Composite calcium salt solid-free completion fluid and preparation method thereof

Abstract

The invention provides a composite calcium salt solid-free completion fluid and a preparation method thereof. The completion fluid is prepared from the following components in percentage by mass: 30-75% of a composite calcium salt weighting agent, 0.2-2.0% of a clay expansion inhibitor, 0.5-3% of a filtrate reducer, 0.2-2.0% of a corrosion inhibitor, 0.1-1.0% of a bactericide, 0.1-1.0% of a pH regulator and the balance of water. The composite calcium salt weighting agent is a mixture of calcium chloride and calcium ammonium nitrate or calcium nitrate, the density of the completion fluid is improved, the crystallization temperature of calcium chloride salt water is reduced, the defects of calcium chloride dissolution heat release and the like are eliminated, and the application range of the low-activity calcium salt completion fluid is widened. A large number of hydrophilic groups exist on a molecular chain of the filtrate loss agent and generate hydration with water, so that a network chain is expanded, water molecules are promoted to permeate into macromolecules, isotropic groups on the network chain repel each other, a cross-linked polymer can bind the water molecules, and a hydrophilic swelling three-dimensional space network structure is formed; the problems that solid-phase-free brine completion fluid is large in filtration loss, large in damage to stratums, capable of blocking the stratums due to scaling and the like are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil drilling and completion fluids, and particularly relates to a composite calcium salt solid-free completion fluid and a preparation method thereof. Background Art

[0002] Completion fluid is a kind of wellbore working fluid and plays a very important role in the drilling technology for protecting reservoirs. With the rapid development of the world's oil industry, higher and newer requirements have been put forward for completion fluid technology in oil and gas field development. Completion fluids are roughly divided into water-based completion fluids, oil-based completion fluids, and mixed completion fluids. In China, water-based completion fluids are mainly used as the leading direction, and the most commonly used water-based completion fluid is solid-free brine fluid, which is prepared with one or more salts and water as the base fluid. However, the density of the brine fluid is greatly affected by temperature, so different types of additives are added to the base fluid according to different formation requirements to prevent formation damage. The additives here include weighting agents, viscosifiers, stabilizers, filtration loss agents, corrosion inhibitors, etc. for compatibility. Among them, the main weighting agents are NaCl, CaCl, and ZnCl 2 as the main additives. The density is generally adjustable between 1.05 and 1.60 g / cm 3 3.

[0003] In the 1990s, the introduction of formate solid-free completion fluid as the main method began in China. According to the types of formate completion fluids, they can be roughly divided into sodium formate, potassium formate, and cesium formate completion fluids. Among them, sodium formate completion fluid is widely used in many oil fields in China due to its relatively low price. Field applications have shown that compared with solid-containing kill fluids, clean brine completion fluids can significantly increase oil well production. Since there are directly available NaCl, KCl, and CaCl 2 solutions in nature for oil and gas well completion, and the sodium salts, potassium salts, and calcium salts at home and abroad are rich in resources and low in price. Therefore, high-density solid-free completion fluids of salts are still widely used in oil and gas field production at home and abroad. However, there are generally problems of relatively low completion fluid density and high construction cost.

[0004] In the prior art, there are also many formate completion fluids and their similar types of completion fluids. For example, Chinese Patent Publication No. CN111154462B discloses a long-acting and stable formate completion fluid and its preparation method and application. The composition components and weight parts are as follows: the formate completion fluid is composed of formate brine, filtration loss reducer, plugging agent, buffer agent, dispersant, defoaming agent, sepiolite, and weighting agent; among them, the formate brine has a density of 1.3 - 1.5 g / cm 3 3, and the completion fluid density is 1.8 - 2.5 g / cm 3, applicable to well completion operations with downhole temperatures of 160 - 200 °C. After aging statically at 200 °C for 15 days, it has good rheology and filtration loss, with a sedimentation degree ≤ 1 N, meeting the well completion requirements. Moreover, its preparation process is simple and easy to operate. However, formate salts have a high cost and are limited in on-site use.

[0005] CN112552886A discloses a variable-density solid-free viscosity-increasing well completion fluid and workover fluid resistant to ultra-high temperature of 180 °C, effectively solving the problems of insufficient temperature resistance and difficulty in increasing density of traditional workover fluids. The workover fluid can resist ultra-high temperature of 180 °C, with the density varying in the range of 1.1 - 1.8 g / cm 3 between, and the corresponding apparent viscosity in the range of 27 - 690 mPa·s, which can meet the requirements of well completion and workover operations for ultra-high temperature and high-pressure oil and gas wells. It is easy to prepare, easy to circulate and backflow, and has strong on-site operability, with good application prospects. However, its composite salts will generate heat during dissolution in clear water, and the mixed solution needs to be cooled to room temperature.

[0006] Literature shows that the commonly used calcium chloride solution as the base fluid of the well completion fluid has a maximum density of 1.40 g / cm 3 , the crystallization temperature of the saturated solution is 18 °C, and the dissolution process of solid calcium chloride releases heat. Even when the dissolution is too fast, it can cause the aqueous solution to boil, resulting in difficulties in on-site construction; formate salt weighting agents increase the cost of the well completion fluid. Especially cesium salts generally must be recycled, and the supporting equipment technology is complex; well completion fluids with salts such as calcium bromide, sodium bromide, and zinc chloride as weighting agents have relatively serious corrosion and scaling, and large environmental risks. For example, a solid-free kill fluid with a density of 1.40 - 1.60 g / cm 3 mainly includes two types of base fluids: NaCl + NaBr base fluid (density 1.51 g / cm 3 ) and MgCl 2 + CaCl 2 + ZnCl 2 base fluid (density 1.40 - 1.60 g / cm 3 , which is harmful to the formation). In on-site applications, the following problems have been found in these two types: (1) serious leakage and scaling, with great environmental harm; (2) the water absorption, crystallization, and corrosion of heavy brine (density greater than 1.60 g / cm 3 ) are large, causing serious corrosion to construction equipment and oil and casing pipes; (3) impurity pollution during transportation and construction processes, as well as other environmental pollutions; (4) there are high-temperature conditions during the preparation and use of composite salt weighting agents, with poor convenience or being unfavorable for on-site safe construction; (5) high cost of high-density brine, etc. Summary of the Invention

[0007] The purpose of the present invention is to provide a composite calcium salt solid-free well completion fluid to overcome the above technical problems existing in the prior art.

[0008] Another object of the present invention is to provide a preparation method of a composite calcium salt solid-free completion fluid. By using the principle of mutual solubility of low-cost composite calcium salts, the density range of the completion fluid is expanded, the construction risk caused by the low crystallization point temperature is overcome, and the completion operation requirements of oil and gas wells with a density range of 1.40 - 1.60 g / cm 3 are met.

[0009] For this purpose, the technical solution provided by the present invention is as follows: A composite calcium salt solid-free completion fluid is composed of the following components by mass percentage: 30 - 75% of a composite calcium salt weighting agent, 0.2 - 2.0% of a clay swelling inhibitor, 0.5 - 3% of a filtrate reducer, 0.2 - 2.0% of a corrosion inhibitor, 0.1 - 1.0% of a bactericide, 0.1 - 1.0% of a pH regulator, and the balance is water. The composite calcium salt weighting agent is 40 - 70%, the clay swelling inhibitor is 0.5 - 1.0%, the filtrate reducer is 1.0 - 2.0%, the corrosion inhibitor is 0.5 - 1.0%, the bactericide is 0.1 - 0.5%, the pH regulator is 0.2 - 0.5%, and the balance is water.

[0010] The composite calcium salt weighting agent is composed of a mixture of calcium chloride and calcium ammonium nitrate or calcium nitrate. Among them, the weight ratio of calcium chloride to calcium ammonium nitrate or calcium nitrate is 90﹕10 - 20﹕80.

[0011] The clay swelling inhibitor is a cationic quaternary ammonium salt, and the cationic quaternary ammonium salt is one of dimethyldiallylammonium chloride, polyepichlorohydrin-dimethylamine or small molecule polyamine.

[0012] The filtrate reducer is an acrylic acid-acrylamide copolymer or N, N'-methylenebisacrylamide, methylene polyacrylamide, a binary copolymer of 2-acrylamido-2-methylpropanesulfonic acid or polymer LVC-39.

[0013] The corrosion inhibitor is an amine corrosion inhibitor or a modified imidazoline type corrosion inhibitor.

[0014] The bactericide is a non-ionic bactericide, and the non-ionic bactericide is glutaraldehyde or benzaldehyde.

[0015] The pH regulator is magnesium oxide, sodium hydroxide or calcium hydroxide.

[0016] A preparation method of a composite calcium salt solid-free completion fluid includes the following steps: Step 1) Add the formulated amount of water into a container, and then add the formulated amount of the composite calcium salt weighting agent, and stir and dissolve thoroughly to mix evenly; Step 2) Add the formulated amount of the filtrate reducer, and stir and dissolve thoroughly to mix evenly; Step 3) Sequentially add the clay swelling inhibitor, corrosion inhibitor, bactericide, and pH regulator in the formula amounts, and stir well to mix evenly to obtain the product.

[0017] The beneficial effects of the present invention are as follows: The composite calcium salt solid-free completion fluid provided by the present invention has a composite calcium salt weighting agent which is a mixture of calcium chloride and calcium ammonium nitrate or calcium nitrate, improving the density of the calcium chloride completion fluid, reducing the crystallization temperature of the calcium chloride brine, eliminating the disadvantages such as heat release during the dissolution of calcium chloride, and expanding the application range of the low-activity calcium salt completion fluid. The filter loss reducer is a binary copolymer, which has the functions of binding water and reducing filter loss, can reduce the free water content in the completion fluid, is salt and calcium resistant, and there are a large number of hydrophilic groups (carboxyl groups, amide groups, and sulfonic acid groups) on the molecular chain. When contacting with water, it will hydrate with water to expand the network chain, promoting the penetration of water molecules into the polymer interior. The mutual repulsion of the same-sex groups on the network chain enables the cross-linked polymer to bind water molecules and form a hydrophilic swelling three-dimensional network structure, solving the problems of large filter loss, great formation damage, and formation scaling and plugging of the solid-free brine completion fluid.

[0018] This composite calcium salt solid-free completion fluid does not contain bromide ions and zinc ions, has no solid-phase particles, low crystallization temperature, low corrosion, low filter loss, and low activity, inhibits clay swelling, reduces the influence of primary clay particles on permeability, causes little damage to the reservoir, and is especially suitable for well completion operations in shale reservoirs or reservoirs with a high content of clay minerals. The completion fluid has good rheology, small circulation friction and pressure loss during the construction process, meets the well completion operation requirements of oil and gas wells with a maximum density of 1.60 g / cm 3 API filtration loss is 16.5 ml, crystallization temperature is -10 °C, reducing non-productive time, reducing reservoir damage, reducing operation risks and production costs, and effectively improving the drilling and well completion operation efficiency. Specific embodiments

[0019] The following specific embodiments illustrate the implementation manners of the present invention, and those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0020] The exemplary implementation manners of the present invention are now introduced. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to disclose the present invention in detail and completely, and to fully convey the scope of the present invention to those skilled in the art. The terms used in the exemplary implementation manners do not limit the present invention.

[0021] Unless otherwise specified, the terms used herein (including technical terms) have the ordinary meaning understood by those skilled in the relevant technical field. Additionally, it can be understood that terms defined in commonly used dictionaries should be construed to have a meaning consistent with the context of their relevant fields, and should not be construed as idealized or overly formal meanings.

[0022] Example 1 This example provides a composite calcium salt solid-free completion fluid, which is composed of the following components in mass percentage: 30 - 75% of composite calcium salt weighting agent, 0.2 - 2.0% of clay swelling inhibitor, 0.5 - 3% of filtrate reducer, 0.2 - 2.0% of corrosion inhibitor, 0.1 - 1.0% of bactericide, 0.1 - 1.0% of pH regulator, and the balance is water. The composite calcium salt weighting agent is composed of a mixture of calcium chloride and calcium ammonium nitrate or calcium nitrate. Among them, the weight ratio of calcium chloride to calcium ammonium nitrate or calcium nitrate is 90﹕10 - 20﹕80.

[0023] The clay swelling inhibitor is a cationic quaternary ammonium salt, and the cationic quaternary ammonium salt is one of dimethyldiallylammonium chloride, polyepichlorohydrin-dimethylamine, or small molecule polyamine.

[0024] It is acrylic-acrylamide copolymer or binary copolymer of N,N'-methylenebisacrylamide, methylene polyacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, or polymer LVC-39.

[0025] The corrosion inhibitor is an amine corrosion inhibitor or a modified imidazoline type corrosion inhibitor.

[0026] The bactericide is a non-ionic bactericide, and the non-ionic bactericide is glutaraldehyde or benzaldehyde.

[0027] The pH regulator is magnesium oxide, sodium hydroxide, or calcium hydroxide.

[0028] Example 2 Based on Example 1, this example provides a composite calcium salt solid-free completion fluid, which is composed of the following components in mass percentage: 40% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 0.5% of filtrate reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water. Preparation process: Add the formulated amount of composite calcium salt weighting agent to the formulated amount of water. After fully stirring and dissolving, add the formulated amount of filtrate reducer, and fully stir and mix evenly. Then, sequentially add the formulated amounts of clay swelling inhibitor, corrosion inhibitor, bactericide, and pH regulator, and mix evenly to obtain the completion fluid.

[0029] In this embodiment, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium ammonium nitrate with a mass ratio of 90:10. The clay swelling inhibitor is dimethyldiallylammonium chloride. The filtrate reducer is polymer LVC-39 (purchased from Henan Haina Petroleum Company). The corrosion inhibitor is a modified imidazoline corrosion inhibitor (the preparation process is shown in Patent CN 102344789 B). The bactericide is glutaraldehyde. The pH regulator is magnesium oxide, and the pH is adjusted to 8-9.

[0030] Among them, calcium ammonium nitrate is a green and efficient fertilizer with the chemical formula 5Ca(NO 3 ) 2 ·NH 4 NO 3 ·10H 2 O. It is in the form of white round granulation and has excellent environmental protection performance. It is miscible with calcium chloride and provides inhibitory calcium ions and ammonium ions to inhibit the swelling of clay particles and reduce the formation pressure transmission. When calcium ammonium nitrate is used in combination with calcium chloride, the technical characteristics and economy of the calcium chloride completion fluid system are retained, and the maximum density of the completion fluid is up to 1.60 g / cm 3 , improving the ability of the completion fluid to balance the formation. At the same time, it overcomes the problem of high crystallization temperature of the calcium chloride saturated solution and reduces the on-site construction risk.

[0031] Example 3 Based on Example 1, this example provides a composite calcium salt solid-free completion fluid, which is composed of the following components in mass percentage: 40% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 0.5% of filtrate reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water. The preparation process is the same as that of Example 2.

[0032] In this embodiment, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium nitrate with a mass ratio of 80:20. The clay swelling inhibitor is polyepichlorohydrin-dimethylamine (PTA). The filtrate reducer is a binary copolymer of N, N'-methylenebisacrylamide (the preparation method is common knowledge). The corrosion inhibitor is an amine corrosion inhibitor dodecylamine. The bactericide is benzaldehyde. The pH regulator is sodium hydroxide.

[0033] Example 4 Based on Example 1, this example provides a composite calcium salt solid-free completion fluid, which is composed of the following components in mass percentage: 50% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 1% of filtrate reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water.

[0034] The preparation process is the same as that of Example 2.

[0035] In this embodiment, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium ammonium nitrate with a mass ratio of 60:40. The clay swelling inhibitor is a small molecule polyamine (coconut oil amide methyl) MEA. The filtration loss reducer is a binary copolymer of methylene polyacrylamide (the preparation method belongs to common knowledge). The corrosion inhibitor is an amine corrosion inhibitor, cetylamine. The bactericide is glutaraldehyde. The pH regulator is sodium hydroxide.

[0036] Example 5 On the basis of Example 1, this embodiment provides a composite calcium salt solids-free completion fluid, which is composed of the following components in mass percentage: 50% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 1% of filtration loss reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water.

[0037] The preparation process is the same as that of Example 2.

[0038] In this embodiment, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium nitrate with a mass ratio of 50:50. The clay swelling inhibitor is dimethyldiallylammonium chloride. The filtration loss reducer is polymer LVC-39 (purchased from Henan Haina Petroleum Company). The corrosion inhibitor is a modified imidazoline corrosion inhibitor (the preparation process can be seen in Patent CN 102344789 B). The bactericide is glutaraldehyde. The pH regulator is magnesium oxide.

[0039] Example 6 On the basis of Example 1, this embodiment provides a composite calcium salt solids-free completion fluid, which is composed of the following components in mass percentage: 60% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 1.5% of filtration loss reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water.

[0040] The preparation process is the same as that of Example 2.

[0041] In this embodiment, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium ammonium nitrate with a mass ratio of 50:50. The clay swelling inhibitor is polyepichlorohydrin-dimethylamine. The filtration loss reducer is a binary copolymer of 2-acrylamido-2-methylpropane sulfonic acid (the preparation method belongs to common knowledge). The corrosion inhibitor is an amine corrosion inhibitor, aniline. The bactericide is glutaraldehyde. The pH regulator is calcium hydroxide.

[0042] Example 7 On the basis of Example 1, this embodiment provides a composite calcium salt solids-free completion fluid, which is composed of the following components in mass percentage: 60% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 1.5% of filtration loss reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water.

[0043] The preparation process is the same as that of Example 2.

[0044] In this embodiment, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium ammonium nitrate with a mass ratio of 40:60. The clay swelling inhibitor is dimethyldiallylammonium chloride. The filtrate reducer is polymer LVC-39 (purchased from Henan Haina Petroleum Company). The corrosion inhibitor is a modified imidazoline corrosion inhibitor (the preparation process is described in Patent CN 102344789 B). The bactericide is benzaldehyde. The pH regulator is magnesium oxide.

[0045] Example 8 Based on Example 1, this embodiment provides a composite calcium salt solids-free completion fluid, which is composed of the following components in mass percentages: 70% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 2% of filtrate reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water.

[0046] The preparation process is the same as that of Example 2.

[0047] In this embodiment, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium ammonium nitrate with a mass ratio of 35:65. The clay swelling inhibitor is polyepichlorohydrin-dimethylamine. The filtrate reducer is a binary copolymer of 2-acrylamido-2-methylpropanesulfonic acid (the preparation method is common knowledge). The corrosion inhibitor is an amine corrosion inhibitor naphthylamine. The bactericide is glutaraldehyde. The pH regulator is magnesium oxide.

[0048] Example 9 Based on Example 1, this embodiment provides a composite calcium salt solids-free completion fluid, which is composed of the following components in mass percentages: 70% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 2% of filtrate reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water.

[0049] The preparation process is the same as that of Example 2.

[0050] In this embodiment, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium ammonium nitrate with a mass ratio of 30:70. The clay swelling inhibitor is dimethyldiallylammonium chloride. The filtrate reducer is an acrylic acid-acrylamide copolymer. The corrosion inhibitor is a modified imidazoline corrosion inhibitor (the preparation process is described in Patent CN 102344789 B). The bactericide is benzaldehyde. The pH regulator is magnesium oxide.

[0051] Example 10 Based on Example 1, this embodiment provides a composite calcium salt solids-free completion fluid, which is composed of the following components in mass percentages: 70% of composite calcium salt weighting agent, 0.8% of clay swelling inhibitor, 2% of filtrate reducer, 0.5% of corrosion inhibitor, 0.2% of bactericide, 0.2% of pH regulator, and the balance is water.

[0052] The preparation process is the same as that of Example 2.

[0053] In this example, the composite calcium salt weighting agent is a mixture of calcium chloride and calcium ammonium nitrate with a mass ratio of 20:80. The clay swelling inhibitor is dimethyldiallylammonium chloride. The filtrate reducer is polymer LVC-39 (purchased from Henan Haina Petroleum Company). The corrosion inhibitor is a modified imidazoline corrosion inhibitor (the preparation process can be found in Patent CN 102344789 B). The bactericide is glutaraldehyde, and the pH regulator is calcium hydroxide.

[0054] Comparative Example: The difference from Example 10 is that the weighting agent is calcium chloride.

[0055] Laboratory experiments: Referring to the national standard "GBT618-2006 General Method for the Determination of Crystallization Point of Chemical Reagents" and the general test methods for drilling and completion fluids, the performance of the completion fluids in Examples 2-10 was tested, and the experimental data are shown in Table 1.

[0056] Table 1 Experimental data of Examples 2-10 Example Density g / cm3 API filtration loss, ml / 30min Crystallization temperature, °C Calcium chloride: Calcium ammonium nitrate 2 1.40 14.6 0 90:10 3 1.405 13.4 -15 80:20 4 1.51 15.2 -12 60:40 5 1.52 15.6 -20 50:50 6 1.55 15.8 -15 50:50 7 1.56 16.6 -18 40:60 8 1.6 16.5 -10 35:65 9 1.605 20.2 -5 30:70 10 1.61 24.6 0 20:80 Comparative example 1.4 -- 18 -- As can be seen from Table 1, the density of the completion fluid in the present invention is 1.40-1.60 g / cm 3 , which is significantly higher than that of the calcium chloride completion fluid; the crystallization temperature decreases (0 to -20 °C) after mixing calcium chloride and calcium ammonium nitrate, overcoming the problem of too high crystallization temperature of calcium chloride solution; the water loss is controlled to 13-25 ml by using the water-binding polymer. The completion fluid of the present invention does not contain bromide ions and zinc ions, has no solid-phase particles, low crystallization temperature, low corrosion, low filtrate loss, low activity, inhibits clay swelling, reduces the influence of primary clay particles on permeability, and causes little damage to the reservoir, and is especially suitable for well completion operations in shale reservoirs or reservoirs with high clay minerals. The materials used are all commonly used industrial materials, with wide sources, low cost, and simple on-site preparation.

[0057] The above examples are only illustrative of the present invention and do not constitute a limitation on the protection scope of the present invention. Any design identical or similar to the present invention falls within the protection scope of the present invention.

Claims

1. A composite calcium salt solid-free completion fluid, characterized in that: The invention is composed of the following components in percentage by mass: 30-75% of a composite calcium salt weighting agent, 0.2-2.0% of a clay expansion inhibitor, 0.5-3% of a filtration reducer, 0.2-2.0% of a corrosion inhibitor, 0.1-1.0% of a bactericide, 0.1-1.0% of a pH regulator, and the balance is water.

2. The composite calcium salt solid-free completion fluid according to claim 1, characterized in that: The composite calcium salt weighting agent is 40-70%, the clay expansion inhibitor is 0.5-1.0%, the filtration loss reducer is 1.0-2.0%, the corrosion inhibitor is 0.5-1.0%, the bactericide is 0.1-0.5%, the pH regulator is 0.2-0.5%, and the balance is water.

3. The composite calcium salt solid-free completion fluid according to claim 1, characterized in that: The composite calcium salt weighting agent is composed of a mixture of calcium chloride and calcium ammonium nitrate or calcium nitrate, wherein the weight ratio of calcium chloride to calcium ammonium nitrate or calcium nitrate is 90:10-20:

80.

4. The composite calcium salt solid-free completion fluid according to claim 1, characterized in that: The clay swelling inhibitor is a cationic quaternary ammonium salt, and the cationic quaternary ammonium salt is one of dimethyldiallylammonium chloride, polyepichlorohydrin-dimethylamine or small molecule polyamine.

5. The composite calcium salt solid-free completion fluid according to claim 1, characterized in that: The fluid loss reducer is an acrylic acid-acrylamide copolymer or a binary copolymer of N,N'-methylenebisacrylamide, methylene polyacrylamide, 2-acrylamide-2-methylpropanesulfonic acid or a polymer LVC-39.

6. The composite calcium salt solid-free completion fluid according to claim 1, characterized in that: The corrosion inhibitor is an amine corrosion inhibitor or a modified imidazoline corrosion inhibitor.

7. The composite calcium salt solid-free completion fluid according to claim 1, characterized in that: The bactericide is a non-ionic bactericide, and the non-ionic bactericide is glutaraldehyde or benzaldehyde.

8. The composite calcium salt solid-free completion fluid according to claim 1, characterized in that: The pH regulator is magnesium oxide, sodium hydroxide or calcium hydroxide.

9. The method for preparing a composite calcium salt solid-free completion fluid according to claim 1, characterized in that: The following steps are involved: Step 1) Add the formulated amount of water into a container, then add the formulated amount of composite calcium salt weighting agent, and stir thoroughly to dissolve and mix evenly; Step 2) Add the formulated amount of fluid loss reducer, stir thoroughly to dissolve and mix evenly; Step 3) Add the clay expansion inhibitor, corrosion inhibitor, fungicide and pH adjuster in the formula amount in sequence, stir and mix thoroughly to obtain the product.