Impulse type deflagration fracturing construction method

Abstract

The invention discloses a pulse type deflagration fracturing construction method. The method comprises the steps that reservoir rock mechanical parameters and the oxygen amount and the combustible gas amount needed by first-time deflagration are determined according to early-stage geological exploration and experimental data; constructing a fractured well from the ground to the reservoir and completing the well; the fracturing pipe column with the detonating device arranged at the toe end is conveyed to the designed position of the well section to be fractured; injecting clear water and a foaming agent into the fractured well; combustible gas and oxygen required by the first detonation fracturing are pumped into the to-be-fractured well section, and combustible gas foam and oxygen foam are formed; igniting foam to complete first detonation fracturing; in the first detonation process, the one-way gas injection valve on the fracturing tubular column is closed; it is ensured that the difference value between the pressure in the fracturing pipe column and the reservoir fracture closing pressure is larger than or equal to 1 MPa; when the difference value between the pressure in the fracturing well and the reservoir fracture closing pressure is smaller than or equal to 1 MPa, a one-way gas injection valve on the fracturing pipe column is opened, combustible gas and oxygen are pumped into the fracturing well, and secondary deflagration fracturing is achieved; and so on, until the pulse type deflagration fracturing construction is completed.

Description

Technical Field

[0001] The invention belongs to the field of oil and gas resource development and relates to a pulse type deflagration fracturing construction method. Background Art

[0002] For unconventional oil and gas resources, low-permeability coal seams account for more than 90%, and the permeability is generally less than 0.1mD. With existing development technology, single-well production is low and gas production is rapidly depleted. Large-scale fracturing is the main technical measure to increase single-well production. However, the morphology and expansion of hydraulic fracturing cracks are limited by tectonic stress, making it difficult to form a complex fracture network. In addition, hydraulic fracturing also has many disadvantages such as high cost and high water resource consumption, which affect the economic development of unconventional oil and gas. Although CO2 dry fracturing or foam fracturing can partially overcome the defects of hydraulic fracturing and the fracture network formed is more complex than that of hydraulic fracturing, it still has a slow pressure increase speed and cannot completely overcome the limitation of tectonic stress. High-energy gas fracturing has the disadvantages of insufficient crack length and small transformation range. The high-energy gas pulse fracturing method mostly uses rocket propellant installed in the fracturing string in advance. After the fracturing string enters the well, it cannot be reloaded. The delay technology can be used to control the formation of 3 to 5 pulse pressure waves, and the transformation range is still small; methane in-situ deflagration fracturing is transformed into a more severe explosion due to the uncontrollable factors of the deflagration of methane and oxygen. The results of public experimental research show that when the methane content per meter in a closed and slender string with a diameter of 0.18 meters is 300 grams, the peak pressure of the deflagration is as high as 280MPa. Therefore, the high peak value fracturing of methane in-situ deflagration fracturing may lead to the formation of compaction and crushing zones in the near-wellbore area of ​​the reservoir, which in turn reduces the permeability of the reservoir. Due to the short operation time of methane in-situ deflagration and gas generation, the length of the cracks produced is only about 10 meters, which is lower than the length of the cracks produced by high-energy gas fracturing using rocket-propelled agents. The transformation range of methane in-situ deflagration fracturing is smaller than that of high-energy gas fracturing, and the methane in the wellbore will be consumed at once during the fracturing process. Even the multi-stage pulse methane in-situ explosion fracturing method can only achieve 2-stage pulses, and the methane cannot be quickly replenished in a short time after it is consumed, and the deflagration cannot be carried out continuously. For this reason, it is urgent to develop a fracturing construction technology that can efficiently transform the reservoir in a large range. Summary of the invention

[0003] In view of the defects and shortcomings of the prior art, the purpose of the present invention is to provide a pulsed deflagration fracturing construction method to solve the technical problems of insufficient crack length and small transformation range caused by existing high-energy gas fracturing and methane in-situ deflagration fracturing.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions to achieve the above problems:

[0005] A pulsed deflagration fracturing construction method comprises the following steps:

[0006] Step 1. Determine the reservoir rock mechanical parameters, the total amount of oxygen required for deflagration fracturing, the amount of oxygen required for the first deflagration, and the amount of combustible gas required for the first deflagration fracturing according to the previous geological exploration and experimental data. The reservoir rock mechanical parameters include the reservoir fracture closure pressure;

[0007] Step 2. Construct a fracturing well from the ground to the reservoir and complete the well;

[0008] Step 3. Send the fracturing string with an initiating device at the toe end to the designed position in the fracturing well and set it;

[0009] Step 4. Inject clear water and foaming agent into the fracturing well through the fracturing string;

[0010] Step 5. Pump the combustible gas and oxygen required for the first deflagration fracturing into the well section to be fractured respectively to generate combustible gas foam and oxygen foam;

[0011] Step 6. Ignite the combustible gas foam and oxygen foam with the initiating device set at the tail end of the fracturing string to complete the first deflagration fracturing;

[0012] During the first deflagration process, the one-way gas injection valve on the fracturing string closes; ensure that the difference between the pressure in the fracturing string and the reservoir fracture closure pressure determined in Step 1 is greater than or equal to 1 MPa by ground pressurization;

[0013] Step 7. After the deflagration ends, when the pressure in the fracturing well drops to a difference less than or equal to 1 MPa from the reservoir fracture closure pressure, open the one-way gas injection valve on the fracturing string and pump combustible gas and oxygen into the fracturing well to achieve secondary deflagration fracturing;

[0014] Step 8. Repeat Step 7 to form multiple deflagrations in the fracturing well to generate a pulsed pressure wave and complete the pulsed deflagration fracturing construction.

[0015] The present invention also has the following technical features

[0016] Specifically, the combustible gas is selected from one or more of methane, ethane, butane, carbon monoxide, and ethylene.

[0017] Furthermore, the fracturing well is a horizontal well.

[0018] Furthermore, the designed position is the tail end of the casing of the second open hole of the horizontal well.

[0019] Furthermore, the amount of combustible gas required for the first deflagration fracturing is determined by the following formula:

[0020] G = μ·L·r 2 ·3P f

[0021] Wherein,

[0022] G is the amount of combustible gas, with the unit of kg;

[0023] μ is a dimensionless coefficient;

[0024] L is the total length of the section to be fractured, with the unit of m;

[0025] r is the wellbore radius of the section to be fractured in the fracturing well, with the unit of m;

[0026] P pro is the reservoir fracture pressure, with the unit of Mpa.

[0027] Furthermore, the fracturing string includes two parallel fracturing strings, and a plurality of groups of check valves are arranged at intervals on the fracturing string, and the opening pressure of each check valve is 1 MPa.

[0028] Furthermore, the amount of combustible gas required for the next deflagration is 70-80% of the amount of combustible gas required for the previous deflagration.

[0029] Furthermore, the foaming agent described in step 4 includes the following raw material components by mass percentage: 2-3% of quaternary ammonium salt gemini surfactant, 0.1-0.3% of sodium dodecylbenzenesulfonate, 0.5-1% of hydroxypropyl guar gum, and the balance is water.

[0030] Compared with the prior art, the present invention has the following technical effects:

[0031] (1) The time taken for the pressure of the method of the present invention to rise from the initial moment of deflagration to the peak pressure is about 200 milliseconds, while the conventional hydraulic fracturing takes 1-10 minutes. Therefore, the pressure rise speed of the method of the present invention is much greater than that of the conventional hydraulic fracturing, which can effectively overcome the stress limitation of the reservoir, form a more complex radial fracture system around the wellbore of the fracturing well, and fully improve the permeability of the near-well reservoir.

[0032] (2) The method of the present invention conducts intermittent gas supply after the first deflagration, realizing pulsed multiple deflagrations. The continuous operation time of the deflagration-generated gas can reach 20 minutes, which is two orders of magnitude longer than the high-energy gas fracturing with an operation time of 22 seconds and the in-situ methane deflagration time. Moreover, the pulsed pressure waves generated by multiple deflagrations can fully create fractures, promote the further extension of fractures, generate a more complex fracture structure, and can generate fractures in a larger range, fully improving the reservoir.

[0033] (3) In the method of the present invention, the products after deflagration are mainly CO2 and water vapor, and the generated CO2 is in a supercritical state, which can weaken the mechanical properties of coal, reduce the fracture initiation pressure and fracture propagation pressure, and the fractures generated by a single wellbore are longer than those of high-energy gas fracturing; supercritical CO2 has extremely high heat conduction efficiency and high diffusivity, can quickly transfer the deflagration heat to the coal seam, and the adsorption capacity of coal for CO2 is stronger than that for methane. With the high temperature generated by deflagration, CO2 can displace methane, which is beneficial to increasing production after fracturing. CO2 is easy to enter the coal pores, becomes weakly acidic with the residual water or formation water in the coal, dissolves some minerals in the coal, and has a certain acidification effect on the coal rock, further improving the permeability. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a flow chart of the method of the present invention;

[0035] Figure 2 is a schematic diagram of the wellbore structure and completion method of the fractured well in Example 1;

[0036] Figure 3 is a schematic diagram of the layout of the fracturing string.

[0037] The reference numerals in the figure indicate:

[0038] 1 - fracturing string, 2 - packer, 3 - initiating device.

[0039] The present invention will be further described in detail below with reference to the drawings and specific embodiments. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described below with reference to the drawings.

[0041] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments, and any equivalent transformation made on the basis of the technical solutions of the present application falls within the protection scope of the present invention.

[0042] The terms "upper", "lower", "front", "rear", "top", "bottom", etc. used in the present invention to indicate the orientation or position relationship are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. "Inner" and "outer" refer to the inside and outside of the corresponding component contour, and the above terms should not be construed as limiting the present invention.

[0043] In the present invention, unconventional oil and gas refers to: shale gas, tight gas, coalbed methane, tight oil, shale oil, oil shale oil, etc.

[0044] The technical concept of the present invention is as follows: by using the fracturing string provided by the present invention, a foaming agent, a combustible gas, and oxygen are injected successively, so that the combustible gas and oxygen are placed in the well section to be fractured in the form of foam. The combustible gas foam and oxygen foam are ignited, and the foam retards the chain reaction during deflagration, reduces the intensity of deflagration, and prolongs the deflagration time. With the pressure generated by deflagration, the first deflagration fracturing is carried out on the reservoir. Since the deflagration pressure rise speed is relatively fast, it can effectively overcome the stress limitation of the reservoir and cause the reservoir to generate a complex radial fracture system. After the first deflagration, the combustible gas and oxygen are intermittently injected to generate pulsed deflagration, continuously pushing the fracture to extend and forming a large-scale fracture network.

[0045] Example 1

[0046] In accordance with the above technical solution, as Figures 1 to 3 shown, in this embodiment, after preliminary exploration, the length of the development area controlled by a single well in the reservoir (total length to be fractured) is 800 m, the width is 600 m, the burial depth is 500 m, the thickness is 10 m, and methane is selected as the combustible gas;

[0047] When fracturing is carried out, a pulsed deflagration fracturing construction method is adopted, including the following steps:

[0048] Step 1: Determine the reservoir rock mechanics parameters, the total amount of oxygen required for deflagration fracturing, the amount of oxygen required for the first deflagration, and the amount of combustible gas required for the first deflagration fracturing according to the preliminary geological exploration and experimental data. The reservoir rock mechanics parameters include the reservoir fracture closure pressure;

[0049] The determined reservoir fracture pressure is 29.6 MPa.

[0050] The amount of methane required for the first deflagration fracturing is determined by the following formula:

[0051] G = μ·L·r 2 ·3P pro

[0052] Wherein,

[0053] G is the amount of combustible gas, with the unit of kg;

[0054] μ is a dimensionless number, which characterizes the relationship between the amount of combustible gas used and the deflagration peak fracturing in a limited space. In the laboratory, a small amount of methane is mixed with oxygen foam, and a deflagration experiment is carried out on a slender pipe string, and it is obtained through linear regression.

[0055] L is the total length of the section to be fractured, with the unit of m;

[0056] r is the wellbore radius of the well section to be fractured in the fracturing well, with the unit of m;

[0057] P pro is the reservoir fracture pressure, with the unit of Mpa.

[0058] μ is a dimensionless coefficient, which is measured in the laboratory within a limited space and specifically includes: mixing a small dose of methane foam and oxygen foam, measuring the relationship between the methane consumption and the peak detonation pressure through a deflagration experiment in a slender pipe column, and obtaining it through linear regression.

[0059] To achieve a better fracturing effect, the peak pressure of the first deflagration is not less than 3 times the reservoir fracture pressure. In this embodiment, μ is taken as 0.197, L = 800 m, the original radius of the fracturing well is 0.12 m, and considering the average wellbore enlargement rate, so r is taken as 0.13 m. The calculated methane consumption G required for the first deflagration is 236.11 kg, and from this, the oxygen amount required for the first deflagration is determined to be 944.45 kg.

[0060] According to the determined chemical equation for the complete combustion of methane, the molar ratio of methane to oxygen for the first deflagration fracturing is determined to be 1:2 to ensure the complete combustion of methane during the deflagration process. A molar ratio of 1:2 can ensure the complete combustion of the gas and also avoid the influence of unburned gas residues on subsequent deflagrations;

[0061] Step 2: Construct and complete a fracturing well from the ground to the reservoir;

[0062] As Figure 2 shown, the fracturing well in this embodiment is a horizontal well, and the horizontal well has a three - opening wellbore structure. Among them, the horizontal well section is arranged in the coal seam, and the first and second openings of the horizontal well use steel casing cementing, and the third opening uses a steel screen pipe to complete the well.

[0063] Specifically, for the first opening of the horizontal well, drill to 5 m below the bedrock and set the casing for cementing; for the second opening, start to build an inclination when drilling to 200 m, drill to a well depth of 500 m, and complete the drilling with a well inclination of 90 degrees and set the casing for cementing; for the third opening, use a φ215.9 mm bit to start horizontal well drilling, drill to a well depth of 1300 m to complete the drilling, and set a steel screen pipe to complete the well.

[0064] Step 3: Send a fracturing string with a detonating device at the toe end to the end of the second - opening casing of the horizontal well for setting and sealing;

[0065] Step 4: Inject clear water and a foaming agent into the fracturing well through the fracturing string;

[0066] Specifically, the foaming agent includes the following raw material components by mass percentage: 2% of quaternary ammonium salt gemini surfactant, 0.3% of sodium dodecylbenzenesulfonate, 0.5% of hydroxypropyl guar gum, and the balance is water.

[0067] Step 5: Pump the combustible gas and oxygen required for the first deflagration fracturing into the well section to be fractured respectively to generate combustible gas foam and oxygen foam;

[0068] During the first deflagration process, the one-way gas injection valve on the fracturing string closes. Meanwhile, the methane and oxygen pressures inside the fracturing string are maintained at 30.6 MPa by the ground pressurizing equipment.

[0069] Step 6: Ignite the combustible gas foam and oxygen foam with the initiating device set at the end of the fracturing string to complete the first deflagration fracturing.

[0070] Step 7: After the deflagration ends, when the pressure of the detonation-generated gas in the fracturing well drops to 29.6 MPa during the first deflagration, the one-way gas injection valve opens, and methane and oxygen are continuously pumped into the fracturing well according to the molar ratio of complete combustion to achieve secondary deflagration fracturing.

[0071] Specifically, since the pressure shock wave generated by the first deflagration has caused the reservoir to fracture, the methane and oxygen consumption for the second deflagration is calculated at 80% of the gas consumption for the first deflagration. Reducing the gas supply by 80% can balance the pressure and equipment safety, preventing the continuous high pressure and high temperature from damaging the fracturing string and screen pipe. After injecting methane and oxygen into the wellbore, under high temperature and high pressure conditions, the mixture will deflagrate again. Since there is already detonation-generated gas mainly composed of carbon dioxide in the wellbore, it can inhibit the deflagration intensity of the mixed gas and extend the action time of the pressure wave. Therefore, the peak pressure of the second deflagration is lower than that of the first deflagration.

[0072] Step 8: Repeat Step 7 to form multiple deflagrations in the fracturing to generate a pulsed pressure wave until the amount of carbon dioxide in the wellbore is sufficient to completely inhibit the combustion of the injected methane and oxygen, completing the pulsed deflagration fracturing construction.

[0073] After multiple deflagrations, the cracks continue to expand, and multiple pulses are superimposed to form a complex fracture network.

[0074] Compared with the traditional in-situ methane deflagration fracturing, the method of the present invention realizes multiple-stage pulses through the intermittent gas supply and carbon dioxide inhibition mechanism, with a more complex fracture structure and higher operation efficiency.

[0075] In summary, the method of the present invention performs intermittent gas supply after the first deflagration, realizing pulsed multiple deflagrations. The continuous operation time of the detonation-generated gas is two orders of magnitude longer than that of the existing in-situ methane explosion deflagration. Moreover, the pulsed pressure waves generated by multiple deflagrations can fully create fractures, promote the further extension of the fractures, generate a more complex fracture structure, and can generate fractures in a larger range, fully transforming the reservoir.

[0076] The embodiments of the present invention have been described above in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the spirit and scope protected by the claims of the present invention, and these all belong to the protection scope of the present invention.

Claims

1. A pulsed deflagration fracturing construction method, characterized in that: The following steps are involved: Step 1: Determine reservoir rock mechanical parameters, the amount of oxygen and combustible gas required for the first deflagration based on previous geological exploration and experimental data, wherein the reservoir rock mechanical parameters include reservoir fracture closure pressure; Step 2: constructing a fracturing well from the ground to the reservoir and completing the well; Step 3, sending the fracturing string with a detonating device at the toe end to the designed position in the fracturing well for sealing; Step 4, injecting clean water and foaming agent into the fracturing well through the fracturing string; Step 5, respectively pumping the combustible gas and oxygen required for the first deflagration fracturing into the well section to be fractured to generate combustible gas foam and oxygen foam; Step 6, igniting the combustible gas foam and oxygen foam with the help of the detonator disposed at the tail end of the fracturing string to complete the first deflagration fracturing; During the first deflagration, the one-way gas injection valve on the fracturing string is closed; the difference between the pressure in the fracturing string and the reservoir fracture closure pressure determined in step 1 is ensured to be greater than or equal to 1 MPa by applying pressure on the ground; Step 7: After the deflagration is over, when the pressure in the fracturing well drops to a value less than or equal to 1 MPa from the closing pressure of the reservoir fracture, the one-way gas injection valve on the fracturing string is opened to pump combustible gas and oxygen required for the next deflagration into the fracturing well to achieve the next deflagration fracturing; Step 8: Repeat step 7 to form multiple deflagrations in the fracturing, generate pulsed pressure waves, and complete the pulsed deflagration fracturing construction.

2. The pulsed deflagration fracturing construction method according to claim 1, characterized in that: The combustible gas is selected from one or more of methane, ethane, butane, carbon monoxide and ethylene.

3. The pulsed deflagration fracturing construction method according to claim 1, characterized in that: The fracturing well is a horizontal well.

4. The pulsed deflagration fracturing construction method according to claim 3, characterized in that: The designed position is the tail end of the secondary casing of the horizontal well.

5. The pulsed deflagration fracturing method according to claim 1, characterized in that: The amount of combustible gas required for the first deflagration fracturing is determined by the following formula: G=μ·L·r 2 ·3P pro in, G is the amount of combustible gas, in kg; μ is a dimensionless coefficient; L is the total length of the section to be fractured, in m; r is the wellbore radius of the section to be fractured in the fractured well, in meters; Ppro is the reservoir fracture pressure, in MPa.

6. The pulsed well deflagration fracturing method according to claim 1, characterized in that: The fracturing string comprises two fracturing strings arranged in parallel, and a plurality of groups of one-way valves are arranged at intervals on the fracturing string, and the opening pressure of each one-way valve is 1 MPa.

7. The pulsed well deflagration fracturing method according to claim 1, characterized in that: The amount of combustible gas required for the next deflagration is 70-80% of the amount of combustible gas required for the previous deflagration.

8. The pulsed well deflagration fracturing method according to claim 1, characterized in that: The foaming agent described in step 4 comprises the following raw material components by mass percentage: 2-3% of quaternary ammonium gemini surfactant, 0.1-0.3% of sodium dodecylbenzene sulfonate, 0.5-1% of hydroxypropyl guar gum, and the balance is water.

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