Carbon dioxide tackifying fracturing fluid as well as preparation method and application thereof

Abstract

The invention discloses a carbon dioxide tackified fracturing fluid which is prepared from the following raw material components in percentage by mass: 2-5% of tackifier and 98-95% of liquid carbon dioxide, and the invention also discloses a preparation method of the carbon dioxide tackified fracturing fluid, which comprises the following steps: selecting 2-5% of tackifier and 98-95% of liquid carbon dioxide; 98-95% of liquid carbon dioxide is stirred at a fixed temperature and pressure to prepare the carbon dioxide tackified fracturing fluid; the invention further discloses application of the carbon dioxide tackifying fracturing fluid in oil and gas field reservoir fracturing with high construction pressure and limited displacement during oil and gas production fracturing, and the problems of large carbon dioxide foam resistance, high construction pressure and limited displacement in the prior art are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of hydraulic fracturing in oil and gas field development, and specifically relates to a carbon dioxide thickening fracturing fluid. The present invention also relates to a preparation method of the above carbon dioxide thickening fracturing fluid, and the present invention also relates to the application of the above carbon dioxide thickening fracturing fluid. Background Art

[0002] The Changqing reservoir has obvious "three lows" characteristics, and the block development generally has problems such as great difficulty in fracturing construction and difficult flowback after fracturing. Compared with water-based fracturing media, carbon dioxide has certain advantages in reducing rock fracture pressure, reducing reservoir damage, and improving flowback after fracturing. The on-site carbon dioxide fracturing construction monitoring results show that the carbon dioxide in the wellbore is in a supercritical state in the early stage and in a liquid state in the middle and late stages; injecting liquid carbon dioxide at a displacement of 2.0 - 4.0 m 3 / min for 140 m 3 After that, the bottom-hole temperature drops from 98 °C to 31 °C, and the phase state of carbon dioxide at the bottom hole changes from a supercritical state to a liquid state; the carbon dioxide in the formation is generally in a supercritical state. Supercritical carbon dioxide not only has the low viscosity and easy diffusibility close to that of a gas, but also has the high density and solubility close to that of a liquid. During the displacement process, it can effectively displace the tight gas in the reservoir through competitive adsorption, and at the same time realize the storage of carbon dioxide. Research has found that by using the special physical and chemical properties of supercritical carbon dioxide and its great advantages in reservoir protection, using supercritical carbon dioxide as a drilling fluid and a fracturing fluid can avoid reservoir damage and solve many problems encountered in the development of tight gas, and is expected to form an integrated development method for drilling, well completion, and production increase of an efficient tight gas reservoir.

[0003] In 2000, the Changqing Oilfield began to conduct research on carbon dioxide fracturing technology. The carbon dioxide foam fracturing has a large frictional resistance, a high construction pressure, and a limited displacement. The on-site actual production data shows that the average frictional resistance coefficient of carbon dioxide foam fracturing is 58.1%, while that of the conventional water-based fracturing fluid is 30%. This shows that the frictional resistance coefficient of the carbon dioxide fracturing fluid is higher than that of the conventional water-based fracturing fluid, and under the condition of constant pressure, the displacement improvement space is small. As a result, the construction difficulty of some wells is great, the single-well economic cost is high, and the economic benefits are affected. Currently, there are problems of large frictional resistance, high construction pressure, and limited displacement of carbon dioxide foam in the existing technology. Summary of the Invention

[0004] The purpose of the present invention is to provide a carbon dioxide thickening fracturing fluid, which solves the problems of large resistance, high construction pressure, and limited displacement of carbon dioxide foam in the existing technology.

[0005] Another purpose of the present invention is to provide a preparation method of the above carbon dioxide thickening fracturing fluid.

[0006] The third object of the present invention is to provide the application of the above carbon dioxide thickening fracturing fluid.

[0007] The technical solution adopted by the present invention is that it is composed of the following raw materials according to mass percentage: 2% - 5% of thickening agent, 98% - 95% of liquid carbon dioxide, and the sum of the mass percentages of the above raw materials is 100%.

[0008] The characteristics of the present invention also lie in that the thickening agent is composed of the following raw materials according to mass percentage: 45% - 50% of acrylamide, 20% - 25% of acrylic acid, 10% - 15% of cationic monomer, 5% - 10% of sulfonic acid group monomer, and 5% - 10% of hydrophilic long-chain monomer, and the sum of the mass percentages of the above raw materials is 100%.

[0009] The pH value of the carbon dioxide thickening fracturing fluid is 5 - 7.

[0010] Another technical solution adopted by the present invention is the preparation method of the carbon dioxide thickening fracturing fluid, which is specifically implemented according to the following steps:

[0011] Step 1: Weigh the following components according to mass percentage: 2% - 5.0% of thickening agent and 95% - 98% of liquid carbon dioxide, and the sum of the mass percentages of the above raw materials is 100%;

[0012] Step 2: Stir the liquid carbon dioxide and the thickening agent, and the stirring time is 19 - 60 mim until the pH value is 5 - 7.

[0013] Step 2 is carried out under the conditions of a temperature of -30 °C and a pressure of not less than 3.5 MPa.

[0014] The stirring rate in Step 2 is 2 m / s - 5 m / s.

[0015] The third technical solution adopted by the present invention is the application of the carbon dioxide thickening fracturing fluid in the fracturing of oil and gas reservoir with high construction pressure and limited displacement in oil production by gas fracturing.

[0016] The beneficial effects of the present invention are as follows: The carbon dioxide thickening fracturing fluid microscopically dredges the pore throats of the reservoir and improves the permeability of the reservoir; macroscopically, it changes the mechanical properties of the formation rock, thereby increasing the complexity and roughness of the artificial fracture surface; the injection of supercritical carbon dioxide increases the scope of fluid influence; the strong inhibition of water-sensitive minerals can eliminate water-sensitive damage, and the extremely low surface tension and anhydrous system avoid reservoir water lock damage, solving the problems of large resistance of carbon dioxide foam, high construction pressure and limited displacement in the prior art. Description of the Drawings

[0017] Figure 1 It is the rheological property test diagram of the carbon dioxide thickening fracturing fluid in Examples 1 - 4 of the present invention. Detailed implementation manners

[0018] The present invention will be described in detail below in conjunction with the specific implementation manners.

[0019] The carbon dioxide thickening fracturing fluid is composed of the following raw materials by mass percentage: 2% - 5% of thickening agent, 98% - 95% of liquid carbon dioxide, and the sum of the mass percentages of the above raw materials is 100%.

[0020] The thickening agent is composed of the following raw materials by mass percentage: 45% - 50% of acrylamide, 20% - 25% of acrylic acid, 10% - 15% of cationic monomer, 5% - 10% of sulfonic acid group monomer, and 5% - 10% of hydrophilic long-chain monomer, and the sum of the mass percentages of the above raw materials is 100%.

[0021] The carbon dioxide thickening fracturing fluid is a colorless transparent liquid, and the pH value of the carbon dioxide thickening fracturing fluid is 5 - 7.

[0022] The present invention also provides a preparation method of the carbon dioxide thickening fracturing fluid, which specifically includes the following steps:

[0023] Step 1: Weigh the following components by mass percentage: select 2% - 5.0% of thickening agent and 95% - 98% of liquid carbon dioxide, and the sum of the mass percentages of the above raw materials is 100%.

[0024] Step 2: Stir the liquid carbon dioxide and the thickening agent at a temperature of -30°C and a pressure of not less than 3.5 MPa, the stirring rate is 2 m / s - 5 m / s, and the stirring time is 19 - 60 min until the pH is 5 - 7.

[0025] The application of the carbon dioxide thickening fracturing fluid in the fracturing of oil and gas reservoir with high construction pressure and limited displacement in oil production by gas fracturing.

[0026] Carbon dioxide is generally in a supercritical state in the formation. When the temperature and pressure are 31.1 °C and 7.38 MPa respectively, carbon dioxide reaches the supercritical state. The properties of supercritical carbon dioxide are between those of gases and liquids, with characteristics such as low viscosity, strong diffusivity, good solubility, and extremely strong flow, penetration, and transmission performance. The threshold pressure for supercritical carbon dioxide jet rock breaking is much smaller than that of water jets, and it can cut and break hard rocks. Therefore, supercritical carbon dioxide jet drilling can achieve a relatively high rate of penetration. Supercritical carbon dioxide has a smaller interfacial tension, better diffusivity, and solubility compared to liquid or gaseous carbon dioxide. After being injected into the reservoir, it can greatly reduce the interfacial tension, significantly improve the mobility ratio, and when dissolved in crude oil, it can increase the volume of crude oil by 10% - 100%, reduce the viscosity, and make it easier to flow, thereby improving the recovery rate of crude oil. Therefore, supercritical carbon dioxide drilling and completion do not damage the reservoir, can effectively avoid near-wellbore formation plugging, protect the oil and gas reservoir, and improve the permeability of the reservoir.

[0027] The mechanism of enhanced oil production by the carbon dioxide thickening fracturing fluid of the present invention is mainly manifested in the following aspects: microscopically, it dredges the pore throats of the reservoir, thereby improving the permeability of the reservoir; macroscopically, it changes the mechanical properties of the formation rock, thereby increasing the complexity and roughness of the artificial fractures; the injection of carbon dioxide increases the fluid influence (swept) range; its strong inhibition of water-sensitive minerals can eliminate water-sensitive damage, and its extremely low surface tension and water-free system avoid reservoir water lock damage.

[0028] Carbon dioxide is in a supercritical state in the reservoir and fractures, with a viscosity of 0.033 mPa·s, strong seepage ability, stable pressure during the construction process, low friction during the co-injection of the base fluid / carbon dioxide, 5 MPa less than that of injecting liquid nitrogen alone, stable construction displacement, ensuring the opening degree of the fractures, and high construction efficiency; through molecular simulation technology, the present invention independently developed a carbon dioxide thickening fracturing fluid, greatly improving the sand-carrying capacity and fracture-forming ability of the carbon dioxide thickening fracturing fluid, and significantly reducing the dosage of conventional fracturing fluid.

[0029] During operation, the prepared carbon dioxide thickening fracturing fluid is injected into the reservoir to make the bottom hole pressure greater than the closure pressure. Considering the comprehensive simulation of the reservoir pressure increase rate and the pressure limit conditions of the pipe string and wellhead, the carbon dioxide construction displacement is finally determined to be 4.0 - 5.0 m 3 / min.

[0030] The proppant is natural sand or artificial high-strength ceramic particles with a certain particle size and gradation. The petroleum proppant enters the formation along with the high-pressure solution and fills the rock fractures. Low-density small-sized ceramic proppants improve the sand-carrying performance and the sand placement profile. The settling velocity of low-density ceramic proppants is reduced by 78% compared to conventional ceramic proppants. The settling velocity of 40 - 70 cm / s is greater than that of 20 - 40 cm / s. The proppant is mainly 40 - 70-mesh ceramic proppants.

[0031] The carbon dioxide thickening fracturing fluid of the present invention independently developed a thickening agent product for the carbon dioxide fracturing fluid system through molecular simulation technology, established a preparation method and application of the carbon dioxide thickening fracturing fluid, greatly improving the sand-carrying capacity and fracture-forming ability of the carbon dioxide thickening fracturing fluid; meeting the on-site requirements of Changqing Oilfield, solving the problems of large frictional resistance of carbon dioxide foam, high construction pressure, and limited displacement, being able to effectively prevent carbon dioxide from penetrating into the formation, significantly improving the liquid efficiency, with small dosage, and being easy to prepare and pump on-site.

[0032] Table 1 Basic properties of carbon dioxide thickening fracturing fluid

[0033] Technical parameters Indicators Appearance Colorless transparent liquid Apparent viscosity / mPa·s 18～26 <![CDATA[Density / g / cm 3 > 1.02～1.08 pH value 5～7 Freezing point / °C ＜-20 Thickening time / min 0.5-5

[0034] Example 1

[0035] For the carbon dioxide thickening fracturing fluid of the present invention, 2% of thickening agent and 98% of liquid carbon dioxide are weighed by mass percentage; the liquid carbon dioxide and the thickening agent are stirred at a temperature of -30°C and a pressure greater than 3.5 MPa, the stirring rate is 2 m / s, and the stirring time is 19 min until the pH is 5 - 7, obtaining the carbon dioxide thickening fracturing fluid A1.

[0036] Example 2

[0037] For the carbon dioxide thickening fracturing fluid of the present invention, 3% of thickening agent and 97% of liquid carbon dioxide are weighed by mass percentage; the liquid carbon dioxide and the thickening agent are stirred at a temperature of -30°C and a pressure greater than 3.5 MPa, the stirring rate is 3 m / s, and the stirring time is 40 min until the pH is 5 - 7, obtaining the carbon dioxide thickening fracturing fluid A2.

[0038] Example 3

[0039] For the carbon dioxide thickening fracturing fluid of the present invention, 4% of thickening agent and 96% of liquid carbon dioxide are weighed by mass percentage; the liquid carbon dioxide and the thickening agent are stirred at a temperature of -30°C and a pressure greater than 3.5 MPa, the stirring rate is 4 m / s, and the stirring time is 60 min until the pH is 5 - 7, obtaining the carbon dioxide thickening fracturing fluid A3.

[0040] Example 4

[0041] For the carbon dioxide thickening fracturing fluid of the present invention, 5% of thickening agent and 95% of liquid carbon dioxide are weighed by mass percentage; the liquid carbon dioxide and the thickening agent are stirred at a temperature of -30°C and a pressure greater than 3.5 MPa, the stirring rate is 5 m / s, and the stirring time is 60 min until the pH is 5 - 7, obtaining the carbon dioxide thickening fracturing fluid A4.

[0042] The viscosity tests, filtration loss coefficient tests, core damage evaluations, and field test effect analyses were respectively carried out on the carbon dioxide thickened fracturing fluids A1 - A4 prepared in Examples 1 - 4, and the analysis results are shown in Tables 2 - 5.

[0043] Table 2 Viscosity Test Results

[0044]

[0045] As can be seen from Table 2, the stable viscosities of the carbon dioxide thickened fracturing fluids A1 - A4 after one hour are 12 mPa·s, 15 mPa·s, 17 mPa·s, and 18 mPa·s respectively, with good sand - carrying capacity. The viscosity of the carbon dioxide thickened fracturing fluid system in the liquid state can reach 90 mPa·s - 120 mPa·s, and the viscosity of the system under supercritical state conditions is 12 mPa·s - 18 mPa·s.

[0046] Table 3 Filtration Loss Coefficient Test Results

[0047]

[0048]

[0049] For the filtration loss test experiment, the results in Table 3 show that: the comprehensive filtration loss coefficient of the prepared carbon dioxide thickened fracturing fluids A1 - A4 for the core is on average 87.8% lower than that of pure carbon dioxide.

[0050] Table 4 Core Damage Evaluation Test Results

[0051]

[0052] From the core damage evaluation results in Table 4, it can be obtained that: the average damage rate of the carbon dioxide thickened fracturing fluids A1 - A4 prepared in the examples to the reservoir core is - 21.9%. The dosage of the thickening agent is small, there is no adsorption, no residue, and the influence on reservoir damage is weak.

[0053] The field test effect analyses were carried out on the prepared carbon dioxide thickened fracturing fluids A1 - A4, and Table 5 can be obtained.

[0054] Table 5 Field Test Effect Analysis Results

[0055]

[0056] The carbon dioxide thickened fracturing fluid provided by the present invention has completed 4 levels of field tests in 3 well - times. As shown in Table 5, the maximum test well depth is 3454 m, the maximum well temperature is 104 °C, the maximum single - well sand addition amount is 20.0 m 3 , the maximum sand ratio is 10.3% (average sand ratio 8.87%), the maximum construction displacement is 4.9 m 3 / min, and the maximum unobstructed flow rate of a single layer is 24.76×104 m 3 / d.

[0057] The rheological properties of the carbon dioxide thickening fracturing fluid are tested as Figure 1 shown. It can be seen that the rheological properties of the carbon dioxide thickening fracturing fluid of the present invention are smaller than those of liquid carbon dioxide, and the viscosity becomes larger.

[0058] The characteristics of the carbon dioxide thickening fracturing fluid are as follows: the liquid efficiency is significantly improved, a stable emulsion system can be formed in liquid or supercritical carbon dioxide, the viscosity of carbon dioxide is greatly increased, the penetration of carbon dioxide into the formation is effectively prevented, and the system efficiency is significantly improved; it is clean and residue-free, can be discharged with carbon dioxide, and has no solid-phase damage to the reservoir and the sand-packed fracture; the construction operation is simple, the dosage is small, and it is easy to prepare and pump on site.

Claims

1. Carbon dioxide thickened fracturing fluid, characterized in that, it is composed of the following raw materials by mass percentage: thickening agent 2% - 5%, liquid carbon dioxide 98% - 95%, and the total mass percentage of the above raw materials is 100%.

2. The carbon dioxide thickened fracturing fluid according to claim 1, characterized in that, the thickening agent is composed of the following raw materials by mass percentage: 45% - 50% acrylamide, 20% - 25% acrylic acid, 10% - 15% cationic monomer, 5% - 10% sulfonic acid group monomer, and 5% - 10% hydrophilic long-chain monomer, and the total mass percentage of the above raw materials is 100%.

3. The carbon dioxide thickened fracturing fluid according to claim 1, characterized in that, the pH value of the carbon dioxide thickened fracturing fluid is 5 - 7.

4. The preparation method of the carbon dioxide thickened fracturing fluid according to any one of claims 1 - 3, characterized in that, it is specifically implemented according to the following steps: Step 1: Weigh the following components by mass percentage: thickening agent 2% - 5.0% and liquid carbon dioxide 95% - 98%, and the total mass percentage of the above raw materials is 100%; Step 2: Stir the liquid carbon dioxide and the thickening agent for 19 - 60 minutes until the pH value is 5 - 7.

5. The preparation method of the carbon dioxide thickened fracturing fluid according to claim 4, characterized in that, in Step 2, the stirring is carried out at a temperature of -30°C and a pressure not less than 3.5 MPa.

6. The preparation method of the carbon dioxide thickened fracturing fluid according to claim 4, characterized in that, the stirring rate in Step 2 is 2 m / s - 5 m / s.

7. The application of the carbon dioxide thickened fracturing fluid according to claim 1 in the fracturing of oil and gas reservoir with high construction pressure and limited displacement in oil production by gas fracturing.

Images