Ultra-low density leak-off workover fluid and method of making same

Abstract

The present application belongs to the technical field of drilling and workover fluid, and specifically discloses an ultra-low density leak-proof workover fluid, which is composed of base fluid and micro-bubble fluid; wherein the base fluid comprises structural stabilizer, viscosity-increasing and filtration-reducing agent, pH regulator, bactericide, corrosion inhibitor, surfactant and water; and the micro-bubble fluid comprises foaming agent, bubble stabilizer and water. The preparation method of the workover fluid is as follows: first, preparing the base fluid; then, preparing the micro-bubble fluid; finally, adding the micro-bubble fluid into the base fluid and mixing to obtain the finished product. The significant effect of the present application is that the workover fluid has good viscosity-increasing and shear-increasing effects, short natural degradation period, small reservoir damage, and can be applied to various oil and gas wells with formation pressure coefficient less than 1.0, well killing and workover operation, can greatly reduce the use amount of conventional working fluid to shorten the flowback period, and has low requirement on production equipment and is easy to implement.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of drilling and workover fluid, in particular to a special working fluid for oil and gas engineering field. BACKGROUND

[0002] In the middle and later stages of oil and gas well development, due to reservoir reconstruction and unsatisfied injection-production ratio, the formation pressure gradually decreases, so that different degrees of leakage often occur in conventional workover operations, which leads to the problems of being unable to workover or high risk of workover. When the leakage of the workover fluid is too large, not only the cost of workover is increased, but also the reservoir is polluted due to the incompatibility between the workover fluid and the formation fluid.

[0003] The workover fluid is mainly prepared based on clean water, and different components such as tackifier, corrosion inhibitor, weighting agent (inorganic salt, organic salt, barite powder), pH adjuster, etc. are added to the clean water to adjust the performance parameters of the workover fluid, so as to achieve different operation purposes. In the prior art, the density of the workover fluid is generally greater than or equal to 1.0 g / cm 3 When it is applied to an oil and gas well with a formation pressure coefficient less than 1.0, leakage of the working fluid due to positive pressure difference will inevitably occur.

[0004] In order to reduce the density of the workover fluid, foaming treatment can be performed. However, the existing foaming technology for the workover fluid adopts the process of adding a foaming agent to the workover fluid base fluid with certain viscosity and shear, and then foaming, so as to reduce the density of the whole system of the workover fluid. In the existing technology, the charging method (mixed gas, explosive gas), and the mechanical foaming method (high-pressure jet, high shear) are commonly used. The charging method has the following shortcomings: in the charging process, the gas bubble floating resistance is large due to the influence of the base fluid viscosity and shear, and the gas bubble is easy to coalesce and become large, thereby leading to slip and breakage. The high-pressure jet has the following shortcomings: the requirement for the pump is high, the conventional centrifugal pump is easy to be gas blocked due to the influence of the gas content of the system, which leads to difficulty in water feeding, and although the plunger pump can effectively overcome the problem of gas blocking, with the further increase of the gas content of the system, the system is easy to be compressed, which leads to the decrease of pump efficiency and limits the lower limit of the system density. The high shear circulation has the following shortcomings: high-speed stirring equipment is required, and in the known similar published patents, this method is mostly used, and the speed requirement is 5000-8000 r / min. This method is easy to realize in the laboratory, but it is difficult to realize in the pilot test, and has strong limitations.

[0005] For example, the Chinese patent with publication number CN115197683A provides a leak-proof low-density microbubble workover fluid and a preparation method thereof; wherein the foaming process of the workover fluid is to add sodium-based bentonite into water, then add bactericide, tackifier, drag reducing agent, foaming agent and foam stabilizer, mix uniformly, and then aerate to achieve the purpose of foaming. However, this foaming method has the following disadvantages: after adding the foaming agent and the foam stabilizer into the workover fluid base fluid, the viscosity and shear of the whole system are further increased, the gas bubbles introduced into the workover fluid are easy to coalesce and become large and slip off, resulting in the increase of the density and the deterioration of the low-density stability. SUMMARY

[0006] To solve the above technical problems, the purpose of the present application is to provide an ultra-low density workover fluid and a preparation method thereof, wherein "ultra-low density" means that the density is less than 1.0 g / cm 3 .

[0007] The main technical solutions adopted to achieve one of the purposes of the present application are as follows:

[0008] An ultra-low density leak-proof workover fluid, characterized by being composed of a base fluid and a microbubble fluid in a weight ratio of 10:1-5;

[0009] The base fluid includes structural stabilizer 0.4-0.6 parts, tackifier and filtration reducer 1-2 parts, pH regulator 0.05-0.1 parts, bactericide 0.05-0.1 parts, corrosion inhibitor 0.05-0.1 parts, surfactant 0.1-0.3 parts, and water 100 parts by weight;

[0010] The microbubble fluid includes foaming agent 0.1-6.0 parts, foam stabilizer 0.1-0.3 parts, and water 100 parts.

[0011] The microbubble fluid is subjected to foaming treatment.

[0012] The bactericide is at least one of formaldehyde and acetaldehyde;

[0013] The surfactant is at least one of fluorocarbon surfactant, alkyl glucoside and fatty acid glyceride.

[0014] The main technical solutions adopted to achieve the second purpose of the present application are as follows:

[0015] A preparation method of an ultra-low density leak-proof workover fluid, comprising the following steps:

[0016] Step one, sequentially adding the required amount of structural stabilizer, tackifier and filtration reducer, pH regulator, bactericide, corrosion inhibitor and surfactant into the required amount of water, and stirring uniformly to obtain a base fluid;

[0017] Step two, add the desired amount of foaming agent and foam stabilizer into the desired amount of water, mix well and then foam to obtain the microbubble liquid;

[0018] Step three, add the microbubble liquid to the base fluid in a weight ratio, mix well to obtain the ultra-low density leak-proof workover fluid.

[0019] By mixing the microbubble liquid after foaming treatment with the base fluid, the density of the workover fluid can be reduced, so that it has the basis of reaching ultra-low density (density less than 1.0 g / cm 3 ) state; at the same time, introducing the high viscosity and shear base fluid into the foaming process can optimize the foaming effect and improve the performance parameters of the workover fluid. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The flowchart of the present application;

[0021] Figure 2 The appearance state comparison chart of the pre-foaming workover fluid and the post-foaming workover fluid when the configuration is completed;

[0022] Figure 3 The appearance state comparison chart of the pre-foaming workover fluid and the post-foaming workover fluid after 8h of standing. DETAILED DESCRIPTION

[0023] The present application is further illustrated below in conjunction with the embodiments and the drawings.

[0024] Examples 1-8:

[0025] An ultra-low density leak-proof workover fluid is mixed from a base fluid and a microbubble liquid; the base fluid includes a structure stabilizer, a viscosity increasing and filtration reducing agent, a pH regulator, a bactericide, a corrosion inhibitor, a surfactant, and water; the microbubble liquid includes a foaming agent, a foam stabilizer, and water; the microbubble liquid is foamed and treated to be in a foam state.

[0026] The components of the workover fluid of examples 1-8 are shown in Table 1 in weight parts:

[0027]

[0028] Examples 9-17:

[0029] A base fluid of an ultra-low density leak-proof workover fluid includes a structure stabilizer, a viscosity increasing and filtration reducing agent, a pH regulator, a bactericide, a corrosion inhibitor, a surfactant, and water;

[0030] The structure stabilizer is a biopolymer; the biopolymer is at least one of xanthan gum, hydroxypropyl guar gum and sodium carboxymethyl cellulose; the viscosity-increasing fluid loss additive is carboxymethyl starch; the pH regulator is at least one of sodium hydroxide, potassium hydroxide and sodium carbonate; the bactericide is at least one of formaldehyde and acetaldehyde; the corrosion inhibitor is at least one of imidazoline corrosion inhibitor, nitric acid corrosion inhibitor and nitrite corrosion inhibitor; and the surfactant is at least one of fluorocarbon surfactant, alkyl glucoside and fatty acid glyceride.

[0031] When the structure stabilizer is polysaccharide biopolymer and the viscosity-increasing fluid loss additive is carboxymethyl starch, both are self-degradable materials and have self-degradation ability; after well completion, the super-low density leak-proof workover fluid entering the formation can be naturally degraded under the action of formation temperature, which has low reservoir damage and does not affect the reservoir and gathering pipeline.

[0032] Specifically, the fluorocarbon surfactant is any one of fluorocarbon surfactant FC-4430, fluorocarbon surfactant FSO-100, fluorocarbon surfactant FS-3100 and fluorocarbon surfactant FC209.

[0033] The alkyl glucoside is alkyl glucoside APG-0814 or alkyl glucoside APG0810.

[0034] The fatty acid glyceride is any one of fatty acid glyceride type 34, fatty acid glyceride type 36, fatty acid glyceride type 38 and fatty acid glyceride type 40.

[0035] Specifically, the ingredient ratio in the base fluid of examples 9-17 is shown in Table 2 by weight parts:

[0036]

[0037] Examples 18-53:

[0038] A microbubble fluid of a super-low density leak-proof workover fluid, comprising a foaming agent, a foam stabilizer and water.

[0039] The foaming agent is at least one of sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, sodium fatty alcohol polyoxyethylene ether sulfate, synthetic fire-fighting foam, fluoroprotein foam and aqueous film-forming foam; and the foam stabilizer is dodecanol.

[0040] The fluoroprotein foam is any one of fluoroprotein foam FP and fluoroprotein foam FFFP; and the aqueous film-forming foam is aqueous film-forming foam AFFF.

[0041] Specifically, the ingredient ratio in the microbubble fluid of examples 18-53 is shown in Table 3 by weight parts:

[0042]

[0043] Embodiments 54-58:

[0044] An ultra-low density leak-proof workover fluid is prepared by mixing a base fluid and a microbubble fluid in a weight ratio; the component proportions of the ultra-low density leak-proof workover fluid of embodiments 54-58 are shown in Table 4:

[0045]

[0046] The base fluid in Table 4 is selected from the base fluid components of any one of embodiments 9-17, and the microbubble fluid is selected from the microbubble fluid components of any one of embodiments 18-53.

[0047] Embodiment 59:

[0048] A preparation method of an ultra-low density leak-proof workover fluid is prepared by the following steps:

[0049] Step one, preparing a base fluid;

[0050] Under normal temperature and pressure conditions, a required amount of a structure stabilizer, a viscosity-increasing fluid loss reducer, a pH regulator, a bactericide, a corrosion inhibitor and a surfactant are sequentially added to a required amount of water, and the mixture is stirred until uniform to obtain the base fluid;

[0051] Step two, preparing a microbubble fluid;

[0052] Under normal temperature and pressure conditions, a required amount of a foaming agent and a bubble stabilizer are sequentially added to a required amount of water, and the mixture is mixed until uniform and then foamed to obtain the microbubble fluid; the density of the microbubble fluid is controlled to be 15 g / L-50 g / L, the particle size is controlled to be 30 μm-200 μm, and the half-life period is controlled to be 30 min-50 min.

[0053] Step three, preparing a workover fluid;

[0054] Under normal temperature and pressure conditions, the microbubble fluid is added to the base fluid, and the mixture is stirred until uniform to obtain the ultra-low density leak-proof workover fluid.

[0055] The workover fluids of embodiments 1-8 and 54-58 are prepared according to the method of this embodiment.

[0056] Embodiment 60:

[0057] An ultra-low density leak-proof workover fluid is composed of a base fluid and a microbubble fluid in a weight ratio of 10:1;

[0058] The base fluid includes, in parts by weight, 0.4 parts of xanthan gum, 1 part of carboxymethyl starch, 0.05 parts of sodium hydroxide, 0.05 parts of formaldehyde, 0.05 parts of imidazoline corrosion inhibitor BP8212, 0.1 parts of fluorocarbon surfactant FC-4430 and 100 parts of water.

[0059] The microbubble liquid comprises 0.1 part of sodium dodecyl sulfate, 0.1 part of sodium dodecyl benzene sulfonate, 0.1 part of dodecanol and 100 parts of water by weight.

[0060] The preparation method of the workover fluid is as follows:

[0061] Step one, configure the base liquid

[0062] All of the xanthan gum, carboxymethyl starch, sodium hydroxide, formaldehyde, imidazoline corrosion inhibitor BP8212 and fluorocarbon surfactant FC-4430 are sequentially added into 100 parts of water, and the base liquid is obtained after uniform stirring;

[0063] Step two, configure the microbubble liquid

[0064] All of the sodium dodecyl sulfate, sodium dodecyl benzene sulfonate and dodecanol are sequentially added into 100 parts of water, and the microbubble liquid is obtained after foaming;

[0065] The density of the microbubble liquid is controlled to be 45 g / L-50 g / L, the particle size is controlled to be 150 μm-200 μm, and the half-life is ≥30 min.

[0066] Step three, configure the workover fluid

[0067] The microbubble liquid is added into the base liquid, and the weight ratio of the base liquid to the microbubble liquid is 10:1; after uniform stirring, the super-low density leak-proof workover fluid with a density of 0.9 g / cm 3 is obtained.

[0068] Example 61:

[0069] A super-low density leak-proof workover fluid is composed of a base liquid and a microbubble liquid with a weight ratio of 10:1;

[0070] The base liquid comprises 0.4 parts of xanthan gum, 1 part of carboxymethyl starch, 0.05 parts of potassium hydroxide, 0.05 parts of acetaldehyde, 0.05 parts of Lan-5 nitric acid cleaning corrosion inhibitor, 0.1 parts of alkyl glucoside APG-0814 and 100 parts of water by weight.

[0071] The microbubble liquid comprises 0.1 part of sodium dodecyl sulfate, 0.1 part of sodium dodecyl benzene sulfonate, 0.5 parts of fatty alcohol polyoxyethylene ether sodium sulfate, 0.15 parts of dodecanol and 100 parts of water by weight.

[0072] The preparation method of the workover fluid is as follows:

[0073] Step one, configure the base liquid

[0074] Into 100 parts of water, add all of the xanthan gum, carboxymethyl starch, carboxymethyl starch, potassium hydroxide, acetaldehyde, Lan-5 nitric acid cleaning inhibitor, alkyl glucoside APG-0814 in turn; after stirring uniformly, the base solution is obtained;

[0075] Step two, configure microbubble liquid

[0076] Into 100 parts of water, add all of the sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, fatty alcohol polyoxyethylene ether sodium sulfate, dodecanol in turn; then foam to obtain the microbubble liquid;

[0077] The density of the microbubble liquid is controlled to be 30g / L-40g / L, the particle size is 100μm-150μm, and the half-life is ≥30min.

[0078] Step three, configure workover fluid

[0079] Add the microbubble liquid into the base solution, and the weight ratio of the base solution to the microbubble liquid is 10:1; after stirring uniformly, the super low density leak-proof workover fluid with a density of 0.85g / cm 3 is obtained.

[0080] Example 62:

[0081] A super low density leak-proof workover fluid is composed of a base solution and a microbubble liquid with a weight ratio of 10:2;

[0082] The base solution includes 0.5 parts of xanthan gum, 1.2 parts of carboxymethyl starch, 0.05 parts of sodium carbonate, 0.05 parts of formaldehyde, 0.05 parts of nitrite inhibitor AQ-700, 0.1 parts of fatty acid glyceride type 34, and 100 parts of water in terms of weight fraction;

[0083] The microbubble liquid includes 0.1 parts of sodium dodecyl sulfate, 0.1 parts of sodium dodecyl benzene sulfonate, 0.5 parts of fatty alcohol polyoxyethylene ether sodium sulfate, 0.5 parts of synthetic fire foam (mass concentration of 6%), 0.2 parts of dodecanol, and 100 parts of water in terms of weight fraction.

[0084] The preparation method of the workover fluid is as follows:

[0085] Step one, configure base solution

[0086] Into 100 parts of water, add all of the xanthan gum, carboxymethyl starch, sodium carbonate, formaldehyde, nitrite inhibitor AQ-700, and fatty acid glyceride type 34 in turn; after stirring uniformly, the base solution is obtained;

[0087] Step two, configure microbubble liquid

[0088] To 100 parts of water, add all of the sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, fatty alcohol polyoxyethylene ether sodium sulfate, synthetic fire foam (mass concentration of 6%) and dodecanol in turn; then foam it to obtain the micro bubble liquid;

[0089] The density of the micro bubble liquid is 25g / L-30g / L, the particle size is 50μm-100μm, and the half-life is ≥40min.

[0090] Step three, configure the workover fluid

[0091] Add the micro bubble liquid to the base fluid, and the weight ratio of the base fluid to the micro bubble liquid is 10:2; after stirring uniformly, the super low density leak-proof workover fluid with a density of 0.75g / cm 3 is obtained.

[0092] Example 63:

[0093] A super low density leak-proof workover fluid is composed of a base fluid and a micro bubble liquid with a weight ratio of 10:4;

[0094] The base fluid includes 0.5 parts of xanthan gum, 1.4 parts of carboxymethyl starch, 0.05 parts of sodium hydroxide, 0.05 parts of formaldehyde, 0.05 parts of imidazoline corrosion inhibitor, 0.1 parts of fluorocarbon surfactant FC-4430, and 100 parts of water by weight;

[0095] The micro bubble liquid includes 0.2 parts of sodium dodecyl sulfate, 0.2 parts of sodium dodecyl benzene sulfonate, 0.8 parts of fatty alcohol polyoxyethylene ether sodium sulfate, 0.5 parts of synthetic fire foam (mass concentration of 6%), 0.5 parts of fluoroprotein foam FFFP (mass concentration of 3%), 0.15 parts of dodecanol, and 100 parts of water by weight.

[0096] The preparation method of the workover fluid is as follows:

[0097] Step one, configure the base fluid

[0098] Add all of the xanthan gum, carboxymethyl starch, sodium hydroxide, formaldehyde, imidazoline corrosion inhibitor, and fluorocarbon surfactant FC-4430 to 100 parts of water in turn; after stirring uniformly, the base fluid is obtained;

[0099] Step two, configure the micro bubble liquid

[0100] Add all of the sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, fatty alcohol polyoxyethylene ether sodium sulfate, synthetic fire foam (mass concentration of 6%), fluoroprotein foam FFFP (mass concentration of 3%), and dodecanol to 100 parts of water in turn; then foam it to obtain the micro bubble liquid;

[0101] The density of the microbubble liquid is controlled to be 20-25 g / L, the particle size is 30-80 um, and the half-life is greater than or equal to 40 min.

[0102] Step three, configure the workover fluid

[0103] Add the microbubble liquid to the base fluid, and the weight ratio of the base fluid to the microbubble liquid is 10:4; after stirring uniformly, an ultra-low density leak-proof workover fluid with a density of 0.72 g / cm 3 is obtained.

[0104] Example 64:

[0105] An ultra-low density leak-proof workover fluid is composed of a base fluid and a microbubble liquid with a weight ratio of 10:5;

[0106] The base fluid includes 0.6 parts of xanthan gum, 1.8 parts of carboxymethyl starch, 0.05 parts of potassium hydroxide, 0.05 parts of acetaldehyde, 0.05 parts of nitric acid corrosion inhibitor, 0.1 parts of alkyl glucoside APG-0814, and 100 parts of water by weight;

[0107] The microbubble liquid includes 1 part of fatty alcohol polyoxyethylene ether sodium sulfate, 0.5 parts of synthetic fire foam liquid (mass concentration of 6%), 0.5 parts of fluoroprotein foam liquid FFFP (mass concentration of 3%), 0.5 parts of aqueous film-forming foam liquid AFFF (mass concentration of 3%), and 0.2 parts of dodecanol, and 100 parts of water by weight.

[0108] The preparation method of the workover fluid is as follows:

[0109] Step one, configure the base fluid

[0110] Add all the xanthan gum, carboxymethyl starch, potassium hydroxide, acetaldehyde, nitric acid corrosion inhibitor, and alkyl glucoside APG-0814 to 100 parts of water in sequence; after stirring uniformly, the base fluid is obtained;

[0111] Step two, configure the microbubble liquid

[0112] Add all the fatty alcohol polyoxyethylene ether sodium sulfate, synthetic fire foam liquid (mass concentration of 6%), fluoroprotein foam liquid FFFP (mass concentration of 3%), aqueous film-forming foam liquid AFFF (mass concentration of 3%), and dodecanol to 100 parts of water in sequence; then foam to obtain the microbubble liquid;

[0113] The density of the microbubble liquid is controlled to be 15-20 g / L, the particle size is 30-80 um, and the half-life is greater than or equal to 50 min.

[0114] Step three, configure the workover fluid

[0115] The microbubble solution was added into the base fluid, and the weight ratio of the base fluid to the microbubble solution was 10:5; after being stirred uniformly, the super-low density leak-proof workover fluid with a density of 0.60 g / cm 3

[0116] Comparative Example 1

[0117] The difference between the comparative example and Example 60 is that the base fluid does not contain the structural stabilizer xanthan gum.

[0118] Comparative Example 2

[0119] The difference between the comparative example and Example 60 is that the base fluid does not contain the viscosity-increasing fluid loss reducer carboxymethyl starch.

[0120] Comparative Example 3

[0121] The difference between the comparative example and Example 60 is that the base fluid does not contain the surfactant fluorocarbon surfactant FC-4430.

[0122] Experimental Example 1: Performance inspection of the base fluid

[0123] Experimental method: the base fluid prepared in Example 60 was allowed to stand for 7 days, and the density, funnel viscosity, initial cut, final cut, apparent viscosity, plastic viscosity, dynamic shear force and surface tension of the base fluid before and after standing for 7 days were tested, and the experimental results are shown in Table 5:

[0124]

[0125] As shown in Table 5, after the base fluid prepared in Example 60 was allowed to stand for 7 days, the structural stabilizer in the base fluid system gradually degraded, the viscosity (apparent viscosity, plastic viscosity and dynamic shear force) decreased, and tended to be consistent with water, but the surfactant component in the base fluid did not fail with the degradation of the structural stabilizer.

[0126] Experimental Example 2: Comprehensive performance inspection of the workover fluid

[0127] Experimental method: the density, funnel viscosity, initial cut, final cut, apparent viscosity, plastic viscosity, dynamic shear force and surface tension of the super-low density leak-proof workover fluid prepared in Examples 60-64 and Comparative Examples 1-3 were tested, and the experimental results are shown in Table 6:

[0128]

[0129] In Table 6, it can be seen from the test results of Examples 60-64 that the workover fluid of the present application has good comprehensive performance indexes.

[0130] ​From the comparison of the test results of Example 60, Comparative Example 1 and Comparative Example 2, it can be seen that, compared with using both the structure stabilizer (xanthan gum) and the viscosity increasing and fluid loss reducing agent (carboxymethyl starch), using only the structure stabilizer (xanthan gum) or the viscosity increasing and fluid loss reducing agent (carboxymethyl starch) can obviously reduce the yield point, apparent viscosity, plastic viscosity and dynamic yield point of the workover fluid. Thus, the structure stabilizer (xanthan gum) and the viscosity increasing and fluid loss reducing agent (carboxymethyl starch) have good compatibility in the workover fluid, and can have a prominent synergistic effect of increasing the yield point. The use of both of them can greatly improve various indexes.

[0131] From the comparison of the test results of Example 60 and Comparative Example 3, it can be seen that, adding an appropriate amount of the surfactant (fluorocarbon surfactant FC-4430) into the workover fluid can effectively reduce the surface tension of the workover fluid, and prevent the reservoir from being "water locked".

[0132] Experimental Example 3: Stability of the workover fluid

[0133] Experimental method: The ultra-low density leak-proof workover fluids prepared in Examples 60-64 and Comparative Example 3 were allowed to stand for 48 h, and the suspension stability of the ultra-low density leak-proof workover fluids before and after standing for 48 h was investigated. The experimental results are shown in Table 7.

[0134] The ultra-low density leak-proof workover fluids prepared in Comparative Examples 1 and 2 were allowed to stand for 12 h, and the suspension stability of the ultra-low density leak-proof workover fluids before and after standing for 12 h was investigated. The experimental results are shown in Table 7.

[0135] In Table 7, the parameters of the workover fluids of Examples 60-64 and Comparative Examples 1, 2 and 3 before standing are consistent with those in Table 6.

[0136]

[0137] The static shear (initial shear and final shear) in the rheological parameters is an important index for evaluating the suspension stability of the ultra-low density leak-proof workover fluid.

[0138] From Table 7, it can be seen that the ultra-low density leak-proof workover fluids obtained in Examples 60-64 have strong suspension stability of micro-bubbles, low rising rate of bubble slip and low density loss rate after standing for 48 h, and can meet the requirements of field construction.

[0139] From the test results of Example 60, Comparative Example 1 and Comparative Example 2, it can be seen that the structure stabilizer (xanthan gum) and the viscosity increasing and fluid loss reducing agent (carboxymethyl starch) have good compatibility, and can synergize with each other when used together, and the workover fluid obtained has a prominent effect of increasing the yield point. However, the yield point of the workover fluid obtained by using only the structure stabilizer (xanthan gum) or the viscosity increasing and fluid loss reducing agent (carboxymethyl starch) cannot meet the requirements of suspending micro-bubbles, and the density stability is poor, the bubble slip speed is fast, and the bubbles have all slipped within 12 h, which cannot meet the requirements of the conventional workover operation cycle.

[0140] From the test results of Example 60 and Comparative Example 3, it can be seen that when no surfactant (fluorocarbon surfactant FC-4430) is added to the workover fluid, the surface tension of the workover fluid is high, which can easily cause "water lock" of the reservoir.

[0141] Experimental Example 4: Application of the ultra-low density leak-proof workover fluid of the present application in an oil and gas well with a formation pressure coefficient less than 1.0.

[0142] The specific use method of the workover fluid in the field: according to the pressure coefficient provided by the geological design, combined with the additional density value corresponding to the oil and gas well control requirement, the density of the kill fluid is determined. Before killing, the reverse washing process is carried out, the single well dosage is confirmed to be 1.5 times the volume of the casing above the pump hanging depth, and during the field construction, the circulation is carried out at a displacement of 400-500 L / min until the inlet and outlet densities are consistent, then the killing operation is stopped, and after observing that there is no abnormality at the wellhead, the next operation is carried out.

[0143] Field test results: 3 well times of field test, 100% efficiency, more than 30 cubic meters of liquid saved on average compared with adjacent wells, and more than 3 days of average recovery period improved.

[0144] The previous workover operations of an oil well showed that the well had serious leakage, and the conventional working fluid (density 1.0-1.03 g / cm 3 ) had an initial return of more than 60 cubic meters after reverse circulation killing operation. The formation pressure coefficient of the well is 0.7, the volume of the wellbore above the pump hanging is 18 cubic meters, and the planned use density of the ultra-low density leak-proof workover fluid is 0.8 g / cm 3 . The well is 30 cubic meters. During the reverse circulation process, the circulating pump pressure continuously increases, the outlet starts to return liquid after 20 cubic meters of circulation, and the inlet and outlet densities are consistent after 25 cubic meters of circulation, both are 0.8 g / cm 3 . Stop the pump and observe for 1 hour, there is no abnormality at the wellhead, then carry out the next operation, and the liquid injection and return are normal during the period. The single well reduces the leakage of working fluid by more than 30 cubic meters, thereby reducing the liquid discharge period.

[0145] Experimental Example 5:

[0146] Under normal temperature and pressure conditions, two portions of the base fluid are prepared according to the formulation of Example 62, each portion being 600 ml; at the same time, two portions of the foaming stock solution are prepared according to the formulation of Example 62. The foaming stock solution is in a state before foaming of the microbubble liquid, and the microbubble liquid is obtained after foaming of the foaming stock solution.

[0147] The first foaming group: one portion of the base fluid and one portion of the foaming stock solution are mixed according to a weight ratio of 10:1, and then air is introduced to foam the mixture to obtain the first foaming workover fluid;

[0148] The second foaming group: air is introduced to foam the other portion of the foaming stock solution to obtain microfoam; and then the microfoam is added to the other portion of the base fluid according to a weight ratio of 10:1 and mixed to obtain the second foaming workover fluid.

[0149] The foaming conditions of the first foaming group and the second foaming group are the same, and the foaming time is the same, which is 15 min. When foaming, a gas pump and a micro-void gas stone can be used to introduce gas into the foaming target, or a foaming machine can be used to foam the foaming target.

[0150] The appearance states of the first foaming workover fluid and the second foaming workover fluid are compared, and the results are shown in Figure 2 , wherein the left beaker contains the first foaming workover fluid, and the right beaker contains the second foaming workover fluid. It can be seen from Figure 2 that the bubble particle size of the first foaming workover fluid is larger, and the bubble particle size of the second foaming workover fluid is smaller.

[0151] The performance parameters of the first foaming workover fluid and the second foaming workover fluid are tested, and the results are shown in Table 8:

[0152]

[0153] It can be seen from Table 8 that the performance parameters of the first foaming workover fluid are relatively poor, and the performance parameters of the second foaming workover fluid are relatively good.

[0154] The appearance states of the first foaming workover fluid and the second foaming workover fluid are compared, and the results are shown in Table 9: Figure 3 , wherein the left beaker contains the first foaming workover fluid, and the right beaker contains the second foaming workover fluid. It can be seen from Figure 3 that the bubble particle size of the first foaming workover fluid is larger, and the bubble particle size of the second foaming workover fluid is smaller.

[0155] The performance parameters of the first foaming workover fluid and the second foaming workover fluid are tested, and the results are shown in Table 9:

[0156]

[0157] It can be seen from Table 9 that the performance parameters of the first foaming workover fluid after standing for 8 h are relatively poor, and the performance parameters of the second foaming workover fluid after standing for 8 h are relatively good.

[0158] In combination with Figure 2 , 3And Table 8, 9 carry out mechanism analysis: in the case that base fluid formula and micro-foam foaming stock solution formula are same, the first foaming workover fluid is due to high viscosity and shear of whole system, which leads to large bubble particle size after foaming, easy to slip and break, and relatively low performance parameters. The foaming stock solution in the second foaming workover fluid has relatively low viscosity and shear, so that the bubble particle size after foaming is small, not easy to slip and break, thereby the second foaming workover fluid can effectively reduce the size of closed pore micro-bubbles, can increase viscosity and shear, increase specific surface area, enhance the effect of increasing viscosity and shear, and prolong the density stability period of foam system, even after standing for 8h, it can still maintain good performance parameters, thereby having the effect of improving the suspension stability of workover fluid.

[0159] The mechanical foaming equipment often has better foaming effect, but the person skilled in the art also knows that: the foaming system with high viscosity and shear is easy to block the foaming net of the mechanical foaming equipment, so the mechanical foaming equipment is more suitable for the foaming system with low viscosity and shear. Therefore, it can be expected that: even if the mechanical foaming equipment is used to foam the foaming stock solution of the first foaming workover fluid and the second foaming workover fluid respectively, the performance parameters of the first foaming workover fluid are not as good as those of the second foaming workover fluid.

[0160] Compared with the prior art, the beneficial effects of the present application are:

[0161] Firstly, the surfactant added in the base fluid of the present application can effectively reduce the surface tension of the ultra-low density leak-proof workover fluid, promote flowback, prevent water lock, and thus reduce the damage of the working fluid to the reservoir;

[0162] Secondly, the ultra-low density leak-proof workover fluid of the present application reduces the density of itself, thereby reducing the static liquid column pressure and tending to the formation pressure, reducing the phenomenon of working fluid entering the reservoir under the action of positive pressure difference; due to its high viscosity and shear characteristics, it can increase the seepage resistance and the "air resistance" effect of micro-bubbles in the reservoir seepage channel, further reduce the leakage amount of the working fluid, and as far as possible reduce the problem of long drainage period caused by leakage.

[0163] Thirdly, the preparation process has low requirements on equipment, is more suitable for batch production operation, and is easy to implement.

[0164] The ultra-low density leak-proof workover fluid of the present application uses a structure stabilizer and a viscosity-increasing and filtration-reducing agent, both of which are polysaccharide biological polymers, have good viscosity-increasing and shear-increasing effects, have short natural degradation period, have small damage to the reservoir, can be applied to various oil and gas wells with formation pressure coefficient less than 1.0, well killing and workover operations, and can greatly reduce the use amount of conventional working fluid, thereby shortening the flowback period.

[0165] Finally, it should be noted that the above description is only for the preferred embodiments of the present application, and those of ordinary skill in the art can make various similar expressions under the inspiration of the present application without departing from the purpose and scope of the present application, and such changes fall within the protection scope of the present application.

Claims

1. An ultra-low density leak-proof well-maintaining fluid, characterized in that... It is composed of a base liquid and a microbubble liquid in a weight ratio of 10:1-5, and its density is less than 1.0 g / cm³. 3 ; The base liquid comprises, by weight, 0.4-0.6 parts of structural stabilizer, 1-2 parts of thickener and filtration loss reducer, 0.05-0.1 parts of pH adjuster, 0.05-0.1 parts of bactericide, 0.05-0.1 parts of corrosion inhibitor, 0.1-0.3 parts of surfactant, and 100 parts of water. The microbubble solution comprises 0.1-6.0 parts of foaming agent, 0.1-0.3 parts of foam stabilizer, and 100 parts of water; The microbubble liquid undergoes a foaming treatment; The bactericide is at least one of formaldehyde and acetaldehyde; The surfactant is at least one of fluorocarbon surfactants, alkyl glucosides and fatty acid glycerides; The foam stabilizer is dodecayl alcohol.

2. The ultra-low density leak-proof well-keeping fluid according to claim 1, characterized in that: The structural stabilizer is a biopolymer.

3. The ultra-low density leak-proof well-keeping fluid according to claim 2, characterized in that: The biopolymer is at least one of xanthan gum, hydroxypropyl guanidine gum, and sodium carboxymethyl cellulose.

4. The ultra-low density leak-proof well-keeping fluid according to claim 1, characterized in that: The thickening and filtration loss reducing agent is carboxymethyl starch.

5. The ultra-low density leak-proof well-keeping fluid according to claim 1, characterized in that: The pH adjuster is at least one of sodium hydroxide, potassium hydroxide, and sodium carbonate.

6. The ultra-low density leak-proof well-keeping fluid according to claim 1, characterized in that: The corrosion inhibitor is at least one of imidazoline corrosion inhibitors, nitrate corrosion inhibitors, and nitrite corrosion inhibitors.

7. The ultra-low density leak-proof well-keeping fluid according to claim 1, characterized in that: The foaming agent is at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, sodium fatty alcohol polyoxyethylene ether sulfate, synthetic fire-fighting foam liquid, fluoroprotein foam liquid, and aqueous film-forming foam liquid.

8. A method for preparing the ultra-low density leak-proof well-keeping fluid according to any one of claims 1-7, characterized in that... Follow these steps: Step 1: Add the required amounts of structural stabilizer, thickener and filtration reducer, pH adjuster, bactericide, corrosion inhibitor and surfactant to the required amount of water in sequence, and stir evenly to obtain the base solution; Step 2: Add the required amount of foaming agent and foam stabilizer to the required amount of water in sequence, mix evenly, and then foam to obtain microbubble liquid; Step 3: Add the microbubble liquid to the base fluid according to the weight ratio, and stir evenly to obtain ultra-low density leak-proof well repair fluid.

9. The method for preparing the ultra-low density leak-proof well-keeping fluid according to claim 8, characterized in that: In step two, the density of the microbubble solution is 15 g / L-50 g / L, the particle size is 30 μm-200 μm, and the half-life is 30 min-50 min.

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