to temperatures of 250° F for periods of up to 32 hours. Toobtain crosslinked starch, a crosslinking agent is reacted with granular starchin an aqueous slurry. The crosslinking reaction is controlled by a Brabender

Starch (Amylose)

Figure 2-2. Starch.

Viscometer test. Typical crosslinked starches are obtained when the initial riseof the viscosity of the product is between 104° and 144° C, and the viscosity ofthe product does not rise above 200 Brabender units at temperatures less than130° C. The crosslinked starch slurry is then drum-dried and milled to obtaina dry product. The effectiveness of the product is checked by the API FluidLoss Test after static aging of sample drilling fluids containing the starch atelevated temperatures. The milled dry product can then be incorporated intothe oil well drilling fluid of the drill site.

Granular Starch and Mica

A fluid loss additive is described that consists of granular starch compo-sition and fine particulate mica [337]. An application comprises a fracturingfluid containing this additive. A method of fracturing a subterranean formationpenetrated by a borehole comprises injecting into the borehole and into contactwith the formation, at a rate and pressure sufficient to fracture the formation, afracturing fluid containing the additive in an amount sufficient to provide fluidloss control.

Depolymerized Starch

Partially depolymerized starch provides decreased fluid losses at muchlower viscosities than the corresponding starch derivatives that have not beenpartially depolymerized [498].

Controlled Degradable Fluid Loss Additives

A fluid loss additive for a fracturing fluid comprises a mixture of natural andmodified starches plus an enzyme [1850]. The enzyme degrades the cc-linkageof starch but does not degrade the P-linkage of guar and modified guar gumswhen used as a thickener. Natural or modified starches are utilized in a preferredratio of 3:7 to 7:3, with optimum at 1:1, and the mix is used in the dry form forapplication from the surface down the well. The preferred modified starchesare the carboxymethyl and hydroxypropyl derivatives. Natural starches may bethose of corn, potatoes, wheat, and soy, and the most preferred is corn starch.Blends include two or more modified starches, as well as blends of natural andmodified starches. Optionally, the starches are coated with a surfactant, suchas sorbitan monooleate, ethoxylated butanol, or ethoxylated nonylphenol, toaid dispersion into the fracturing fluid.

A fluid loss additive is described [1849] that helps achieve a desired frac-ture geometry by lowering the spurt loss and leak-off rate of the fracturingfluid into the surrounding formation by rapidly forming a filter-cake with lowpermeability. The fluid loss additive is readily degraded after the completion

of the fracturing process. The additive has a broad participate size distributionthat is ideal for use in effectively treating a wide range of formation porositiesand is easily dispersed in the fracturing fluid. The fluid loss additive comprisesa blend of modified starches or blends of one or more modified starches andone or more natural starches. These blends have been found to maintain in-jected fluid within the created fracture more effectively than natural starches.The additive is subject to controlled degradation to soluble products by a natu-rally proceeding oxidation reaction or by bacterial attack by bacteria naturallypresent in the formation. The oxidation may be accelerated by adding oxidizingagents such as persulfates and peroxides.

Guar

A hydrophobically modified guar gum can be used as an additive for drilling,completion, or servicing fluids [93,94]. The modified gum is used together withpolymers or reactive clay.

Hydroxypropylguar Gum

Hydroxypropylguar gum gel can be crosslinked with borates [1227], ti-tanates, or zirconates. Borate-crosslinked fluids and linear hydroxyethyl-cellulose gels are the most commonly used fluids for high-permeability fracturetreatments. This is for use for hydraulic fracturing fluid under high-temperatureand high-shear stress.

Succinoglycan

Succinoglycan is a biopolymer. It has been shown to possess a combinationof desirable properties for fluid loss control [1069,1070]. These include ease ofmixing, cleanliness, shear-thinning rheology, temperature-insensitive viscositybelow its transition temperature (Tm), and an adjustable transition temperature(Tm) over a wide range of temperatures. Being a viscous fluid, succinoglycanrelies solely on viscosity to reduce fluid loss. It does not form a hard-to-removefilter-cake, which can cause considerable formation damage. Based on thesefindings, succinoglycan has been used successfully as a fluid loss pill beforeand after gravel packing in more than 100 offshore wells. Calculations based onlaboratory-measured rheology and field experience have shown succinoglycanto be effective in situations in which hydroxyethylcellulose is not. Fluid loss,even over 40 barrels/hr, was reduced to several barrels per hour after applicationof a properly designed succinoglycan pill. Most wells experienced no problemin production after completion.

Hydroxyethylcellulose

Carboxymethylcellulose

Figure 2-3. Hydroxyethylcellulose, carboxymethylcellulose.

Succinoglycan can be degraded with an internal acid breaker [222]. Theformation damage that results from the incomplete back-production of vis-cous fluid loss control pills can be minimized if a slow-acting internal breakeris employed. In particular, core-flow tests have indicated that combining asuccinoglycan-based pill with a hydrochloric acid internal breaker enables afluid loss system with sustained control followed by delayed breakback andcreates only low levels of impairment. To describe the delayed breaking of thesuccinoglycan/hydrochloric acid system, a model, based on bond-breakingrate, has been used. With this model, it is possible to predict the change ofthe rheologic properties of the polymer as a function of time for various for-mation temperatures, transition temperatures of the succinoglycan, and acidconcentrations. The model can be used to identify optimal formulations ofsuccinoglycan and acid breaker on the basis of field requirements, such as thetime interval over which fluid loss control is needed, the overbalance pressurea pill should be able to withstand, and the brine density required.

Polyether-Modified Polysaccharides

Compositions containing mixtures of metal hydroxides a polysaccharide,partially etherified with hydroxyethyl and hydroxypropyl groups, are used asfluid loss additives for aqueous, clay-mineral-based drilling muds [1437].

Scleroglucan

A combination of graded calcium carbonate particle sizes, a nonionic poly-saccharide of the scleroglucan type, and a modified starch, has been claimedfor use as a fluid loss formulations [915]. It is important that the calcium car-bonate particles are distributed across a wide size range to prevent filtration

or fluid loss into the formation. Because the filter-cake particles do not invadethe wellbore due to the action of the biopolymer and starch, no high-pressurespike occurs during the removal of the filter-cake.

The rheologic properties of the fluid allow it to be used in a number ofapplications in which protection of the original permeable formation is desir-able. The applications include drilling, fracturing, and controlling fluid lossesduring completion operations, such as gravel packing or well workovers.

Gellan (Xanthan, Sderoglucan, or Wellan)

It has been found that gellan has good characteristics as a filtrate reducerin water-based drilling fluids [515,516]. Preferential use is made of nativegellan, which has a considerable gelling capacity and good solubility. It shouldbe noted that native gellan contains cellular debris or other insoluble residue.Xanthan gum has been used extensively in the oil industry as a viscosifier forvarious applications [1327]. Deacetylated xanthan gum is used in guar-freecompositions instead of guar [1064].

Synthetic Polymers

Polyhydroxyacetic Acid

A low-molecular-weight condensation product of hydroxyacetic acid withitself or compounds containing other hydroxy acid, carboxylic acid, or hydroxy-carboxylic acid moieties has been suggested as a fluid loss additive [164]. Pro-duction methods of the polymer have been described. The reaction productsare ground to 0.1 to 1500 \i particle size. The condensation product can be usedas a fluid loss material in a hydraulic fracturing process in which the fracturingfluid comprises a hydrolyzable, aqueous gel. The hydroxyacetic acid conden-sation product hydrolyzes at formation conditions to provide hydroxyaceticacid, which breaks the aqueous gel autocatalytically and eventually providesthe restored formation permeability without the need for the separate additionof a gel breaker [315-317,329].

Lignosulfonates

Polymer of Monoallylamine

A water-soluble polymer of monoallylamine can be used in conjunction witha sulfonated polymer, such as a water-soluble lignosulfonate, condensed naph-thalene sulfonate, or sulfonated vinyl aromatic polymer, to minimize fluid lossfrom the slurry during well cementing operations [1510,1511]. The polymer

Allylamine

Figure 2-4. Monoallylamine.

of monoallylamine may be a homopolymer or a copolymer and may be cross-linked or uncrosslinked. These components react with each other in the pres-ence of water to produce a gelatinous material that tends to plug porous zonesand to minimize premature water loss from the cement slurry to the formation.

Polyphenolic Materials for Oil-Based Drilling Fluids

Organophilic polyphenolic materials for oil-based drilling fluids have beendescribed [407]. The additives are prepared from a polyphenolic materialand one or more phosphatides. The phosphatides are phosphoglycerides ob-tained from vegetable oils, preferably commercial lecithin. Humic acids, ligno-sulfonic acid, lignins, phenolic condensates, tannins; the oxidized, sulfonated,or sulfomethylated derivatives of these polyphenolic materials may serve aspolyphenolic materials.

A fluid loss additive is described that uses graded calcium carbonate particlesizes and a modified lignosulfonate [917]. Optionally, a thixotropic polymer,such as a wellan or xanthan gum polymer, is used to keep the CaCC>3 andlignosulfonate in suspension. It is important that the calcium carbonate parti-cles are distributed across a wide size range to prevent filtration or fluid lossinto the formation. Furthermore, the lignosulfonate must be polymerized toan extent effective to reduce its water solubility. The modified lignosulfonate(lignin sulfonate) is necessary for the formation of a filter-cake essentially onthe surface of the wellbore. Because the filter-cake particles do not invade thewellbore due to the action of the modified lignosulfonate, no high-pressurespike occurs during the removal of the filter-cake, which would indicate dam-age of the formation and wellbore surface. The additive is useful in fracturingfluids, completion fluids, and workover fluids.

Tests showed that a fluid loss additive on a base of a sulfonated tannic-phenolic resin is effective for fluid loss control at high temperature and pressure,and it exhibits good resistance to salt and acid [868].

Grafting to Lignin and Lignite

In hydraulic cement slurries, fluid loss additives based on sulfonated orsulfomethylated lignins have been described.

Sulfonated or sulfomethylated lignins are reacted with phenol-blockingreagents, such as ethylene oxide, propylene oxide, or butylene oxide [1571].

The fluid loss and thickening time characteristics of the cement slurry isaltered, either by increasing the molecular weight of the lignin by cross-linking with formaldehyde or epichlorohydrin or by adding agents such assodium sulfite, sodium metasilicate, sodium phosphate, and sodium naphtha-lene sulfonate.

Another method is animation with a polyamine and an aldehyde [ 1567]. Theformulation also contains sodium carbonate, sodium phosphate, sodium sulfite,sodium metasilicate, or naphthalene sulfonate. The sulfonated or sulfomethyl-ated aminated lignin shows less retardation (shorter thickening time) than asulfonated or sulfomethylated lignin without the attached amine.

Lignite can be grafted with synthetic comonomers to obtain lignite fluidloss additives [873]. Comonomers can be AMPS, N,N-dimethylacrylamide,acrylamide, vinylpyrrolidone, vinylacetate, acrylonitrile, dimethylaminoethylmethacrylate, styrene sulfonate, vinyl sulfonate, dimethylaminoethyl meth-acrylate methyl chloride quaternary, and acrylic acid and its salts.

Various polymers, for example, lignin, lignite, derivatized cellulose poly-vinyl alcohol, polyethylene oxide, polypropylene oxide, and polyethylene-imine, can be used as a backbone polymer onto which some other groupscan be grafted [650]. The grafted pendant groups can be AMPS, acrylonitrile,N,N-dimethylacrylamide, acrylic acid, N,N-dialkylaminoethylmethacrylate,and their salts. The backbone polymer makes 5% to 95% by weight of thegraft polymer, and consequently the pendant groups are in the range of 5% to95% by weight of the graft polymer.

A polymeric composition for reducing fluid loss in drilling muds and wellcement compositions is obtained by the free radical-initiated polymerizationof a water-soluble vinyl monomer in an aqueous suspension of lignin, modifiedlignins, lignite, brown coal, and modified brown coal [705,1847]. The vinylmonomers can be methacrylic acid, methacrylamide, hydroxyethyl acrylate,hydroxypropyl acrylate, vinylacetate, methyl vinyl ether, ethyl vinyl ether,N-methylmethacrylamide, N,N-dimethylmethacrylamide, vinyl sulfonate, andadditional AMPS. In this process a grafting process to the coals by chaintransfer may occur.

Graft copolymers and other polymers are prepared in a way that is com-mon in polymerization techniques [1894]. For example, they are made byproviding a foamed, aqueous solution of water-soluble monomeric material,initiating polymerization by adding an initiator, exothermically polymerizing

Methylvinylether Ethylvinylether

Figure 2-5. Methyl vinyl ether, ethyl vinyl ether.

the monomeric material to form a foamed gel, and comminuting the gel. Prefer-ably, the polymerization temperature is held below 60° C for at least the first10 minutes of the polymerization and then rises exothermically to a highertemperature. Graft copolymers that can be made by this technique and thatare of particular value as fluid loss additives are formed from a polyhydroxypolymer, a sulfonate monomer, acrylamide, and acrylic acid.

The vinyl grafted wattle tannin [872] comprises a wattle tannin grafted withAMPS and small amounts of acrylamide. The wattle tannin is present in anamount between 2% and 14% by weight. The AMPS is present in an amountbetween 98% and 84% accordingly.

Latex

A thermally stable drilling fluid system includes an additive that comprisesstyrene-butadiene copolymers having an average molecular weight greaterthan approximately 500,000 Dalton, wherein the drilling fluid system exhibitsfluid loss control at high-temperature (350° F) and high-pressure conditions.The drilling fluid may be either all-oil- or water-in-oil-based. In each case,gas oil is preferably used, although mineral oils with low aromatic content,synthetic oils, or vegetable oils are suitable alternatives. The all-oil fluid furthercomprises an organophilic clay (hectorite, bentonite, and mixtures thereof).

Optionally, a surfactant that behaves as an emulsifier and a wetting agentand a weighting agent (calcium carbonate, barite, hematite, and mixturesthereof) are included. Suitable emulsifiers and wetting agents include sur-factants; ionic surfactants such as fatty acids, amines, amides, and organicsulfonates; and mixtures of any of these with nonionic surfactants such asethoxylated surfactants. The water-in-oil emulsion may consist of an oil phase,a water phase (salt or fresh), a surfactant, a weighting agent, and salts or elec-trolytes [125,819,820].

Poly vinyl Alcohol

Partially hydrolyzed polyvinylacetate polymer (PVA), a crosslinker for thepolymer, and other additives such as calcium sulfate can be used in cementingcasing strings [1253]. PVA is not totally water-soluble below 500C, but is,instead, water swellable. It is believed that the individual PVA particles swelland soften to form small gel-balls in the slurry. These gel-balls deform byflattening and become a part of the filter-cake, greatly reducing the filter-cake permeability, thus giving good fluid loss control. Because PVA is nottotally water soluble, it does not significantly increase the slurry viscosity.Furthermore, PVA does not delay the setting of the cement, and it has high-temperature properties that are relatively insensitive to external conditions.

PVA can be crosslinked with a crosslinker present in a molar concentra-tion, relative to monomer residues, of 0.01% to 1.0%. The crosslinker maybe formaldehyde, acetaldehyde, glyoxal, glutaraldehyde, maleic acid, oxalicacid, dimethylurea, polyacrolein, diisocyanate, divinyl sulfonate, or a chlorideofadiacid[89-91].

Humic Acid Derivatives

Polysulfonated humic acid resin is a drilling fluid filtrate loss additive com-posed of three mud additives: sulfonated chromium humate, sulfonated phe-nolic resin, and hydrolytic ammonium polyacrylate [1716]. Field applicationand effectiveness of polysulfonated humic acid resin, especially in extra-deepwells and in sylvite and undersaturated salt muds, have been described. Poly-sulfonated humic acid resin resists high temperature, salt concentration, andcalcium contamination, as well. A polysulfonated humic acid resin-treateddrilling fluid has stable properties and good rheologic characteristics and canimprove cementing quality.

Oil-Based Well Working Fluids

Adducts of aminoethylethanolamine and polyethylenepolyamines withhumic acid-containing materials and fatty acids [1400] are useful as fluidloss additives in oil-based drilling fluids [854].

In addition, a fluid loss additive for oil-based drilling fluids, which con-sists of fatty acid compounds and lignite or humic acid, an oil-soluble oroil-dispersible amine or amine salt with phosphoric acid, or an aliphatic amideor hydroxyamide [392], has been described.

Polyethyleneimine

A liquid fluid loss-reducing additive for well cementing compositions con-sists of water, polyethyleneimine, an alkali metal salt of alkylbenzene sulfonicacid, and an alkali metal salt of naphthalene sulfonic acid, condensed withformaldehyde [231]. The polyethyleneimine has a molecular weight of 40,000to 60,000 Dalton and is present in an amount of 50% to 55% by weight ofthe additive. The alkali metal salt of the alkylbenzene sulfonic acid is sodiumdodecylbenzene sulfonate and is present in an amount of 3% to 4% by weightof the additive. The alkali metal salt of the naphthalene sulfonic acid that iscondensed with formaldehyde is sodium naphthalene sulfonate. The sodiumnaphthalene sulfonate-formaldehyde condensation product has a molecularweight of 1400 to 2400 Dalton, and the condensation product is present inan amount of 3% to 4% by weight of the additive. The alkyl group of thealkylbenzene sulfonic acid salt contains from 8 to 16 carbon atoms.

Acrylics

Copolymers of N-Vinyl-2-pyrrolidone and Sodium-2-acrylamido-2-methylpropane Sulfonate

Homopolymers and copolymers from amido-sulfonic acid or salt contain-ing monomers can be prepared by reactive extrusion, preferably in a twinscrew extruder [1660]. The process produces a solid polymer. Copolymersof acrylamide, N-vinyl-2-pyrrolidone, and sodium-2-acrylamido-2-methyl-propane sulfonate have been proposed to be active as fluid loss agents. Anothercomponent of the formulations is the sodium salt of naphthalene formaldehydesulfonate [207]. The fluid loss additive is mixed with hydraulic cements in suit-able amounts.

A fluid loss additive for hard brine environments has been developed [1685],which consists of hydrocarbon, an anionic surfactant, an alcohol, a sulfonatedasphalt, a biopolymer, and optionally an organophilic clay, a copolymer ofN-vinyl-2-pyrrolidone and sodium-2-acrylamido-2-methylpropane sulfonate.Methylene-bis-acrylamide can be used as a crosslinker [1398]. Crosslinkingimparts thermal stability and resistance to alkaline hydrolysis.

Terpolymers and Tetrapolymers

Terpolymers and tetrapolymers have been proposed as fluid loss additivesfor drilling fluids [1676,1679]. The constituent monomers are a combinationof nonionic monomers and ionic monomers.

The nonionic monomer can be acrylamide, N,N-dimethylacrylamide,N-vinyl-2-pyrrolidone, N-vinyl acetamide, or dimethylamino ethyl methacryl-ate. Ionic monomers are AMPS, sodium vinyl sulfonate, and vinylbenzenesulfonate. The terpolymer should have a molecular weight between 200,000to l,000,000Dalton.

A formulation consisting of 2-acrylamido-2-methylpropane sulfonic acid,acrylamide, and itaconic acid has been proposed [676]. Such polymers areused as fluid loss control additives for aqueous drilling fluids and are advanta-geous when used with lime- or gypsum-based drilling muds containing solublecalcium ions.

For seawater muds, another example [139] is a copolymer of 10% byweight AMPS and 90% by weight acrylic acid in its sodium salt form. Thepolymers have an average molecular weight between 50,000 and 1,000,000Dalton.

A terpolymer from a family of intramolecular polymeric complexes(i.e., polyampholytes), which are terpolymers of acrylamide-methyl styrenesulfonate-methacrylamido propyltrimethylammonium chloride [106,1418],has been reported.

A terpolymer formed from ionic monomers AMPS, sodium vinyl sulfonateor vinylbenzene sulfonate itaconic acid, and a nonionic monomer, for exam-ple, acrylamide, N,N-dimethylacrylamide, N-vinylpyrrolidone, N-vinyl acet-amide, and dimethylaminoethyl methacrylate, is used as a fluid loss agent in oilwell cements [1562]. The terpolymer should have a molecular weight between200,000 and 1,000,000 Daltons. The terpolymer comprises AMPS, acrylamide,and itaconic acid. Such copolymers also serve in drilling fluids [1892].

A tetrapolymer consisting of 40 to 80 mole-percent of AMPS, 10 to 30mole-percent of vinylpyrrolidone, 0 to 30 mole-percent of acrylamide, and0 to 15 mole-percent of acrylonitrile was also a suggested as a fluid lossadditive [1061]. Even at high salt concentrations, these polymers yield high-temperature-stable protective colloids that provide for minimal fluid loss underpressure.

Similar copolymers with N-vinyl-N-methylacetamide as a comonomer havebeen proposed for hydraulic cement compositions [669]. The polymers consistof AMPS in an amount of 5% to 95%, vinylacrylamide in an amount of 5% to95%, and acrylamide in an amount of 0% to 80%, all by weight. The polymersare effective at well bottom-hole temperatures ranging from 200° to 500° Fand are not adversely affected by brine. Terpolymers of 30 to 90 mole-percentAMPS, 5 to 60 mole-percent of styrene, and residual acrylic acid are alsosuitable for well cementing operations [253].

Copolymer ofAcrylamide/Vinyl Imidazole

A fluid loss additive useful in cementing oil and gas wells is a blend[423,424,1015] of a copolymer of acrylamide/vinyl imidazole. The secondcomponent in the blend is a copolymer of vinylpyrrolidone and the sodiumsalt of vinyl sulfonate. Details are given in Table 2-2. The copolymers aremixed together in the range of 20:80 to 80:20. Sodium or potassium saltsor a sulfonated naphthalene formaldehyde condensate can be used as adispersant.

Table 2-2Copolymer Blends for Fluid Loss [423,424,1015]

Molecular weightCopolymer Composition (Dalton)

Acrylamide/vinyl imidazole 95:5 to 5:95 100,000 to 3,000,000Vinylpyrrolidone and the 80:20 to 20:80 100,000 to 3,000,000

sodium salt of vinyl sulfonate

N-Vinylpyrrolidone/Acrylamide Random Copolymer

An N-vinylpyrrolidone/acrylamide random copolymer (0.05% to 5.0% byweight) is used for cementing compositions [371, 1076]. Furthermore,a sulfonate-containing cement dispersant is necessary. The additive can beused in wells with a bottom-hole temperature of 80° to 300° F. The fluid lossadditive mixture is especially effective at low temperatures, for example, below100° F and in sodium silicate-extended slurries.

Copolymer of N-Vinylpyrrolidone and a Salt of Styrenesulfonic Acid

For aqueous cement slurries a copolymer of N-vinylpyrrolidone and a saltof styrenesulfonic acid has been proposed [1585]. A naphthalene sulfonic acidsalt condensed with formaldehyde serves as a dispersant.

Copolymer ofAcrylamide and N-Vinylamide

The fluid loss control of aqueous, clay-based drilling mud compositions isenhanced by the addition of a hydrolyzed copolymer of acrylamide and anN-vinylamide [402]. The copolymer, which is effective over a broad rangeof molecular weights, contains at least 5 mole-percent of the N-vinylamideunits, which are hydrolyzed to N-vinylamine units. The copolymers can bemade from various ratios of N-vinylamide and acrylamide by using commonradical-initiated chain growth polymerization techniques.

N-vinylamide can be polymerized by the inverse emulsion polymeriza-tion technique [1050]. The polymers of that monomer are used in cementingcompositions for oil and gas wells. The method for preparing the inverse, orwater-in-oil, emulsion involves colloidally dispersing an aqueous solution con-taining 10% to 90% by weight water-soluble N-vinylamide in a hydrocarbonliquid, using a surfactant with a hydrophilic-lipophilic balance value between4 and 9. The weight ratio of a monomer-containing aqueous solution to hydro-carbon liquid is preferably from 1:2 to 2:1. To initiate the polymerization, anazo-type free radical initiator is used. The resultant high-molecular-weightpolymer emulsion has a low viscosity ranging from 2 to less than 10 cP at 15%solids (60 rpm Brookfield and 20° C), thus eliminating problems of solutionviscosity that arise when the polymer is prepared by a solution polymerizationprocess.

Copolymer ofStyrene with 2-Acrylamido-2-methylpropane Sulfonic Acid

Copolymers of styrene with AMPS having fluid loss capabilities for use inwell cementing operations have been described [254]. The styrene is present

Figure 2-6. 2-Acryiamido-2-methylpropane sulfonic acid.

in an amount of 15 to 60 mole-percent, and the AMPS is present in an amountof 40 to 85 mole-percent. The polystyrene units are not hydrophilic, so AMPSwill affect solubility in water.

Copolymers of Acrylic Acid and Itaconic Acid

Copolymers of mainly acrylic acid and 2% to 20% by weight of itaconicacid are described as fluid loss additives for aqueous drilling fluids [138]. Thepolymers have an average molecular weight between 100,000 and 500,000Dalton and are water dispersible. The polymers are advantageous when usedwith muds containing soluble calcium and muds containing chloride ions, suchas seawater muds.

Copolymers of 2-Acrylamido-2-methylpropane Sulfonic Acid

Copolymers from the monomers AMPS, diallyldimethylammonium chlo-ride (DADMAC), N-vinyl-N-methylacetamide (VIMA), acrylamides, andacrylates are particularly useful for fluid loss additives [824]. The molecularweights of the copolymers range from 200,000 to 1,000,000 Dalton. The co-polymers are used in suspensions of solids in aqueous systems, includingsaline, as water binders. In these systems, the water release to a formation issubstantially reduced by the addition of one or more of these copolymers.

A suitable formulation for filtration reducers, good temperature resistance(over 200° C), and good tolerance to salts and calcium compounds is a copoly-mer of AMPS and other vinyl monomers [HOl].

Poly-N-vinyl Lactam

An additive described as reducing the water loss and enhancing other prop-erties of well-treating fluids in high-temperature subterranean environmentsconsists of polymers or copolymers from N-vinyl lactam monomers or vinyl-containing sulfonate monomers. Organic compounds like lignites, tannins, andasphaltic materials are added as dispersants [175].

Phthalimide

Figure 2-7. Phthalimide.

Phthalimide as a Diverting Material

Phthalimide has been described as a diverting material, or fluid loss addi-tive, for diverting aqueous treating fluids, including acids, into progressivelyless permeable portions of a subterranean formation [485]. The additive alsoreduces the fluid loss to the formation of an aqueous or hydrocarbon treat-ing fluid utilized, for example, in fracturing treatments. The use of the ma-terial depends on the particle size of the material that is used. Phthalimidewill withstand high formation temperatures and can be readily removed fromthe formation by dissolution in the produced fluids or by sublimation at el-evated temperatures. The material is compatible either with other formationpermeability-reducing materials or formation permeability-increasing mate-rials. The phthalimide particles act by sealing off portions of a subterraneanformation by blocking off the fissures, pores, channels, and vugs that grantaccess to the formation from the wellbore penetrating the formation.

Tall Oil Pitch

A fluid loss additive for well drilling fluids consists of air-blown tall oilpitch, which has a softening point (ring and ball) of 100° to 165° C. Tall oilpitch is available as the residue from the distillation of tall oil. Its solubilityis low in fatty acids and high in fatty esters, higher alcohols, and sterols.Blowing air through tall oil pitch at an elevated temperature partially oxidizesand polymerizes the material and drives off volatiles. Blowing reduces thevolume of the pitch by 30% and increases the viscosity and the softeningpoint. The softening point of the resultant blown pitch is therefore a measureof the degree of oxidation-polymerization that has occurred. It has been foundthat optimal properties as a fluid loss additive are given by blown tall oil pitchesthat have a softening point between 125° and 130° C [1851].

Figure 2-8. Acrylic acid, acrylamide, methacrylic acid, methacrylamide,hydroxyethyl acrylate, N,N-dimethylmethacrylamide, dimethylaminoethylmethacrylate, N-methylmethacrylamide.

Dimethylaminoethyl methacrylate N-Methylmethacrylamide

Hydroxyethyl acrylate N,N-Dimethylmethacrylamide

Methacrylic acid Methacrylamide

Acrylic acid Acrylamide

Table 2-3Summary of Formulations of Fluid Loss Additives

Composition References

Lignite or humic acid oil-soluble or oil-dispersible amine [392]or amine salt with phosphoric acid, or an aliphaticamide or hydroxyamide, and fatty acid compounds

Adducts of aminoethylethanolamine and [854]polyethylenepolyamines with humic acid-containingmaterials and fatty acidsOb'D

Polysulfonated humic acid resin-sulfonated chromium humate, [1716]sulfonated phenolic resin, and hydrolytic ammoniumpolyacrylateCDHT

Polyvinylacetate partially hydrolyzed crosslinker: [89,90,1253]formaldehyde, acetaldehyde, glyoxal, glutaraldehyde,maleic acid, oxalic acid, dimethylurea, polyacrolein,diisocyanate, divinyl sulfonate, calciumsulfateCHT

Sodium dodecylbenzene sulfonate and naphthalene [231]sulfonic acid salt condensed with formaldehyde

Hydrocarbon, an anionic surfactant, an alcohol, a [1685]sulfonated asphalt, biopolymer, organophilic clay,copolymer of N-vinyl-2-pyrrolidone, andsodium-2-acrylamido-2-methylpropane sulfonateSB

Copolymers of acrylamide, N-vinyl-2-pyrrolidone, [207]and sodium-2-acrylamido-2-methylpropanesulfonate; dispersant is sodium salt of naphthaleneformaldehyde sulfonatec

Combination of nonionic monomers and ionic monomers [1676,1679]acrylamide, N,N-dimethylacrylamide, N-vinylpyrrolidone,N-vinyl acetamide, or dimethylamino ethyl methacrylate,2-acrylamido-2-methylpropane sulfonic acid, sodium vinylsulfonate, and vinylbenzene sulfonate

Copolymer of 2-acrylamido-2-methylpropane sulfonic acid, [676]acrylamide, and itaconic acidAqD

Copolymer of 2-acrylamido-2-methylpropane sulfonic acid, [1376]vinylphosphonic acid,diallyldimethylammonium chlorides

Copolymer 2-acrylamido-2-methylpropane sulfonic [139]acid and acrylic acid sodium salt

Copolymers of acrylamide-methyl styrene sulfonate- [1418]methacrylamido propyltrimethylammonium chloride

Copolymer from 2-acrylamido-2-methylpropane [ 1562]sulfonic acid, itaconic acid, and acrylamide0

Copolymer of 2-acrylamido-2-methylpropane [ 1061 ]sulfonic acid, vinylpyrrolidone, acrylamide, andacrylonitrileHTSB

Continued.

Table 2-3 (continued)

Composition References

Copolymers with N-vinyl-N-methylacetamide, acrylamide, [669]and2-acrylamido-2-methylpropane sulfonic acid c 'H T S B

Terpolymers of 2-acrylamido-2-methylpropane [253]sulfonic acid, styrene, and acrylic acidc

Copolymer of acrylamide and vinyl imidazole, copolymer of [423,424,1015]N-vinylpyrrolidone, and the sodium salt of vinyl sulfonate;dispersants are sodium or potassium salts or a sulfonatednaphthalene formaldehyde condensatec

Copolymer of N-vinylpyrrolidone-acrylamideC'LT [371]Copolymer of N-vinylpyrrolidone and a salt of styrene- [1585]

sulfonic acid; dispersant is naphthalene sulfonic acid saltc

Hydrolyzed copolymer of acrylamide and an [402]N-vinylamideA^CbD

Copolymers of styrene with 2-acrylamido-2-methylpropane [254]sulfonic acidc

Styrene sulfonate polymersHT [105]Copolymers of acrylic acid and itaconic ac id A q D S [138]Copolymers 2-acrylamido-2-methylpropane sulfonic acid, [824]

diallyldimethylammonium chloride, N-vinyl-N-methyl-acetamide (VIMA), acrylamides, and acrylates

Polyanionic cellulose and sulfonate-containing polymer [812]Copolymer of 2-acrylamide-2-methyl propane sulfonic acid [1101]

and other vinyl monomersHTSB

Polymers or copolymers from N-vinyl lactam monomers or [175]vinyl-containing sulfonate monomers; lignites, tannins,and asphaltic materials are added as dispersantsHT

Tall oil pitch [1851]Admixtures of carboxymethylhydroxyethylcellulose or [252]

copolymers and copolymer salts of N,N-dimethylacryl-amide, AMPS, together with a copolymer of acrylicacid/AMPSC'HT

Hydroxyethylcellulose, degree of substitution of 1.1 to [1473]1.6Aq'D

Starch, crosslinkedAqDHT [632]Starch, granular mica, fine particulate starches, mixture of [337]

natural and modified plus an enzyme modified:carboxymethyl and hydroxypropyl derivatives^

Natural corn, potatoes, wheat, and soy coating with a [1850]surfactant sorbitan monooleate, ethoxylated butanol,or ethoxylated nonylphenol, to aid dispersionFF

Guar gum, hydrophobically modified; used together with [93]polymers or reactive clayA q D

Succinoglycan [ 1069,1070]

Table 2-3 (continued)

Composition References

Polyether-modified polysaccharides; mixtures of metal [1437]hydroxides polysaccharide partially etherified withhydroxyethyl and/orhydroxypropyl groupsAqCbD

Scleroglucan, starch, modified calcium carbonate particles [915]GellanA£i'D [515]Hydroxyacetic acid, low-molecular-weight condensation [164]

product^Polymonoallylamine, crosslinked or uncrosslinked ligno- [1510,1511]

sulfonate, condensed naphthalene sulfonate, or sulfonatedvinyl aromatic polymer; components react with each other inthe presence of water to produce a gelatinous material0

Polyphenolic materials prepared from a polyphenolic material [407](humic acids, lignosulfonic acid, lignins, phenoliccondensates, tannins) andphosphatides (lecithin)ObD

Tannic-phenolicresin, sulfonatedHTHPSB [868]Lignins, sulfonated or sulfomethylated; reaction products with [1571]

ethylene oxide, propylene oxide, butylene oxidec

Lignite grafted with synthetic comonomers; comonomers can [873]be AMPS, dimethylacrylamide, acrylamide,vinylpyrrolidone, vinylacetate, acrylonitrile,dimethylaminoethyl methacrylate, styrene sulfonate, vinylsulfonate, dimethylaminoethyl methacrylate methyl chloridequaternary, and acrylic acid and its salts

Lignin, brown coal; polymer of methacrylic acid, methacryl- [705,1847]amide, hydroxyethyl acrylate, hydroxypropyl acrylate, vinylacetate, methyl vinyl ether, ethyl vinyl ether, N-methylmeth-acrylamide, N,N-dimethylmethacrylamide, vinyl sulfonate,or 2-acrylamido-N-methylpropane sulfonic acid; free radicalpolymerization of a water-soluble vinyl monomer in an aque-ous suspension of coalsDC

Wattle tannin, vinyl grafted AMPS and acrylamide [872]lS-endo-borneol, camphor, iodine, P-carotene, lycopene, [1048,1049]

cholesterol, lanosterol, and agnosterolCopolymer with vinylamide morpholineAqD [1773]Lignite and GilsoniteHTHP [419]Peanut hulls [625-627]

Aq, Aqueous; C, cementing; Cb, clay based; D, drilling fluids; FF, fracturing fluids; HP, high pressureapplication; HT, high-temperature application; LT, low temperature; Ob, oil based; S, seawater mud; SB, saltand brine tolerant.