（2） The working principle of fluid loss additive of Zoranoc is a kind of chemical agent which can reduce the filtration rate of drilling fluid. Most of the filtrate reducer are water-soluble polymer compounds, which can reduce the filtration of drilling fluid through the following ways.

A . Stabilizing colloidal particles as mentioned above, clay particles in drilling fluid should have proper size distribution and more fine clay particles. On the one hand, the drilling fluid with large filtration loss has more coarse particles, on the other hand, fine particles have been flocculated into coarse particles, and there are less sol particles (< 100plm) in the system. The filter cake formed by the drilling fluid with more coarse particles and less fine particles is loose and porous, so the filtration loss is large. Filtrate reducer is a kind of macromolecular compound which can dissociate negative charge group in water. On the one hand, it can adsorb on the clay surface to form an adsorption layer to prevent the clay particles from flocculating; on the other hand, it can stabilize the fine particles separated under the action of drilling fluid circulation stirring by adsorption, and no longer bond into large particles. In this way, a sufficient proportion of fine particles can be ensured, so that the drilling fluid can form a thin and dense filter cake and reduce the filtration loss. This effect is also known as glue protection. It should be noted that the concentration of the fluid loss additive made by Zoranoc in the drilling fluid must be high enough to surround the broken clay particles. On the one hand, it will bring high negative charge density to the surface of clay particles, increase the f potential and increase the repulsion force between particles; on the other hand, due to the hydration of hydration groups, a thick hydration film is formed, which makes it difficult for clay particles to merge and grow. If the concentration of the fluid loss additive is lower than the concentration required for protection, the fluid loss additive will not only have no protective effect on the colloidal particles, but also make the clay particles easier to agglomerate.

B . Increase the viscosity of the filtrate. According to the static filtration equation, the filtrate loss is inversely proportional to the second power of the filtrate viscosity of drilling fluid. Polymer fluid loss additive can increase the viscosity of filtrate and reduce the filtration rate. However, it should be noted that the increase of viscosity will reduce the ROP. Therefore, it is generally required that the viscosity of fluid loss additive should not be greatly increased.

A.  The plugging effect of filtrate reducer is a kind of high polymer fluid loss additive. Its molecular size is in the range of colloidal particles. Adding these additives can increase the content of colloidal particles in drilling fluid, and they can block the pores of filter cake. These macromolecules seal pores in two ways. Let the diameter of polymer random coil be d. , pore diameter D,. If 0.3d, < D. The polymer can be retained in the pores to form incomplete physical plugging and reduce the permeability of filter cake, which is called trapping. If D. >d. The polymer can not enter the pore, but can block the pore entrance, which is called plugging, and can also reduce the filtration amount.

（3） Common filtrate reducer

A . Carboxymethyl cellulose (CMC) is a kind of natural macromolecular compound. Its structure is poly (p-glucose). The hydroxyl groups in the molecular chain form intramolecular and intermolecular hydrogen bonds and crystallize, so it is insoluble in water. Water soluble modified cellulose can be produced after modification. Carboxymethyl cellulose (CMC) is a modified product of cellulose. Its sodium salt is widely used in oil field, also known as sodium carboxymethyl cellulose. Its molecular structure is: (sodium carboxymethyl fiber system) CMC is a linear water-soluble polymer. Its two main performance indexes are relative molecular weight and degree of substitution (the number of hydrogen in hydroxyl group of each dehydrated sugar unit is replaced by carboxymethyl). The higher the molecular weight of CMC, the higher the viscosity of aqueous solution. In industry, CMC is divided into three grades according to the viscosity of its aqueous solution: CMC with high viscosity, at 25. The viscosity of 1% aqueous solution is 400-500 MPa · s at C, and it is not used as a fluid loss reducer generally; CMC with medium viscosity is 50-270m1% · s when the viscosity of 2% aqueous solution is 25%, which can be used as fluid loss reducer of general drilling fluid, which can not only reduce the filtration rate but also increase the viscosity of drilling fluid; CMC with low viscosity is 25. When the viscosity of 2% aqueous solution is less than 50 MPa · s at C, it can be used as a filtrate reducer for weighting drilling fluid to avoid excessive viscosity. The degree of substitution is the main factor to determine the water solubility of CMC. Only when the degree of substitution is greater than 0.5, CMC is easily soluble in water. The higher the degree of substitution, the better the water solubility. CMC, as a drilling fluid treatment agent, has a good effect when the degree of substitution is 0.6-0.9. CMC is a kind of filtrate reducer with strong salt and temperature resistance, and also has certain calcium resistance. It is suitable for the preparation of sea water drilling fluid, saturated salt water drilling fluid and calcium treatment drilling fluid. When drilling ultra deep wells over 5000m in China, CMC can be used as filtrate reducer to obtain better results. CMC can reduce the filtration rate by stabilizing colloidal particles. CMC dissociates the long chain polyvalent anion in drilling fluid, and the hydroxyl group in the chain forms hydrogen bond adsorption with the oxygen atom on the surface of clay particles. A part of carboxyl group generates electrostatic attraction between aluminum ions at the edge of bond breaking, and a part of carboxyl group forms a hydration layer on the surface of clay particles through hydration, and increases the f potential on the surface of clay particles. Due to the long molecular chain of CMC, one molecule can adsorb multiple clay particles at the same time, forming a mixed network structure with clay particles, avoiding the mutual adhesion of clay particles, which greatly improves the coalescence stability of clay particles and is conducive to the formation of dense and tough filter cake. In addition, CMC can increase the viscosity of filtrate and reduce the filtrate loss.

B . Humic acid and its derivatives humic acid are the products of partial decomposition of biological residues in the presence of air and water, and are natural polymer compounds that can be extracted from peat, lignite or some soils. Humic acid is not a single compound, but a mixture of hydroxyl aromatic carboxylic acids with different molecular sizes and structures. It can be divided into three components by different solvents: elemental analysis shows that the chemical composition of humic acid is generally C: 55% ～ 65%; H: 5.5% ～ 6.5%; 0:25% ～ 35%; N: 3% ～ 4%; and a small amount of S and P. The relative molecular weights of humic acid components are also quite different. The molecular weight of fulvic acid is 300-400, that of brown humic acid is 2000-20000, and that of black humic acid is 104-105. The chemical structure of humic acid is very complex and not very clear. It is generally believed that it is a large complex composed of several similar structural units, each of which is composed of nuclear, bridge and active groups. The main functional groups of humic acid are carboxyl, carbonyl, hydroxyl, methoxy and ether bonds. The structural fragments of humic acid and its modified products can be expressed as: XOX / C, cl-t, Zui / CI | o, where x = h, y = h, humic acid; X = k, y = h, potassium humate, k-hm; X = Na, y = h, sodium humate, coal alkali agent, Na HM; X = Na, y = N02, sodium nitrohumate, Na NHM; X = Na, y = ch: so, Na, sodium sulfomethylhumate, Na smhm. The following humic acid derivatives are mainly used as filtrate reducer. 1) Coal alkali agent (Na HM) is prepared from lignite with appropriate amount of caustic soda and water. The main effective component is sodium humate. The content of humic acid in lignite is 20% ～ 80%. Humic acid is difficult to dissolve in water, but easy to dissolve in alkali solution to form sodium humate. The ratio of coal alkali agent prepared on site is: Brown alkali: caustic soda: water = 15: (1-3): (50-200); the specific ratio depends on the humic acid content of lignite and the actual use conditions. Because the basic framework of humic acid is carbon chain and aromatic ring structure, coal alkali has good thermal stability. Laboratory experiments show that for fresh water drilling fluid, at 200. The filtrate reduction performance is basically unchanged when C is kept at constant temperature for 24h. However, the salt and calcium resistance of coal alkali agent is poor. Because humic acid contains more functional groups which can be adsorbed with clay particles, especially ortho bisphenol hydroxyl group, and sodium carboxyl group with strong hydration effect, sodium humate has both filtration reducing effect and dilution effect. The filtration reduction mechanism of coal alkali agent lies in that the anionic macromolecular sodium humate with various functional groups is adsorbed on the surface of clay particles to form an adsorption hydration layer, and at the same time, it improves the f potential of clay particles, thus greatly increasing the coalescence stability of clay particles, keeping the clay particles in the drilling fluid in a multi-stage dispersion state, which is easy to form dense filter cake. In particular, the high viscosity and elasticity of the adsorption hydration membrane of clay particles can make the filter cake more compact and reduce the filtration loss. In addition, it can increase the viscosity of filtrate and reduce the filtration loss. 2) Chromium lignite chrome lignite (chromium humic acid) is the reaction product of dichromate and lignite. The mass ratio of humic acid to dichromate is 3:1 or 4:1. The mixture of the two is at 80. The effective component is chromium humic acid. The reaction includes two steps: oxidation and chelation. Oxidation enhances the hydrophilicity of humic acid. At the same time, dichromate is reduced to Cr3 +, and Cr3 + is chelated with oxidized humic acid or humic acid. Chromium humic acid has higher solubility in water, and its salt resistance and calcium resistance are better than that of sodium humate. Chromium humic acid has both filtration reducing and dilution effects. Especially when it is used with iron chromium salt, it has good synergistic effect. The "chromium humic acid active agent drilling fluid", which is composed of ferric chromium salt, chromium humic acid and surfactant, has good thermal stability and anti collapse effect. It has been used in 6280m high temperature deep well and collapsible formation. 3) Nitro humic acid nitro humic acid.

C . Resin filtrate reducer 1) sulfomethyl phenolic resin SMP and SP are the commodity codes of sulfomethyl phenolic resin, and their synthetic routes are slightly different. The synthesis of SMP is that formaldehyde reacts with phenol under acidic condition (pH = 3-4) to form linear phenolic resin with appropriate molecular weight, and then sulfonated methyl reagent is added in alkaline condition to carry out sulfomethylation reaction. The products with higher sulfonation degree and higher relative molecular weight can be obtained by properly controlling the reaction conditions. The reaction is as follows: the synthesis of sojnasp is that phenol and formaldehyde, sodium sulfite or sodium bisulfite are fed at one time, and sulfonated and methylated at the same time under alkaline conditions to form sulfomethyl phenolic resin. Sulfomethyl phenolic resin is a kind of water-soluble irregular linear polymer. Its molecular structure is mainly composed of benzene ring, methylene bridge and C-S bond. The phenolic hydroxyl group is the adsorption group, and the sulfomethyl group is the hydrophilic group. It has strong salt resistance and thermal stability, and its temperature resistance can reach 200-2200c. 2) Sulfonated lignin sulfonated methyl phenolic resin condensate (SLSP) is a kind of water-soluble linear polymer copolymer. The first step is to produce sulfomethyl phenolic resin by condensation of phenol, formaldehyde and sodium bisulfite under the catalysis of alkali. Then, in the presence of formaldehyde solution and sodium hydroxide, it was dehydrated and condensed with sulfonated lignin (pulp waste liquid) by heating reflux. After drying, SLSP product was obtained. SLSP has good thermal stability, salt and calcium resistance. The drilling fluid treated with SLSP is 150-180. After high temperature C, the filtration rate changes little. Due to the large number of sulfonic acid groups in its molecules, it is not easy to produce dehydration and salting out when a large number of sodium, calcium or magnesium ions are encountered. The molecular chain of SLSP contains hydroxyl groups, which can adsorb hydrogen bonds with oxygen on clay particles. Sulfonic groups can thicken the solvated water film on the surface of clay particles and increase the f potential, thus improving the coalescence stability of clay particles. Because SLSP can stabilize colloidal particles and increase the viscosity of drilling fluid, the filtration loss is reduced. 3) Sulfonated lignite resin the trade name of sulfonated lignite resin is resinex, which is composed of 50% sulfonated lignite and 50% sulfomethyl phenolic resin. The product is soluble in water and can be used in various water-based drilling fluids with pH = 7-14. It is a salt and temperature resistant fluid loss additive. The temperature resistance can reach 230 in brine drilling fluid. C. The highest salt resistance is 1.1 × 105mg / L, and the drilling fluid performance is stable when the calcium content is 2000mg / L. It does not increase the viscosity of drilling fluid while reducing the fluid loss, especially in high-density drilling fluid. The permeability of drilling fluid cake treated with sulfonated lignite resin is very low, which is beneficial for stabilizing wellbore, preventing sticking and not blocking oil and gas reservoir.

D . Modified starch modified starch, such as carboxymethyl starch, hydroxyethyl starch and hydroxypropyl starch, plays an important role in drilling fluid additives, and its consumption is second only to iron chromium salt and lignite products, ranking third. Carboxymethyl starch is a product obtained by the reaction between carboxymethyl starch and etherifying agent chloroacetic acid on the basis of pre gelatinized starch, washing, dehydration, drying, crushing and screening; hydroxyethyl starch is produced by reacting with chloroethanol or ethylene oxide on the basis of pre gelatinized starch; hydroxypropyl starch is prepared by reacting with propylene oxide on the basis of pre gelatinized starch. Their structures are as follows: the filtration mechanism of modified starch is similar to that of sodium carboxymethyl cellulose. Due to a large number of hydroxyl groups, glycoside bonds and ether bonds in the molecules, they can adsorb hydrogen bonds with oxygen or hydroxyl groups on clay particles; strong hydration groups can thicken the solvation film on the surface of clay particles, and increase the f potential; starch has a strong hydration group which can make the solvation film on the surface of clay particles thicker and f potential increase The molecular chain has spiral structure and high relative molecular weight, which can adsorb several clay particles to form a space grid structure, which is also conducive to improving its coalescence stability; the viscosity of free water in drilling fluid can be increased and the filtration effect of filter cake can be reduced by adding modified starch into drilling fluid.

E . Acrylamide polymer acrylamide polymer is a kind of copolymer which contains acrylamide (AM) chain. 1) Hydrolyzed polyacrylonitrile (HPA) is a kind of high molecular compound which is polymerized by acrylonitrile. Hydrolyzed polyacrylonitrile (hpan) is a product obtained by alkali hydrolysis of waste silk from acrylic fiber production. The structure of hpan is as follows: - - CH2 -- ch specialty CH2 -- ch -- ch -- 7licoonaconh2cn (hydrolyzed polyacrylonitrile). Hydrolyzed polyacrylonitrile can be regarded as a terpolymer of sodium acrylate, acrylamide and acrylonitrile in structure. The nitrile and amide groups in the molecular chain are the adsorption groups, and the sodium carboxyl group is the hydration group. The unhydrolyzed nitrile group can be hydrolyzed into amide group or sodium carboxyl group under the condition of high temperature and alkalinity at the bottom of the well, so that the effect of downhole high temperature on the whole molecular chain can be alleviated, so its temperature resistance can be improved. The performance of hydrolyzed polyacrylonitrile for drilling fluid treatment is mainly determined by its molecular weight and degree of hydrolysis. Hpan with higher relative molecular weight has stronger fluid loss reduction ability, and increases the viscosity of drilling fluid more obviously; hpan with lower molecular weight has weaker fluid loss reducing ability and no obvious viscosity increasing effect. The hydrolytic polyacrylonitrile is resistant to 240-250. C high temperature, salt resistance is also strong, but calcium resistance is weak, encountered high concentration of calcium chloride solution will form flocculent precipitation. 2) Among all kinds of water-soluble polymers, polyacrylamide (PAM) is a synthetic product with excellent performance and low price. It has good thickening, resistance reduction, filtration loss reduction, flocculation and other functions. Its main disadvantages are: poor shear stability, poor salt resistance, poor thermal stability, which can be improved by chemical modification methods. The modification of PAM is mainly to develop functional monomers and then copolymerize with am to prepare polymers with new functions. Among the drilling fluid additives, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) is the most attractive modified monomer. Amps can be regarded as a derivative of am, and its structure is as follows: Xiaoqian H3 | L. Because of the strong anionic water-soluble sulfonic acid group, hydrophilic amide group and unsaturated double bond in the AMPS molecule, it has excellent properties. Sulfonic acid group makes it salt resistant, acid resistant, alkali resistant and thermal stable; amide group makes it have good hydrolysis stability, and active double bond makes it have polymerization performance. Amps, am and other functional monomers can form binary copolymers, terpolymers and even quaternary copolymers, which are widely used in oil fields. Among these copolymers, the introduction of different monomers can make the copolymers have different properties. The nitrile group, sulfo group and pyrrolidone group have good salt and temperature resistance properties, the amide group has good adsorption performance, and the carboxyl group has good hydration function.