Carbon dioxide working solution and preparation method therefor, and drainage gas recovery method for low-permeability gas reservoir

Abstract

The present invention relates to the technical field of gas field development. Disclosed are a carbon dioxide working solution and a preparation method therefor, and a drainage gas recovery method for a low-permeability gas reservoir. The carbon dioxide working solution comprises: liquid carbon dioxide, an entrainer, a foaming agent and a foam stabilizer, wherein the weight ratio of the liquid carbon dioxide : the entrainer : the foaming agent : the foam stabilizer is 100: (1-3) : (0.1-2) : (0.1-1.5); and the entrainer comprises an alcohol auxiliary agent and a solvent. The carbon dioxide working solution provided in the present invention has good water-carrying performance, foaming performance and foam-stabilizing performance, wherein the foaming efficiency is high, the foaming rate is greater than 75%, the foam is stable, and the half-life period of foam drainage is longer than 9 min. In a rock core throughput simulation experiment of the working solution provided in the present invention, a reduction degree of the water saturation degree of a rock core of 53% or above can be realized, the maximum reduction degree of the water saturation degree of the rock core is 80.1%, and the water saturation degree of a near-well area of a low-permeability gas reservoir can be significantly reduced, thereby effectively achieving water control and yield increase of a gas field old well.

Description

Carbon dioxide working fluid and preparation method thereof, and method for draining and recovering gas from low-permeability gas reservoirs

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Chinese patent application 202311274207.3, filed on September 28, 2023, entitled “Carbon dioxide working fluid, preparation method thereof, and method for drainage and gas production in low permeability gas reservoirs,” the contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the technical field of gas field development, and in particular to a carbon dioxide working fluid and a preparation method thereof, and a method for draining and recovering gas from a low-permeability gas reservoir. Background Art

[0004] Low-permeability gas reservoirs, as a future energy source with great potential for oil and gas exploration and development, play a vital role in the global energy structure. However, most gas reservoirs exhibit low production and associated water production during the development process. For example, in the Jingbian gas field, which has a long history of exploration and development, high water saturation in the near-wellbore area and severe liquid accumulation in the wellbore during the middle and late stages of development were caused by factors such as the inrush of side and bottom water and pressure decay, resulting in a decline in production or even shutdown. In addition, in recent years, measures to tap the potential of old wells in gas fields have mainly included layer inspection and perforation repair, repeated fracturing, bridge plug deployment, reservoir unblocking in the near-wellbore area, and well repair. While these measures have achieved certain results, they still suffer from pressure deficits in the old layers, large filtration losses during well pressure, low post-pressure flowback rates, and severe residual liquid retention in the formation, which can easily lead to water sensitivity and water lock damage.

[0005] For low-yield, low-efficiency, water-breaking gas reservoirs, water-displacement and gas recovery techniques are currently commonly used to control water and increase gas production. This technique involves injecting a certain proportion of foaming agent into the wellbore. The agitation of the natural gas flow reduces the density of the accumulated liquid and the surface tension of the formation water, transforming the bottomhole liquid into a foamy, easily lifted fluid, which is then carried to the surface by the gas. However, during reservoir development, residual liquid is severely retained in the formation, resulting in high water production in gas wells and increased water saturation near the wellbore. This foaming and gas recovery technique can only remove accumulated liquid from the wellbore but cannot be used to reduce water saturation near the wellbore.

[0006] Therefore, providing a material for reducing the water saturation in the near-well area for old wells in low-permeability gas reservoirs is of great significance for controlling water and increasing production in old wells in low-permeability gas fields, and facilitating further high-quality and high-efficiency development of low-permeability gas reservoirs.

[0007] Summary of the Invention

[0008] In order to overcome the problem of reduced gas production in low-permeability gas reservoirs during the middle and late stages of development due to high water saturation in the near-well area, the present invention provides a carbon dioxide working fluid and a preparation method thereof, as well as a method for water drainage and gas production in low-permeability gas reservoirs. These methods can significantly reduce the water saturation in the near-well area of ​​low-permeability gas reservoirs, thereby achieving water control and increased production in old wells in gas fields.

[0009] In order to achieve the above-mentioned object, the present invention provides a carbon dioxide working fluid in a first aspect, comprising: liquid carbon dioxide, an entrainer, a foaming agent and a foam stabilizer;

[0010] Wherein, the weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100: (1-3): (0.1-2): (0.1-1.5);

[0011] The entrainer includes an alcohol auxiliary agent and a solvent.

[0012] A second aspect of the present invention provides a method for preparing a carbon dioxide working fluid, comprising:

[0013] (1) performing a first mixing of an entrainer, a foaming agent, and a foam stabilizer to obtain a mixed solution;

[0014] (2) performing a second mixing of the mixed liquid and liquid carbon dioxide to obtain a carbon dioxide working liquid;

[0015] The weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100: (1-3): (0.1-2): (0.1-1.5); the entrainer includes an alcohol auxiliary agent and a solvent.

[0016] The third aspect of the present invention provides a carbon dioxide working fluid prepared by the method described in the second aspect.

[0017] A fourth aspect of the present invention provides a method for water drainage and gas production in a low-permeability gas reservoir, comprising: injecting the carbon dioxide working fluid described in the first or third aspect into a gas production well of the low-permeability gas reservoir, allowing the working fluid to enter a high-water-content gas-producing formation through the gas production well for foaming, and then returning the carbon dioxide foam generated by the foaming to the surface, where the carbon dioxide foam is used to carry away water in a near-wellbore area of ​​the high-water-content gas-producing formation;

[0018] Wherein, the water content of the high-water-content gas-producing formation is greater than 80% by volume;

[0019] The permeability of the high-water-content gas-producing formation is less than 1 md, preferably 0.1-0.5 md.

[0020] The carbon dioxide working fluid provided by the present invention uses liquid carbon dioxide as its main functional component. Under the combined action of the entrainer, foaming agent, and foam stabilizer contained in it, it can increase the solubility of carbon dioxide in water by changing the phase behavior of carbon dioxide, thereby increasing the water carrying capacity of carbon dioxide. The working fluid also has excellent foaming properties, generating foam at low shear rates, with high foaming efficiency, a foaming rate greater than 75%, stable foam, and a foam liquid separation half-life greater than 9 minutes, further enhancing the water carrying capacity of the working fluid. Core huff-and-puff simulation experiments using this working fluid have achieved a reduction in core water saturation of more than 53%, with a maximum reduction of 80.1%. It can significantly reduce the water saturation in the near-wellbore area of ​​low-permeability gas reservoirs, effectively achieving water control and production increase in old wells in gas fields. DETAILED DESCRIPTION

[0021] 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, and these numerical ranges should be considered to be specifically disclosed herein.

[0022] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0023] A first aspect of the present invention provides a carbon dioxide working fluid comprising: liquid carbon dioxide, an entrainer, a foaming agent, and a foam stabilizer;

[0024] Wherein, the weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100: (1-3): (0.1-2): (0.1-1.5);

[0025] The entrainer includes an alcohol auxiliary agent and a solvent.

[0026] According to the present invention, the carbon dioxide working fluid comprises liquid carbon dioxide, the primary functional component, which exhibits excellent miscibility with fluids, particularly water, and can dissolve large amounts of water. The entrainer increases carbon dioxide's water solubility by altering its phase behavior. The foaming agent and foam stabilizer facilitate the foaming of the working fluid, forming a large amount of stable foam with a foaming rate exceeding 75% and a foam separation half-life exceeding 9 minutes, further enhancing its water solubility while simultaneously reducing its overall density, facilitating water lifting and carrying. The synergistic effects of these components impart the working fluid with excellent water solubility and efficient, stable foaming properties, enabling efficient water carrying at temperatures between 80°C and 120°C and 10-15 MPa. Core huff-and-puff simulations using this working fluid have demonstrated a reduction in core water saturation of over 53%, with a maximum reduction of 80.1%. If the components and proportions of the carbon dioxide working fluid do not meet the specified conditions of the present invention, a working fluid with these effects will not be achieved.

[0027] According to the present invention, in the carbon dioxide working fluid, on the basis of satisfying the above-mentioned quantitative relationship, preferably, the weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100: (1.5-2): (1-1.5): (0.8-1), which can make the carbon dioxide working fluid have better water solubility and foaming and stabilizing properties.

[0028] According to the present invention, the liquid carbon dioxide refers to carbon dioxide in liquid form under low temperature and high pressure. The present invention has a broad definition of the liquid carbon dioxide, which can be carbon dioxide in liquid form obtained by any means, preferably liquid carbon dioxide with a purity of ≥99%.

[0029] Normally, due to the strong polarity of water molecules, the interaction between water molecules and carbon dioxide is weak. In the present invention, under the combined action of the alcohol auxiliary agent and the solvent, the entrainer can change the phase behavior of carbon dioxide at a certain temperature and pressure by forming interactions such as van der Waals forces and hydrogen bonds between the entrainer and water, thereby increasing the solubility of carbon dioxide in water.

[0030] According to the present invention, preferably, the content of the alcohol auxiliary agent in the entrainer is no more than 50 wt %. Further preferably, the weight ratio of the alcohol auxiliary agent to the solvent in the entrainer is 1:(1-1.5), which can improve the water solubility of the carbon dioxide working fluid.

[0031] According to the present invention, the alcohol auxiliary agent can be a monohydric alcohol or a polyhydric alcohol, for example, at least one selected from methanol, ethanol, propanol, ethylene glycol, triethylene glycol, propylene glycol, glycerol, and dipropylene glycol. Preferably, the alcohol auxiliary agent is a polyhydric alcohol.

[0032] According to a preferred embodiment of the present invention, the alcohol auxiliary agent is at least one selected from ethylene glycol, glycerol, and dipropylene glycol. Further preferably, the alcohol auxiliary agent is a mixture of ethylene glycol, glycerol, and dipropylene glycol in a weight ratio of 1: (0.8-1.8): (0.5-1.5).

[0033] According to the present invention, the solvent can be selected from alcoholamine compounds, carboxylic acid compounds and aldehyde compounds, preferably at least one of monoethanolamine, diethanolamine, triethanolamine, formic acid, acetic acid, propionic acid, formaldehyde and acetaldehyde.

[0034] According to a preferred embodiment of the present invention, the solvent is selected from triethanolamine, acetic acid and acetaldehyde. Further preferably, the solvent is a mixture of triethanolamine, acetic acid and acetaldehyde in a weight ratio of 1: (0.8-1.8): (1-2).

[0035] According to the present invention, the foaming agent can make the working fluid -1 ) to generate carbon dioxide foam, thereby better entering the reservoir to dissolve water, while reducing the density of the working fluid, which is conducive to lifting and carrying water. Preferably, the foaming agent is a non-ionic surfactant.

[0036] According to the present invention, the nonionic surfactant can be selected from at least one of 1-polyoxyethylene phenol-3-octyl, 1-polyoxyethylene phenol-3-nonyl, 1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-octyl, 1-polyoxypropylene phenol-3-nonyl, 1-polyoxypropylene phenol-3-dodecyl, 1-polyoxybutylene phenol-3-octyl, 1-polyoxybutylene phenol-3-nonyl and 1-polyoxybutylene phenol-3-dodecyl, preferably at least one of 1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-dodecyl and 1-polyoxybutylene phenol-3-dodecyl.

[0037] According to a preferred embodiment of the present invention, the foaming agent is further preferably a mixture of 1-polyoxyethylenephenol-3-dodecyl, 1-polyoxypropylenephenol-3-dodecyl and 1-polyoxybutylenephenol-3-dodecyl in a weight ratio of 1:(0.8-1.5):(0.8-1.5).

[0038] According to the present invention, the foam stabilizer can stabilize the liquid film. It has high interfacial desorption energy and, in synergy with a surfactant, can adsorb on the surface of the foam liquid film to form a stable adsorption layer. This layer slows down the thinning of the liquid film by hindering the flow of the liquid phase within the liquid film, thereby inhibiting the coalescence of bubbles and achieving the purpose of stabilizing the foam. Preferably, the foam stabilizer is selected from nano-metal oxide particles and / or cellulose nanofibers.

[0039] According to the present invention, preferably, the average particle size of the nano metal oxide particles is 10-40 nm, which is conducive to better adsorption of the foam liquid film, plays a barrier protection role for the foam, increases the surface elasticity of the foam, weakens the drainage effect of the liquid film, and reduces the bubble coalescence rate, thereby making the carbon dioxide foam more stable and having a longer life.

[0040] In the present invention, the average particle size refers to D50, which can be measured by a laser particle size analyzer.

[0041] According to the present invention, preferably, the diameter of the cellulose nanofibers is 3-8 nm and the length is 5-10 μm, which is conducive to better adsorption of the foam liquid film, plays a barrier protection role for the foam, increases the surface elasticity of the foam, weakens the drainage effect of the liquid film, and reduces the coalescence rate of bubbles, thereby making the carbon dioxide foam more stable and having a longer life.

[0042] In the present invention, the diameter and length of the cellulose nanofibers can be measured using a transmission electron microscope.

[0043] According to the present invention, preferably, the nano metal oxide particles can be selected from at least one of nano titanium dioxide particles, nano aluminum oxide particles, nano zirconium dioxide particles, nano silicon dioxide particles, nano zinc oxide particles, nano chromium oxide particles and nano manganese oxide particles.

[0044] According to the present invention, preferably, the cellulose nanofibers may be at least one selected from the group consisting of carboxymethylated cellulose nanofibers, ethyl cellulose nanofibers, hydroxyethyl cellulose nanofibers, hydroxypropyl methylcellulose nanofibers and cellulose acetate butyrate nanofibers.

[0045] According to a preferred embodiment of the present invention, the foam stabilizer is a mixture of nano-metal oxide particles and cellulose nanofibers, and more preferably a mixture of nano-titanium dioxide particles, nano-aluminum oxide particles and hydroxyethyl cellulose nanofibers in a weight ratio of 1: (0.5-1.5): (1-2).

[0046] In the present invention, the foaming rate is calculated by the following formula: Foaming rate % = (AB) / A × 100%

[0047] Wherein, A represents the foaming volume (mL), and B represents the initial volume of the foaming liquid (mL); wherein, the “initial volume of the foaming liquid” refers to the volume of the liquid before foaming; the “foaming volume” is determined according to the method specified in the Petroleum and Natural Gas Industry Standard SY / T 7494-2020 of the People’s Republic of China.

[0048] In the present invention, the foam liquid separation half-life is determined by the method specified in the Petroleum and Natural Gas Industry Standard SY / T 7494-2020 of the People's Republic of China.

[0049] In the present invention, a core huff-and-puff simulation experiment is conducted on the carbon dioxide working fluid to evaluate its effect of reducing the water saturation of the core (or formation). The process of the core huff-and-puff simulation experiment is as follows:

[0050] (1) Determination of core dimensions: Using a vernier caliper, the diameter and length of a cylindrical core plunger (about 25 mm in diameter) that has been cleaned (oil and salt removed) and dried are measured three times. After each measurement, the core is rotated 120° about the line connecting the centers of the two bottom surfaces of the plunger before the next measurement. The average of the three diameter and length measurements is taken as the final diameter and length measurement value of the core, and the total volume of the core is calculated, which is recorded as V1.

[0051] (2) Calibration of core analysis instrument: Set the helium test pressure to 1.38 MPa and calibrate the Boyle's law core analysis instrument using a stainless steel standard volume block;

[0052] (3) Core pore volume determination: The core obtained in step (1) was placed in the sample cell of a calibrated Boyle's law core analyzer, and the helium pressure was set to 1.38 MPa. The volume of the dry core particles was measured and recorded as V2. The pore volume of the core (recorded as V3) = V1 - V2;

[0053] (4) Core saturation: The core whose pore volume was measured in step (3) was saturated with formation water or standard brine under vacuum for 48-72 hours. The water adhering to the surface of the core was absorbed with filter paper and then tightly wrapped with plastic wrap for use.

[0054] (5) Determination of water saturation of saturated core: Remove the plastic wrap from the core treated in step (4) and measure the particle volume of the saturated core (denoted as V4) according to the method in step (3). The water saturation of the saturated core (denoted as μ1) = (V4-V2) / V3×100%;

[0055] (6) Core throughput simulation: The core whose water saturation has been determined in step (5) is placed in a high-pressure (no higher than 30 MPa) sealed container, and the core is soaked in carbon dioxide working fluid for 2-3 h (temperature is 80-120 ° C, pressure is 10-15 MPa), and then the pressure is slowly released at a rate of less than 0.5 MPa / min. After the pressure is released, the core is taken out, and the adhesion on the surface of the core is absorbed with filter paper, and then tightly wrapped with plastic wrap for use;

[0056] (7) Determination of water saturation of the core after huff and puff: Remove the plastic wrap from the core soaked in the carbon dioxide working solution in step (6), and measure the particle volume of the core after huff and puff (denoted as V5) according to the method in step (3). The water saturation of the core after huff and puff (denoted as μ2) = (V5-V2) / V3×100%;

[0057] (8) Calculation of the degree of reduction in core water saturation (denoted as μ2): μ3 = μ1 - μ2.

[0058] According to a most preferred embodiment of the present invention, in the carbon dioxide working fluid, the alcohol auxiliary agent in the entrainer is a mixture of ethylene glycol, glycerol and dipropylene glycol in a weight ratio of 1: (0.8-1): 1, the solvent is a mixture of triethanolamine, acetic acid and acetaldehyde in a weight ratio of 1: (1-1.5): (1.2-1.5), and the weight ratio of the alcohol auxiliary agent to the solvent is 1: (1.2-1.5); the foaming agent is -polyoxyethylene phenol-3-dodecyl, 1 The carbon dioxide working fluid of this most preferred embodiment has further improved water solubility and efficient and stable foaming performance, with a foaming rate greater than 82wt%, a foaming liquid half-life greater than 12 minutes, and the ability to efficiently carry water at conditions of 80-120°C and 10-15MPa. In a core huff-and-puff simulation experiment, it can achieve a core water saturation reduction of more than 70%, thereby demonstrating a more excellent effect in reducing water saturation in the near-wellbore region of low-permeability gas reservoirs.

[0059] The carbon dioxide working fluid provided by the present invention can be stored under any conditions that can make the carbon dioxide liquid, preferably stored under the conditions of -20 to -15°C and 1.8-2.5 MPa until used.

[0060] A second aspect of the present invention provides a method for preparing a carbon dioxide working fluid, comprising:

[0061] (1) performing a first mixing of an entrainer, a foaming agent, and a foam stabilizer to obtain a mixed solution;

[0062] (2) performing a second mixing of the mixed liquid and liquid carbon dioxide to obtain a carbon dioxide working liquid;

[0063] The weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:100:(1-3):(0.1-2):(0.1-1.5); the entrainer includes an alcohol auxiliary agent and a solvent.

[0064] The present invention provides a method for preparing a carbon dioxide working fluid. The method comprises the following steps: first preparing a mixed liquid by preparing an entrainer, a foaming agent, and a foam stabilizer; then mixing the obtained mixed liquid with liquid carbon dioxide to form a working fluid having liquid carbon dioxide as a main functional component; and controlling the feeding amounts of the raw materials to satisfy the aforementioned quantitative relationship. The prepared carbon dioxide working fluid has excellent water solubility and efficient and stable foaming performance, a foaming rate of greater than 75%, a foam liquid separation half-life of greater than 9 minutes, and the ability to efficiently carry water under conditions of 80-120° C. and 10-15 MPa. In a core huff-and-puff simulation experiment, the method can achieve a core water saturation reduction of more than 53%, with a maximum core water saturation reduction of 80.1%. The method can be used to effectively reduce the water saturation in the near-wellbore area of ​​a low-permeability gas reservoir, and has an excellent water control and production-increasing effect on old wells in gas fields.

[0065] According to the present invention, in the method for preparing the carbon dioxide working fluid, the feeding amount of each raw material, on the basis of satisfying the above-mentioned quantitative relationship, preferably, the feeding weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100: (1.5-2): (1-1.5): (0.8-1), which can make the prepared carbon dioxide working fluid have better water solubility and foaming and stabilizing properties.

[0066] According to the present invention, in the method for preparing the carbon dioxide working fluid, the liquid carbon dioxide refers to carbon dioxide in liquid form under low temperature and high pressure. The present invention has a broad definition of the liquid carbon dioxide, and can be carbon dioxide in liquid form obtained by any means, preferably liquid carbon dioxide with a purity of ≥99%.

[0067] According to the present invention, in the method for preparing a carbon dioxide working fluid, the entrainer includes an alcohol auxiliary agent and a solvent. Under the combined action of the alcohol auxiliary agent and the solvent, the entrainer can, at a certain temperature and pressure, alter the phase behavior of carbon dioxide through interactions such as van der Waals forces and hydrogen bonds formed between the entrainer and water, thereby increasing the solubility of carbon dioxide in water.

[0068] According to the present invention, in the method for preparing the carbon dioxide working fluid, preferably, the content of the alcohol auxiliary agent in the entrainer is no more than 50 wt %. Further preferably, the weight ratio of the alcohol auxiliary agent to the solvent in the entrainer is 1:(1-1.5), which can improve the water solubility of the prepared carbon dioxide working fluid.

[0069] According to the present invention, in the method for preparing the carbon dioxide working fluid, the alcohol auxiliary agent may be a monohydric alcohol or a polyhydric alcohol, for example, at least one selected from methanol, ethanol, propanol, ethylene glycol, triethylene glycol, propylene glycol, glycerol, and dipropylene glycol. Preferably, the alcohol auxiliary agent is a polyhydric alcohol.

[0070] According to a preferred embodiment of the present invention, the alcohol auxiliary agent is at least one selected from ethylene glycol, glycerol, and dipropylene glycol. Further preferably, the alcohol auxiliary agent is a mixture of ethylene glycol, glycerol, and dipropylene glycol in a weight ratio of 1: (0.8-1.8): (0.5-1.5).

[0071] According to the present invention, in the method for preparing the carbon dioxide working fluid, the solvent can be selected from alcoholamine compounds, carboxylic acid compounds and aldehyde compounds, preferably at least one of monoethanolamine, diethanolamine, triethanolamine, formic acid, acetic acid, propionic acid, formaldehyde and acetaldehyde.

[0072] According to a preferred embodiment of the present invention, the solvent is selected from at least one of triethanolamine, acetic acid and acetaldehyde. Further preferably, the solvent is a mixture of triethanolamine, acetic acid and acetaldehyde in a weight ratio of 1: (0.8-1.8): (1-2).

[0073] According to the present invention, in the method for preparing the carbon dioxide working fluid, the foaming agent can make the working fluid -1 ) to generate carbon dioxide foam, thereby better entering the reservoir to dissolve water, while reducing the density of the working fluid, which is conducive to lifting and carrying water. Preferably, the foaming agent is a non-ionic surfactant.

[0074] According to the present invention, the nonionic surfactant can be selected from at least one of 1-polyoxyethylene phenol-3-octyl, 1-polyoxyethylene phenol-3-nonyl, 1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-octyl, 1-polyoxypropylene phenol-3-nonyl, 1-polyoxypropylene phenol-3-dodecyl, 1-polyoxybutylene phenol-3-octyl, 1-polyoxybutylene phenol-3-nonyl and 1-polyoxybutylene phenol-3-dodecyl, and is further preferably at least one of 1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-dodecyl and 1-polyoxybutylene phenol-3-dodecyl.

[0075] According to a preferred embodiment of the present invention, the foaming agent is further preferably a mixture of 1-polyoxyethylenephenol-3-dodecyl, 1-polyoxypropylenephenol-3-dodecyl and 1-polyoxybutylenephenol-3-dodecyl in a weight ratio of 1:(0.8-1.5):(0.8-1.5).

[0076] According to the present invention, in the method for preparing the carbon dioxide working fluid, the foam stabilizer can stabilize the liquid film and play a role in stabilizing the foam. Preferably, the foam stabilizer is selected from nano-metal oxide particles and / or cellulose nanofibers.

[0077] According to the present invention, preferably, the average particle size of the nano metal oxide particles is 10-40 nm, which is conducive to better adsorption on the foam liquid film, plays a barrier protection role on the foam, increases the surface elasticity of the foam, weakens the drainage effect of the liquid film, and reduces the bubble aggregation rate, thereby making the carbon dioxide foam more stable and having a longer life.

[0078] According to the present invention, preferably, the diameter of the cellulose nanofibers is 3-8 nm and the length is 5-10 μm, which is conducive to better adsorption of the foam liquid film, plays a barrier protection role for the foam, increases the surface elasticity of the foam, weakens the drainage effect of the liquid film, and reduces the bubble aggregation speed, thereby making the carbon dioxide foam more stable and having a longer life.

[0079] According to the present invention, preferably, the nano metal oxide particles can be selected from at least one of nano titanium dioxide particles, nano aluminum oxide particles, nano zirconium dioxide particles, nano silicon dioxide particles, nano zinc oxide particles, nano chromium trioxide and nano manganese trioxide.

[0080] According to the present invention, preferably, the cellulose nanofibers may be at least one selected from the group consisting of carboxymethylated cellulose nanofibers, ethyl cellulose nanofibers, hydroxyethyl cellulose nanofibers, hydroxypropyl methylcellulose nanofibers and cellulose acetate butyrate nanofibers.

[0081] According to a preferred embodiment of the present invention, the foam stabilizer is a mixture of nano-metal oxide particles and cellulose nanofibers, and more preferably a mixture of nano-titanium dioxide particles, nano-aluminum oxide particles and hydroxyethyl cellulose nanofibers in a weight ratio of 1: (0.5-1.5): (1-2).

[0082] According to the present invention, in the method for preparing the carbon dioxide working fluid, the first mixing method is not particularly limited, as long as the entrainer, foaming agent, and foam stabilizer are thoroughly mixed to form a uniform mixed liquid. Preferably, the first mixing is performed at a stirring rate of 500-3000 rpm for a mixing time of 1-5 minutes.

[0083] According to a preferred embodiment of the present invention, for the first mixing step, the entrainer and foam stabilizer are preferably first mixed, and then the foaming agent is added and mixed to obtain a uniform mixed solution. This mixing method can achieve a more uniform dispersion of the foam stabilizer, thereby improving the water solubility and foaming and stabilizing properties of the resulting carbon dioxide working fluid.

[0084] According to the present invention, in the method for preparing the carbon dioxide working fluid, the second mixing method is not particularly limited, as long as the mixed liquid and liquid carbon dioxide are thoroughly mixed to produce a uniform carbon dioxide working fluid. During the second mixing process, the carbon dioxide is maintained in a liquid state by controlling the temperature and pressure. Preferably, the second mixing conditions include a temperature of -20 to -15°C and a pressure of 1.8 to 2.5 MPa.

[0085] Preferably, the second mixing is performed at a stirring rate of 500-3000 rpm, and the mixing time is 1-5 min.

[0086] The third aspect of the present invention provides a carbon dioxide working fluid prepared by the method described in the second aspect.

[0087] According to the present invention, the carbon dioxide working fluid prepared by the method described in the second aspect has the same composition and properties as the carbon dioxide working fluid described in the first aspect of the present invention, and will not be described in detail here.

[0088] A fourth aspect of the present invention provides a method for water drainage and gas production in a low-permeability gas reservoir, comprising: injecting the carbon dioxide working fluid described in the first or third aspect into a gas production well of the low-permeability gas reservoir, allowing the working fluid to enter a high-water-content gas-producing formation through the gas production well for foaming, and then returning the carbon dioxide foam generated by the foaming to the surface, where the carbon dioxide foam is used to carry away water in a near-wellbore area of ​​the high-water-content gas-producing formation;

[0089] Wherein, the water content of the high-water-content gas-producing formation is greater than 80% by volume;

[0090] The permeability of the high-water-content gas-producing formation is less than 1 md, preferably 0.1-0.5 md.

[0091] For the purposes of this disclosure, the near-wellbore region refers to the area within 5 meters of the wellbore where the pressure drops sharply on the pressure drop funnel curve from bottomhole pressure to formation pressure in a low-permeability gas reservoir. Because low-permeability gas reservoirs generally have low porosity, low permeability, and high water content, liquid accumulation is highly likely to form in the near-wellbore region, leading to reduced or even shutdown gas well production.

[0092] The present invention provides a method for draining and recovering gas from a low-permeability gas reservoir. The method utilizes the carbon dioxide working fluid of the present invention and employs a huff-and-puff process to bring formation water from the near-wellbore region of the low-permeability gas reservoir to the surface. An entrainer modifies the phase behavior of carbon dioxide to increase the solubility of the carbon dioxide working fluid in water, allowing the carbon dioxide working fluid to carry more formation water from the near-wellbore region during injection and return. A foaming agent generates carbon dioxide foam during low-velocity / low-shear-rate flowback in the reservoir, bringing the carbon dioxide working fluid into the reservoir and facilitating flowback. A foam stabilizer stabilizes the foam, making the carbon dioxide foam more stable and having a longer lifespan. This method can achieve a reduction in water saturation of more than 53% in the near-wellbore region of a low-permeability gas reservoir, with a maximum reduction of 80.1% achieved, effectively controlling water and increasing production in older wells in gas fields.

[0093] The present invention will be described in detail below through examples. In the following examples and comparative examples, unless otherwise specified, conventional methods are used; the reagents and materials described are all commercially available unless otherwise specified.

[0094] Nano-titanium dioxide particles: average particle size 30 nm, purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd.

[0095] Nano-alumina particles: average particle size 25 nm, purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd.

[0096] Nano-zirconium dioxide particles: average particle size 30 nm, purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd.

[0097] Carboxymethylated cellulose nanofibers: 6 nm in diameter and 8 μm in length, purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd.

[0098] Hydroxyethyl cellulose nanofibers with a diameter of 8 nm and a length of 10 μm were purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd.

[0099] Example 1

[0100] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1.2; wherein the alcohol auxiliary agent is ethylene glycol, glycerol and dipropylene glycol mixed in a weight ratio of 1:0.8:1; and the solvent is triethanolamine, acetic acid and acetaldehyde mixed in a weight ratio of 1:1:1.2) and a foam stabilizer (nano-titanium dioxide particles, nano-alumina particles and hydroxyethyl cellulose nanofibers mixed in a weight ratio of 1:0.8:1.5) are stirred at a stirring rate of 2000 rpm for 3 min, and then a foaming agent (1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-dodecyl and 1-polyoxybutylene phenol-3-dodecyl mixed in a weight ratio of 1:0.8:1) is added and stirred for 2 min to obtain a uniform mixed solution;

[0101] (2) Inject liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turn on the stirrer and adjust the stirring rate to 1000 rpm, and simultaneously inject the above mixed solution into the liquid carbon dioxide using a high-pressure pump (during the injection process, the temperature is controlled at -18°C and the pressure is 2.1 MPa). After the injection is completed, stir and mix for 5 minutes to obtain a carbon dioxide working solution (denoted as S1);

[0102] In S1, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:1.8:1.2:1.

[0103] Example 2

[0104] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1.5; wherein the alcohol auxiliary agent is ethylene glycol, glycerol and dipropylene glycol mixed in a weight ratio of 1:1:1; and the solvent is triethanolamine, acetic acid and acetaldehyde mixed in a weight ratio of 1:1.5:1.5) and a foam stabilizer (nano-titanium dioxide particles, nano-alumina particles and hydroxyethyl cellulose nanofibers mixed in a weight ratio of 1:1.5:1.5) are stirred at a stirring rate of 2000 rpm for 5 minutes, and then a foaming agent (1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-dodecyl and 1-polyoxybutylene phenol-3-dodecyl mixed in a weight ratio of 1:1:1) is added and stirred for 2 minutes to obtain a uniform mixed solution;

[0105] (2) Inject liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turn on the stirrer and adjust the stirring rate to 1000 rpm, and simultaneously inject the above mixed solution into the liquid carbon dioxide using a high-pressure pump (during the injection process, the temperature is controlled at -18°C and the pressure is 2.1 MPa). After the injection is completed, stir and mix for 5 minutes to obtain a carbon dioxide working solution (denoted as S2);

[0106] In S2, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:1.8:1.2:1.

[0107] Example 3

[0108] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1; wherein the alcohol auxiliary agent is a mixture of ethylene glycol, glycerol and dipropylene glycol in a weight ratio of 1:0.8:0.5; and the solvent is a mixture of triethanolamine, acetic acid and acetaldehyde in a weight ratio of 1:0.8:1) and a foam stabilizer (nano-titanium dioxide particles, nano-alumina particles and hydroxyethyl cellulose nanofibers are mixed in a weight ratio of 1:0.5:1) are stirred at a stirring rate of 2000 rpm for 2 min, and then a foaming agent (1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-dodecyl and 1-polyoxybutylene phenol-3-dodecyl are added and stirred for 2 min to obtain a uniform mixed solution;

[0109] (2) Inject liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turn on the stirrer and adjust the stirring rate to 1000 rpm. Simultaneously, use a high-pressure pump to inject the above mixed solution into the liquid carbon dioxide (during the injection process, control the temperature to -18°C and the pressure to 2.1 MPa). After the injection is completed, stir and mix for 5 minutes to obtain a carbon dioxide working solution (denoted as S3);

[0110] In S3, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:1.5:1:0.8.

[0111] Example 4

[0112] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1.5; wherein the alcohol auxiliary agent is ethylene glycol, glycerol and dipropylene glycol mixed in a weight ratio of 1:1.8:1.5; and the solvent is triethanolamine, acetic acid and acetaldehyde mixed in a weight ratio of 1:1.8:2) and a foam stabilizer (nano-titanium dioxide particles, nano-alumina particles and hydroxyethyl cellulose nanofibers mixed in a weight ratio of 1:1.5:2) are stirred at a stirring rate of 2000 rpm for 3 min, and then a foaming agent (1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-dodecyl and 1-polyoxybutylene phenol-3-dodecyl mixed in a weight ratio of 1:1.5:1.5) is added and stirred for 2 min to obtain a uniform mixed solution;

[0113] (2) Injecting liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turning on the agitator and adjusting the stirring rate to 1000 rpm, while using a high-pressure pump to inject the above mixed solution into the liquid carbon dioxide (during the injection process, the temperature was controlled at -18°C and the pressure was 2.1 MPa). After the injection is completed, stirring and mixing is carried out for 5 minutes to obtain a carbon dioxide working solution (denoted as S4);

[0114] In S4, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:2:1.5:1.

[0115] Example 5

[0116] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1; wherein the alcohol auxiliary agent is ethanol and the solvent is diethanolamine) and a foam stabilizer (nano-zirconium dioxide particles) are stirred and mixed at a stirring rate of 2000 rpm for 2 minutes, and then a foaming agent (1-polyoxyethylene phenol-3-octyl) is added and stirred and mixed for another 2 minutes to obtain a uniform mixed solution;

[0117] (2) Injecting liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turning on the stirrer and adjusting the stirring rate to 1000 rpm, while simultaneously injecting the above mixed solution into the liquid carbon dioxide using a high-pressure pump (during the injection process, the temperature was controlled at -18°C and the pressure was 2.1 MPa). After the injection is completed, stirring and mixing is performed for 5 minutes to obtain a carbon dioxide working solution (denoted as S5);

[0118] In S5, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:1:0.1:0.1.

[0119] Example 6

[0120] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1.5; wherein the alcohol auxiliary agent is a mixture of methanol, propylene glycol and dipropylene glycol in a weight ratio of 1:1.5:1.5; and the solvent is a mixture of monoethanolamine, formic acid and formaldehyde in a weight ratio of 1:1.5:1.8) and a foam stabilizer (a mixture of nano zinc oxide particles, nano zirconium dioxide particles and hydroxypropyl methylcellulose nanofibers in a weight ratio of 1:1.3:1.8) are stirred at a stirring rate of 2000 rpm for 3 min, and then a foaming agent (a mixture of 1-polyoxyethylene phenol-3-octyl, 1-polyoxypropylene phenol-3-nonyl and 1-polyoxybutylene phenol-3-dodecyl in a weight ratio of 1:1.5:1.8) is added and stirred for 2 min to obtain a uniform mixed solution;

[0121] (2) Injecting liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turning on the agitator and adjusting the stirring rate to 1000 rpm, while using a high-pressure pump to inject the above mixed solution into the liquid carbon dioxide (during the injection process, the temperature was controlled at -18°C and the pressure was 2.1 MPa). After the injection is completed, stirring and mixing is carried out for 5 minutes to obtain a carbon dioxide working solution (denoted as S6);

[0122] In S6, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:3:2:1.5.

[0123] Example 7

[0124] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1.5; wherein the alcohol auxiliary agent is ethanol, ethylene glycol and propylene glycol mixed in a weight ratio of 1:1.5:1.5; and the solvent is diethanolamine, propionic acid and acetaldehyde mixed in a weight ratio of 1:1.5:1.8) and a foam stabilizer (nano-zinc oxide particles, nano-zirconium dioxide particles and hydroxypropyl methylcellulose nanofibers mixed in a weight ratio of 1:1.3:1.8) are stirred at a stirring rate of 2000 rpm for 3 min, and then a foaming agent (1-polyoxyethylene phenol-3-octyl, 1-polyoxypropylene phenol-3-nonyl and 1-polyoxybutylene phenol-3-dodecyl mixed in a weight ratio of 1:1.5:1.8) is added and stirred for 2 min to obtain a uniform mixed solution;

[0125] (2) Injecting liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turning on the stirrer and adjusting the stirring rate to 1000 rpm, while simultaneously injecting the above mixed solution into the liquid carbon dioxide using a high-pressure pump (during the injection process, the temperature was controlled at -18°C and the pressure was 2.1 MPa). After the injection is completed, stirring and mixing is performed for 5 minutes to obtain a carbon dioxide working solution (denoted as S7);

[0126] In S7, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:3:2:1.5.

[0127] Example 8

[0128] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1.5; wherein the alcohol auxiliary agent is a mixture of methanol, propylene glycol and dipropylene glycol in a weight ratio of 1:1.5:1.5; and the solvent is a mixture of monoethanolamine, formic acid and formaldehyde in a weight ratio of 1:1.5:1.8) and a foam stabilizer (a mixture of nano zinc oxide particles, nano zirconium dioxide particles and hydroxypropyl methylcellulose nanofibers in a weight ratio of 1:1.3:1.8) are stirred at a stirring rate of 2000 rpm for 3 min, and then a foaming agent (a mixture of 1-polyoxyethylene phenol-3-octyl, 1-polyoxypropylene phenol-3-nonyl and 1-polyoxybutylene phenol-3-dodecyl in a weight ratio of 1:1.5:1.8) is added and stirred for 2 min to obtain a uniform mixed solution;

[0129] (2) Injecting liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turning on the stirrer and adjusting the stirring rate to 1000 rpm, while using a high-pressure pump to inject the above mixed solution into the liquid carbon dioxide (during the injection process, the temperature was controlled at -18°C and the pressure was 2.1 MPa). After the injection is completed, stirring and mixing is carried out for 5 minutes to obtain a carbon dioxide working solution (denoted as S8);

[0130] In S8, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:1:2:1.5.

[0131] Example 9

[0132] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:0.8; wherein the alcohol auxiliary agent is a mixture of methanol, propylene glycol and dipropylene glycol in a weight ratio of 1:1.5:1.5; and the solvent is a mixture of monoethanolamine, formic acid and formaldehyde in a weight ratio of 1:1.5:1.8) and a foam stabilizer (a mixture of nano zinc oxide particles, nano zirconium dioxide particles and hydroxypropyl methylcellulose nanofibers in a weight ratio of 1:1.3:1.8) are stirred at a stirring rate of 2000 rpm for 3 min, and then a foaming agent (a mixture of 1-polyoxyethylene phenol-3-octyl, 1-polyoxypropylene phenol-3-nonyl and 1-polyoxybutylene phenol-3-dodecyl in a weight ratio of 1:1.5:1.8) is added and stirred for 2 min to obtain a uniform mixed solution;

[0133] (2) Injecting liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turning on the stirrer and adjusting the stirring rate to 1000 rpm, while using a high-pressure pump to inject the above mixed solution into the liquid carbon dioxide (during the injection process, the temperature was controlled at -18°C and the pressure was 2.1 MPa). After the injection is completed, stirring and mixing is carried out for 5 minutes to obtain a carbon dioxide working solution (denoted as S9);

[0134] In S9, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:3:2:1.5.

[0135] Example 10

[0136] (1) An entrainer (an alcohol auxiliary agent and a solvent are mixed in a weight ratio of 1:1.5; wherein the alcohol auxiliary agent is a mixture of methanol, propylene glycol and dipropylene glycol in a weight ratio of 1:1.5:1.5; and the solvent is a mixture of monoethanolamine, formic acid and formaldehyde in a weight ratio of 1:1.5:1.8), a foam stabilizer (a mixture of nano zinc oxide particles, nano zirconium dioxide particles and hydroxypropyl methylcellulose nanofibers in a weight ratio of 1:1.3:1.8) and a foaming agent (a mixture of 1-polyoxyethylene phenol-3-octyl, 1-polyoxypropylene phenol-3-nonyl and 1-polyoxybutylene phenol-3-dodecyl in a weight ratio of 1:1.5:1.8) are stirred together at a stirring rate of 2000 rpm for 5 min to obtain a uniform mixed solution;

[0137] (2) Injecting liquid carbon dioxide (99% purity, -18°C, 2.1 MPa) into the high-pressure visualization container, turning on the stirrer and adjusting the stirring rate to 1000 rpm, while using a high-pressure pump to inject the above mixed solution into the liquid carbon dioxide (during the injection process, the temperature was controlled at -18°C and the pressure was 2.1 MPa). After the injection is completed, stirring and mixing is carried out for 5 minutes to obtain a carbon dioxide working solution (recorded as S10);

[0138] In S10, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:3:2:1.5.

[0139] Comparative Example 1

[0140] The method of Example 6 was followed, except that no entrainer was added in step (1). The foam stabilizer and foaming agent were mixed at a stirring rate of 2000 rpm for 5 minutes to obtain a uniform mixed solution. The other steps, conditions, and parameters were the same as those of Example 6 to obtain a carbon dioxide working solution (denoted as D1).

[0141] In D1, the weight ratio of liquid carbon dioxide: foaming agent: foam stabilizer is 100:2:1.5.

[0142] Comparative Example 2

[0143] The method of Example 6 was followed, except that the proportions of liquid carbon dioxide, entrainer, foaming agent, and foam stabilizer were changed. Other steps, conditions, and parameters were the same as those of Example 6, to obtain a carbon dioxide working solution (denoted as D2).

[0144] In D2, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:3.5:2.5:2.

[0145] Comparative Example 3

[0146] The method of Example 6 was followed, except that the entrainer was obtained by mixing methanol, propylene glycol, and dipropylene glycol in a weight ratio of 1:1.5:1.5. The other steps, conditions, and parameters were the same as those of Example 6 to obtain a carbon dioxide working fluid (denoted as D3);

[0147] In D3, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:3.5:2.5:2.

[0148] Comparative Example 4

[0149] The method of Example 6 was followed, except that the entrainer was prepared by mixing monoethanolamine, formic acid, and formaldehyde in a ratio of 1:1.5:1.8 by weight. The other steps, conditions, and parameters were the same as those of Example 6 to obtain a carbon dioxide working fluid (denoted as D4).

[0150] In D4, the weight ratio of liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100:3.5:2.5:2.

[0151] Test Case

[0152] 1. Foaming rate and foam liquid half-life test

[0153] A high-pressure visual container containing carbon dioxide working solutions S1-S10 and D1-D4 prepared in Examples 1-10 and Comparative Examples 1-4 was heated to 90° C., and the pressure in the container was controlled to 15 MPa. The working solutions were stirred at a stirring rate of 500 rpm for 5 minutes to foam, and the foaming rate and foam separation half-life of each working solution were measured.

[0154] The results are shown in Table 1.

[0155] Table 1

[0156] As can be seen from Table 1, the carbon dioxide working fluid provided by the present invention has excellent foaming and foam stabilizing properties, with a foaming rate greater than 75% and a foam liquid half-life greater than 9 minutes. Among them, the foaming rate and half-life of working fluids S1-S4 are better than those of working fluids S5 and S6. Working fluids S1 and S2 have higher foaming rates and longer foam liquid half-lives, indicating that the composition of the carbon dioxide working fluid under the preferred conditions can bring about better foaming and foam stabilizing effects; the foaming rate and half-life of working fluids S7 and S8 are similar to those of working fluid S6, indicating that within the range of entrainer type and dosage defined by the present invention, only changing the material type or dosage of the entrainer has limited effect on the foam performance; compared with working fluid S6, working fluid S9 has a higher proportion of alcohol additives in S9, which makes the foaming rate and half-life of working fluid S9 inferior to those of S6; working fluid S10 is improved compared to working fluid S6. By changing the order of adding materials and mixing the entrainer, foam stabilizer and foaming agent together, the foam expansion rate and half-life of S10 were both lower than those of S6. This may be because the dispersion effect of the foam stabilizer in the working liquid S10 was not as good as that produced by the working liquid S6; no entrainer was added to the working liquid D1, but since the foaming agent and foam stabilizer were the same as those of the working liquid S6, the foaming performance of the working liquid D1 was still comparable to that of S6; the amount of foaming agent and foam stabilizer in the working liquid D2 was higher than that of S6, which made the foaming rate and half-life of the working liquid D2 longer; working liquids D3 and D4 used different entrainers compared to S6, but since the foaming agent and foam stabilizer were the same as those of the working liquid S6, the foaming performance of D3 and D4 was still comparable to that of S6.

[0157] 2. Core huff-and-puff simulation experiment

[0158] Core huff-and-puff simulation experiments were conducted on the carbon dioxide working fluids S1-S10 and D1-D4 prepared in Examples 1-10 and Comparative Examples 1-4 to evaluate their effectiveness in reducing core water saturation, thereby simulating and evaluating the effectiveness of the working fluids in reducing water saturation in the near-wellbore area of ​​a low-permeability gas reservoir.

[0159] The core huff-and-puff simulation experiment process is as follows:

[0160] (1) Determination of core dimensions: Using a vernier caliper, the diameter and length of a cylindrical core plunger (about 25 mm in diameter) that has been cleaned (oil and salt removed) and dried are measured three times. After each measurement, the core is rotated 120° about the line connecting the centers of the two bottom surfaces of the plunger before the next measurement. The average of the three diameter and length measurements is taken as the final diameter and length measurement value of the core, and the total volume of the core is calculated, which is recorded as V1.

[0161] (2) Calibration of core analysis instrument: Set the helium test pressure to 1.38 MPa and calibrate the Boyle's law core analysis instrument using a stainless steel standard volume block;

[0162] (3) Core pore volume determination: The core obtained in step (1) was placed in the sample cell of a calibrated Boyle's law core analyzer, and the helium pressure was set to 1.38 MPa. The volume of the dry core particles was measured and recorded as V2. The pore volume of the core (recorded as V3) = V1 - V2;

[0163] (4) Core saturation: The core whose pore volume was measured in step (3) was saturated with formation water (salt content and salinity in formation water are shown in Table 2) under vacuum for 48 h. The adhering water on the surface of the core was absorbed with filter paper and then tightly wrapped with plastic wrap for use.

[0164] (5) Determination of water saturation of saturated core: Remove the plastic wrap from the core treated in step (4) and measure the particle volume of the saturated core (denoted as V4) according to the method in step (3). The water saturation of the saturated core (denoted as μ1) = (V4-V2) / V3×100%;

[0165] (6) Core throughput simulation: The core whose water saturation has been determined in step (5) is placed in a high-pressure (10 MPa) sealed container, and the core is soaked in carbon dioxide working fluid for 2 h (temperature is 80-120 ° C, pressure is 10-15 MPa), and then the pressure is slowly released at a rate of less than 0.5 MPa / min. After the pressure is released, the core is taken out, and the adhesion on the surface of the core is absorbed with filter paper, and then tightly wrapped with plastic wrap for use;

[0166] (7) Determination of water saturation of the core after huff and puff: Remove the plastic wrap from the core soaked in the carbon dioxide working solution in step (6), and measure the particle volume of the core after huff and puff (denoted as V5) according to the method in step (3). The water saturation of the core after huff and puff (denoted as μ2) = (V5-V2) / V3×100%;

[0167] (8) Calculation of the degree of reduction in core water saturation (denoted as μ2): μ3 = μ1 - μ2.

[0168] The evaluation results are shown in Table 3.

[0169] Table 2

[0170] Table 3

[0171] Note: The permeabilities of SU15-5, SM58-61, M46-37 and L102-25 cores are 0.23 md, 0.41 md, 0.36 md and 0.17 md, respectively.

[0172] As shown in Table 3, the working fluids S1-S10 provided by the present invention can achieve a core water saturation reduction of more than 53% in the core huff-and-puff simulation experiment, with the highest core water saturation reduction achieved being 80.1%. This indicates that the working fluids provided by the present invention can significantly reduce the water saturation in the near-wellbore region of low-permeability gas reservoirs, thereby achieving water control and increased production in old gas field wells. Among them, working fluids S1-S4 are more effective in reducing core water saturation than working fluids S5 and S6, indicating that the carbon dioxide working fluid composition under optimal conditions can achieve a better effect in reducing core water saturation. Compared with S6, working fluid S9 has a higher proportion of alcohol additives in S9, resulting in a lower foaming rate and half-life than S6, which in turn reduces the effect of S9 in reducing core water saturation compared to S6. In particular, although the foaming and foam stabilizing properties of the working fluid D1 are comparable to those of S6, D1 does not contain an entrainer, which significantly reduces the water solubility of D1, resulting in D1 being significantly worse than S1-S10 in reducing the water saturation of the core; the ratio of the entrainer, foaming agent, and foam stabilizer in the working fluid D2 does not meet the working fluid composition of the present invention, and its effect of reducing the water saturation of the core is also significantly worse than S1-S10; the working fluids D3 and D4 do not use the composite entrainer of the present invention, and their effect of reducing the water saturation of the core is also significantly worse than S1-S10.

[0173] 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 carbon dioxide working fluid, characterized in that: include: Liquid carbon dioxide, entrainers, foaming agents and foam stabilizers; Wherein, the weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100: (1-3): (0.1-2): (0.1-1.5); The entrainer includes an alcohol auxiliary agent and a solvent.

2. The carbon dioxide working fluid according to claim 1, wherein: The weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100: (1.5-2): (1-1.5): (0.8-1).

3. The carbon dioxide working fluid according to claim 1 or 2, wherein: The weight ratio of the alcohol auxiliary agent to the solvent is 1:(1-1.5).

4. The carbon dioxide working fluid according to claim 3, wherein: The alcohol auxiliary agent is selected from at least one of methanol, ethanol, propanol, ethylene glycol, triethylene glycol, propylene glycol, glycerol, and dipropylene glycol, preferably at least one of ethylene glycol, glycerol, and dipropylene glycol; And / or, the solvent is selected from at least one of alcoholamine compounds, carboxylic acid compounds and aldehyde compounds, preferably at least one of monoethanolamine, diethanolamine, triethanolamine, formic acid, acetic acid, propionic acid, formaldehyde and acetaldehyde, and more preferably triethanolamine, acetic acid and acetaldehyde.

5. The carbon dioxide working fluid according to any one of claims 1 to 4, wherein: The foaming agent is a nonionic surfactant.

6. The carbon dioxide working fluid according to claim 5, wherein: The nonionic surfactant is selected from at least one of 1-polyoxyethylene phenol-3-octyl, 1-polyoxyethylene phenol-3-nonyl, 1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-octyl, 1-polyoxypropylene phenol-3-nonyl, 1-polyoxypropylene phenol-3-dodecyl, 1-polyoxybutylene phenol-3-octyl, 1-polyoxybutylene phenol-3-nonyl and 1-polyoxybutylene phenol-3-dodecyl, preferably at least one of 1-polyoxyethylene phenol-3-dodecyl, 1-polyoxypropylene phenol-3-dodecyl and 1-polyoxybutylene phenol-3-dodecyl.

7. The carbon dioxide working fluid according to any one of claims 1 to 4, wherein: The foam stabilizer is selected from nano metal oxide particles and / or cellulose nanofibers.

8. The carbon dioxide working fluid according to claim 7, wherein: The average particle size of the nano metal oxide particles is 10-40 nm; And / or, the cellulose nanofibers have a diameter of 3-8 nm and a length of 5-10 μm.

9. The carbon dioxide working fluid according to claim 8, wherein: The nano-metal oxide particles are selected from at least one of nano-titanium dioxide particles, nano-aluminum oxide particles, nano-zirconium dioxide particles, nano-silicon dioxide particles, nano-zinc oxide particles, nano-chromium trioxide particles and nano-manganese trioxide particles; And / or, the cellulose nanofibers are selected from at least one of carboxymethylated cellulose nanofibers, ethyl cellulose nanofibers, hydroxyethyl cellulose nanofibers, hydroxypropyl methyl cellulose nanofibers and cellulose acetate butyrate nanofibers.

10. A method for preparing a carbon dioxide working fluid, comprising: (1) performing a first mixing of an entrainer, a foaming agent, and a foam stabilizer to obtain a mixed solution; (2) performing a second mixing of the mixed liquid and liquid carbon dioxide to obtain a carbon dioxide working liquid; Wherein, the weight ratio of the liquid carbon dioxide: entrainer: foaming agent: foam stabilizer is 100: (1-3): (0.1-2): (0.1-1.5); The entrainer includes an alcohol auxiliary agent and a solvent.

11. The preparation method according to claim 10, wherein: The first mixing includes: firstly mixing the entrainer and the foam stabilizer, and then adding the foaming agent to mix.

12. The preparation method according to claim 10 or 11, wherein: The second mixing conditions include: a temperature of -20 to -15°C and a pressure of 1.8 to 2.5 MPa.

13. The preparation method according to claim 10 or 11, wherein: The weight ratio of the alcohol auxiliary agent to the solvent is 1:(1-1.5); And / or, the foaming agent is a nonionic surfactant; And / or, the foam stabilizer includes nano metal oxide particles and / or cellulose nanofibers.

14. A carbon dioxide working fluid obtained by the method according to any one of claims 10 to 13.

15. A method for recovering gas from a low-permeability gas reservoir by draining water, comprising: Injecting the carbon dioxide working fluid described in any one of claims 1 to 9 and 14 into a gas production well of a low-permeability gas reservoir, and then foaming the carbon dioxide foam generated by the foaming into the ground, and using the carbon dioxide foam to carry out water in the near-well area of ​​the high-water-content gas-producing formation; Wherein, the water content of the high-water-content gas-producing formation is greater than 80% by volume; The permeability of the high water-content gas-producing formation is less than 1 md, preferably 0.1-0.5 md.

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