Foam plugging slurry, preparation method of foam plugging slurry, foam plugging slurry system and application

Abstract

The present invention relates to the field of oil drilling and discloses a foam plugging slurry, a preparation method for the foam plugging slurry, a foam plugging slurry system, and applications. The foam plugging slurry comprises: a cement-based slurry, a foaming agent, and nitrogen filling; wherein the foaming agent is no more than 5 parts by weight per 100 parts by weight of the cement-based slurry; and the nitrogen filling is added in an amount by volume such that the density of the foam plugging slurry is reduced by 3-30% compared to the cement-based slurry. The foam plugging slurry of the present invention can flexibly adjust its density according to the inflation ratio and is suitable for variable well leakage conditions on site.

Description

Technical Field

[0001] The present invention relates to the field of oil drilling, and in particular to a foam plugging slurry, a preparation method of the foam plugging slurry, a foam plugging slurry system and applications. Background Art

[0002] During the oil drilling and cementing process, well leakage is often encountered, resulting in a large amount of drilling fluid and other working fluids being lost into the formation. In addition, it takes a long time to plug the leak and withstand the pressure after plugging, which greatly increases the drilling cost.

[0003] Usually, the plugging operation is to pump a certain concentration of plugging slurry containing plugging particles of different particle sizes into the leakage layer, and gradually plug the leakage layer through technical measures such as static plugging, pressure-holding plugging or while drilling plugging. When the leakage rate of the local formation exceeds a certain scale (severe leakage or even loss return), the above plugging effect will be greatly reduced or even completely ineffective.

[0004] After conventional plugging slurry enters the formation, it is unable to stay in the leaking zone due to its high density under the influence of formation pressure, resulting in leakage at the leaking zone and plugging failure. To solve the problem of cement plugs not being able to stay in formations with low pressure bearing capacity, the density of the cement plugs must be reduced as much as possible. However, conventional cement plugs are generally limited by the requirements of lightening materials and the performance of the slurry entering the well. Maintaining a low density while maintaining a certain slurry performance is difficult, resulting in the current density of cement slurries entering the well being unable to be sufficiently low.

[0005] CN103074042A discloses a drilling fluid and a method for preparing the drilling fluid, and discloses a chemical foaming method that relies on chemical agent reactions to generate nitrogen foam. However, the method is limited by the mixing process and compatibility with other slurries, and the foaming amount and foam stabilization performance are also limited.

[0006] Therefore, it is necessary to design a plugging slurry specifically for the leaking layer with obvious leakage, and combine it with a certain plugging process to achieve a good plugging effect. Summary of the Invention

[0007] The purpose of the present invention is to overcome the problem that conventional plugging slurries in the prior art have unsatisfactory plugging effects, and to provide a foam plugging slurry and a preparation method of a foam plugging slurry, a foam plugging slurry system and an application. The foam plugging slurry has a gas-liquid-solid three-phase flow, a simple and reliable formula, and is easy to use. It meets the low density of the slurry while taking into account the various properties of the slurry.

[0008] In order to achieve the above-mentioned object, the first aspect of the present invention provides a foam plugging slurry, wherein the foam plugging slurry comprises: a cement-based slurry, a foaming agent and nitrogen filling;

[0009] The foaming agent is no more than 5 parts by weight relative to 100 parts by weight of cement-based slurry; the nitrogen gas is added in an amount by volume such that the density of the foam plugging slurry is reduced by 3-30% compared with the cement-based slurry.

[0010] A second aspect of the present invention provides a method for preparing a foam plugging slurry, comprising the following steps:

[0011] (1) Prepare cement-based slurry;

[0012] (2) adding a foaming agent to the cement-based slurry and stirring evenly to obtain a plugging slurry intermediate;

[0013] (3) filling gas into the plugging slurry intermediate to obtain foam plugging slurry;

[0014] Wherein, relative to 100 parts by weight of cement-based slurry, the foaming agent is no more than 5 parts by weight;

[0015] Preferably, the nitrogen gas is added in an amount by volume such that the density of the foam plugging slurry is reduced by 3-30% compared with the cement-based slurry.

[0016] A third aspect of the present invention provides a foam plugging slurry, which is prepared by the preparation method described in the second aspect.

[0017] The fourth aspect of the present invention provides the aforementioned foam plugging slurry system, comprising the foam plugging slurry described in the first aspect or the third aspect and optional conventional plugging slurry;

[0018] Wherein, relative to 100 parts by weight of the foam plugging slurry, the conventional plugging slurry is no more than 100 parts by weight, preferably 10-30 parts by weight.

[0019] A fifth aspect of the present invention provides an application of the aforementioned foam plugging slurry or the aforementioned foam plugging slurry system in a shale gas well.

[0020] Through the above technical solution, the beneficial technical effects achieved by the present invention are as follows:

[0021] (1) The foam plugging slurry of the present invention can flexibly adjust the density according to the inflation ratio, and is suitable for the variable leakage well conditions on site;

[0022] (2) The foam plugging slurry of the present invention has unique compressibility, is easier to enter the leaking layer, and has a better plugging effect than conventional plugging slurry;

[0023] (3) The foam plugging slurry preparation process of the present invention is simple and low-cost. DETAILED DESCRIPTION

[0024] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0025] A first aspect of the present invention provides a foam plugging slurry, the foam plugging slurry comprising: a cement-based slurry, a foaming agent and filled nitrogen;

[0026] The foaming agent is no more than 5 parts by weight relative to 100 parts by weight of cement-based slurry; the nitrogen gas is added in an amount by volume such that the density of the foam plugging slurry is reduced by 3-30% compared with the cement-based slurry.

[0027] According to the present invention, preferably, the nitrogen gas is added in an amount by volume such that the density of the foam plugging slurry is reduced by 5-20% compared with the cement-based slurry.

[0028] In the present invention, the gas volume is controlled by temperature and pressure, which are in turn determined by the maximum temperature and leakage pressure of the leaky zone in each well. Therefore, the required nitrogen volume is the amount required to reduce the density of the original cement slurry by 3-30% (under the maximum temperature and leakage pressure of the leaky zone), preferably by 5-20%. Based on the required nitrogen volume at this temperature and pressure, the nitrogen charging capacity of the ground nitrogen charging equipment (generally referred to as the construction nitrogen charging capacity) is inferred through the real nitrogen state equation.

[0029] In the present invention, for example, the density of the cement-based slurry is set to 1.88 g / cm 3 The maximum temperature of the leaking zone is 90°C, and the well loss pressure is 35 MPa. At this temperature and pressure, the nitrogen injection volume required to reduce the cement slurry density by 3%-30% is 0.03-0.53 parts by volume (based on the cement slurry volume). Based on this required nitrogen injection volume at this temperature and pressure, and inversely deduced from the true nitrogen state equation, the ground nitrogen injection volume is 7-124 parts by volume (based on the cement slurry volume). This means that the ground nitrogen injection equipment requires 7-124 parts by volume of nitrogen injection per unit volume of cement slurry (based on the cement slurry volume), thereby obtaining the foam plugging slurry required by the present invention.

[0030] According to the present invention, the foaming agent is 2-3 parts by weight, and the foaming agent is an anionic surfactant foaming agent, preferably selected from at least one of sodium dodecylbenzenesulfonate, α-olefin sulfonate, α-sulfo fatty acid methyl ester and sodium N-oleoyl N-methyl taurate.

[0031] In some specific embodiments of the present invention, the foaming agent plays a role in foaming and stabilizing foam. When the foaming agent introduces gas under mechanical force, it can effectively reduce the surface tension of the liquid, has a high foaming multiple, good foam stability, can be kept without defoaming for a long time, has fine foam, and has good compatibility with the medium used.

[0032] In the present invention, compared with the use of chemical agents for foaming, it is difficult to control the amount of foaming and the uniformity of foaming. Excessive foaming will destroy the mechanical properties of the base slurry, and insufficient foaming cannot meet the foaming requirements. The present invention adopts mechanical action physical foaming, which can flexibly respond to different application conditions.

[0033] According to the present invention, the cement-based slurry comprises, by weight, 100 parts of cement, 1-10 parts of fluid loss reducer, 0-3 parts of retarder, 0-5 parts of early strength agent, 0-0.2 parts of thickener, and 30-70 parts of water.

[0034] According to the present invention, preferably, the cement-based slurry comprises, by weight, 100 parts of cement, 3-8 parts of fluid loss additive, 0.1-2 parts of retarder, 0.25-1 parts of early strength agent, 0.05-0.1 parts of thickener, and 35-60 parts of water.

[0035] In the present invention, preferably, the density of the foam plugging slurry is 1.2-1.7 g / cm 3 .

[0036] In the present invention, the cement is pure cement and / or low-density ash mixed with a density-reducing material. The density-reducing material is not particularly limited and can be any conventional density-reducing material in the art. Preferably, the cement is selected from at least one of buoyant beads, hollow glass microspheres, microsilica, and fly ash.

[0037] In the present invention, the fluid loss additive is an emulsion fluid loss additive and / or a water-soluble polymer fluid loss additive. The use of the above fluid loss additive can control the loss of the liquid phase to the permeability boundary in the cement slurry, thereby maintaining an appropriate water-cement ratio of the cement slurry.

[0038] In the present invention, the emulsion-based fluid loss additive is a latex-based fluid loss additive and / or a resin-based fluid loss additive, and the water-soluble polymer fluid loss additive is selected from at least one of cellulose and its derivatives, acrylamide and its derivatives, and polyvinyl alcohol-based fluid loss additives. Preferably, the fluid loss additive is acrylamide and / or 2-acrylamido-2-methylpropanesulfonic acid.

[0039] In the present invention, the early strength agent is selected from at least one of an electrolyte inorganic salt early strength agent, a water-soluble organic polymer early strength agent, and an organic compound and inorganic salt composite early strength agent. In the present invention, the electrolyte inorganic salt early strength agent is selected from at least one of sulfates, sulfate complexes, nitrates, nitrites, and chlorides, and the water-soluble organic polymer early strength agent is selected from at least one of triethanolamine, formates, acetates, and propionates. The use of these early strength agents can accelerate the development of early strength in concrete.

[0040] In the present invention, preferably, the early strengthening agent is sodium nitrate and / or triethanolamine.

[0041] In the present invention, the type of thickener is not particularly limited and can be any conventional thickener in the art, such as at least one selected from inorganic thickeners, cellulose thickeners, natural polymer thickeners and their derivatives, synthetic polymer thickeners, and complex organometallic compound thickeners. The use of such thickeners can increase the viscosity of the cement slurry system.

[0042] In the present invention, the inorganic thickener is selected from at least one of organic bentonite, diatomaceous earth, attapulgite and silicone gel; preferably, the cellulose thickener is selected from at least one of methyl cellulose, hydroxypropyl methyl cellulose and hydroxyethyl cellulose; preferably, the natural polymer and its derivative thickener is selected from at least one of starch, gelatin, casein, guar gum, xanthan gum and soy protein gum; preferably, the synthetic polymer thickener is selected from at least one of polyacrylamide, polyvinyl alcohol and polyacrylic acid; preferably, the complex organic metal compound thickener is amino alcohol complex titanate.

[0043] In the present invention, the retarder is selected from at least one of hydroxycarboxylic acids, aminocarboxylic acids and salts thereof, polyols and derivatives thereof, glucose, sucrose and derivatives thereof, molasses and derivatives thereof, phosphates, metaphosphates, borax and sodium fluorosilicate. The state of the retarder is not particularly limited and may be in liquid or solid phase. The use of the above-mentioned retarder can reduce the cement hydration rate and hydration heat and prolong the setting time.

[0044] In the present invention, the water is common well water, river water, river water, rainwater, etc. that does not contain obvious minerals and does not affect the basic properties of the cement slurry after mixing.

[0045] According to the present invention, there is no particular limitation on the stirring speed, and those skilled in the art can make an adaptive selection as needed. Preferably, the stirring speed is 5-1000 rpm, preferably 100-500 rpm. At the above stirring speed, uniform mixing of the foam plugging slurry can be achieved.

[0046] The present invention has developed a special foam plugging slurry, that is, on the basis of conventional density and low-density cement-based slurry, a foaming agent is added and a certain proportion of nitrogen is filled in under mechanical action to further reduce the slurry density and form a foam plugging slurry with uniform foam. In this way, it is possible to achieve the goal of injecting plugging cement (plugging cement slurry) into strata with low pressure bearing capacity with almost no leakage, thereby ensuring the success rate of plugging.

[0047] A second aspect of the present invention provides a method for preparing a foam plugging slurry, comprising the following steps:

[0048] (1) Prepare cement-based slurry;

[0049] (2) adding a foaming agent to the cement-based slurry and stirring evenly to obtain a plugging slurry intermediate;

[0050] (3) filling nitrogen into the plugging slurry intermediate to obtain foam plugging slurry;

[0051] Wherein, relative to 100 parts by weight of cement-based slurry, the foaming agent is no more than 5 parts by weight;

[0052] Preferably, the volume of the nitrogen added is such that the density of the foam plugging slurry decreases by 3-30% compared to when no nitrogen is added.

[0053] In some specific embodiments, the gas density is approximately calculated based on the real gas state equation or obtained by looking up published experimental data tables for gases at different temperatures and pressures. For example, at temperature T1 and pressure P1, the gas density is D1 and the cement slurry density is C1. Then, if n unit volumes of gas are added to each unit volume of cement slurry at that temperature and pressure and mixed evenly, the density of the uniform gas-liquid-solid three-phase flow foam plugging slurry is approximately F1(T1, P1) = (n×D1+C1) / (n+1). As n increases, the amount of gas added increases, and the density of the foam plugging slurry decreases.

[0054] In some specific embodiments of the present invention, the nitrogen filling process in step (3) includes:

[0055] (3-1) Simulation calculation: Under the highest temperature and leakage pressure of the leaking layer, a certain volume of nitrogen is charged into the plugging slurry intermediate (which can be a sample liquid that meets the aforementioned composition of the present invention but in a smaller amount) to obtain a foam plugging slurry. The density of the foam plugging slurry is reduced by 3-30%, preferably 5-20%, compared with the density of the cement-based slurry. That is, the density of the foam plugging slurry is 70-97%, preferably 80-95% of the density of the cement-based slurry. The density of the formed foam plugging slurry approximately satisfies the following relationship: F(T,P)=(n×D+C) / (n+1). For example, at the highest temperature T1 and leakage pressure P1 of the leaking layer, the nitrogen density is D1 and the cement-based slurry density is C1. Then, the cement-based slurry is filled with n unit volumes of nitrogen at this temperature and pressure and mixed evenly to obtain a foam plugging slurry. The density of the foam plugging slurry is 70-97% of the density of the base liquid, preferably 80-95%. Based on this, the required nitrogen amount at the highest temperature and leakage pressure of the leaking layer is obtained. Then, the nitrogen filling amount required for the ground (temperature and ground pressure under the ground environment) is reversed according to the real nitrogen state equation to obtain the construction nitrogen filling amount;

[0056] (3-2) Using a special nitrogen filling device, nitrogen is filled into the plugging slurry intermediate according to the construction nitrogen filling amount obtained above (the amount used meets the construction nitrogen filling amount required for the plugging slurry intermediate) to obtain foam plugging slurry.

[0057] The density of nitrogen is different at different temperatures and pressures. It can be approximately calculated based on the real gas state equation or obtained by looking up published experimental data tables of nitrogen at different temperatures and pressures.

[0058] According to the present invention, preferably, the foaming agent is 2-3 parts by weight relative to 100 parts by weight of the cement-based slurry.

[0059] According to the present invention, preferably, the volume of the nitrogen added is such that the density of the foam plugging slurry is reduced by 5-20% compared to when no nitrogen is added.

[0060] According to the present invention, the cement-based slurry in step (1) comprises, by weight, 100 parts of cement, 1-10 parts of fluid loss additive, 0-3 parts of retarder, 0-5 parts of early strength agent, 0-0.2 parts of thickener, and 30-70 parts of water. Furthermore, the cement-based slurry comprises, by weight, 100 parts of cement, 3-8 parts of fluid loss additive, 0.1-2 parts of retarder, 0.25-1 parts of early strength agent, 0.05-0.1 parts of thickener, and 35-60 parts of water.

[0061] In some embodiments, after the cement-based slurry is prepared in step (1), various properties of the cement-based slurry are evaluated at the highest temperature and leakage pressure at the depth of the leaking layer, including cold and hot slurry rheology (cold and hot slurry six-speed rheometer tail turn value ≥10), API water loss (30min water loss <100mL), thickening curve (the curve is normal, meeting the safety time and construction conditions for downhole pumping to the leaking layer) and compressive strength (compressive strength >3MPa after curing for 48 hours at the lowest temperature and leakage pressure of the leaking layer). If the cement-based slurry meets the above properties, it meets the requirements for further preparation of foam plugging slurry.

[0062] In some embodiments, according to the bearing capacity of the formation of the leakage zone, a cement slurry of a certain volume and density is prepared to cover the thickness of the leakage zone (the density of the foam plugging slurry at the leakage zone in the wellbore is required to be low enough, and the slurry pressure at this point is equivalent to the leakage pressure of the formation at this point, so that this section of cement slurry can stay and wait for solidification and strength to be developed. Otherwise, the cement slurry will leak at the leakage zone, and will be completely or partially lost, resulting in plugging failure). Then, a certain cement slurry injection process is combined with the pumping to the leakage zone to wait for solidification for 24-72 hours. After the cement slurry is thoroughly thickened and strengthened, a cement plug with a certain strength bonded to the formation is formed. At this time, the cement plug completely blocks and seals the leakage channels such as pores, cracks and holes in the original leakage zone. After drilling down and sweeping away the cement plug in this section of the wellbore, the well wall of this section of the leakage zone is strengthened by cement solidification, which can greatly alleviate or eliminate the leakage condition of this section of the leakage zone, so that normal drilling can continue.

[0063] A third aspect of the present invention provides a foam plugging slurry, which is prepared by the preparation method described in the second aspect.

[0064] The fourth aspect of the present invention provides the aforementioned foam plugging slurry system, comprising the foam plugging slurry described in the first aspect or the third aspect and optional conventional plugging slurry;

[0065] Wherein, relative to 100 parts by weight of the foam plugging slurry, the conventional plugging slurry is no more than 100 parts by weight, preferably 10-30 parts by weight.

[0066] In the present invention, the foam plugging slurry is mixed with conventional plugging slurry to further improve the plugging performance of the foam plugging slurry. When plugging cement plugs (plugging cement slurry) in formations with low pressure bearing capacity, there is basically no leakage, thus ensuring the success rate of plugging.

[0067] A fifth aspect of the present invention provides an application of the aforementioned foam plugging slurry or the aforementioned foam plugging slurry system in a shale gas well.

[0068] The present invention is described in detail below through examples and comparative examples. In the following examples and comparative examples, where specific conditions are not specified, the experiments were conducted under conventional conditions or those recommended by the manufacturer. Reagents and instruments used, where the manufacturer is not specified, are commercially available, either conventional or custom-made products.

[0069] Example 1

[0070] (1) Set the maximum temperature of the leaking layer section to 90°C and the overburden pressure to 35 MPa. Prepare cement slurry in the following proportions: 100 parts by weight of cement, 3 parts of fluid loss reducer, 2 parts of retarder, 0.25 parts of early strength agent, 0.05 parts of thickener, and 50 parts of water. The cement density is 1.88 g / cm 3 , the specific formula is shown in Table 1;

[0071] (2) adding 3 parts by weight of a foaming agent to the cement-based slurry and stirring at a speed of 100 rpm for 3 minutes to obtain a plugging slurry intermediate;

[0072] (3) Assume that the ground nitrogen filling equipment fills the plugging slurry intermediate with nitrogen 50 times the volume of the base slurry to obtain foam plugging slurry (1 atmosphere, 20°C). The foam plugging slurry of this density is equivalent to the formation loss pressure of this leaking layer after superimposing all the upper pressures on this leaking layer.

[0073] Example 2

[0074] The foam plugging slurry was prepared according to the steps of Example 1. The specific formula is shown in Table 1.

[0075] Example 3

[0076] The foam plugging slurry was prepared according to the steps of Example 1. The specific formula is shown in Table 1.

[0077] Example 4

[0078] The foam plugging slurry was prepared according to the steps of Example 1. The specific formula is shown in Table 1.

[0079] Example 5

[0080] The foam plugging slurry was prepared according to the steps of Example 1. The specific formula is shown in Table 1.

[0081] Table 1

[0082]

[0083] Comparative Example 1

[0084] The foam plugging slurry was prepared according to the set conditions and steps of Example 1, except that it was not inflated with gas. The specific formula is shown in Table 2.

[0085] Comparative Example 2

[0086] The foam plugging slurry was prepared according to the set conditions and steps of Example 1. The specific formula is shown in Table 2.

[0087] Comparative Example 3

[0088] The foam plugging slurry was prepared according to the set conditions and steps of Example 1, except that no foaming agent was added. The specific formula is shown in Table 2.

[0089] Comparative Example 4

[0090] The foam plugging slurry was prepared according to the set conditions and steps of Example 1, except that no retarder was added. The specific formula is shown in Table 2.

[0091] Comparative Example 5

[0092] The foam plugging slurry was prepared according to the steps of Example 1, except that no fluid loss additive was added. The specific formula is shown in Table 2.

[0093] Table 2

[0094]

[0095] Test Example 1

[0096] The various performance indicators of the cement-based slurries of the examples and comparative examples were tested, and the results are shown in Table 3. Rheology of hot and cold slurries The hot and cold slurries were read using a six-speed rotary viscometer;

[0097] API water loss is measured using a high-temperature and high-pressure water loss instrument in accordance with API standards;

[0098] The thickening curve was obtained using a pressurized thickening instrument according to conventional industry practices;

[0099] Compressive Strength The compressive strength of cement specimens cured for 48 hours was tested using a pressure testing machine.

[0100] Table 3

[0101]

[0102] Test Example 2

[0103] The performance parameter measurement results of the foam plugging slurry of the embodiment and the comparative example are shown in Table 4.

[0104] Table 4

[0105]

[0106] As can be seen from Table 4, the foam plugging slurries of Examples 1 and 2 and the cement-based slurry performance meet the safety time and construction conditions for downhole pumping to the leaking layer. The foam plugging slurries at the leaking layer are uniform bubbly flows without leakage, and the bubbles are stably distributed without being free. The slurry has good stability and has a better plugging effect than the foam plugging slurry of the comparative example.

[0107] Test Example 3

[0108] The plugging performance of the foam plugging slurries of Example 1 and Example 2 was tested, and the results are shown in Table 5.

[0109] Table 5

[0110]

[0111] As can be seen from Table 5, the foam plugging slurries of Examples 1 and 2 of the present invention were used in the Nanchuan shale gas field in Chongqing. The formula and preparation method of the foam plugging slurry of the present invention were applied. The plugging effect of the conventional plugging scheme in the early stage was not ideal. Finally, the foam plugging slurry was used to plug the leaking layer, and the plugging effect was significant. The plugging was successful in one time, saving a lot of plugging costs.

[0112] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A foam plugging slurry, characterized in that: The foam plugging slurry comprises: cement-based slurry, foaming agent and filled nitrogen; The foaming agent is not more than 5 parts by weight relative to 100 parts by weight of the cement-based slurry; the nitrogen gas is added in an amount by volume such that the density of the foamed plugging slurry is reduced by 3-30% compared to the cement-based slurry; The cement-based slurry comprises, by weight, 100 parts of cement, 1-10 parts of fluid loss reducer, 0-3 parts of retarder, 0-5 parts of early strength agent, 0-0.2 parts of thickener, and 30-70 parts of water.

2. The foam plugging slurry according to claim 1, wherein: The foaming agent is 2-3 parts by weight.

3. The foam plugging slurry according to claim 1, wherein: The nitrogen gas added is calculated by volume, and the amount of nitrogen added is such that the density of the foam plugging slurry is reduced by 5-20% compared with the cement-based slurry.

4. The foam plugging slurry according to claim 1, wherein: The cement-based slurry comprises, by weight, 100 parts of cement, 3-8 parts of fluid loss reducer, 0.1-2 parts of retarder, 0.25-1 parts of early strength agent, 0.05-0.1 parts of thickener, and 35-60 parts of water.

5. A method for preparing a foam plugging slurry, characterized in that: The following steps are involved: (1) Prepare cement-based slurry; (2) Adding a foaming agent to the cement-based slurry and stirring evenly to obtain a plugging slurry intermediate; (3) Nitrogen is injected into the plugging slurry intermediate to obtain foam plugging slurry; Wherein, relative to 100 parts by weight of cement-based slurry, the foaming agent is no more than 5 parts by weight; The nitrogen gas is added in an amount by volume such that the density of the foam plugging slurry is reduced by 3-30% compared to the cement-based slurry; The cement-based slurry in step (1) comprises, by weight, 100 parts of cement, 1-10 parts of fluid loss reducer, 0-3 parts of retarder, 0-5 parts of early strength agent, 0-0.2 parts of thickener, and 30-70 parts of water; The fluid loss additive is an emulsion fluid loss additive and / or a water-soluble high molecular polymer fluid loss additive; The early strength agent is selected from at least one of electrolyte inorganic salt early strength agents, water-soluble organic polymer early strength agents and organic compound and inorganic salt composite early strength agents.

6. The preparation method according to claim 5, wherein The cement-based slurry comprises, by weight, 100 parts of cement, 3-8 parts of fluid loss reducer, 0.1-2 parts of retarder, 0.25-1 parts of early strength agent, 0.05-0.1 parts of thickener, and 35-60 parts of water.

7. The preparation method according to claim 5, wherein Relative to 100 parts by weight of the cement-based slurry, the foaming agent is 2-3 parts by weight.

8. The preparation method according to claim 5, wherein The nitrogen gas is added in an amount by volume, and the amount of nitrogen added is such that the density of the foam plugging slurry is reduced by 5-20% compared with the cement-based slurry.

9. The preparation method according to claim 5, wherein The foaming agent in step (2) is a surfactant foaming agent.

10. The preparation method according to claim 9, wherein The foaming agent in step (2) is selected from at least one of sodium dodecylbenzenesulfonate, α-olefin sulfonate, α-sulfo fatty acid methyl ester and sodium N-oleoyl-N-methyl taurate.

11. The preparation method according to claim 5, wherein The stirring speed is 50-1000 rpm.

12. The preparation method according to claim 11, wherein The stirring speed is 100-500 rpm.

13. A foam plugging slurry, characterized in that: The foam plugging slurry is prepared by the preparation method described in any one of claims 5-12.

14. A foam plugging slurry system, characterized in that: The system comprises the foam plugging slurry according to any one of claims 1 to 4 and 13 and optionally conventional plugging slurry; Wherein, relative to 100 parts by weight of the foam plugging slurry, the conventional plugging slurry is no more than 100 parts by weight.

15. The foam plugging slurry system according to claim 14, wherein: Relative to 100 parts by weight of the foam plugging slurry, the conventional plugging slurry is 10-30 parts by weight.

16. Use of the foam plugging slurry according to any one of claims 1 to 4 and 13 or the foam plugging slurry system according to claims 14 to 15 in a shale gas well.

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