A system and method for in-situ conversion of carbon dioxide in shale by explosive fracturing

Abstract

The application discloses a system and method for shale in-situ transformation carbon dioxide fracturing and blasting. The system comprises a wellhead injection valve, a central pipe, a plurality of flow guide pipes and a plurality of packers, the central pipe is provided with a plurality of hot water or steam injection holes, and each flow guide pipe is provided with a plurality of liquid carbon dioxide injection holes. The application uses the in-situ transformation principle, injects hot water or steam to rapidly heat the shale reservoir, uses the strong impact force generated by the gasification of liquid carbon dioxide when heated to promote the extension and expansion of the near-wellbore in the form of a fracture network. Compared with the existing carbon dioxide fracturing and blasting technology, the heating mode of the method can heat the reservoir, is easy to operate, simple and feasible, and greatly increases the complexity of the fracture network and the reservoir permeability. The method provides technical support for reducing the fracturing cost of shale oil and gas reservoirs and safely and greenly directing the fracturing.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of reservoir oil and gas stimulation engineering, and relates to an oil and gas reservoir engineering method for reducing invalid injection cycles and fracturing costs and improving fracturing efficiency through shale in-situ conversion fracturing systems, in particular to a system and method for shale in-situ conversion carbon dioxide blasting fracturing. BACKGROUND

[0002] In order to solve the problems of low fracture creation efficiency and high fracturing fluid injection cost of shale oil and gas reservoirs, scholars actively explore new methods, and it is of great significance to develop safe, green and low-damage reservoir directional fracturing technology. Carbon dioxide blasting technology is originally a physical blasting technology developed to avoid explosion accidents caused by explosive blasting. The technical principle is to use the strong thrust generated by a large amount of gas generated when carbon dioxide is converted from liquid to gas to achieve the purpose of breaking reservoir rocks. This technology has small blasting vibration and noise, does not produce harmful gases, and has a higher safety factor. With the in-depth research, carbon dioxide blasting technology is more and more favored by domestic and foreign scholars, and the application range is expanded to the resource exploitation fields of low-permeability oil and gas reservoirs and coalbed methane. After oil and gas reservoir perforation, how to stimulate fracturing cracks in different directions at low cost, safety and environmental protection to maximize the extension, and ultimately improve the complexity of the near-wellbore fracture network, is the research focus of the current stimulation technology.

[0003] Henan Polytechnic University (Patent No. CN201910634740) proposes a carbon dioxide blasting hydraulic fracturing stimulation method for the case that most of the fractures in the near-wellbore zone of low-pressure and low-permeability reservoirs are wide and long, and no complex fracture network is formed, resulting in poor stimulation effect. The invention electrically connects the carbon dioxide blasting device and the wellhead blasting control system through a cable, and applies the blasting control system to develop oil and gas well blasting operations, improve fracturing operation efficiency and reduce fracturing operation cost, but the blasting device process of the invention is complex. The existing carbon dioxide blasting technology mostly uses a cable or an electromagnetic coil to directly heat the device containing liquid carbon dioxide, which consumes a lot of electricity, has high cost, and is prone to safety accidents. It is currently a difficult point in the research of carbon dioxide blasting fracturing technology to heat and gasify liquid carbon dioxide in the reservoir at the lowest possible cost, safety and reliability.

[0004] In-situ transformation heating technology is widely used. In addition to electric heating, convection heating technology and radiation heating technology have also been developed. Through the slow heat conduction method, a large number of high-power electric heaters are arranged in the unfractured formation to heat the formation. Through the high-temperature fluid convection, the formation is heated at a relatively fast speed. Through the underground radio frequency transmitter, the formation water is heated. The heating efficiency of this technology is high, the heating is uniform, and the heating speed is fast. The above reservoir heating technologies are mainly used in the injection and production technology of low-permeability reservoirs, and the effect of improving the recovery efficiency is obvious, but the research on the carbon dioxide explosion fracturing technology of shale oil and gas reservoirs is relatively less. SUMMARY

[0005] The purpose of the present application is to provide a system and method for in-situ transformation of carbon dioxide explosion fracturing of shale. For the fracturing cracks of shale oil and gas reservoirs perpendicular to the horizontal wellbore, the in-situ transformation principle and carbon dioxide explosion technology are used to promote the rapid expansion and gasification of liquid carbon dioxide in a short time. The gasified carbon dioxide diffuses to the deep part of the shale reservoir and explodes, so that the fluid channel is extended in the form of a fracture network. The system and method for in-situ transformation of carbon dioxide explosion fracturing of shale provided by the present application can increase the number of fractures near the wellbore, increase the fracture network density, and expand the swept range of the injected medium, thereby providing an important technical basis for the fracturing and production increase of shale oil and gas reservoirs. The purpose of the present application is achieved by the following technical solutions.

[0006] A system for in-situ transformation of carbon dioxide explosion fracturing of shale, characterized in that it comprises a wellhead injection valve, a central pipe, a plurality of flow guide pipes and a plurality of packers. The central pipe is provided with a plurality of hot water or steam injection holes. Each flow guide pipe is provided with a plurality of liquid carbon dioxide injection holes. When the system is working, hot water or steam is first injected into the shale reservoir through the central pipe via the wellhead injection valve. The original fractures of the reservoir are filled with hot water or steam. As the amount of injected hot water or steam increases, the temperature of the reservoir is continuously rising. Then, liquid carbon dioxide is injected through the flow guide pipes. The liquid fluid migrates to the deep part of the reservoir along the fractures. When the liquid carbon dioxide encounters the heated high-temperature reservoir or fluid, it rapidly expands and gasifies due to the heat, and rapidly diffuses along the fracturing cracks, secondary cracks and natural cracks. The gasified carbon dioxide gradually expands and produces a strong impact force, which causes an explosion in the reservoir fractures, thereby realizing the in-situ transformation of carbon dioxide explosion fracturing of shale.

[0007] Further, the hot water or steam injection hole is located at a natural fracture development position, and the fracture development position is determined by static geological data and numerical simulation results, and the liquid carbon dioxide injection hole is located at a position adjacent to the natural fracture. When the system of this form is working, the wellhead injection valve is opened, hot water or steam is injected into the fracture development position through the central pipe, the hot water or steam diffuses radially along the developed fracture to the surrounding, the reservoir temperature gradually rises, and a part of the originally closed and filled natural fractures are excited, expanded and connected to form a new flow space; then liquid carbon dioxide is injected into the position adjacent to the horizontal well section through the flow guide pipe, and the liquid carbon dioxide migrates to the deep shale formation, and when it meets the high temperature reservoir or fluid, carbon dioxide burst fracturing occurs. The hot water or steam is injected into the horizontal well fracture development section through the central pipe, and the liquid carbon dioxide is injected into the adjacent well section through the flow guide pipe, which plays a role in accurate directional fracturing, and ensures that the liquid carbon dioxide does not meet the hot water or steam in the pipe string to cause explosion.

[0008] Further, the packers are arranged at equal intervals, and the central pipe in the adjacent two packers is provided with a hot water or steam injection hole, and the flow guide pipe is provided with a liquid carbon dioxide injection hole. When the system of this form is working, the wellhead injection valve is opened, hot water or steam is injected through the central pipe, and then liquid carbon dioxide is injected in different directions at the same horizontal well section. This form of system does not need to consider the fracture development degree, only the length of the interval packer needs to be arranged at equal intervals. At the same time, the liquid carbon dioxide is injected into each horizontal well section of this form of system, and the scale of carbon dioxide burst fracturing is relatively increased, and the fracture network near the well is more complex.

[0009] A method for in-situ conversion of carbon dioxide burst fracturing of shale, characterized in that it comprises the following steps:

[0010] (1) injecting hot water or steam into the reservoir, and the high temperature fluid diffuses to the shale reservoir, and the original fractures of the reservoir are filled with hot water or steam; as the amount of injected hot water or steam increases, the temperature of the reservoir continuously rises;

[0011] (2) injecting liquid carbon dioxide into the reservoir, and the liquid carbon dioxide meets the high temperature reservoir or fluid, and rapidly expands to produce gasification explosion;

[0012] (3) the explosion produces a strong impact force, when the impact force increases to the rock fracture pressure, the rock will produce a crack, the new crack promotes the heating range and heating speed of the subsequent liquid carbon dioxide to continuously increase, and the range of explosion also expands, and the number and density of the cracks near the well change in quality, and the cracks are extended and expanded in the form of complex cracks.

[0013] Further, considering the fracture development degree, the fracture development condition is determined according to static geological data and numerical simulation results, step (1) injecting hot water or steam in the natural fracture development zone of the horizontal well section, and step (2) injecting liquid carbon dioxide in the position adjacent to the natural fracture.

[0014] Further, without considering the fracture development degree, the injection section is arranged at equal intervals in the horizontal well section, step (1) injecting hot water or steam in each injection section, and step (2) injecting liquid carbon dioxide in each injection section.

[0015] The present application adopts the hot water injection or steam injection technology, and extends the channel of the reservoir fluid near the well in the form of a fracture on the basis of the principle and method of the existing carbon dioxide blasting technology. The innovation of the present application lies in that the in-situ conversion principle is used, the hot water injection or steam injection technology makes the shale reservoir rapidly heat up, the strong impact force generated by the gasification of liquid carbon dioxide when heated is utilized to make the network of fractures near the well extend in the form of a fracture network. Compared with the existing carbon dioxide fracturing and blasting technology, the heating mode of the method realizes heating in the reservoir, is highly operable, simple and easy to implement, and greatly increases the complexity of the fracture network and the reservoir permeability. The method provides technical support for reducing the fracturing cost of shale oil and gas reservoirs and safely and greenly directing the fracturing.

[0016] The present application has the following beneficial technical effects: (1) a system and method for carbon dioxide blasting and fracturing through a horizontal well multi-section are provided, which can effectively expand the injection sweep range, improve the reservoir permeability, and increase the fracture density. (2) The liquid carbon dioxide is gasified in the reservoir by increasing the reservoir temperature, which solves the problem of high power consumption and high cost in the prior art that only the device containing liquid carbon dioxide can be electrically heated. (3) The shale between sections can be tightly connected by using the natural fracture development section and converted into a favorable flow channel, and the propagation speed of the injected hot water or steam can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a device schematic diagram of example 1.

[0018] Figure 2 is a device schematic diagram of example 2.

[0019] Figure 3 is a method flowchart of the present application.

[0020] Reference signs: 1 - center tube, 2 - flow guide tube, 3 - packer, 4 - hot water or steam injection hole, 5 - liquid carbon dioxide injection hole. IMPLEMENTATION

[0021] The technical solutions of the present application will be described clearly and completely below with reference to the drawings in the specification. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all the other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application. Embodiment 1

[0022] A system for in-situ conversion of carbon dioxide fracturing of shale, as shown in Figure 1 Figure, including wellhead injection valve (not shown in the figure), a central tube 1, 4 diversion pipe 2 and a plurality of packers 3, the central tube 1 is provided with a plurality of hot water or steam injection hole 4, each diversion pipe 2 is provided with a plurality of liquid carbon dioxide injection hole 5. Hot water or steam injection hole 4 is located in the natural fracture development position, the fracture development position is determined by static geological data and numerical simulation results, liquid carbon dioxide injection hole 5 is located in the adjacent position of natural fracture. Embodiment 2

[0023] A system for in-situ conversion of carbon dioxide fracturing of shale, as shown in Figure 2 Figure, including wellhead injection valve (not shown in the figure), a central tube 1, 4 diversion pipe 2 and a plurality of packers 3, the central tube 1 is provided with a plurality of hot water or steam injection hole 4, each diversion pipe 2 is provided with a plurality of liquid carbon dioxide injection hole 5. The packer 3 is arranged at intervals of small section, the central tube 1 in the adjacent two packers is provided with hot water or steam injection hole 4, and the diversion pipe 2 is provided with liquid carbon dioxide injection hole 5. Embodiment 3

[0024] A method for in-situ conversion of carbon dioxide fracturing of shale, comprising the following steps:

[0025] (1) considering the degree of fracture development, determining the fracture development according to static geological data and numerical simulation results, injecting hot water or steam into the natural fracture development zone of the reservoir horizontal well section, the high-temperature fluid diffuses to the shale reservoir, and the original fracture of the reservoir is filled with hot water or steam; with the increase of the amount of injected hot water or steam, the temperature of the reservoir is continuously increased;

[0026] (2) injecting liquid carbon dioxide into the adjacent position of the natural fracture of the reservoir, the liquid carbon dioxide encounters high-temperature reservoir or fluid, and is rapidly expanded and gasified to produce explosion;

[0027] (3) the explosion produces strong impact force, when the impact force increases to the rock fracture pressure, the rock will produce cracks, the new cracks promote the heating range and heating speed of the subsequent liquid carbon dioxide to increase continuously, and the range of explosion expands, and the number and density of cracks near the well change in quality, and are extended and expanded in the form of complex cracks. Example 4

[0028] A method for shale in-situ conversion carbon dioxide blasting fracturing, comprising the following steps:

[0029] (1) Without considering the degree of crack development, inject section is set at equal interval small section distance of horizontal well section, hot water or steam is injected into each injection section of reservoir, high temperature fluid diffuses to shale reservoir, original crack of reservoir is filled with hot water or steam; with the increase of hot water or steam injection amount, reservoir temperature is continuously increased;

[0030] (2) Liquid carbon dioxide is injected into each injection section of reservoir, liquid carbon dioxide meets high temperature reservoir or fluid, is heated and rapidly expands to produce explosion;

[0031] (3) Explosion produces strong impact force, when the impact force increases to rock breaking pressure, rock produces crack, new crack promotes the heating range and heating speed of subsequent liquid carbon dioxide to be continuously increased, the range of explosion is also expanded, the number and density of cracks near well are qualitatively changed, and are extended and expanded in the form of complex cracks.

[0032] Although the embodiments of the present application have been shown and described above, it should be understood by those skilled in the art that the above embodiments are exemplary and cannot be understood as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments without departing from the principles and purposes of the present application within the scope of the present application. The protection scope of the present application is defined by the claims and equivalent technical solutions thereof.

Claims

1. A method for in-situ conversion of shale into carbon dioxide by explosive fracturing, characterized in that: A shale in-situ conversion carbon dioxide explosive fracturing system is used, comprising a wellhead injection valve, a central pipe, several flow tubes, and multiple packers. The central pipe is provided with multiple hot water or steam injection holes, and each flow tube is provided with multiple liquid carbon dioxide injection holes. The hot water or steam injection holes are located at locations where natural fractures have developed, as determined by static geological data and numerical simulation results. The liquid carbon dioxide injection holes are located adjacent to the natural fractures. The packers are arranged at equal intervals and in small sections. The central pipes of two adjacent packers are provided with hot water or steam injection holes, and the flow tubes are provided with liquid carbon dioxide injection holes. The method comprises the following steps: (1) Injecting hot water or steam into the reservoir causes the high-temperature fluid to diffuse into the shale reservoir, and the original fractures in the reservoir are filled with hot water or steam. As the amount of hot water or steam injected increases, the reservoir temperature continues to rise. (2) Liquid carbon dioxide is injected into the reservoir. When the liquid carbon dioxide encounters a high-temperature reservoir or fluid, it expands rapidly due to the heat and vaporizes, causing an explosion. (3) The explosion generates a strong impact force. When the impact force increases to the rock fracture pressure, cracks will appear in the rock. The new cracks will cause the subsequent heating range and heating rate of liquid carbon dioxide to continue to increase, and the range of the explosion will also expand. The number and density of cracks near the wellbore will undergo qualitative changes and will be extended and expanded in the form of complex cracks.

2. The method according to claim 1, characterized in that Considering the degree of fracture development, the fracture development is determined based on static geological data and numerical simulation results. Step (1) injects hot water or steam into the natural fracture development zone of the horizontal well section, and step (2) injects liquid carbon dioxide into the position adjacent to the natural fracture.

3. The method according to claim 1, characterized in that Regardless of the degree of fracture development, injection sections are set at equal intervals in the horizontal well section. Step (1) injects hot water or steam into each injection section, and step (2) injects liquid carbon dioxide into each injection section.

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