CN109826606B - Method and device for intermittent in-situ exploitation of oil shale by alternately fracturing high-temperature fluid and low-temperature fluid - Google Patents

Abstract

The present application provides a method and device for intermittently fracturing oil shale in situ by alternating high and low temperature fluids. The method includes the following steps: fracturing an oil shale reservoir with a high temperature fluid, so as to make the oil shale reservoir An artificial main fracture of predetermined length is created. The high temperature fluid is stopped from fracturing the oil shale reservoir, the high temperature fluid is allowed to heat the oil shale reservoir, and after the first predetermined time is reached, the high temperature fluid is flown back. Liquid nitrogen is used to fracturing oil shale reservoirs to generate secondary fractures on both sides of artificial main fractures, while liquid nitrogen gasification produces nitrogen gas, which in turn produces multi-level fractures on both sides of secondary fractures. The wellhead is closed, and after reaching a second predetermined time, the wellhead is opened to flow back nitrogen, so that a series of interconnected fracture networks are formed in the oil shale reservoir. The present application can not only realize the transformation of kerogen in oil shale into oil and gas resources, but also can effectively improve the permeability of oil shale reservoir, and can realize large-scale in-situ mining of oil shale.

Description

Method and device for intermittent in-situ production of oil shale by alternating fracturing of high and low temperature fluids

technical field

The application belongs to the technical field of hydraulic fracturing of oil shale reservoirs, and in particular relates to a method and a device for in-situ production of oil shale using alternating fracturing discontinuities of high and low temperature fluids.

Background technique

At present, my country's oil shale mining technology is relatively backward, and oil shale development basically adopts above-ground dry distillation technology. The oil yield is low, it is difficult to meet the large-scale development and application, and the pollution problem is prominent. Therefore, relevant scholars have carried out research on the underground dry distillation technology of oil shale to promote the large-scale development and utilization of oil shale. Because the oil shale is relatively tight and has extremely low permeability, it is difficult for the recoverable oil and gas resources generated by the kerogen in the oil shale reservoir to enter the wellbore through the reservoir after being pyrolyzed underground. layers are remodeled. Hydraulic fracturing technology is currently the most commonly used method for effective stimulation of unconventional oil and gas reservoirs. Therefore, hydraulic fracturing technology can be used to stimulate oil shale reservoirs. Oil shale reservoirs are generally buried deep. Under the action of high temperature and high pressure in the formation, oil shale will show thermoplasticity. Therefore, the hydraulic fractures generated by conventional hydraulic fracturing in oil shale reservoirs are relatively simple, and the effect of reservoir stimulation is relatively simple. not good.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned defects of the prior art, the technical problem to be solved by the present invention is to provide a method and device for in-situ production of oil shale by alternating fracturing of high and low temperature fluids, which can not only realize kerogen transformation in oil shale For oil and gas resources, it can effectively improve the permeability of oil shale reservoirs, and can realize large-scale in-situ mining of oil shale.

The concrete technical scheme of the present invention is:

The invention provides a method for producing oil shale in situ by alternating fracturing of high and low temperature fluids, comprising the following steps:

fracturing the oil shale reservoir with high temperature fluid, so that the oil shale reservoir produces artificial main fractures of predetermined length;

Stop the high temperature fluid fracturing the oil shale reservoir, make the high temperature fluid heat the oil shale reservoir, and after reaching a first predetermined time, flow back the high temperature fluid;

Fracturing the oil shale reservoir with liquid nitrogen, so that secondary fractures are generated on both sides of the artificial primary fracture, and nitrogen gas is generated by gasification of liquid nitrogen, thereby generating multi-level fractures on both sides of the secondary fracture;

The wellhead is closed, and after reaching a second predetermined time, the wellhead is opened to flow back nitrogen, so that the oil shale reservoir forms a series of interconnected fracture networks.

In a preferred embodiment, after the oil shale reservoir forms a series of interconnected fracture networks, the oil shale reservoir is first produced.

In a preferred embodiment, after the oil shale reservoir is produced for the first time, the oil shale reservoir is continuously fractured with high temperature fluid, so that the artificial main fracture continues to extend forward;

Stop the high temperature fluid fracturing the oil shale reservoir, make the high temperature fluid heat the oil shale reservoir, and after reaching a first predetermined time, flow back the high temperature fluid;

Fracturing the oil shale reservoir with liquid nitrogen, so that secondary fractures are generated on both sides of the artificial main fracture extending forward, and nitrogen gas is generated by the gasification of liquid nitrogen, thereby generating multi-level fractures on both sides of the secondary fracture;

closing the wellhead, and after reaching the second predetermined time, opening the wellhead to flow back nitrogen, so that the oil shale reservoir further forms a network of interconnected fractures;

After the oil shale reservoir further forms an interconnected fracture network, the oil shale reservoir is produced for the second time.

In a preferred embodiment, the predetermined length is determined according to the content of oil-producing substances in the oil shale reservoir.

In a preferred embodiment, the temperature of the liquid nitrogen is -196°C.

In a preferred embodiment, the temperature of the high temperature fluid is between 300-500°C.

In a preferred embodiment, the high temperature fluid is superheated steam.

In a preferred embodiment, the first predetermined time depends on the pyrolysis rate of kerogen in the oil shale reservoir.

In a preferred embodiment, the second predetermined period of time is 7 days.

In addition, the present invention also provides a device for in-situ production of oil shale with alternating high and low temperature fluid fracturing and discontinuous in-situ production of oil shale using any of the above-mentioned methods for alternately fracturing and intermittently producing oil shale with high and low temperature fluids, including:

a first fracturing module configured to fracturing an oil shale reservoir with a high temperature fluid to create an artificial primary fracture of a predetermined length in the oil shale reservoir;

a heating module configured to stop the high temperature fluid fracturing the oil shale reservoir, make the high temperature fluid heat the oil shale reservoir, and after reaching a first predetermined time, flow back the high temperature fluid;

The second fracturing module is configured to use liquid nitrogen to fract the oil shale reservoir, so as to generate secondary fractures on both sides of the artificial main fracture, and simultaneously gasify the liquid nitrogen to generate nitrogen, thereby causing the secondary fractures There are multi-level cracks on both sides of the crack;

The communication module is configured to close the wellhead, and after reaching the second predetermined time, open the wellhead and flow back nitrogen gas, so that the oil shale reservoir forms a series of interconnected fracture networks.

In addition, the present invention also provides a device for intermittently fracturing oil shale in situ alternately with high and low temperature fluids, comprising a memory and a processor, wherein a computer program is stored in the memory, and when the computer program is executed by the processor, the The following steps: the method for producing oil shale in situ by alternate fracturing of high and low temperature fluids as described in any of the above.

By the above technical solutions, the beneficial effects of the present application are:

The invention adopts high temperature fluid fracturing, which can not only generate long artificial fractures, but also heat the oil shale to promote the kerogen in the oil shale around the fractures to be converted into oil and gas resources. Using low-temperature fluid fracturing can generate secondary fractures and/or multi-level fractures on both sides of the formed fractures, providing necessary seepage channels for the exploitation of oil and gas resources. After each high and low temperature fluid fracturing is completed, the exploitation is carried out for a period of time, which can avoid the pollution of the formed oil and gas resources caused by the high and low temperature fluid injected again.

The method and device for in-situ production of oil shale by utilizing high and low temperature fluids alternately fracturing discontinuities provided by the present invention can not only realize the transformation of kerogen in oil shale into oil and gas resources, but also effectively improve the permeability of oil shale reservoirs , through the alternating fracturing of high and low temperature fluids, the stimulation range of oil shale reservoirs can be continuously increased, and the recovery rate can be continuously improved by intermittent mining, which provides a theoretical basis for realizing large-scale in-situ mining of oil shale.

With reference to the following description and drawings, specific embodiments of the present application are disclosed in detail, indicating the manner in which the principles of the present application may be employed. It should be understood that the embodiments of the present application are not thereby limited in scope. Embodiments of the present application include many changes, modifications and equivalents within the spirit and scope of the appended claims.

Features described and/or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, in combination with, or instead of features in other embodiments .

It should be emphasized that the term "comprising/comprising" when used herein refers to the presence of a feature, integer, step or component, but does not exclude the presence or addition of one or more other features, integers, steps or components.

Description of drawings

The drawings described herein are for explanatory purposes only and are not intended to limit the scope of the present disclosure in any way. In addition, the shapes and proportions of the components in the figures are only schematic and are used to help the understanding of the present application, and do not specifically limit the shapes and proportions of the components of the present application. Under the teachings of the present application, those skilled in the art can select various possible shapes and proportions according to specific conditions to implement the present application. In the attached image:

1 is a flow chart of a method for producing oil shale in situ by alternating high and low temperature fluid fracturing intermittently according to an embodiment of the application;

2 is a block diagram of a device for intermittently producing oil shale in situ by alternating high and low temperature fluid fracturing according to an embodiment of the application;

3 is a schematic diagram of an artificial fracture perpendicular to the wellbore direction in the high-temperature oil shale reservoir of the present invention;

FIG. 4 is a schematic diagram of the liquid nitrogen-assisted fracturing multi-stage fracture of the present invention.

Reference numerals of the above drawings: 1, artificial main fracture; 2, wellbore; 3, multi-level fracture; 4, fracturing string; 5, secondary fracture; 6, tertiary fracture; 7, quaternary fracture; 8 , n-level cracks.

Detailed ways

The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

It should be noted that when an element is referred to as being "disposed on" another element, it can be directly on the other element or intervening elements may also be present. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or intervening elements may also be present. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for the purpose of illustration only and do not represent the only embodiment.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this application belongs. The terms used herein in the specification of the present application are for the purpose of describing particular embodiments only, and are not intended to limit the present application. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

As shown in FIG. 1 , an embodiment of the present invention provides a method for intermittently fracturing oil shale in situ by alternating high and low temperature fluids. The method includes the following steps:

S1: fracturing an oil shale reservoir with a high-temperature fluid, so that an artificial main fracture 1 of a predetermined length is generated in the oil shale reservoir;

S2: stop the high-temperature fluid fracturing the oil shale reservoir, make the high-temperature fluid heat the oil shale reservoir, and flow back the high-temperature fluid after reaching a first predetermined time;

S3: fracturing the oil shale reservoir with liquid nitrogen, so that secondary fractures 5 are generated on both sides of the artificial primary fracture 1, and then multi-level fractures 3 are generated on both sides of the secondary fracture 5;

S4: The wellhead is closed, and after the second predetermined time is reached, the wellhead is opened to flow back nitrogen, so that the oil shale reservoir forms a series of interconnected fracture networks.

In this embodiment, as shown in FIG. 3 and FIG. 4 , firstly, high-temperature fluid is injected into the wellbore 2 by using the fracturing string 4 to fract the oil shale reservoir. The high temperature fluid can be superheated steam, and its temperature can be between 300-500°C, so that the oil shale reservoir will generate an artificial main fracture 1 with a predetermined length. Under normal circumstances, the artificial main fracture 1 is perpendicular to the direction of the minimum main in-situ stress of the formation. The predetermined length needs to be determined according to the content of oil-producing substances such as kerogen in the oil shale reservoir. For example, if the reservoir has a higher oil-producing material content, the predetermined length may be shorter. However, the minimum requirement of the predetermined length is that the recoverable oil and gas resources obtained after the reservoir is fracturing with high and low temperature fluids of the predetermined length have the exploitation value.

Then, the pump needs to be stopped to stand still, so that the high-temperature fluid heats the surrounding oil shale reservoir, and the heating process needs to be maintained for a first predetermined time, which can fully heat the oil shale around the artificial main fracture 1 and promote the oil shale in the oil shale. Kerogen is converted into oil and gas resources. After reaching the first predetermined time, the high-temperature fluid needs to be flowed back to avoid the phase transition of the high-temperature fluid caused by the ultra-low temperature action of the subsequently injected liquid nitrogen, resulting in the blockage of the artificial main fracture 1 . It should be noted that the first predetermined time depends on the pyrolysis rate of kerogen in the oil shale reservoir.

After the high temperature fluid flowback is completed, the oil shale reservoir is fracturing with liquid nitrogen. After the liquid nitrogen can be injected into the wellbore 2 through the fracturing string 4, it will enter the artificial main fracture 1, and the two sides of the artificial main fracture 1 Under the ultra-low temperature of liquid nitrogen (-196 °C), the high-temperature oil shale will generate huge thermal stress, causing the high-temperature oil shale (300 °C) on both sides to rupture and produce secondary fractures5. At the same time, the high-pressure nitrogen gas generated by the liquid nitrogen gasification promotes the re-extension of the secondary crack 5, thereby causing multi-level cracks 3 on both sides of the secondary crack 5. The multi-level cracks 3 may include tertiary cracks 6 around the secondary cracks 5, quaternary cracks 7 around the tertiary cracks 6, and so on, n-level cracks 8 around the (n-1) level cracks, where n≥ 2, and n is a positive integer. For example, in a special case, when n=2, the (n-1)-level fracture is the artificial primary fracture 1, and the n-level fracture 8 is the secondary fracture 5. It should be emphasized that at this time, under the action of liquid nitrogen, the artificial main crack 1 will also slightly extend forward, and the secondary crack 5 and the multi-level crack 3 also occur simultaneously.

Finally, it is necessary to close the wellhead and let it stand. After the liquid nitrogen is completely converted into nitrogen, it will further promote fracture extension. After reaching the second predetermined time (about 7 days), the wellhead is opened to flow back nitrogen, which can avoid nitrogen pollution of oil and gas resources. It can also reduce the pressure in the fracture network and facilitate the flow of oil and gas resources in the fracture network. This can eventually form a series of interconnected complex seam networks.

After a series of interconnected fracture networks are formed in the oil shale reservoir, the oil shale reservoir needs to be exploited for the first time, so as to avoid the re-injected high and low temperature fluids from polluting the formed oil and gas resources. The pollution here includes not only the contamination of oil and gas resources by foreign substances such as nitrogen and high-temperature fluids, but also the influence of temperature on the composition of the oil and gas resources that have been generated, and the oil and gas will be removed when the fracturing material is re-injected with high-low temperature fluids after fracturing. Carrying out resources will cause the loss of oil and gas resources.

After the first exploitation of the oil shale reservoir, the above steps are repeated, and the oil shale reservoir is continuously fracturing with high temperature fluid, so that the artificial main fracture 1 continues to extend forward. Then stop the high temperature fluid fracturing the oil shale reservoir, make the high temperature fluid heat the oil shale reservoir, and after reaching the first predetermined time, the high temperature fluid is flowed back. Next, fracturing the oil shale reservoir with liquid nitrogen, so that new secondary fractures 5 are generated on both sides of the artificial main fracture 1 extending forward, and simultaneously the liquid nitrogen is gasified to generate nitrogen, thereby causing new secondary fractures Multi-level cracks 3 are generated on both sides of the crack 5 . Finally, the wellhead is closed, and after reaching the second predetermined time, the wellhead is opened to flow back nitrogen, so that the oil shale reservoir further forms a network of interconnected fractures. After the oil shale reservoir further forms an interconnected fracture network, the oil shale reservoir is produced for the second time. The use of intermittent mining can minimize the occurrence of the above-mentioned pollution. The intermittent mining here includes the first mining, the second mining, and the mining mode in the subsequent steps repeated if necessary.

Further, the above steps can be repeated again, and the oil shale can be produced in situ by alternate fracturing of high and low temperature fluids, which can continuously increase the stimulation range of the oil shale reservoir and continuously improve the recovery rate of the oil shale.

The method for in-situ production of oil shale using alternating fracturing of high and low temperature fluids as described above, wherein, after each oil and gas exploitation is completed, it is necessary to inject high temperature fluid for fracturing again, and the new artificial main fracture 1 will be in the existing artificial fracture. On the basis of the main fracture 1, it is extended again, so that the scope of reservoir stimulation can be enlarged. Then the liquid nitrogen is injected for fracturing again, and secondary fractures 5 and/or multi-level fractures 3 will be generated on both sides of the new artificial main fracture 1, which can increase the seepage channels of oil and gas resources.

In the method of the present invention for fracturing intermittently in situ oil shale by alternating high and low temperature fluids, there are two main reasons for first injecting high temperature fluid and then injecting liquid nitrogen for fracturing. First, injecting high-temperature fluid first can heat the oil shale reservoir, so that the oil shale in the fracturing area can reach a higher temperature, which can not only promote the transformation of kerogen in the oil shale into recoverable oil and gas resources, but also enable the oil shale to reach a higher temperature. A temperature difference that is very different from the liquid nitrogen temperature can be obtained, so that the thermal stress generated during the liquid nitrogen fracturing process can be increased, and more branched fractures can be generated.

Second, if liquid nitrogen is injected for fracturing first, many small fractures with random directions will be generated around the wellbore. When high-temperature fluid is injected later for fracturing, the fracturing fluid will first enter the liquid nitrogen fracturing to obtain small fractures and expand along the small fractures. , it is not easy to form a long artificial main fracture 1, resulting in a small fracturing area. Injecting high-temperature fluid for fracturing first can obtain long artificial main fractures 1 first, and then injecting liquid nitrogen can generate small fractures around artificial main fractures 1, which can not only increase the reformed area, but also improve the fracturing effect.

Moreover, the artificial main fracture 1 obtained by the method for intermittently producing oil shale in situ by alternating high and low temperature fluid fracturing of the present application has few branch fractures on both sides, and the length is easy to control, while the length of the artificial fracture obtained directly by liquid nitrogen fracturing is difficult to achieve Take control. This is because the high temperature fluid is used to fracturing the oil shale reservoir first, and the expansion of artificial fractures is mainly controlled by the three-dimensional stress of the formation. The length of artificial fractures is estimated by the liquid displacement, on-site construction pressure curve, and mechanical properties of formation rock. The artificial fractures obtained by direct fracturing with liquid nitrogen include the extension of the original main artificial fracture 1 and the generation of multi-level branched fractures. The cause of cracks is mainly the effect of thermal stress and fluid pressure.

In addition, based on the same inventive concept, the embodiments of the present invention also provide a device for intermittently fracturing and intermittently producing oil shale in situ with high and low temperature fluids, as described in the following embodiments. Because the principle of a device for intermittently producing oil shale by alternating high and low temperature fluid fracturing is similar to that of a method for intermittently producing oil shale in situ by alternating high and low temperature fluid fracturing. For the implementation of the device for in-situ production of oil shale, reference may be made to the implementation of the method for intermittent in-situ production of oil shale by alternating high and low temperature fluid fracturing, and the repetition will not be repeated. As used below, the term "unit" or "module" may be a combination of software and/or hardware that implements a predetermined function. Although the apparatus described in the following embodiments is preferably implemented in software, implementations in hardware, or a combination of software and hardware, are also possible and contemplated.

As shown in FIG. 2 , an embodiment of the present application also provides a device for intermittently fracturing and intermittently producing oil shale in situ with high and low temperature fluids. The device includes:

The first fracturing module 101 is configured to fract the oil shale reservoir with high temperature fluid, so that the oil shale reservoir generates artificial main fractures 1 of predetermined length.

The heating module 102 is configured to stop the high temperature fluid from fracturing the oil shale reservoir, to cause the high temperature fluid to heat the oil shale reservoir, and to flow back the high temperature fluid after a first predetermined time.

The second fracturing module 103 is configured to use liquid nitrogen to fract the oil shale reservoir, so as to generate secondary fractures 5 on both sides of the artificial main fracture 1, and simultaneously gasify the liquid nitrogen to generate nitrogen gas, and further Multi-level cracks 3 are generated on both sides of the secondary crack 5 .

The communication module 104, which is configured to close the wellhead, open the wellhead after a second predetermined time period, and flow back nitrogen gas to form a series of interconnected fracture networks in the oil shale reservoir.

In a preferred embodiment, the apparatus further includes a first production module configured to perform a first production run on the oil shale reservoir after the oil shale reservoir forms a series of interconnected fracture networks mining.

In a preferred embodiment, the device further comprises:

The third fracturing module is configured to continue fracturing the oil shale reservoir with high temperature fluid after the first production of the oil shale reservoir, so that the artificial main fracture 1 continues to develop toward the oil shale reservoir. front extension.

The continuing heating module is configured to stop the high temperature fluid fracturing the oil shale reservoir, and cause the high temperature fluid to heat the oil shale reservoir, and after a first predetermined time, flow back the high temperature fluid.

a fourth fracturing module, which is configured to use liquid nitrogen to fract the oil shale reservoir, so as to generate secondary fractures 5 on both sides of the forward-extending artificial main fracture 1, and simultaneously gasify the liquid nitrogen to generate nitrogen, Further, multi-level cracks 3 are generated on both sides of the secondary crack 5 .

The continuous communication module is configured to close the wellhead, and after reaching the second predetermined time, open the wellhead and flow back nitrogen gas, so that the oil shale reservoir further forms an interconnected fracture network.

The second production module is configured to perform a second production on the oil shale reservoir after the oil shale reservoir is further formed with an interconnected fracture network.

In a preferred embodiment, the predetermined length is based on the content of oil-producing substances in the oil shale reservoir.

In a preferred embodiment, the temperature of the liquid nitrogen is -196°C.

In a preferred embodiment, the temperature of the high temperature fluid is between 300-500°C.

In a preferred embodiment, the high temperature fluid is superheated steam.

In a preferred embodiment, the first predetermined time depends on the rate of kerogen pyrolysis in the oil shale reservoir.

In a preferred embodiment, the second predetermined period of time is 7 days.

In addition, an embodiment of the present invention also provides a device for intermittently fracturing oil shale in situ by alternating high and low temperature fluids, which includes a memory and a processor, and a computer program is stored in the memory, and the computer program is processed by the When the device is executed, the following steps are implemented: the method for intermittently producing oil shale in situ by alternating fracturing of high and low temperature fluids according to any one of the above.

In another embodiment, a software is also provided, and the software is used to execute the technical solutions described in the above embodiments and preferred embodiments.

In another embodiment, a storage medium is also provided, and the above-mentioned software is stored in the storage medium, and the storage medium includes but is not limited to: an optical disk, a floppy disk, a hard disk, a rewritable memory, and the like.

From the above description, it can be seen that the embodiments of the present invention achieve the following technical effects: the present invention adopts high-temperature fluid fracturing, which can not only generate long artificial fractures, but also heat the oil shale to promote the oil shale around the fractures. of kerogen into oil and gas resources. Using low-temperature fluid fracturing can generate secondary fractures and/or multi-level fractures on both sides of the formed fractures, providing necessary seepage channels for the exploitation of oil and gas resources. After each high and low temperature fluid fracturing is completed, the exploitation is carried out for a period of time, which can avoid the pollution of the formed oil and gas resources caused by the high and low temperature fluid injected again.

The method and device for in-situ production of oil shale by utilizing high and low temperature fluids alternately fracturing discontinuities provided by the present invention can not only realize the transformation of kerogen in oil shale into oil and gas resources, but also effectively improve the permeability of oil shale reservoirs , through the alternating fracturing of high and low temperature fluids, the stimulation range of oil shale reservoirs can be continuously increased, and the recovery rate can be continuously improved by intermittent mining, which provides a theoretical basis for realizing large-scale in-situ mining of oil shale.

Obviously, those skilled in the art should understand that each module or each step of the above-mentioned embodiments of the present invention may be implemented by a general-purpose computing device, and they may be centralized on a single computing device, or distributed in multiple computing devices. network, they can optionally be implemented with program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, and in some cases, can be different from the The illustrated or described steps are performed in order, either by fabricating them separately into individual integrated circuit modules, or by fabricating multiple modules or steps of them into a single integrated circuit module. As such, embodiments of the present invention are not limited to any particular combination of hardware and software.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, various modifications and changes may be made to the embodiments of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (8)

1. A method for alternately fracturing discontinuous in-situ extraction oil shale by high and low temperature fluids is characterized by comprising the following steps:

fracturing an oil shale reservoir by using high-temperature fluid to enable the oil shale reservoir to generate artificial main fractures with preset lengths;

stopping the high-temperature fluid from fracturing the oil shale reservoir, enabling the high-temperature fluid to heat the oil shale reservoir, and returning the high-temperature fluid after the first preset time is reached;

fracturing the oil shale reservoir by adopting liquid nitrogen to generate secondary cracks on two sides of the artificial main crack, and simultaneously gasifying the liquid nitrogen to generate nitrogen so as to generate multi-stage cracks on two sides of the secondary cracks;

closing the well mouth, opening the well mouth after the second preset time is reached, and back-discharging nitrogen so as to enable the oil shale reservoir to form a series of communicated seam nets; after the oil shale reservoir forms a series of interconnected seam nets, performing first mining on the oil shale reservoir;

after the oil shale reservoir is mined for the first time, continuously fracturing the oil shale reservoir by using high-temperature fluid so as to enable the artificial main fracture to continue to extend forwards;

stopping the high-temperature fluid from fracturing the oil shale reservoir, enabling the high-temperature fluid to heat the oil shale reservoir, and returning the high-temperature fluid after the first preset time is reached;

fracturing the oil shale reservoir by adopting liquid nitrogen to generate secondary cracks on two sides of the artificial main crack extending forwards, and simultaneously gasifying the liquid nitrogen to generate nitrogen so as to generate multi-stage cracks on two sides of the secondary cracks;

closing the well mouth, opening the well mouth after the second preset time is reached, and back-discharging nitrogen so as to further form a mutually communicated seam network in the oil shale reservoir;

after the oil shale reservoir further forms a communicated seam network, performing secondary exploitation on the oil shale reservoir;

the temperature of the high-temperature fluid is between 300 and 500 ℃.

2. The method of claim 1, wherein the predetermined length is determined by the amount of oleaginous matter in the oil shale reservoir.

3. The method of alternately fracturing interrupted in situ recovery oil shale with high and low temperature fluids according to claim 1, wherein the temperature of the liquid nitrogen is-196 ℃.

4. The method of alternately fracturing interrupted in situ recovery oil shale with high and low temperature fluids according to claim 1, wherein the high temperature fluid is superheated steam.

5. The method of alternately fracturing interrupted in situ mined shale with high and low temperature fluids according to claim 1, wherein the first predetermined time is dependent upon a rate of kerogen pyrolysis in the oil shale reservoir.

6. The method of alternately fracturing interrupted in situ recovery oil shale with high and low temperature fluids according to claim 1, wherein the second predetermined time is 7 days.

7. An apparatus for alternately fracturing an interrupted in situ mined oil shale by high and low temperature fluids using the method for alternately fracturing an interrupted in situ mined oil shale as claimed in any one of the preceding claims, comprising:

a first fracturing module configured to fracture an oil shale reservoir with a high temperature fluid to cause the oil shale reservoir to produce artificial primary fractures of a predetermined length;

a heating module configured to stop the high temperature fluid from fracturing the oil shale reservoir and cause the high temperature fluid to heat the oil shale reservoir, the high temperature fluid being flowback after reaching a first predetermined time;

the second fracturing module is configured to fracture the oil shale reservoir by adopting liquid nitrogen so as to generate secondary fractures on two sides of the artificial main fracture, and simultaneously gasify the liquid nitrogen to generate nitrogen gas so as to generate multi-stage fractures on two sides of the secondary fractures;

the communication module is configured to close the well mouth, open the well mouth after a second preset time is reached, and back discharge nitrogen so that the oil shale reservoir forms a series of communicated seam nets;

the temperature of the high-temperature fluid is between 300 and 500 ℃.

8. An apparatus for alternate high and low fluid fracturing of discontinuous in situ produced oil shale, comprising a memory and a processor, the memory having stored therein a computer program which when executed by the processor performs the steps of: a method of alternately fracturing intermittent in situ mined oil shale by high and low temperature fluids as claimed in any of claims 1 to 6.

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